Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session AA: Turbulent Boundary Layers: Experiments I
Chair: Ivan Marusic, University of MelbourneRoom: 101A
Sunday, November 22, 2009 8:00AM  8:13AM 
AA.00001: ICET  International Collaboration on Experiments in Turbulence: Coordinated Measurements in High Reynolds Number Turbulent Boundary Layers from Three Wind Tunnels H. Nagib, A. Smits, I. Marusic, P.H. Alfredsson Zero pressure gradient (ZPG) boundary layers are one of the canonical, wallbounded, turbulent flows that have been the focus of experimental and analytical investigations for several decades. Over the past few years, four groups have focused on systematic comparison between several measurement techniques and three facilities. Two closed return wind tunnels with ZPG boundary layers developed on a plate suspended near the midheight of the test section (at KTH and IIT), and an open return facility with a large and long test section and a boundary layer developing along its floor (at the University of Melbourne), are used for these coordinated efforts. The development length of the boundary layers and the freestream velocity in the three facilities range from 5.5 to 22 m, and from 10 to 60 m/s, respectively. Various arrangements for adjustable test section ceilings are employed to generate ZPG boundary layers over the range of momentum thickness Reynolds numbers from 11,000 to 70,000. Oil film interferometry (OFI) is employed to directly measure the wall shear stress, and various sizes of Pitot probes and types of hotwire sensors are used to measure wallnormal velocity profiles at different locations and freestream velocities. Mean velocity, turbulence statistics and integral parameters are examined. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AA.00002: Accurate and Independent Measurements of WallShear Stress in Turbulent Flows J.D. R\"uedi, R. Duncan, S. Imayama, K. Chauhan Oil Film Interferometry (OFI) is used to directly measure the wallshear stress in the high Reynolds number turbulent boundary layers from three facilities used for ICET. Various optical arrangements were utilized to collect the digital images generated on transparent plugs integrated into the boundary layer surface. Testsection free stream velocities ranging from 10 to 60 m/s and development lengths from 5.5m to 21 m, resulted in friction velocities varying from 0.35 to 1.65 m/s, corresponding to boundary layer thicknesses varying by factors of nearly four. Silicon oils with viscosities from 20 to 1000 cSt were employed in the measurements, with multiple oils used for several of the test conditions. A reference temperature measurement was used in all three facilities and for the calibration of the oils as a function of temperature in four different laboratories using two types of viscometers. The processing of the images was carried out using several approaches and compared for consistency of the results. Results of the skin friction coefficient from the three wind tunnels are examined and compared as a function of the displacement thickness Reynolds number, as determined from hotwire and Pitot probe profiles at comparable conditions, and are found to be accurately represented by the logarithmic Rotta relation. The various uncertainties and the final accuracy of this type of measurement are discussed. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AA.00003: Pitot probe corrections for measurements in turbulent boundary layers Jason Monty, Sean Bailey, Marcus Hultmark, Beverley McKeon Mean velocity measurements over a range of Reynolds number have been taken in zeropressuregradient, flat plate turbulent boundary layers using Pitot probes of varied diameter. Three worldclass boundary layer facilities were involved in this investigation, ensuring the results are not facilitydependent. Different methods of correcting Pitot probe data were compared to each other and to a concurrent study where hotwire measurements provided mean velocity data. It was found that there was very little difference in the commonly used shear corrections, although improvements could be made in the nearwall corrections and a modification to the correction is proposed. The applicability of a turbulence correction is investigated with the final, fully corrected pitot probe measurements compared with hotwire measurements, demonstrating excellent agreement overall between the two. This study confirms the accuracy of pitot probes for mean flow analysis in turbulent shear flows. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AA.00004: Challenges in Hot Wire Measurements in WallBounded Turbulent Flows R. Oerlue, N. Hutchins, T. Kurian, A. Talamelli Despite the rapid development of optical velocimetry methods (like LDV, PIV, etc.) the hotwire anemometer remains the main instrument used in wind tunnel studies of turbulence. To obtain precise results close to walls in turbulent boundary layers, requires the user to have accurate procedures for a good calibration at low velocities, knowledge of effects of blockage and heat conduction to the wall, and how spatial resolution influences the results. We have carried out measurements in three different wind tunnels (at KTH, Univ. Melbourne and IIT) with various hotwire probes (stubbed and stubless, as well as straight and boundary layer type) operated with commercially available and homemade anemometer systems. The use of different facilities enabled measurements at similar Reynolds numbers, but with different free stream velocities, resulting in a wide range of viscous scales for the hotwire sensor lengths. The results indicate that poor spatial resolution influences the measured fluctuating velocity distribution well into the overlap region and clarifies controversial aspects regarding the scaling of the nearwall peak and the apparent existence of an outer peak in the rms distribution. The mean velocity within the buffer region has been found to be affected by probe geometry and size, an influence that is especially important when correcting for the absolute wall position by means of common correction methods. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AA.00005: Mean Flow Measurements with Pitot Probes in High Reynolds Number Boundary Layers S. Bailey, M. Hultmark, R. Duncan, B. Mckeon Mean velocity profiles were measured in zeropressuregradient turbulent boundary layers using a variety of Pitot probes in three facilities at five different Reynolds numbers. The results were analyzed to verify the Reynolds number similarity between the two facilities and the scaling of the integral parameters was found to be consistent with the results from other studies. A linear relationship was found between $Re_{\theta}$ and $Re_{\tau}$ and used to determine a new friction factor relationship based on momentum thickness. Different methods used to determine the von K\'{a}rm\'{a}n constant were compared. When the friction velocity was determined using the new relationship, a logarithmic region between $y^+=300$ and $y/\delta=0.1$ was observed for $Re_{\theta}>15000$. A von K\'{a}rm\'{a}n constant of 0.40 was found for this region. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AA.00006: Mean Flow Measurements with Hot Wires in High Reynolds Number Boundary Layers R. Duncan, N. Hutchins, A. Segalini, J. Monty Mean flow measurements of high Reynolds number, Zero Pressure Gradient (ZPG) turbulent boundary layers are presented from the ICET data set using hot wire anemometry. The measurements were performed at momentum thickness Reynolds numbers in the range from 11,000 to 70,000, and compared to Pitot probe measurements at the same conditions. Various wire diameters, sensing lengths, probe designs and construction techniques are used, as well as different anemometer setups, in each of the facilities. Mean flow similarity between the three facilities is shown to be well within expected experimental uncertainty and ZPG layer manifestations, both when examining mean velocity profiles and integral parameters. The results reinforce the need for accurate near wall velocity and position measurements, as well as consistent analysis of physical and instrumentation biases. Various approaches are used to determine parameters such as the shape factor, the logarithmic overlapregion parameters, and the wake or outer flow parameters. Parameters extracted from the hotwire profiles and those based on Pitot probe data are also compared and discussed in light of past experience with both instruments in different wall bounded flow experiments. Finally, consistency of the results is examined between the profile data and the skin friction behavior with Reynolds number, as measured by oil film interferometry. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AA.00007: Turbulence Measurements with Hot Wires in High Reynolds Number Boundary Layers J.H.M. Fransson, N. Hutchins, R. Oerlue, M. Chong During the last decade there has been a renewed interest in the
scaling of turbulent boundary layers, especially with regard to
the mean and fluctuation velocity distributions. Recently the
ICET team carried out velocity measurements in three different
wind tunnels (at KTH, Univ. Melbourne and IIT) for overlapping
Reynolds numbers in the range $11,000 
Sunday, November 22, 2009 9:31AM  9:44AM 
AA.00008: Study of turbulent boundary layer structures using Tomographic PIV Qi Gao, Ellen Longmire, Cecilia OrtizDuenas TomographicPIV was applied to investigate vortical structures in the logarithmic region of turbulent boundary layers. Measurements were performed in a water channel facility with $\delta \approx $110 mm for Re$_{\tau}\approx $2400 and 2900. Laser sheets with thickness up to 7mm were aligned parallel to the bounding surface. Four cameras with 2k x 2k pixels were placed in a rectangular array facing the measurement volume with tilt angle $\sim $30\r{ }to the wall normal direction. Magnification was $\sim $0.05 mm/pixel. The resulting measurement volumes were 0.8$\delta $ x 0.8$\delta $ in the streamwise and spanwise directions and 0.065$\delta $ or 120 viscous units in the wallnormal direction. Correlations were performed on 64$^{3}$ voxel volumes with 75{\%} overlap yielding a vector spacing of 25$^{3}$ viscous units. The data were probed using swirl strength and direction as well as convection velocity to identify and characterize relatively large scale eddies and structures within the volumes. The results will be discussed and compared with results at similar wallnormal locations in lower Reynolds number DNS channel (Re$_{\tau }$=590, 934 of Moser et al., 1999 and del \'Alamo et al., 2004) and wind tunnel (Re$_{\tau}$=1160) flows. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AA.00009: Tomographic Particle Image Velocimetry Measurements of a High Reynolds Number Turbulent Boundary Layer Callum Atkinson, Michel Stanislas, Julio Soria Streamwise/wallparallel volumes in the buffer region of a turbulent boundary layer at Re$_{\theta }$ =7800 and 11800 are measured using a 4 camera (2048 $\times $ 2048 px) tomographic particle image velocimetry (TomoPIV) system in the turbulent boundary layer wind tunnel at the Laboratoire de M\'{e}canique de Lille (LML). Measurement volumes of 1200 $\times $ 180 $\times $ 1200 pixels are achieved, the large boundary layer provided by this tunnel ($\delta \sim $ 0.3 m) resulting in volumes of 470$^{+} \times $ 70$^{+} \times $ 470$^{+}$ and 920$^{+} \times $ 140$^{+} \times $ 920$^{+}$ wall units, respectively. The quality of the data acquired by this technique is assessed based on the mean velocity profile, velocity fluctuations, velocity power spectrum and the fluctuating divergence. Streaks and streamwise vortices are examined and an attempt is made to classify the flow using the invariants of the full velocity gradient tensor. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AA.00010: Flow Structures and Effects of Spatial Resolution on Turbulence Statistics in Rough Wall Turbulent Channel Flow Joseph Katz, Jiarong Hong, Michael Schultz PIV data obtained in the roughness sublayer of a turbulent channel flow is used for examining effects of spatial resolution on the magnitude, distribution, and trends of Reynolds stresses. Starting with a vector spacing of 63um (912 wall units), for roughness consisting of 0.45mm high pyramids at Re$_{\tau}$=34005418, spatial filtering of data causes major reduction in the magnitude of Reynolds stresses in the roughness sublayer. Although these reductions extend to well above the log layer, they increase with decreasing distance from the wall, especially for terms involving the wallnormal velocity fluctuation component, but also for the streamwise component. As expected, these effects increase with filter size, and are much higher for 2D filters in comparison to 1D ones. Consequently, trends of Reynolds stresses, and even mean flow profile vary significantly with filter properties. Spatial energy spectra and distributions of 2D swirling strength show the increasing role of small scale eddies on 2$^{nd}$ order statistics as the wall is approached, which is attenuated by filtering. [Preview Abstract] 
Session AB: Turbulent Shear Layers
Chair: Kenneth Christensen, University of IllinoisRoom: 101B
Sunday, November 22, 2009 8:00AM  8:13AM 
AB.00001: Evaluation of Similarity Theory for Weakly Swirling Jets using LDA Measurements Richard Semaan, Jonathan Naughton A similarity theory for weakly swirling jets developed by Ewing in 1999 is considered using recent experimental measurements. Momentum conserving jets with swirl numbers varying from 0 to 0.4 were measured using 3D Laser Doppler Anemometry. This range ensured that none of the jets exhibited vortex breakdown. High quality measurements of the mean velocities and Reynolds stresses allow for a comprehensive evaluation of the theory. Despite the rapid decay of the swirl velocity relative to the axial velocity, the jet behavior is shown to be consistent with Ewing's theory even in the limited axial region where the swirl velocity is still significant. This is demonstrated through use of the experimental data to consider the constraints that are required for and the scaling quantities that result from the development of the similarity theory. The similarity theory governing weakly swirling jets is shown to hold for a range swirl numbers and thus provides another canonical flow for validating models. This theory is particularly useful for validating models that attempt to capture curvature effects. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AB.00002: Coherent structures in uniformly sheared turbulence Christina Vanderwel, Stavros Tavoularis Uniformly sheared flow with a turbulence Reynolds number $Re_{\lambda}$ in the range from 100 to 140 has been generated in a water tunnel and its instantaneous structure has been examined using flow visualization, LDV and PIV. The flow was found to consist of regions with nearly uniform velocity and separated by relatively strong shear layers containing large vortices. The concentration of vortices and the probability distribution functions of their directions of rotation, strengths, sizes and shapes in three characteristic planes of the flow have been determined. These results, as well as highspeed scans of injected dye patterns, show that horseshoeshaped vortices are prevalent in this flow. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AB.00003: The Low Speed Intermittent Region of a Single Stream Shear Layer (SSSL) John Foss, Jason Peabody External intermittency: intermittent presence of vortical fluid, exists on the high and the low speed sides of a SSSL. From Corrsin and Kistler (1954) it is understood that the vortical fluid is bounded by a viscous superlayer (VSL) that is welldefined given zero vorticity in the entrained fluid. That condition is met for the SSSL flow of Morris and Foss (2003). The smoke trace from an incense stick provides a convenient technique to mark the 3D locations of the VSL. Specifically, the vorticity at the VSL ``shreds'' the end of the trace; the shredding is captured by two cameras with calibrated xy=f(z) and yz=g(x) image planes. Complementary transverse vorticity measurements were acquired in the spatial domain indicated by the ``shredding.'' Interpreted results will be presented. Corrsin, S. and Kistler, A.L. (1954) ``The Free Stream Boundaries of Turbulent Flows,'' NACA TN 3133. Morris, S.C. and Foss, J.F. (2003) ``Turbulent boundary layer to singlestream shear layer: the transition region'', Jour. Fluid Mechanics, 494, pp. 187221. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AB.00004: An Experimental Study of Circular and Rectangular Heated Jets in CrossFlow B.E. Johnson, G. Elliott, K.T. Christensen Heated plumes of fluid emanating from both flush and raised stacks occur in a wide variety of technologicallyrelevant applications and the dispersion of this heated, and sometimes contaminated, fluid is therefore of practical concern. To this end, a detailed series of experiments are under way to document the downstream development of heated jets of both circular and rectangular crosssection emanating into a crossflow with the intent of constructing an experimental database for validation of ongoing largeeddy simulations. Multiple crossstream velocities as well as various jet exit velocities and temperatures are under study to understand the plume development both in the near and farfield. The jet flow is driven by a highpressure regenerative blower and heated with an opencoil heater while the crossflow is provided by an open circuit wind tunnel. Mean temperature fields and streamwise velocity fields are measured using a rake of Pitotstatic probes and thermocouples at select locations downstream of the jet. Of particular interest, the impact of jet exit temperature, velocity and crosssection on the downstream development of the heated plume is explored. Similarities and differences between flush and raised stack configurations under identical experimental conditions will also be highlighted. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AB.00005: Open Loop Forced Experimental Investigation of Optical Beam Propagation through a Free Shear Layer Casey Fagley, Stefan Siegel, Jurgen Seidel, Thomas McLaughlin The performance of airborne platforms emitting or receiving light beams is severely hampered by the flow field around the turret mounted on the air vehicle. From a fluid dynamics point of view, the flow separating from the turret develops large, coherent structures. From an optical point of view, these structures due to their associated density variations, cause large optical distortions since the index of refraction is a function of density. The goal of this research is to reduce optical distortions by mitigating these structures using feedback flow control. A blowing and suction slot along the top of the backwards facing step allows for actuation of the flow field. A study varying open loop forcing frequency and amplitude of the actuation signal shows the influence on the natural shedding frequency responsible for the large coherent structures. Initial findings support that forcing with more than twice the natural frequency does not produce structures that increase the optical path difference (OPD), measured by a Malley probe, while hot film measurements show that structures at the forcing frequency are present. The differences between these OPD and hot film measurements will be demonstrated and supported with simulation results. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AB.00006: Spectral analysis of ``homogenized'' PIV data of axisymmetric jet turbulence Maja W\"{a}nstr\"{o}m, William K. George, Knud Erik Meyer This paper concerns the estimation of onedimensional spatial spectra from a set of 10,580 twocomponent velocity fields in a plane cut along the centerline axis of a turbulent air jet. The jet nozzle diameter $d =1cm$ and the fieldofview extends from $30d$ to $100d$ and covers at least four jet halfwidths. The Reynolds number based on nozzle diameter is $Re = 20, 000$. Following [1], PIV data of the growing flow was mapped and resampled into streamwise homogeneous similarity coordinates, $\eta =r/\delta_{1/2}(x)$ and $\xi = \ln[(xx_o)/D]$. Inevitable spatial variation in attainable dynamic range was overcome by the extensive number of statistically independent samples. Studies of the effects of window length and type, filtering, sampling noise and data resampling on the spectral energy distribution are performed and discussed. Finally, the spectral estimates are compared to the temporal estimates of [2], which are obtained by hotwire using Taylor's Hypothesis.\\[4pt] [1] Ewing, D. et al.\ J. Fluid Mech., 577, 309330, 2007.\\[0pt] [2] Frohnapfel, B. MultiPoint Similarity of the Axisymmetric Turbulent Far Jet and Its Implication for the POD. Master Thesis, U. Erlangen 2003. [Preview Abstract] 

AB.00007: ABSTRACT WITHDRAWN 
Sunday, November 22, 2009 9:31AM  9:44AM 
AB.00008: Measurements in a High Reynolds Number Wake Juan Jimenez, Marcus Hultmark, Alexander Smits Experiments were conducted in the Princeton/ONR HRTF windtunnel with air pressurized up to 220atm. The wake of a DARPA SUBOFF submarine model was measured at 5 different downstream locations for Reynolds numbers from $1\times 10^6$ to $70\times 10^6$. For all Reynolds numbers studied, the mean velocity distribution is selfsimilar from 3 diameters, $D$, downstream for the side where the support is not located. In contrast, selfsimilarity in the Reynolds stresses is not reached at the furthest downstream location ($x/D=15$). The nondimensional fluctuations are Reynolds number dependent for all measured Reynolds numbers. The energy spectra reveal two peaks in the nearwake. The lower wavenumber peak corresponds to a Strouhal number based on diameter and freestream velocity of about 0.2, suggesting that it is associated with an azimuthal or helical shedding mode in the wake. The peak decays with downstream distance, suggesting that this mode might play a parital role in the approach to selfsimilarity of the turbulent stresses [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AB.00009: Flow Field of ThreeDimensional Turbulent Wall Jets Mark Tachie, Martin Agelinchaab A wall jet is formed when a jet of fluid is directed tangentially along a wall. Wall jets can be two or threedimensional. Threedimensional wall jets (3DWJs) are complex flows whose structures are still not well understood despite the extensive studies on this subject. For example, the mechanism responsible for their more rapid lateral spread rate than in the wallnormal spread rate is not well understood. Velocity measurements of 3DWJs were conducted using particle image velocimetry. The 3DWJs were formed by jets exiting a $d$ = 7 mm inside diameter circular pipe (143$d$ in length) placed to flush the test section floor. The Reynolds numbers based on the jet exit velocities and jet exit diameters were 5000, 10000 and 20000. The detailed flow fields of the 3DWJs were examined in terms of mean velocities and onepoint turbulence statistics. In view of the wide range of length and temporal scales that are present in turbulent flows, multipoint turbulence statistics such as twopoint velocity correlations and proper orthogonal decomposition are used to document the salient features of the 3DWJs. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AB.00010: Structure of threedimensional turbulent offset jets with small offset distances Martin Agelinchaab, Mark Tachie An offset jet is a jet that discharges into a medium above a wall which is offset by a certain distance. The ``Coanda effect'' forces the offset jet to deflect towards the wall and eventually attaches itself to the wall. The only detailed study of threedimensional offset jets (3DOJs) did not report the flow field in the region from the jet exit to the point where the jet attaches itself to the wall. In this region flow reversal is expected. Velocity measurements of 3DOJs were conducted using particle image velocimetry. The 3DOJs have different jet exit offset distances ($h)$ normalized by the jet exit diameter ($d)$ of $h$/$d$ = 0.5 to 4. The Reynolds numbers based on the jet exit velocities and jet exit diameters were 5000, 10000 and 20000. The detailed flow fields of the 3DOJs were examined in terms of mean velocities, and onepoint turbulence statistics. In view of the wide range of length and temporal scales that are present in turbulent flows, multipoint turbulence statistics such as twopoint velocity correlations and proper orthogonal decomposition are used to document the salient features of 3DOJs. [Preview Abstract] 
Session AC: Turbulence Simulations I
Chair: Yves Dubief, University of VermontRoom: 101C
Sunday, November 22, 2009 8:00AM  8:13AM 
AC.00001: Largeeddy simulation of the flow over twodimensional dunes M. Omidyeganeh, U. Piomelli, A.M. da Silva When a fluid flows over a mobile sand bed the sediment transport generated by the interaction of the flow field with the bed often results in the periodic deformation of the bed in the form of dunes. Dunes generally reach an equilibrium shape, and slowly propagate in the direction of the flow, as sand is lifted in the highshear regions, and redeposited in the separatedflow area. Our aim is to attempt to connect the flowfield characteristics with the bed deformation. As a first step, we perform large eddy simulation of the flow over a typical dune geometry at laboratory scale (the Reynolds number based on the average channel height and mean velocity is 18,900). We consider three dunes, with different heights (relative to the channel depth) but equal wavelengths, using approximately ten million grid points. The mean flow shows a recirculation region downstream of the dune crest, whose extent increases with dune height. After reattachment the shear stress becomes high, confirming that sediment is lifted up in this region. The Reynolds stresses are higher in the shear layer, where the high spanwise vorticity gives rise to coherent vortices. The budgets of turbulent kinetic energy show that, in addition to production and dissipation, the diffusion terms play an important role. In the reattachment region, diffusion and dissipation are more significant. The mean flow advection is important at the beginning of the shear layer. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AC.00002: Large Eddy Simulation study of the fully developed windturbine array boundary layer Charles Meneveau, Marc Calaf, Johan Meyers When wind turbines are deployed in large arrays, their efficiency decreases due to complex interactions among themselves and with the atmospheric boundary layer (ABL). For wind farms whose length far exceeds the height of the ABL, a fully developed flow regime can be established. Such a fully developed windturbine array boundary layer may be studied computationally using periodic boundary conditions in the horizontal direction. A suite of Large Eddy Simulations in which wind turbines are modeled using the classic ``drag disk'' concept are performed, in order to quantify the vertical transport of momentum and kinetic energy across the boundary layer. LES for various wind turbine arrangements, loading factors, and surface roughness are performed. Horizontally averaged statistics are documented. Results are compared with models for effective roughness length scales experienced by the ABL. This scale is often used to parameterize wind turbine arrays in models of atmospheric flow at regional or global scales. Based on the observed trends, a modified model is proposed showing improvements in the predicted effective roughness heights. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AC.00003: Coherent structures in a turbulent flow over an urban canopy made of cubical obstacles Stefano Leonardi, Ian Castro The study of turbulent heat or mass transport is of special interest in engineering, especially for heat exchangers. For instance, roughness elements (turbulators) are usually placed on the walls of the internal channels of a turbine blade to enhance the heat transfer. In the present paper, DNSs are carried out for passive heat transport in a turbulent channel flow with $\Lambda$ shape square ribs for w/k = 1, 3, 7, 15 (w being the pitch, k the height of the ribs turbulators. The angle of inclination of the lambda shape turbulators is 45 degrees. Numerical results show that $\Lambda$ shape square ribs are more efficient than square ribs in maximizing the heat transfer. The configuration with w/k=3 presents the largest heat flux. The increase in the heat transfer is due to a secondary motion which is generated by the $\Lambda$ shape turbulators. Two counter rotating vortices above the square ribs transport the heat out of the wall into the center of the channel. The distribution of the heat flux coefficient is not uniform in the channel and leads to temperature gradients at the wall. The total drag of the $\Lambda$ shape turbulators is larger than that over a smooth wall due to an increase of form drag. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AC.00004: Direct numerical simulation of turbulent flow over a surface mounted obstacle Nikolaos Malamataris The direct numerical simulation of turbulent flow over a surface mounted obstacle is studied as a numerical experiment that takes place in a wind tunnel. For this reason, the incompressible, three dimensional, transient NavierStokes equations for Newtonian fluids are solved directly using Galerkin finite elements. The Reynolds number defined with respect to the height of the obstacle is in the range of $10^5$. The results include instantaneous streamline patterns that show the vortex shedding phenomenon and the flapping of the recirculation bubble downstream the obstacle. Energy spectra are studied along with Eulerian autocorrelation coefficients, longitudinal and lateral coefficients that yield the chaotic behavior of turbulence. The computer code developed for this work is a parallel program written in Fortran 90 that uses the MPIparadigm and runs in distributed memory systems. Movies are shown where both streaklines and instantaneous streamlines are depicted that clearly demonstrate the transient characteristics of this prototype separated flow. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AC.00005: Largeeddy simulation of flow over a multielement airfoil Donghyun You An accurate prediction of turbulent flow over a multiple element highlift airfoil configuration remains a challenge to computational fluid dynamics. Maximum lift, drag, and pitching moment are difficult to accurately predict especially in the presence of flow separation on one or more of the airfoil elements. In this study, we investigate turbulent flow over a MD30P30N highlift configuration using largeeddy simulation. The MD30P30N configuration consists of three elements: a slat, a main airfoil, and a flap. Four different attack angles, 16$^\circ$, 19$^\circ$, 21$^\circ$, and 24$^\circ$, are considered while deflection angles of the slat and flap are fixed to 30$^\circ$. The Reynolds number is $9\times 10^6$ based on the mountedwing chordlength and freestream velocity. Simulation results obtained on a 54 millionelement mesh agree well with experimental data in terms of pressure distribution, velocity profiles, and transition location. A grid sensitivity study is performed to identify the resolution effects on the prediction of flow transition, wakes, and turbulent boundary layers. Accurate prediction of laminartoturbulence transition on the slat surface and downstream evolution of the slat wake is found to be crucial for the global accuracy of the simulation. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AC.00006: Numerical Simulation of TurbulenceInduced Bedform Initiation YiJu Chou, Oliver Fringer Bedform initiation induced by nearwall turbulence structures on a sand bed is studied using largeeddy simulation. Due to the dilute sediment concentration during the simulation, transport of sediment is modeled with the Eulerian method. A secondorder accurate arbitrary LagrangianEulerian scheme is implemented that allows flow simulation over evolving bedforms. With a bed elevation model based on conservation of sediment mass to calculate changes in bed elevation, the present numerical model enables detailed observation of bedform instability caused by nearwall turbulence. It is found that the streak structure on the bed surface appears as the initial bed perturbation due to sediment erosion by turbulent sweeps, which in turn induce small pileup at the downstream end of the inrush zone where the sweep diminishes. The continuous growth of the small sediment pileup leads to the formation of bed defects, which alter the flow condition and the spatial distribution of nearbed sediment erosion. As a consequence, merging of multiple bed defects leads to the formation of ripple marks, which are a common bedform pattern in the subaqueous environment. The simulation results reveal interactions between turbulent structures and the sand bed and demonstrate the importance of the resolved turbulence in the simulation of bedform initiation. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AC.00007: Direct numerical simulation of surface ablation by turbulent convection Ryan Crocker, Yves Dubief, Christopher White Rapid erosion by a turbulent flow creates complex flow/surface phenomena arising from the evolving surface topography and its interaction with a turbulent flow that transports the erosive agent onto the surface. The nonequilibrium nature of the problem poses major challenges to current turbulent models and boundary conditions used in direct numerical simulation (DNS) algorithms. A generalized algorithm for turbulent erosion processes based on levelset and immersed boundary methods has been developed in a DNS flow solver to investigate the action of various erosive agents (heat, particles, chemical species) on erodible surfaces. This algorithm is applied to the ablation of a slab of ice by natural and forced convection of water. The study focuses on the characterization of the surface topography in relation to the evolution of coherent structures in the flow, as ablation proceeds. [Preview Abstract] 

AC.00008: ABSTRACT WITHDRAWN 

AC.00009: ABSTRACT WITHDRAWN 
Sunday, November 22, 2009 9:57AM  10:10AM 
AC.00010: Comparing DNS and Experiments of Subcritical Flow Past an Isolated Surface Roughness Element Charles Doolittle, David Goldstein Results are presented from computational and experimental studies of subcritical roughness within a Blasius boundary layer. This work stems from discrepancies presented by Stephani and Goldstein (AIAA Paper 2009585) where DNS results did not agree with hotwire measurements. The near wake regions of cylindrical surface roughness elements corresponding to roughnessbased Reynolds numbers \textit{Re}$_{k}$ of about 202 are of specific concern. LaserDoppler anemometry and flow visualization in water, as well as the same spectral DNS code used by Stephani and Goldstein are used to obtain both quantitative and qualitative comparisons with previous results. Conclusions regarding previous studies will be presented alongside discussion of current work including grid resolution studies and an examination of vorticity dynamics. [Preview Abstract] 
Session AD: Flow Control I
Chair: Luciano Castillo, Rensselaer Polytechnic InstituteRoom: 101D
Sunday, November 22, 2009 8:00AM  8:13AM 
AD.00001: Shock Wave/Boundary Layer Interaction Control in a Supersonic Inlet Nathan Webb, Edgar Caraballo, Jesse Little, JinHwa Kim, Mo Samimy A shock wave/boundary layer interaction (SWBLI) occurs in a supersonic mixed compression inlet. The SWBLI could cause boundary layer separation resulting in adverse consequences such as reduced pressure recovery and nonuniform fan loading. Boundary layer bleed is currently used to prevent separation, which incurs a significant performance penalty. We have recently used Localized Arc Filament Plasma Actuators (LAFPAs) with high amplitude and wide bandwidth to control the SWBLI in a Mach 1.9 flow. The preliminary results are promising and show excellent potential for this technique. These actuators may affect the SWBLI by two mechanisms: manipulation of flow instabilities and/or streamwise vorticity generation. Particle image velocimetry measurements have confirmed that instability manipulation is the key to the LAFPAs' ability to significantly energize the boundary layer in the interaction region. The streamwise vorticity effects are currently being investigated. The LAFPAs have been tested at varying frequency, duty cycle, and mode to determine the parameters with the maximum effectiveness. Supported by AFRL and AFOSR. [Preview Abstract] 

AD.00002: ABSTRACT WITHDRAWN 
Sunday, November 22, 2009 8:26AM  8:39AM 
AD.00003: Flow Mechanisms Leading to Separation Control over Embedded Hexagonal Cavities Amy Lang, Blake Melnick Digital Particle Image Velocimetry was used to measure the flow over an array of hexagonal cavities, with a focus on discerning the effect of cavity orientation on the adjacent boundary layer flow. Timeaveraged velocity profiles above the cavities were measured under transitioning and turbulent boundary layer conditions. Two flow mechanisms leading to separation control, produced by the presence of the embedded cavities, were considered and will be discussed: (1) the presence of a partial slip velocity, produced by the embedded vortices forming within the cavities, on the adjacent boundary layer flow; and (2) turbulence augmentation close to the surface leading to a greater momentum exchange with the higher momentum, outer boundary layer region. The Reynolds stresses over the hexagonal cavities were thus compared to those over the flat plate under turbulent conditions to attempt to discern the effect of cavity orientation on turbulence augmentation. Results will discuss how these flow mechanisms lead to higher momentum in the boundary layer close to the wall as compared to a flat plate. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AD.00004: Flow mechanisms induced by 2D transverse cavities leading to separation control Drew Smith, Amy Lang, Leah Mendelson Cavities embedded in the surface of an object moving through a fluid can help delay flow separation by imposing partial slip velocities and augmenting the turbulence of the boundary layer. Furthermore, recent experiments have shown that embedded cavity surface geometries may have their greatest effect on separation control at the point where the flow initially encounters them. This study investigated whether this effect is observed on a model with twodimensional, transverse grooves embedded in the surface. The embedded cavities were mounted as a fullspan section within a flat plate model in a lowspeed water tunnel. Digital Particle Image Velocimetry was used to obtain flow data which were compared to that obtained over a flat surface. An analysis of the boundary layer profiles and Reynolds stresses at multiple locations on the model was conducted. Special attention was paid to the changes in these characteristics with streamwise distance. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AD.00005: Long Lasting Modifications to Karman Vortex by a Single Pulse of DBD Plamsa KwingSo Choi, Tim Jukes We discovered a unique phenomenon whereby the vortex shedding and force fluctuations on a circular cylinder in cross flow are halted for a considerably long duration by applying a single, shortduration pulse of DBD plasma close to flow separation points. This period of flow modifications is equivalent to over 150 times that of the plasma excitation. We believe this is due to the induced vortex by a shortduration plasma pulse that interacts with the Karman vortex formation. As a result, the drag and lift fluctuations are reduced by 8{\%} and 40{\%}, respectively. This corresponds to the powersaving ratio of nearly 1200, or the energy efficiency of more than 50{\%}. Experiments were conducted in a wind tunnel, where the freestream velocity was 4.6 m/s and the turbulence intensity was 0.5{\%}. The Reynolds number based on the diameter of circular cylinder was 15000. A single asymmetric DBD plasma actuator was placed on a circular cylinder at 75$^{\circ}$, where a pulse of DBD plasma with a short duration (5{\%} of the vortex shedding period) was applied at a high ac voltage (7.4 kV peaktopeak, 33 kHz). Simultaneously with the force measurements using a twocomponent dynamic force balance, the global flow field in the near wake of a circular cylinder was studied using a timeresolved PIV system. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AD.00006: Scalnig of transient lift response to actuation in a 3D separated flow Tim Colonius, David Williams, Gilead Tadmor, Wes Kerstens, Vien Quach, Seth Buntain The transient lift response of a separated flow to short duration (pulsed) blowing is studied on a low Reynolds number, semicircularplanform, flatplate wing. Actuators were distributed along the leading edge of the wing. The pulse duration, amplitude (supply pressure), and freestream speed were varied in the experiments. We identify two nondimensional parameters governing the response, and use the data to find functional forms for the lift coefficient increment. We show that the lift coefficient increment is nearly independent of the pulse duration and increases (solely) with the square root of the supplypressure coefficient up to a saturation. We also find that the shape of the lift response curve is similar to that produced in other experiments with different airfoils and actuators. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AD.00007: Optimized Control of Vortex Shedding from an Inclined Flat Plate Won Tae Joe, Tim Colonius, Doug MacMynowski Optimal control theory is combined with the numerical simulation of an incompressible viscous flow to control vortex shedding in order to maximize lift. A twodimensional flat plate model is considered at a high angle of attack and a Reynolds number of 300. Actuation is provided by unsteady mass injection near the trailing edge and is modeled by a compact body force. The adjoint of the linearized perturbed equations is solved backwards in time to obtain the gradient of the lift to changes in actuation (the jet velocity), and this information is used to iteratively improve the controls. We investigate how features of the optimized waveform modify the vortex shedding and lead to higher lift, and compare the results with sinusoidal control. In order to obtain a practically implementable control scheme, the optimized waveform is also implemented in a simple closedloop controller where the control signal is shifted or deformed periodically to adjust to the (instantaneous) frequency of the lift fluctuations. The feedback utilizes a narrowband filter and an Extended Kalman Filter to robustly estimate the phase of vortex shedding and achieve phaselocked, high lift flow states. Finally, the sensitivity of the flow to the phase shift and other features of the optimized waveform are presented. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AD.00008: Separation Control in a 3D Diffuser using Plasma Actuators Sven Grundmann, John K. Eaton Control experiments were conducted for the fullyturbulent flow in a 3D diffuser with an expansion ratio of 4.8. The uncontrolled flow for the same diffuser has a stable, threedimensional separation zone which begins as a slender bubble in one corner before spreading across the entire width of the diffuser, giving the opportunity to develop and test active separation control devices. Dielectricbarrier discharge actuators were used to actively control the flow separation with the goal of improving the pressure recovery. The most effective control was achieved using spanwise acting plasma actuators in the inlet section of the diffuser which create streamwise vortices. The pressure recovery can be clearly improved or degraded depending on whether the actuators are operated pulsed or continuously. Parameter studies showed the dependence of the pressure recovery along the diffuser wall on the actuator operating parameters, including the modulation frequency and duty cycle. Velocity profile measurements in the inlet and outlet planes of the diffuser show the creation of the streamwise vortices and their influence on the uniformity of the velocity in the end of the diffuser. Frequency spectra taken in the exit plane using a hotwire probe show the influence of the operating parameters on the diffuser flow. A closedloop control circuit for the automated adaption of the operating parameters is being tested. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AD.00009: Separation Control on a Cascade of Airfoils using Pulsed Vortex Generator Jets R.J. Volino, M.B. Ibrahim, O. Kartuzova Flow through a row of airfoils will separate if the loading (lift) on each airfoil is too high. This can happen if the airfoil turning angle or the spacing between airfoils is too high. If the boundary layers separate, the actual flow turning and lift drop, and aerodynamic losses increase. In applications, such as the flow through turbines, high lift airfoils are desirable, as the same power generation can be achieved using fewer airfoils, thereby saving weight and cost. Advances in understanding of separation and transition have led to high lift airfoils without separation problems, but further increases in loading will likely require flow control. In the present study, flow through a linear cascade of very high lift low pressure turbine airfoils is controlled using pulsed vortex generator jets. Without flow control there is a large unclosed separation bubble at low Reynolds numbers. Separation causes a 20\% drop in lift and increases losses by up to a factor of seven. Transition of the separated shear layer does not guarantee reattachment. Vortex generator jets with very low mass flow successfully control the separation if the jet velocity and pulsing frequency are sufficiently high. Experimental pressure distributions and phase averaged velocity and turbulence results will be presented. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AD.00010: Flow Separation Control over a Highlift Airfoil using Multiple DBD Plasma Actuators Jesse Little, Munetake Nishihara, Igor Adamovich, Mo Samimy This work continues an ongoing experimental study on the efficacy of plasma actuators for controlling flow separation on a highlift airfoil. Previous results showed that a single dielectric barrier discharge (DBD) plasma actuator at the shoulder of a simple trailing edge flap can be effective for enhancing lift by increasing momentum transport between the freestream and separated region through amplification of largescale structures at low frequencies. This work examines the ability of multiple actuators to further generate and amplify flow structures using pulsed actuation with variable phase. Multiple actuators are also operated at high frequency in an effort to reattach the separating boundary layer with quasisteady plasma induced flow. Results show that low frequency pulsed forcing requires less power input and generates greater lift increases than high frequency actuation, but has a penalty of increased fluctuating pressure loads on the flap. These studies constitute necessary steps in the development and implementation of plasma actuators for control of flow separation at velocities associated with takeoff and landing applications in transport aircraft. [Preview Abstract] 
Session AE: Biofluids I: General I  Biological Systems
Chair: Silas Alben, Georgia Institute of TechnologyRoom: 101E
Sunday, November 22, 2009 8:00AM  8:13AM 
AE.00001: Fluid Friction and Fungal Spore Ejection Joerg Fritz, Agnese Seminara, Marcus Roper, Anne Pringle, Michael Brenner A wide range of fungal species in the phylum Ascomycota uses the forcible ejection of microscopic spores to disperse and to cover new territory, triggered by the breakdown of osmolytes in the sack containing the spores (the ascus). The spores experience very high aerodynamic drag due to their small size and need to attain high velocities to leave the boundary layer of still air around the fruiting body. Here we address the efficiency of conversion of osmotic pressure to the kinetic energy of the spore, and in particular its dependence on the design of the ascus and the hole (the socalled apical ring) from where the spores leave the ascus. We present a fluid mechanical model of the ejection process, which predicts that the hole the apical ring should have specific properties, in order to minimize frictional and pressure losses and maximize the ejection velocity. We compare these predictions to measurements of apical ring properties across the phylum. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AE.00002: Dynamics of a nuclear invasion Marcus Roper, Anna Simonin, N. Louise Glass Filamentous fungi grow as a network of continuous interconnected tubes, containing nuclei that move freely through a shared cytoplasm. Wild fungi are frequently chimerical: two nuclei from the same physiological individual may be genetically different. Such internal diversity can arise either from spontaneous mutations during nuclear division, or by nuclear exchange when two individuals fuse, sharing their resources and organelles to become a single individual. This diversity is thought to be essential to adaptation in plant pathogens, allowing, for instance, an invading fungus to present many different genetic identities against its host's immune response. However, it is clear that the presence of multiple genetic lineages within the same physiological individual can also pose challenges  lineages that are present in growing hyphal tips will multiply preferentially. Nuclei must therefore be kept well mixed across a growing front. By applying models developed to describe mixing of fluids in microfluidic reactors to experimental observations of lineage mixing in a growing \emph{Neurospora crassa} colony, we show how this mixing is achieved. In particular we analyze the individual contributions from interdigitation of hyphae and from nuclear transport. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AE.00003: A model for wetting and evaporation of a postblink precorneal tear film Daniel Anderson, Katlyn Winter, Richard Braun We examine a onedimensional hydrodynamic model derived using lubrication theory for the evolution of a postblink precorneal tear film that includes evaporation of the aqueous layer and a wetting corneal surface. The evaporation model includes the effects of conjoining pressure and predicts the existence of an equilibrium adsorbed fluid layer that serves as a model for a wetting corneal surface/mucin layer. The dewetting rates predicted by the model are in qualitative agreement with experimental measurements. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AE.00004: The ant raft Nathan Mlot, David Hu, Solomon Equabai To survive floods, fire ants link their arms together to assemble a raft with their own bodies. Because ants are nearly as dense as water, this cooperative behavior requires that a portion of the ant colony must sacrifice itself by remaining underwater to support the colony's weight. Surprisingly, few ants drown during this process due to a striking metamorphosis of the raft: as we show using timelapse photography, the raft morphs from a spherical to a pancake shape. This pancake configurationa monolayer of floating ants supporting their dry counterpartsallows all ants to both breathe and remain united as a colony. Data is presented in the form of the dimensions and the rates of formation of the ant raft. We use the statics of small floating bodies to account for the equilibrium raft size as a function of the initial mass and density of the ants. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AE.00005: Flight of the smallest insects Laura Miller, Arvind Santhanakrishnan, Tyson Hedrick, Alice Robinson A vast body of research has described the complexity of flight in insects ranging from the fruit fly, \textit{Drosophila melanogaster}, to the hawk moth, \textit{Manduca sexta}. Over this range of scales, flight aerodynamics as well as the relative lift and drag forces generated are surprisingly similar. The smallest flying insects (Re$\sim $10) have received far less attention, although previous work has shown that flight kinematics and aerodynamics can be significantly different. In this presentation, we have used a threepronged approach that consists of measurements of flight kinematics in the tiny insect \textit{Thysanoptera} (thrips), measurements of flow velocities using physical models, and direct numerical simulations to compute lift and drag forces. We find that drag forces can be an order of magnitude larger than lift forces, particularly during the clap and fling motion used by all tiny insects recorded to date. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AE.00006: Legged locomotion on sand Chen Li, Paul Umbanhowar, Haldun Komsuoglu, Daniel Koditschek, Daniel Goldman To understand how and why animals modulate foot kinematics to achieve effective locomotion on granular media, we study the speed of a sixlegged robot with cshaped legs, SandBot, moving on granular media for varying volume fraction, $\phi$, limb frequency, $f$, and gait timing parameters\footnote{Li et. al, PNAS, \textbf{106}, 3029, 2009}. Speed is determined by step length which in turn depends on limb penetration. At low $f$ and high $\phi$ penetration is small, step length is large, and SandBot advances with a rotary walking gait in which clegs rotate about their centers by slipping relative to stationary grains. In the opposite extreme, grains cannot support the robot; its underside always contacts the ground and it advances slowly via thrust generated as the clegs translate through the grains. For varied gait parameters, high speeds are only observed in a small area of parameter space. A yield stress based model predicts the speed and reveals that performance is maximized when gait parameters minimize limb acceleration and interference, and limbs utilize the solidification properties of the media. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AE.00007: Aquatic mapping using hydrodynamic pressure sensing Roland Bouffanais, Dick K.P. Yue Pressure sensing is instrumental to most animals and organisms living in an aquatic environment: for instance fish at human scale through their lateral line and amoeba at microscale through mechanodetection at their surface. It also represents for underwater vehicles an alternative way of sensing the fluid environment when visual and acoustic sensing are limited. To assess the effectiveness of hydrodynamic sensing we propose a framework applicable to both high and lowReynolds number flows corresponding to typical fluid environment encountered by macro and microswimmers respectively. In this framework both the forward and inverse problem corresponding to the object shape detection are presented. The forward mapping relies on a general solution of the pressure field expanded as an infinite series. The detection problem corresponds to the inverse problem which consists in determining some of the necessary coefficient of the expansion based on a noisy pressure signal over the limited length of the mechanosensing device. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AE.00008: The hummingbird's tongue: a selfassembling syphon John Bush, Francois Peaudecerf, David Quere We present the results of a combined experimental and theoretical investigation of the drinking technique of the hummingbird. Its long, thin tongue is dipped into nectar approximately 20 times per second. With each insertion, fluid rises along the length of the tongue through capillary action. While the tongue is open in crosssection, resembling a sliced straw, experiments demonstrate that surface tension serves to close it, with the tongue's zipping front corresponding to the rising meniscus. Supporting theoretical and analogue experimental models of this novel, natural example of capillary origami are developed and explored. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AE.00009: Why Do Elephants Flap Their Ears? Moise Koffi, Latif Jiji, Yiannis Andreopoulos It is estimated that a 4200 kg elephant generates as much as 5.12 kW of heat. How the elephant dissipates its metabolic heat and regulates its body temperature has been investigated during the past seven decades. Findings and conclusions differ sharply. The high rate of metabolic heat coupled with low surface area to volume ratio and the absence of sweat glands eliminate surface convection as the primary mechanism for heat removal. Noting that the elephant ears have high surface area to volume ratio and an extensive vascular network, ear flapping is thought to be the principal thermoregulatory mechanism. A computational and experimental program is carried out to examine flow and heat transfer characteristics. The ear is modeled as a uniformly heated oscillating rectangular plate. Our computational work involves a threedimensional time dependent CFD code with heat transfer capabilities to obtain predictions of the flow field and surface temperature distributions. This information was used to design an experimental setup with a uniformly heated plate of size 0.2m x 0.3m oscillating at 1.6 cycles per second. Results show that surface temperature increases and reaches a steady periodic oscillation after a period of transient oscillation. The role of the vortices shed off the plate in heat transfer enhancement will be discussed. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AE.00010: The effect of shear and flow separation on out of plane growth in biological films Derek Rinderknecht, Mory Gharib Shear stress and flow separation are important physical cues initiating biofouling in many biological systems examples are the formation of plaques in the cardiovascular system and the accumulation of algae or other contaminants on the hulls of ships. To examine the effect of unsteady flow on the local shear profile and flow separation location and their relationship to the growth of thin biofilms, an experiment was constructed consisting of an open ended box with two opposing cylindrical half rounds located along the midline of the top and bottom faces. This chamber when mounted on a traverse is capable of creating steady, oscillatory and pulsatile flow profiles. A parametric study consisting of LIF dye experiments and PIV was conducted to examine the affect of unsteady flow amplitude and frequency on flow separation. Empirical velocity fields were analyzed using Lagrangian Coherent Structures to determine the impact of the unsteady flow profile on boundaries to transport within the flow. Results show the existence of three distinct flow regimes where the size and number of recirculations present depend on the frequency and amplitude of the oscillation. The flow was also seeded with algae and the apparent effects of flow separation and time periodic shear on outofplane biological growth will be discussed. [Preview Abstract] 
Session AF: Microfluidics: Devices I
Chair: Michael Shelley, New York UniversityRoom: 101F
Sunday, November 22, 2009 8:00AM  8:13AM 
AF.00001: The hydrodynamic mobility of chiral colloidal aggregates Eric Keaveny, Michael Shelley A recent advance in colloidal technology [Zerrouki \emph{et al.}, Nature \textbf{455}, 380 (2008)] uses magnetic aggregation to enable the formation of micronscale particle clusters with helical symmetry from doublets composed of two micronscale beads of different radii bonded together by a magnetic cement. Such selfassembled structures offer a means of controllable transport and separation in a low Reynolds number environment using externally applied magnetic or electric fields. We identify two necessary conditions that reveal further parameterized expressions that describe the positions of the beads in an aggregrate as a function of size ratio of the two beads composing the doublets. With the geometry of the structure known, we perform hydrodynamic calculations to ascertain entries of the mobility matrix for the aggregate and establish the relationship between the applied torque about the helical axis and translations parallel to this direction. For larger values of the particle radius ratio the coupling between rotations and translations changes sign as the number of doublets in the aggregate increases indicating that the clusters possess a more complex superhelical structure. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AF.00002: Actuated cilial layers regulate deposition of microscopic solid particles Rajat Ghosh, Gavin A. Buxton, O. Berk Usta, Anna C. Balazs, Alexander Alexeev We use computational modeling to examine the threedimensional interactions between oscillating, synthetic cilia and microscopic solid particles in a fluidfilled microchannel. The synthetic cilia are elastic filaments that are tethered to a substrate and are actuated by a sinusoidal force, which is applied to their free ends. The cilia are arranged in a square pattern and a neutrally buoyant particle is initially located between these filaments. Our computational studies reveal that depending on frequency of the beating cilia, the particle can be either driven downwards toward the substrate or driven upwards and expelled into the fluid above the cilial layer. This behavior mimics the performance of biological cilia used by certain marine animals to extract suspended food particles. The findings uncover a new route for controlling the deposition of microscopic particles in microfluidic devices. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AF.00003: Sizebased sorting of microparticles using microbubble streaming Cheng Wang, Shreyas Jalikop, Sascha Hilgenfeldt Oscillating microbubbles driven by ultrasound have shown great potential in microfluidic applications, such as transporting particles and promoting mixing [13]. The oscillations generate secondary steady streaming that can also trap particles. We use the streaming to develop a method of sorting particles of different sizes in an initially wellmixed solution. The solution is fed into a channel consisting of bubbles placed periodically along a side wall. When the bubbles are excited by an ultrasound piezoelectric transducer to produce steady streaming, the flow field is altered by the presence of the particles. This effect is dependent on particle size and results in sizebased sorting of the particles. The effectiveness of the separation depends on the dimensions of the bubbles and particles as well as on the ultrasound frequency. Our experimental studies are aimed at a better understanding of the design and control of effective microfluidic separating devices. Ref: [1] P. Marmottant and S. Hilgenfeldt, Nature 423, 153 (2003). [2] P. Marmottant and S. Hilgenfeldt, Proc. Natl. Acad. Science USA, 101, 9523 (2004). [3] P. Marmottant, J.P. Raven, H. Gardeniers, J. G. Bomer, and S. Hilgenfeldt, J. Fluid Mech., vol.568, 109 (2006). [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AF.00004: Whistling Bubbles: Allfluidic linear frequency sweep generators Manu Prakash, Matthias Klauser Complex biochemical systems are often composed of networks of interacting elements with spatial and temporal dynamics. Unlike ever so prevalent electronic debugging tools, automated tools to chemically probe these systems are in their infancy. Here we demonstrate one such novel tool  an allfluidic frequency sweep generator capable of linear and exponential sweeps from a few Hz to KHz response. This is accomplished by coupling a flowfocusing geometry driven by negative pressure ( FF), a nonlinear resistor and retraction dynamics of fluid threads in microchannels. The device exhibits various modes including accordions and bursts with negative ramps to generate the desired sweep. The sweep parameters including f$_{min}$, f$_{max}$ and T$_{off}$ can be programmed by knobs including the hydrostatic pressure at inlet geometries. Our work highlights the importance of exploiting dynamics of drops and bubbles in fluidic networks to engineer desired function. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AF.00005: High Speed motion generated by an oscillating microfiber Qi (Rick) He, Bian Qian, Shane Woody, Bethany Woody, Kenneth Breuer We present detail regarding the flow field generated by the highspeed motion of a long, thin elastic fiber immersed in a viscous fluid. The fiber, made of glass, or carbon, is approximately 1 mm in length, has a diameter of 7 microns, and is immersed in water, seeded with submicron tracer particles. The fiber is oscillated back and forth at 32 kHz with a peaktopeak tip amplitude of approximately 10 microns. The resultant flow field is measured using microPIV, imaged at high speed using an intensified high speed camera, capable of taking data up to 12kHz. Symmetric vortices around the tip are generated by the steady streaming effect, with fluid velocities approaching 1 m/s, and shear rates close to $10^5$ $s^{1}$. The vortices have a strong threedimensional structure due to the presence of the substrate below the moving tip, as well as the axial variation of the fiber amplitude. In addition to quantifying the fluid motion, the mixing of fluid and the dispersion, and accumulation of particles due to the fiber motion is measured and discussed. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AF.00006: Multi  directional Electrophoretic Positioning of Charged Drops B.S. Hamlin, I.K. Emerson, J.W. Gage, W.D. Ristenpart We demonstrate a multidirectional electrophoretic technique to provide precise control over the position, trajectory and velocity of a charged object. Two or more pairs of electrodes, oriented along different directions, are periodically activated and deactivated to move the charged object along a desired trajectory. Two perpendicular sets of electrodes, for example, can induce the charged object to take a series of sudden $90^\circ$ turns. We derive scaling expressions for the rates of acceleration and deceleration as the electric field direction is switched, and we corroborate the expressions experimentally for water droplets in oil. Surprisingly, we observe that the electrode geometry significantly alters the trajectory of charged drops, even in the absence of an applied electric field. The results have broad implications for any system that requires dynamic threedimensional positioning, including labonachip devices and electrophoretic displays. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AF.00007: A theoretical model of electrophoretic biomolecule separation in periodic nanofilter arrays Zi Rui Li, Nicolas Hadjiconstantinou, Gui Rong Liu, Jongyoon Han, Yu Zong Chen, JianSheng Wang We present a theoretical model describing Ogston sievingpore size comparable to or larger than the characteristic molecular dimensionof rigid isotropic and anisotropic biomolecules in nanofluidic filters, comprising of a periodic array of alternating deep and shallow regions. We obtain onedimensional analytical results and twodimensional numerical solutions of a driftdiffusion description, which captures the interplay between the driving electric force, entropic barrier and molecular diffusion. The analytical solution yields explicit results for the effective mobility and trapping time, and shows that the configurational entropy, which arises from the reduction of available configurations in the confined space of the nanochannel, dominates the resulting separation behavior. Our results are in line with experimental observations, and elucidate the effects of field strength, device geometry and entropic barrier height, providing a robust tool for the design and optimization of nanofilter/nanopore systems. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AF.00008: Colon Cancer Cell Separation by Dielectrophoresis Fang Yang, Xiaoming Yang, H. Jiang, P. Wood, W. Hrushesky, Guiren Wang Separation of cancer cells from the other biological cells can be useful for clinical cancer diagnosis and cancer treatment. In this presentation, conventional dielectrophoresis (cDEP) is used in a microfluidic chip to manipulate and collect colorectal cancer HCT116 cell, which is doped with Human Embryonic Kidney 293 cells (HEK 293). It is noticed that, the HCT116 cell are deflected to a side channel from a main channel clearly by apply electric field at particular AC frequency band. This motion caused by negative DEP can be used to separate the cancer cell from others. In this manuscript, chip design, flow condition, the DEP spectrum of the cancer cell are reported respectively, and the separation and collection efficiency are investigated as well. The sorter is microfabricated using plastic laminate technology. /abstract This work has been financially supported by the NSF RII funding (EP [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AF.00009: Simulation of Taylor bubble flow in Tshaped microchannel by MPS method Shaoshan Rong, Bin Chen Moving Particle Semiimplicit (MPS) method uses particles and their interactions to simulate incompressible flow and it is a promising meshless method for multiphase flow simulation. In order to capture the interface in microscale channel, Taylor bubble flow in a Tshaped microchannel is simulated in this paper. Firstly available surface tension and wettability model are improved and validated by simulating the droplet vibration and static shapes of a droplet attached on the solid surface, respectively. Afterwards, by discretizing the liquid and gas phases into moving particles with different density, bubble slug generation in Tshaped microchannel is reproduced by MPS method with above models. The good agreement between numerical simulation with visualization experiment confirmed the capacity of MPS for the microscale twophase flow. Finally, bubble generation mechanism is revealed by the velocity field of typical squeezing and shearing regime. The influences of viscosity, surface tension and contact angle on the bubble slug length are also discussed in detail. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AF.00010: A Microscale MultiInlet Vortex Nanoprecipitation Reactor: Turbulence Measurement and Simulation Janine Chungyin Cheng, Michael G. Olsen, Rodney O. Fox Microreactors capable of generating turbulent flow are used in Flash NanoPrecipitation, a novel approach to produce functional nanoparticles. Microreactor design and optimization can be greatly enhanced by developing reliable computational models. Scalar mixing in a multiinlet microvortex reactor has been previously studied and a RANS simulation with a scalar mixing model had been successfully validated for fully turbulent flow. To further validate the flow field, the reactor, operated in flow regimes from laminar to turbulent, was investigated using microscopic particle image velocimetry and computational fluid dynamics. In the experiments, instantaneous velocity fields were recorded on three different planes in the reactor and flow statistics such as the mean velocity and turbulent kinetic energy were computed for comparison with flow simulations. Laminar simulations were performed for the lowReynoldsnumber cases, and largeeddy simulations (LES) were performed for higherReynoldsnumber cases. The simulation results were in good agreement with experiments for all cases, demonstrating the accuracy of the simulation models in the laminarturbulent transition range. [Preview Abstract] 
Session AG: Free Surface Flows I
Chair: Alexandra Techet, Massachusetts Institute of TechnologyRoom: 101G
Sunday, November 22, 2009 8:00AM  8:13AM 
AG.00001: Resolving the unsteady deceleration and forces after water entry by low massratio spheres B.P. Epps, T.T. Truscott, R.R. LaFoy, A.H. Techet The dynamics of water entry are significantly affected by surface coating and mass ratio, over a range of moderate impact velocities and sphere diameters. A hydrophobic sphere creates a subsurface vapor cavity whereas a hydrophilic sphere does not, and the forces acting on the sphere after entry depend upon whether or not this cavity is formed. Using high speedvideo, subpixelaccurate image processing techniques, and a smoothing spline method to find the derivatives of position data, we find the unsteady forces acting on spheres during the water entry event. Our data reveals that as mass ratio decreases from O(10 to 1), the sphere's deceleration becomes highly nonlinear, since inertial forces cease to dominate over hydrodynamic forces. The unsteady deceleration is affected by vortex shedding in the nocavity case, whereas in the cavity forming case, forces are affected by cavity growth and collapse, and vortex shedding is inhibited until after pinchoff. PIV sequences taken in both cavityforming and noncavityforming impact cases highlight the vortex shedding onset and can be used to gain further insight into the sphere dynamics. Ultimately, this work emphasizes the need to accurately account for unsteady effects in modeling the post impact dynamics of spheres, especially as mass ratios approach unity. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AG.00002: The water exit of buoyant spheres Tadd Truscott Results of a combined experimental and theoretical investigation of a buoyant sphere as it passes through the free surface are presented. When a buoyant sphere ascends through a fluid column various behaviors are exhibited (e.g. vortex induced oscillations, etc.) and are a function of release depth and mass ratio. Using high speed dye visualization and particle image velocimetry to reveal the flow behavior, the conditions of release for which maximum height occurs and the governing parameters for optimum water exit are examined. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AG.00003: Wave resistance Marie Le Merrer, Frederic Chevy, Elie Raphael, Christophe Clanet, David Quere The more viscous, the more slippery! This is what happens when millimetric liquid nitrogen drops are thrown at the surface of water or viscous oil. Because these drops float on a cushion of vapor, the resistance to the motion mainly arises from the formation of waves, which mostly occurs on liquids of low viscosity. The wave resistance is very low, of the order of ten micronewtons. However, we could measure it from the slow deceleration of the drops as they move along the surface. We were able to show that this force increases very strongly (in a quasidiscontinuous fashion) when the velocity becomes higher than 23 cm/s, that is, when a stationary wake can exist. We also studied the effect of the bath viscosity, which damps the waves and hence reduces the drag. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AG.00004: Laserproduced microjets Sigurdur Thoroddsen, K. Takehara, T.G. Etoh, C.D. Ohl We use ultrahighspeed imaging to characterize the formation of a microjet when a laserproduced shock hits a bubble sitting under a free surface. The bubble is formed inside a sessile drop, sitting on a glass slide and buoyancy drives it to its top. The jetting is forced by an Nd:YAGlaser pulse of about 30 mJ, focused by a microscope objective sitting under the glass plate. The jet is initiated when the shock hits the curved bottom of the bubble. It emerges out of a bottom crown and has a very regular shape. For water the jets are a few microns in size and can emerge at over 200 m/s. In intermediate viscosity liquids the jetting can be even faster and can emerge at over 500 m/s, depending on the depth of the laser focus. Jets can even be produced in pure glycerin where they emerge at about 100 m/s. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AG.00005: Dragout of bubbles by a plate withdrawn from a liquid bath Justin Kao, Andrea Blakemore, Anette Hosoi We report work on a new aspect of the classic LandauLevich problem of liquid dragout by a moving plate, namely, interaction of bubbles with the coating flow. Due to the cheerios effect, bubbles present in a liquid bath may gather at menisci such as those caused by a partiallysubmerged plate. Our experiments show that a critical withdrawal speed exists. Below this speed, bubbles are in stable equilibrium and remain stationary in the lab frame as the plate is withdrawn. Above the critical speed, no stable equilibrium exists and bubbles are drawn up onto the plate, with obvious consequences for the uniformity of the resulting coating. We examine the dependence of the critical speed on fluid properties. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AG.00006: Laminar circular hydraulic jumps without separation Ratul Dasgupta, Gaurav Tomar, Rama Govindarajan The traditional inviscid criterion for the occurrence of a planar, standing hydraulic jump is to have the Froude number decrease downstream and go through a value of 1 at some location. Here, upstream propagating, smallamplitude, long, nondispersive gravity waves are trapped, and nonlinear steepening is said to result in a neardiscontinuous height profile, but it is not clear how. Such a condition on the Froude number is shown in the present axisymmetric NavierStokes computations to hold for a circular jump as well. The relevance of nonlinear steepening to a circular jump is therefore a question we wish to answer. In circular jumps, moreover, a region of recirculation is usually observed underneath the jump, underlining the importance of viscosity in this process. This led Tani (J. Phys. Soc. Japan, 1949) to hypothesise that boundarylayer separation was the cause of the circular jump. This hypothesis has been debated extensively and the possibility of circular jumps without separation hinted at. In our simulations, we are able to obtain circular hydraulic jumps without any flow separation. This, and the necessity or otherwise of viscosity in jump formation will be discussed. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AG.00007: Study of Air Entrainment by a Horizontal Plunging Liquid Jet Mario Trujillo, Suraj Deshpande, Xiongjun Wu, Georges Chahine The process of air entrainment following the impact of an initially horizontal circular water jet on a pool of water has been studied computationally and experimentally. It has been found that the entrainment of air cavities in the near field region is periodic, not continuous as reported in earlier studies. The simulations are based on a VolumeofFluid methodology with interfacial compression using a modified version of the open source utilities, OpenFoam. Close agreement with experiments is reported on the creation of cavities in the near field, where air entrainment occurs. The period of entrainment is found to be proportional to g, and a simplified closedform solution for this periodic event is presented. An overall physical picture of the mechanisms leading to bubble formation is given. The far field, which is characterized by the presence of small bubbles is only partially resolved computationally. Comparisons against velocity data are performed in this region leading to adequate qualitative agreement. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AG.00008: Viscous rivulet flow over trenches Thomas Ward, G.M. Homsy The dynamic interfacial behavior of an advancing rivulet encountering a trench of square crosssection is studied experimentally at low capillary and Reynolds numbers. Trench depths vary from slightly smaller than to slightly larger than the capillary length. The fluids are a glycerol/water mixture and a silicone oil, representing a partially wetting and a nearly complete wetting fluid, respectively, and the rivulet interface is observed using low speed CCD imaging. A rich variety of phenomenon is observed in this range of depths suggesting that trench wetting is greatly affected by a combination of geometry and dynamic contact angle. The dynamics are characterized by measuring the local film height as a function of time and are compared with the theory of Gramlich et al. (Phys. Fluid, 2004). In spite of the fact that the theory is 2D while the experiments are 3D, remarkably good qualitative agreement is observed for large trench depths and partially wetting fluids. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AG.00009: Temporal statistics of a meandering rivulet Peter Vorobieff, Keith Mertens, Vakhtang Putkaradze Meandering of a rivulet on an inclined, partially wetting surface can be triggered or suppressed by introducing small flowrate fluctuations or, correspondingly, by reverting to a constant flow rate. Here we study a rivulet continuously meandering with flowrate fluctuations present. Image sequences of the plane of the meander reveal a spatially and temporally resolved picture of the process, covering spatial scales from millimeters to meters and time scales from seconds to hours. For a given coordinate in the direction downstream from the origin of the rivulet, we construct time histories of the location of the rivulet centerline. Statistics of these time histories show an interesting correspondence with the spatial meandering statistics known from earlier work (such as the absence of a dominant wavelength), while also possibly manifesting previously unobserved trends at short time scales. It is also noteworthy that even a modest volume fraction of solid particles in the flow can radically alter the behavior of the rivulet, producing a stationary (pinned) meandering pattern. [Preview Abstract] 
Session AH: Drops I: Impact
Chair: John Saylor, Clemson UniversityRoom: 101H
Sunday, November 22, 2009 8:00AM  8:13AM 
AH.00001: Thin Sheet Formation in Viscous Splash Michelle Driscoll, Sidney Nagel Ambient air is crucial for creating a splash on smooth dry surfaces for both viscous and inviscid liquids.\footnote{L. Xu, \textit{Phys. Rev. E} \textbf{75}, 056316 (2007); L. Xu \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{94}, 184505 (2005).} In a viscous splash, the drop initially spreads in the form of a thick lamella until $t_{ejt}$ at which time it emits a thin fluid sheet. We have previously shown that $t_{ejt}$ is set by the ambient pressure and the liquid viscosity, and shows only a weak dependence on drop impact velocity and surface tension.\footnote{M. Driscoll \textit{et al.}, \textit{DFD 2008} BAPS.2008.DFD.AG.5} We have measured the thickness of the ejected sheet using absorption measurements of a dyed liquid drop. The ejected sheet has a thickness $\sim10 ~ \mu m$ that is approximately a tenth the thickness of the lamella preceding it. Using highresolution, highspeed photography we have observed that as the ejected sheet expands, air bubbles are entrained into the trailing lamella. The bubble size increases as the lamella velocity decreases. Air entrainment ceases at a critical lamella velocity, $v_c \sim 1.2 ~ m/s$, which appears to be independent of drop impact velocity as well as the ambient pressure. At the critical velocity, the bubble radius is approximately 30 $\mu m$. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AH.00002: The dependence of Mesler entrainment on Weber number and drop axis ratio J.R. Saylor, B.H. Mills, F.Y. Testik The impact of a water drop on a flat water surface can result in a variety of subsurface bubble formation events. Under certain conditions, the impact results in the formation of a large number of micronscale bubbles, often referred to as Mesler entrainment. An experimental study is presented revealing that the existence of Mesler entrainment depends on both the drop Weber number and the drop axis ratio. Specifically, Mesler entrainment was observed for Weber numbers greater than 8 and less than 26. Within this range, the occurrence of Mesler entrainment was more frequent for axis ratios close to unity, that is for spherical drops. Drops of a prolate or oblate shape showed significantly less frequent Mesler entrainment. The working fluid for all experiments was water with a constant concentration of the soluble surfactant Triton X100. This was done to avoid the influence of contaminating surfactants which tend to accrue when pure water is used as the working fluid. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AH.00003: A computational study of high speed droplet impact Toshiyuki Sanada, Keita Ando, Tim Colonius When a droplet impacts a solid surface at high speed, the contact periphery expands very quickly and liquid compressibility plays an important role in the initial dynamics and the formation of lateral jets. Impact results in high pressures that can damage the surface. In this study, we numerically investigated a high speed droplet impacts on a solid wall. The multicomponent Euler equations are computed by a FVWENO scheme with an HLLC Riemann solver [Johnsen {\&} Colonius, J. Comp. Phys. (2006)] that accurately captures shocks and interfaces. Stiffened equation of state is employed to model of gas, liquid and solid components. In order to compare the available theory and experiments, 1D, 2D and axisymmetric solutions are obtained. The generated pressures, shock speeds, and the lateral jetting mechanism are investigated. In addition, the effect of target compliance is evaluated. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AH.00004: Events before droplet splashing on a solid surface Shreyas Mandre, Madhav Mani, Michael Brenner A high velocity impact between a liquid droplet and a solid surface produces a splash. Classical observations traced the origin of this splash to a thin sheet of fluid ejected near the impact point, though the fluid mechanical mechanism leading to the sheet is not known. Mechanisms of sheet formation have heretofore relied on initial contact of the droplet and the surface. In this paper, we theoretically and numerically study the events within 1 $\mu$s of contact. The droplet initially tries to contact the substrate by either draining gas out of a thin layer or compressing it, with the local behavior described by a self similar solution of the governing equations. This similarity solution is not asymptotically consistent: forces that were initially negligible become relevant and dramatically change the behavior. Depending on the radius and impact velocity of the droplet, we show that the solution is overtaken by either the surface tension of the liquidgas interface or viscous forces in the liquid. At low impact velocities surface tension stops the droplet from impacting the surface, whereas at higher velocities viscous forces become important before surface tension. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AH.00005: Oscillating Effects on the Bubble Induced by A Free Falling Drop AnBang Wang, C.C. Kuan, P.H. Tsai The impact of a droplet on a liquid pool can result in different fantastic phenomena. Many investigations have been conducted since decades; however, none has been studied for the effects of oscillating drop on the big bubble induced by the impacting droplet since Worthington (1908). In the present study, big bubble induced by the droplet impact has been experimentally studied and systematically analyzed. Effects of impact velocity, drop size, oscillation parameters and depth of target liquid have been investigated and discussed. New characteristic regimes in the V (impact velocity)d (diameter of droplet)map have been discovered. Two geometry parameter oscillation parameters sharpnessratio and offsetratio of the freefalling droplet have been found to be the most important controlling parameters. Their results are revealed and compared in this study. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AH.00006: Drop impact on sand: from donut to pie Giles Delon, Stephane Dorbolo, Nicolas Vandewalle, Herve Caps We have studied the impact of water drops onto granular layers. Depending on the impact energy, various shapes are observed for the resulting craters. Experimental parameters that have been considered are : the size of the millimetric droplets; the height of the free fall, ranging from $1.5$~cm to $100$~cm; and, the depth of the granular layers, ranging from tenth of millimeters to a few centimeters. As the drop is impacting the sand layer, energy is dissipated and a splash of sand occurs. Meanwhile, surface tension, inertia and viscosity compete, leading to strong deformations of the drop which depend on the experimental conditions. Just after the drop enters into contact with the sand, imbibition takes place and increases the apparent viscosity of the fluid. Soon, the drop motion is stopped by this process. Images and fastvideo recordings of the impact allowed us to draw scaling laws for the crater morphology and size. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AH.00007: Suppressing Viscous Drop Splashing with Surface Roughness Ariana StrandburgPeshkin, Michelle Driscoll, Sidney Nagel The splashing of a liquid drop on a smooth, dry surface depends on a host of factors: the speed, surface tension, viscosity and size of the drop, but also, surprisingly, the pressure and molecular weight of the surrounding gas.\footnote{ L. Xu, W.W. Zhang, S.R. Nagel, PRL 94, 184505 (2005).}$^,$\footnote{ L. Xu, PRE 75, 056316 (2007).} In the case of a viscous drop splashing on a smooth surface, a thin sheet of fluid is first ejected from the rim of the expanding drop and then breaks up into droplets to form a splash.\footnote{M. Driscoll et al., BAPS DFD AG.00005 (2008).} When the surface is rough, different behavior, known as prompt splashing, may also be observed.$^{2,}$\footnote{ L. Xu, L. Barcos, and S.R. Nagel, PRE 76 066311 (2007).} Here we explore the splashing of a viscous liquid as the surface roughness, $R_a$, is varied. We find that a small degree of roughness, $R_a < 1 ~ \mu m$, can completely suppress the thinsheet ejection occurring on smooth surfaces. The degree of roughness necessary for this suppression decreases with increasing viscosity. In some cases, the roughness is great enough to suppress the thin sheet, but insufficient to produce a prompt splash, thus suppressing the splash entirely. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AH.00008: The effects of wind on the impact of a single drop on a water surface Xinan Liu The impact of single water drops on a water surface was studied experimentally in a wind tunnel. Water drops were generated from a needle oriented vertically from the top surface of the wind tunnel test section. The wind speed ranged from 0 to 10.0 m/s. After leaving the needle, the drops move downward due to gravity and downstream due to the effect of the wind, eventually hit a shallow pool of water on the bottom of the test section. The drop impacts were backlit with a halogen lamp and photographed with a highspeed movie camera at 1,000 frames per second. It is shown that the water drop obliquely impacts the water surface and the impingement angle relative to vertical increases with increasing wind speed. After the drop hits the water surface, a chain of secondary drops are formed and move in the leeward direction. This is followed by a stalk formation at the location of the water drop impact. It is found that the shape of the secondarydrop chain and the appearance of the stalk are markedly affected by wind speed. The effects of wind speed and initial drop size on a number of parameters, including the number, diameter and total mass of secondary drops were investigated. The dynamics of secondary drops in the presence of wind are discussed. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AH.00009: Droplet impact on a porous substrate: a capillary tube model Hang Ding, Theo Theofanous The dynamics of impacting (spreading, penetrating) a droplet on a porous substrate, modeled by an array of capillary tubes, is studied numerically using diffuse interface methods. The absorption rate depends on the diameter ratio of the capillary tube to the droplet, wettability, and liquid properties. The flow dynamics is resolved by solving the NavierStokes equations and interface capturing is governed by the CahnHilliard equation. Contactangle hysteresis is included (Ding{\&}Spelt 2008) and the stress singularity at moving contact lines is relieved using a diffuse interface model (Seppecher 1996; Jaqcmin 2000). The model is validated by studying the evolution of a droplet initially resting on a porous substrate and by comparison to dropimpact experiments involving just one capillary tube (Kogan et al 2008). Comparisons with analytical solutions and results available in the literature (e.g. Hilpert {\&} BenDavid 2009) are presented. Through parametric simulations over relevant ranges of Reynolds and Ohnesorge numbers and contact angles, impact regime maps are derived. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AH.00010: Impact of Droplets on a Vertical Capillary Tube Aline Delbos, Elise Lorenceau, Olivier Pitois, Mich\`ele Adler We experimentally study whether it is possible to force liquid impregnation of a porous media using the kinetics energy of an impacting drop. We study impregnation at the local scale and only consider a unique pore, a vertical glass capillary tube with thick walls. The forced impregnation is achieved with droplet of water impacting with an initial velocity on the tube. We focus both on forced impregnation of hydrophilic or hydrophobic capillary tubes. For small impact velocities, the classical results of impregnation are recovered. For large impact velocities, we observe new regimes due to the initial kinetics energy of the droplet. In particular, a liquid index disconnected from the upper part of the drop, which spreads on the horizontal flat surface, is observed both for the hydrophilic and hydrophobic tubes. To quantify the efficiency of the forced impregnation, we answer the following questions i) what is the volume of liquid eventually trapped within the porosity? ii) How deep this liquid is located in the pore? [Preview Abstract] 
Session AJ: Bubbles I: Cavitation
Chair: Roger Arndt, University of MinnesotaRoom: 101I
Sunday, November 22, 2009 8:00AM  8:13AM 
AJ.00001: Numerical simulation of shock/bubblecloud interaction problems Keita Ando, Tim Colonius, Christopher Brennen The interaction of a shock wave with a dilute bubble cloud is computed using a continuum twophase model incorporating the effect of a distribution of nuclei sizes. The bubble dynamics are evaluated using a RayleighPlessettype equation including the effects of heat transfer, liquid viscosity and compressibility. A finitevolume WENO scheme coupled with an approximate HLLC Riemann solver is developed to solve the shock problems. Linear and shock wave propagation through a onedimensional bubble screen is computed and the effect of phase cancellations among the differentsized bubbles is quantified. The size distribution in the screen is found to increase the cushioning of the shock loading. Computations of shock/bubblecloud interaction in two dimensions are also presented. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AJ.00002: Numerical Simulations of Bubble Dispersion over a Hydrofoil Shuang Zhu, Andrew Ooi, Hugh Blackburn, Brendon Anderson The production and entrainment of bubbles in ship wakes is not completely understood despite the fact that it has many practical applications. For example, bubbles trapped in the large vortical structures in the ship wake can form clusters that are able to persist for large distances leaving a long trail of bubbles, which increases the ship's signature; an important consideration in the defence environment. The fundamental mechanisms behind the complicated bubbly flow can be understood using data from numerical simulations. The objective of the study is to investigate the accuracy of current stateofart numerical models for simulating bubbly flows. A spectral elementFourier code will be used to carry out direct numerical simulations (DNS) with Lagrangian particle tracking to study the interaction of the upstream bubble distribution with a hydrofoil at different angles of attack and Reynolds numbers, and the effect on the resulting downstream bubble distribution. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AJ.00003: Partial Cavity Flows at High Reynolds Numbers Simo Makiharju, Brian Elbing, Andrew Wiggins, David Dowling, Marc Perlin, Steven Ceccio Partial cavity flows created for friction drag reduction were examined on a largescale. Partial cavities were investigated at Reynolds numbers up to 120 million, and stable cavities with frictional drag reduction of more than 95{\%} were attained at optimal conditions. The model used was a 3 m wide and 12 m long flat plate with a plenum on the bottom. To create the partial cavity, air was injected at the base of an 18 cm backwardsfacing step 2.1 m from the leading edge. The geometry at the cavity closure was varied for different flow speeds to optimize the closure of the cavity. Cavity gas flux, thickness, frictional loads, and cavity pressures were measured over a range of flow speeds and air injection fluxes. Highspeed video was used extensively to investigate the unsteady three dimensional cavity closure, the overall cavity shape and oscillations. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AJ.00004: Ventilated Supercavities Ellison Kawakami, Roger Arndt The topic of supercavitation is of considerable interest to drag reduction and/or speed augmentation in marine vehicles. Supercavitating vehicles need to be supplied with an artificial cavity through ventilation until they accelerate to conditions at which a natural supercavity can be sustained. A study has been carried out in the highspeed water tunnel at St. Anthony Falls Laboratory to investigate some aspects of the flow physics of such a supercavitating vehicle. During the present experimental work, the ventilated supercavity formed behind a sharpedged disk was investigated utilizing several different configurations. Results regarding cavity shape, cavity closure and ventilation requirements versus cavitation number and Froude number are presented. Additionally, effects related to flow choking in a water tunnel test section are discussed. Results obtained are similar in character to previously reported results, but differ significantly in measured values. Cavity shape, particularly aft of the maximum cavity diameter, is found to be a strong function of the model support scheme chosen. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AJ.00005: Effect of the cavity closure condition on the flow of liquid around a supercavitating wedge Anna Zemlyanova, Yuri Antipov The problem for a one nonsymmetric supercavitating wedge in a jet is considered. The single and doublespiralvortex models proposed by Tulin are used to describe the flow of the liquid at the rear part of the cavity. Both problems are solved in a closed form using the methods of complex analysis. The models are compared with respect to different parameters of the flow. It is obtained that the flow around the wedge, in the front part of the cavity and the lift and drag coefficients are not affected by the choice of the model. On the other hand, the flow at the tail part of the cavity and the length of the cavity depend strongly on the chosen model. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AJ.00006: Tip vortex cavitation suppression via mass injection Harish Ganesh, Natasha Chang, Ryo Yakushiji, Steven Ceccio Tip vortex cavitation (TVC) suppression by mass injection in the core of the vortex was studied with an elliptical planform hydrofoil NACA66 modified in a recirculating water tunnel of known nuclei distribution. The chord based Re was O(106) for all experiments. Water and Polyox WSR 301 solution for a range of concentrations (10 to 500pmm) and relative flow rates (Qjet / Qcore of 0.033 to 0.27) were injected. Also, different injection port size and angle of attack were studied. It was found that the TVC suppression effect was different for inception and desinence. The baseline (no injection) inception cavitation number was more than the average negative pressure coefficient, Cp of the vortex, while mass addition reduced the inception cavitation number to approximately the Cp value. TVC desinence for the baseline case was found to match the estimated Cp value and polymer injection provided some cavitation suppression. Flow measurements were made to understand the underlying physics of TVC. The mechanisms and scalability that lead to TVC suppression by mass injection are discussed. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AJ.00007: Dynamic of cavitation bubble in a flowing liquid with a pressure gradient Marc Tinguely, Mohamed Farhat In the present study, a high energy pulsed laser is used to generate a millimetric cavitation bubble within a water flow over a symmetric hydrofoil. The bubble is initiated at different locations in the vicinity of the hydrofoil leading edge. A high speed camera is used to observe the motion of the bubble as it travels along the hydrofoil suction side. Besides the standoff parameter, we have found that the pressure gradient plays a major role on bubble dynamic and subsequent phenomena. For a specific initial location of the bubble, the microjet is no more directed towards the hydrofoil surface, as commonly observed in still water. In this case, we have also observed a spectacular behaviour of the cavity rebound, which migrates towards the solid surface despite of the outward direction of the microjet. This result differs from the behaviour of a bubble near a solid surface in water at rest or water flowing uniformly since the microjet is normally directed toward the solid. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AJ.00008: Cavitation induced by high speed impact of a solid surface on a liquid jet Mohamed Farhat, Marc Tinguely, Mathieu Rouvinez A solid surface may suffer from severe erosion if it impacts a liquid jet at high speed. The physics behind the erosion process remains unclear. In the present study, we have investigated the impact of a gun bullet on a laminar water jet with the help of a high speed camera. The bullet has a flat front and 11 mm diameter, which is half of jet diameter. The impact speed was varied between 200 and 500 ms$^{1}$. Immediately after the impact, a systematic shock wave and high speed jetting were observed. As the compression waves reflect on the jet boundary, a spectacular number of vapour cavities are generated within the jet. Depending on the bullet velocity, these cavities may grow and collapse violently on the bullet surface with a risk of cavitation erosion. We strongly believe that this transient cavitation is the main cause of erosion observed in many industrial applications such as Pelton turbines. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AJ.00009: Cavitation Bubble in Shear Flow Sadegh Dabiri, William Sirignano, Daniel Joseph In the orifice of liquid injectors at high pressure, cavitation occurs behind the sharp corners, where a strong pressure drop is present due to quick change in the flow direction. In addition, a high level of shear is present inside the boundary layer. Therefore, it is important to understand the influence of the shear on the cavitation. In this study, the deformation of a cavitation bubble in shear and extensional flows is numerically investigated. The NavierStokes equations are solved to observe the threedimensional behavior of the bubble as it grows and collapses. During the collapse phase of the bubble, two reentrant jets are observed on two sides of the bubble due to interaction of the bubble with the background flow. Reentrant jets with enough strength could breakup the bubble into smaller bubbles. Post processing of the results is done to cast the disturbance by the bubble on the liquid velocity field in terms of spherical harmonics. It is found that a quadrupole moment is created in addition to the monopole source. As the bubble collapses regions of high vorticity are created near the bubble interface. [Preview Abstract] 
Session AK: Multiphase Flows I
Chair: Pavlos Vlachos, Virginia Polytechnic Institute and State UniversityRoom: 101J
Sunday, November 22, 2009 8:00AM  8:13AM 
AK.00001: Dynamic Characteristics at the Interface of Underwater Round Gas Jets Chris Weiland, Pavlos Vlachos The gasliquid interface characteristics of round gasjets submerged in water was studied across a wide range of Mach numbers (0.41.9). High speed shadowphotography was used to image the gas jet and the interface was tracked from the digital images for all points in space and time. The results show how the interface characteristics are governed by buoyancy to momentum driven flow as the Mach number increases. The jet penetration, defined as the maximum length a continuous gas jet occupies 99 percent of the time, increases with the injection Mach number. The penetration is related to the compressible jetting length, defined as the distance from the orifice where the momentum and buoyancy forces are balanced, and signifies a change in the jet behavior spatially from a momentum to buoyancy driven flow. The interface motion is computed as a function of the Mach number and the distance downstream from the orifice. These results indicate the most unsteady jetting process near the orifice occurs at Mach 1, presumably due to the formation of a shock cell structures. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AK.00002: A numerical study of air layer drag reduction phenomenon on a flat plate Dokyun Kim, Parviz Moin The objective of the present study is to predict and understand the air layer drag reduction (ALDR) phenomenon. Recent experiments (Elbing et al. JFM 2008) have shown large net drag reductions if air is injected beyond a critical rate at the wall. The stability analysis and numerical simulations are performed to investigate mechanisms of ALDR on a flat plate using the same geometry as in the experiment. The linear stability of airliquid interface is investigated by solving the OrrSommerfeld equations, and numerical simulations of twophase flow have been performed to describe the evolution of airwater interface. The stability analysis shows that the air flow rates, Reynolds number, Weber number, and Froude number are important parameters determining the stability of the air layer. In laminar boundary layer, it is observed from the numerical simulations that the Froude number is the key to the stability of the air layer. The presentation will include a new and very efficient numerical method for twophase flow calculations used in this study. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AK.00003: Lagrangian statistics for bubbles in a turbulent boundary layer Michael Mattson, Krishnan Mahesh We are developing the simulation capability for bubbly flows in complex geometries using unstructured grids and an EulerLagrangian methodology. In the Lagrangian bubble model, the bubbles are treated as a dispersed phase in the carrier fluid, and individual bubbles are point particles governed by an equation for bubble motion. For this talk, direct numerical simulation is used to solve the NavierStokes equations for a spatiallyevolving turbulent boundary layer ($Re_{\theta}=1430$) and bubbles are injected into the nearwall region. The bubbly suspension is dilute and oneway coupled equations are used. The temporal evolution of the bubble dispersion and probability density functions of the bubble forces will be presented, with emphasis on the role Stokes number and injection location play in determining bubble behavior. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AK.00004: A comprehensive subgrid air entrainment model for Reynoldsaveraged simulations of freesurface bubbly flows Jingsen Ma, Assad A. Oberai, Donald A. Drew, Richard T. Lahey, Jr, Mark C. Hyman The simulation of free surface bubbly flows using a twofluid model remains challenging in part due to the lack of a comprehensive air entrainment model that can predict the location and rate of air entrainment for a wide range of flows. In this study we derive one such model and implement it into a computational multiphase fluid dynamics (CMFD) framework that solves the Reynoldsaveraged twofluid equations. The subgrid air entrainment model is derived from a simple argument that the wave action near the air/water interface causes it to ingest air bubbles and they are entrained into the liquid if their downward velocity exceeds that of the interface. This yields a simple expression for the rate of entrainment as a product of the downward gradient of the liquid velocity near the free surface and the turbulent kinetic energy. We have tested the performance of this model and CMFD in simulating the bubbly flow due to a plunging liquid jet, in a hydraulic jump and around a fullscale naval surface ship by comparing with experimental data. It's found that in each case the subgrid air entrainment model and the twofluid modeling approach yields accurate results. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AK.00005: On bubble clustering and energy spectra in pseudoturbulence Julian Martinez Mercado, Daniel Chehata Gomez, Dennis van Gils, Chao Sun, Detlef Lohse We performed 3DParticle Tracking (3DPTV) and Phase Sensitive Constant Temperature Anemometry in pseudoturbulence to investigate bubble clustering and to obtain the mean bubble rise velocity, distributions of bubble velocities, and energy spectra at dilute gas concentrations. To characterize the clustering the pair correlation function $G(r,\theta)$ is calculated. The deformable bubbles with equivalent bubble diameter $d_b=45$ mm are found to cluster within a radial distance of a few bubble radii with a preferred vertical orientation. This vertical alignment is present at both small and large scales. The large number of datapoints and the non intrusiveness of PTV allowed to obtain wellconverged Probability Density Functions (PDFs) of the bubble velocity. The PDFs have a nonGaussian form for all velocity components and intermittency effects can be observed. The energy spectrum of the liquid fluctuations decays with a power law of $3.2$, different from the $\approx 5/3$ found for homogeneous isotropic turbulence. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AK.00006: How does interfacial rheology govern soap bubble cluster dynamics? Sylvie CohenAddad, AnneLaure Biance, Reinhard Hohler Aqueous foams are concentrated dispersions of gas bubbles in a soapy solution. These complex fluids exhibit solidlike or liquidlike mechanical behaviors, depending on the applied shear. When it is increased beyond a yield strain, neighbor switching bubble rearrangements called T1 events are triggered and plastic flow sets in. We study experimentally the dynamics of such strain induced T1s in 3D bubble clusters that we consider as model systems of 3D foams. To determine the hydrodynamics and physicochemistry that set the duration of T1s, we use foaming solutions of a wide range of well characterized bulk and interfacial rheological properties. At low shear rates, the T1 duration is set by a balance between surface tension and surface viscous forces in qualitative agreement with previous studies of T1s in 2D foams [1] and we present a simple physical model that explains our 3D findings. Moreover, above a characteristic shear rate, rearrangement dynamics are driven by the applied strain. By combining all our results, we link the transition from intermittent to continous flow dynamics in foams to the rheology of the gasliquid interfaces. \\[4pt] [1] M. Durand, H. A. Stone, \textit{Phys. Rev. Lett.} \textbf{97}, 2226101 (2006). [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AK.00007: Measurements of the fluctuating liquid velocity of a bidisperse suspension of bubbles rising in a vertical channel Juan Carlos Serrano, Santos Mendez, Roberto Zenit Experiments were performed in a vertical channel to study the
behaviour of a
bidisperse suspension of bubbles. Bubbles were produced using
capillaries of
two distinct inner diameters. The capillaries are small enough to
generate
bubbles in the range of 1 to 6 mm in diameter. Using water and
waterglycerin
mixtures, the vertical component of the fluctuating liquid
velocity was
obtained using a flying hot wire anemometer technique. The system is
characterized by the dimensionless Reynolds and Weber numbers in
the range of
$22 
Sunday, November 22, 2009 9:31AM  9:44AM 
AK.00008: Determination of Hydrodynamic Parameters on TwoPhase Flow Gas  Liquid in Pipes with Different Inclination Angles Using Image Processing Algorithm Gustavo Montoya, Mar\'Ia Valecillos, Carlos Romero, Dosinda Gonz\'ales In the present research a digital image processingbased automated algorithm was developed in order to determine the phase's height, hold up, and statistical distribution of the drop size in a twophase system waterair using pipes with 0\r{ }, 10\r{ }, and 90\r{ } of inclination. Digital images were acquired with a high speed camera (up to 4500fps), using an equipment that consist of a system with three acrylic pipes with diameters of 1.905, 3.175, and 4.445 cm. Each pipe is arranged in two sections of 8 m of length. Various flow patterns were visualized for different superficial velocities of water and air. Finally, using the image processing program designed in Matlab/Simulink$^{\textregistered}$, the captured images were processed to establish the parameters previously mentioned. The image processing algorithm is based in the frequency domain analysis of the source pictures, which allows to find the phase as the edge between the water and air, through a Sobel filter that extracts the high frequency components of the image. The drop size was found using the calculation of the Feret diameter. Three flow patterns were observed: Annular, ST, and ST{\&}MI. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AK.00009: Bubble Size Control Mechanisms and Effect on Flow Regime Thomas Shepard, Paul Strykowski Research has been conducted in an effort to understand the impacts of different control mechanisms on bubble size during air injection into a liquid crossflow. The motivation for this work is to gain better control during the bubble formation and coalescence processes in order to reliably generate bubbles of different sizes. In this study air is injected through a porous plate into an electrolyte solution flowing through an adjustable geometry channel. The control mechanisms considered include the pore size in the porous plate, the channel height (and thus shear rate) at the injection site, and the electrolyte concentration. The effects of the controls are studied for a range of channel pressures (1060 psi) and gas to liquid mass flowrate ratios (0.0010.005). By varying the different controls, bubbles are generated with a mean diameter of 801600 microns while keeping the pressure and gas to liquid mass flowrate constant. An additional outcome of the research is the demonstrated effect that bubble size has on the transition from bubbly flow to slug flow. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AK.00010: Disturbances to AirLayer SkinFriction Drag Reduction at High Reynolds Numbers David Dowling, Brian Elbing, Simo Makiharju, Andrew Wiggins, Marc Perlin, Steven Ceccio Skin friction drag on a flat surface may be reduced by more than 80{\%} when a layer of air separates the surface from a flowing liquid compared to when such an air layer is absent. Past largescale experiments utilizing the US Navy's Large Cavitation Channel and a flatplate test model 3 m wide and 12.9 m long have demonstrated air layer drag reduction (ALDR) on both smooth and rough surfaces at water flow speeds sufficient to reach downstreamdistancebased Reynolds numbers exceeding 100 million. For these experiments, the incoming flow conditions, surface orientation, air injection geometry, and buoyancy forces all favored air layer formation. The results presented here extend this prior work to include the effects that vortex generators and free stream flow unsteadiness have on ALDR to assess its robustness for application to oceangoing ships. Measurements include skin friction, static pressure, airflow rate, video of the flow field downstream of the injector, and profiles of the flowing airwater mixture when the injected air forms bubbles, when it is in transition to an air layer, and when the air layer is fully formed. From these, and the prior measurements, ALDR's viability for fullscale applications is assessed. [Preview Abstract] 
Session AL: CFD I: Methods
Chair: Elias Balaras, University of MarylandRoom: 200A
Sunday, November 22, 2009 8:00AM  8:13AM 
AL.00001: A divergence preserving Adaptive Mesh Refinement strategy for viscous incompressible flows M. Vanella, E. Balaras Structured adaptive mesh refinement (SAMR) concentrates computational resources (i.e. grid points) in highgradient regions of the flow, while maintaining most of the desirable properties of structured Cartesian solvers. Whenever the computational grid is locally refined/derefined the flow variables in SAMR calculations need to maintain certain conservation properties during restriction or prolongation operations. Restriction refers to the transfer of a flow variable from a grid at a fine level of refinement to an underlying grid at a coarser level, while prolongation is the data transfer in the inverse direction. Of particular interest in S AMR applications in viscous incompressible flows are divergencepreserving prolongation operators of a vector field. When the mesh refinementderefinement procedure is applied after the predictor step of the fractional step integration scheme, divergence preservation for prolongation is crucial to avoid spurious pressure oscillations and additional errors on the computed flow field. In this work we propose method for divergencepreserving prolongation applicable to nested grids that differ by a factor of two in terms of resolution. The accuracy of the method is evaluated on prototypical laminar flows, like the TaylorGreen vortex problem and flow around a cylinder. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AL.00002: Towards a Fully Adaptive MeshFree Method for Solving Viscous Incompressible Flows Paritosh Mokhasi, Dietmar Rempfer A fully adaptive meshfree method based on radial basis functions (RBF) is proposed for numerically solving the Navier Stokes equations. The scheme is based on the method of lines wherein the spatial derivatives are approximated using a differential quadrature approach. The solution is progressed in time using a fractional step method with pressure correction. To demonstrate its flexibility, the 2D driven cavity problem is solved in the Eulerian and semiLagrangian framework using radial basis functions. We further demonstrate, via a 1D spatio temporal example, that using RBFs adaptively enables one to produce highly accurate results. Finally, we present algorithms for solving a large class of fluid dynamics problems using radial basis functions. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AL.00003: A spectral multidomain penalty method model for high Reynolds number incompressible flows Jorge EscobarVargas, Peter Diamessis We present our latest results towards the development of a spectral multidomain penalty methodbased incompressible NavierStokes solver for high Reynolds number stratified turbulent flows in doubly nonperiodic domains. Temporal discretization of the governing equations is based on three fractional steps (explicit advancement of nonlinear terms and implicit treatment of pressure and viscous terms). The spatial discretization uses a Legendre collocation approach in discontinuous quadrilateral subdomains. Numerical stability is enabled through a penalty scheme, spectral filtering and appropriately defined dealiasing. The conditioning of the linear system associated with the discretized Poisson equation for the pressure is analyzed in detail. In addition, the efficiency of various preconditioning strategies such as diagonal and block Jacobi, finite difference, and additive Schwartz are investigated. Finally, the efficiency and accuracy of the Navier Stokes solver are assessed through application to select test cases. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AL.00004: Wavelet regularization of the 2D incompressible Euler equations Romain Nguyen van Yen, Marie Farge, Kai Schneider We examine the viscosity dependence of the solutions of twodimensional NavierStokes equations in periodic and wallbounded domains, for Reynolds numbers varying from $10^3$ to $10^7$. We compare the NavierStokes solutions to those of the regularized twodimensional Euler equations. The regularization is performed by applying at each time step the waveletbased CVS filter {\it (Farge et al., Phys. Fluids, 11, 1999)}, which splits turbulent fluctuations into coherent and incoherent contributions. We find that for Reynolds $10^5$ the dissipation of coherent enstrophy tends to become independent of Reynolds, while the dissipation of total enstrophy decays to zero logarithmically with Reynolds. In the wallbounded case, we observe an additional production of enstrophy at the wall. As a result, coherent enstrophy diverges when Reynolds tends to infinity, but its time derivative seems to remain bounded independently of Reynolds. This indicates that a balance may have been established between coherent enstrophy dissipation and coherent enstrophy production at the wall. The Reynolds number for which the dissipation of coherent enstrophy becomes independent on the Reynolds number is proposed to define the onset of the fullyturbulent regime. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AL.00005: A Multigrid Accelerated HighOrder Compact Fractional Step Method for Unsteady Incompressible Viscous Flows Omer San, Anne Staples An efficient highorder compact scheme is presented for computing unsteady incompressible viscous flows. The scheme is constructed on a staggered Cartesian grid. Using the fractional step framework, the NavierStokes equations are advanced in time with the secondorder AdamsBashforth method without considering the pressure terms in the predictor step. The velocity field is then corrected so that the continuity equation is satisfied through a pressure Poisson equation. Since the efficiency of the fractional step method depends on the Poisson solver, a Mehrstellenbased Vcycle multigrid acceleration is implemented in the solution of the Poisson equation to enhance the computational efficiency. The method is validated by simulating a decaying TaylorGreen vortex. The results show that the method has high resolving efficiency, drastically reduced computational time, and highorder accuracy, making it applicable for the simulation of complex turbulent flows. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AL.00006: Vortexinduced vibrations of a long flexible cylinder in transitional and turbulent flows Remi Bourguet, George Karniadakis, Michael Triantafyllou The flow past a flexible cylinder subject to VortexInduced Vibrations (VIV) is investigated by direct numerical simulation at low and moderate Reynolds numbers (Re$=1001000$). The cylinder of large spanwise extension ($\ge 200$ diameters) is pinned and hinged at both ends and its central part is free to move in all directions under the effect of fluidstructure interaction. The cylinder dynamic is governed by a beamcable equation. The influence of Reynolds number and structural parameters such as tension, bending stiffness and mass ratio, on VIV amplitudes and characteristic frequencies is quantified. The relationship between hydrodynamic efforts, structure motion and vortex shedding pattern is examined during the transition to turbulence. In particular, modifications of the alternating shedding pattern related with specific VIV conditions are analyzed in respect to the appearance of space/time irregularities in the structure response. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AL.00007: A novel computational method to determine the dynamics of a lipid bilayer vesicle in a viscous flow David Salac, Michael Miksis Models of lipid bilayer vesicle motion require that both the local area element of the interface and the volume enclosed by the interface be conserved. Here we present a novel levelset computational method to predict the dynamics of a vesicle under the influence of an external viscous fluid. The fluid both inside and outside the vesicle is governed by the NavierStokes equations. We impose both the volume and area constraint by implementing a novel splitting scheme. Similar to standard pressurecorrection methods for the NavierStokes equations, which require the velocity field to be divergence free, we solve a variable coefficient pressurePoisson equation with Neumann boundary conditions to ensure volume conservation. We also impose the constraint that the velocity field must be divergence free on the moving interface. This necessitates the solution of an additional partial differential equation. This equation and the needed boundary conditions will be presented. Numerical examples of the scheme and convergence checks will also be presented. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AL.00008: Two layer fluid stress analysis during airway closure ChengFeng Tai, David Halpern, James Grotberg The airways are lined with a film consisting of two immiscible liquids, a serous layer and a more viscous mucus layer. Due to a surface tension driven instability, a liquid plug can form that obstructs the passage of air along the airways provided the ratio of the film thickness to the tube radius is greater than a critical value $\sim $0.12. In this study, we assume that the liquid layers are Newtonian, the surface tension is constant at the interfaces and the aircore phase is passive. We solve the NavierStokes and continuity equations subject to interfacial stress conditions and kinematic boundary conditions numerically using a finite volume approach in conjunction with a sharp interface method for the interfaces. Surface tension, viscosity and film thickness ratios can be altered by disease, and their influence on the closure instability is investigated. Results show that the shear and normal stresses along the airway walls can be strong enough to injure airway epithelial cells. We acknowledge support from the National Institutes of Health grant number NIH HL85156. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AL.00009: Feasibility of using Hybrid Wavelet Collocation  Brinkman Penalization Method for Shape and Topology Optimization Oleg V. Vasilyev, Mattia Gazzola, Petros Koumoutsakos In this talk we discuss preliminary results for the use of hybrid wavelet collocation  Brinkman penalization approach for shape and topology optimization of fluid flows. Adaptive wavelet collocation method tackles the problem of efficiently resolving a fluid flow on a dynamically adaptive computational grid in complex geometries (where grid resolution varies both in space and time time), while Brinkman volume penalization allows easy variation of flow geometry without using bodyfitted meshes by simply changing the shape of the penalization region. The use of Brinkman volume penalization approach allow seamless transition from shape to topology optimization by combining it with level set approach and increasing the size of the optimization space. The approach is demonstrated for shape optimization of a variety of fluid flows by optimizing single cost function (time averaged Drag coefficient) using covariance matrix adaptation (CMA) evolutionary algorithm. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AL.00010: Uncertain shape optimization for dense gas flows Pietro Marco Congedo, Christophe Corre, JeanMarc Martinez Uncertain shape optimization is a fascinating but challenging task. Our work explores some key issues in uncertain optimization and proposes a strategy to obtain a more reliable solution at a moderate computational cost. The steady transonic inviscid flow of a dense gas over an airfoil is considered and a shape optimization performed to minimize the airfoil's drag coefficient. Three sources of uncertainties are accounted for : thermodynamic model, freestream conditions and geometry. The combined effect of these uncertainties is analyzed to get the average and variance of the drag coefficient, that are both minimized during the optimization. Preliminary stochastic simulations based on polynomial chaos expansions yield the most influent uncertain parameters; several optimization strategies are then studied, with an original combination of response surfaces and metamodels, to obtain robust optimal solutions for a limited number of flow computations. [Preview Abstract] 
Session AM: Reacting Flows I
Chair: Heinz Pitsch, Stanford UniversityRoom: 200B
Sunday, November 22, 2009 8:00AM  8:13AM 
AM.00001: Flamelet model for supersonic combustion Vincent Terrapon, Heinz Pitsch, Rene Pecnik The vast majority of computational work in supersonic turbulent combustion has so far relied on simplified/reduced mechanisms and the explicit transport of the involved species. Such approaches require then closure of the chemical source term in the species transport equation. An alternative approach is based on the flamelet concept which assumes that the chemical time scales are shorter than the turbulent time scales so that the flame can be approximated as onedimensional. However, the implementation of the flamelet model is based on a low Mach number assumption, explaining the still very limited number of studies of high speed flows using this approach. Since supersonic speed and compressibility effects play an important role at supersonic speeds, the flamelet implementation has been reformulated where temperature is not any longer given by a chemistry table but computed from the total energy and the tabulated species mass fractions, thus, better accounting for compressibility effects. The model is applied to the combustor of the HyShot II vehicle and results are compared to experiment measurements and simulation data. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AM.00002: LES/PDF Modeling of Soot Evolution in Turbulent Flames Venkatramanan Raman, Michael Mueller, Guillaume Blanquart, Heinz Pitsch Modeling soot evolution is turbulent flames is a complex problem due to the nonlinear interactions between the soot particles and the gasphase turbulent combustion process. In this work, we develop a transported probability density function (PDF) approach for soot description in the context of large eddy simulation (LES) based combustion modeling. The soot number density is described using the bivariate VS (volumesurface) approach. The number density evolution equation is discretized using the hybrid method of moments technique, where the first four moments of the number density function are solved. In the transported PDF approach, the joint subfilter distribution of the gasphase thermochemical scalars and the soot moments are evolved using a Lagrangian MonteCarlo approach. The LESPDF approach is validated using a piloted diffusion flame experiment. Results indicate that the PDF approach predicts delayed inception of soot particles and lower soot volume fraction as compared to the pure LES approach. While the soot volume fraction along the centerline of the jet is overpredicted by the simulation, the radial distribution is underpredicted as compared to the experiments. Further, the influence of mixing model on soot evolution is discussed. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AM.00003: Simulation of an ethyleneair jet flame with soot and radiation modeling Jeffrey Doom, Joseph Oefelein Large eddy simulation of an ethyleneair diffusion flame and supporting direct numerical simulations are presented. A reduced mechanism recently developed by Wang et al. is used (22 species, 107 reactions) and a systematic study is performed which compares the reduced mechanism to the original full mechanism ({\it USC Mech Version II}: 111 species, 784 reactions). A series of calculations are then validated by comparing results with CHEMKIN, Lignell et al. ({\it Combust. Flame 2007}) and the premixed experiments from Bhargava \& Westmoreland ({\it Combust. Flame 1998}). The baseline soot model employed is from Leung et al ({\it Combust. Flame 1991}) and accounts for nucleation, growth, oxidation and coagulation. This model is coupled through source terms as a function of $C_2 H_2$, $C O$, $O_2$ and $H_2$. The first two moments are considered to account for the number density and soot mass per volume. Initially the radiation model assumes an optically thin medium in a manner consistent with Lignell et al. Results associated with the soot model will be presented along with comparisons with experimental data. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AM.00004: Direct Quadrature Method of Moments for LESbased Modeling of Supersonic Combustion Pratik Donde, Heeseok Koo, Venkat Raman The LES/transported probability density function (PDF) model has been successfully used for predictive modeling of turbulent combustion in lowspeed flows. The PDF approach evolves the jointdistribution of the gasphase thermochemical composition and is ideally suited for supersonic flows, where conservedscalar approaches are not valid due to the compressible nature of the flow. In lowspeed flows, the highdimensionality of the PDF transport equation is handled through the use of MonteCarlo based stochastic methods. However, the presence of shocks and large density and pressure gradients pose significant challenges in the use of these stochastic methods for highspeed flows. In this work, we propose a direct quadrature method of moments (DQMOM) approach, which is a fully Eulerian method for solving the PDF transport equation. Here, the subfilter PDF is discretized in terms of a finite number of delta functions, each characterized by a weight and an abscissa. Eulerian transport equations for these quantities are similar in structure to scalar transport equations and can be solved using finitevolume/finite difference approaches. Here, the accuracy of the DQMOM approach and the numerical implementation of this method using shockcapturing schemes are discussed. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AM.00005: Effects of Acoustic Excitation on Bluffbody Stabilized Premixed Reacting Flows Vaidyanathan Sankaran, Robert Erickson, Marios Soteriou Bluff body stabilized flames are used in numerous combustion applications to enable stable burning at high speeds. These confined flames are susceptible to acoustic excitation arising due to the confinement that can lead to thermoacoustic instabilities which are detrimental to the operability of the combustion device. In this study, we formulate a computational approach for the simulation of this phenomenon that is based on the one way coupling of an acoustic solution to a low Mach number but dilatational reacting flowfield. The latter is simulated with a purely Lagrangian and grid free approach that captures the rotational flowfield using the discrete Vortex Method and the reacting field by a kinematical solution of the Gequation. Earlier studies using this flow simulation approach have shown that the unsteady interactions, such as the transition from the asymmetric VonKarman shedding to the more symmetric shedding structure present when reaction occurs can be captured accurately. Flame response to longitudinal acoustic waves is simulated and results are to be discussed in the context of transfer functions of heat release response to acoustic velocity excitation. Dominant mechanisms by which the flame responds to acoustics will also be identified. Finally, results are to be contrasted to those from analytical models that are in use in thermoacoustic studies today. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AM.00006: The Interaction of HighSpeed Turbulence with Flames Alexei Poludnenko, Elaine Oran Interaction of flames with turbulence occurs in systems ranging from chemical flames on Earth to thermonuclear burning fronts in supernovae. We present results of a systematic study of the dynamics and properties of turbulent flames formed under the action of highspeed turbulence in stoichiometric hydrogenair mixture. Numerical simulations were performed using the massively parallel reactiveflow code AthenaRFX. Here we discuss (1) global properties of the turbulent flame in this regime (flame width, speed, etc.); (2) the internal structure of the flame brush; and (3) the internal structure of the flamelets folded inside the flame brush. We demonstrate that, in the case of hydrogen, turbulence does not affect the internal flame structure essentially for all subsonic turbulent intensities. We address the relative role of largescale and smallscale motions on global and local properties of the turbulent flame. We also consider the processes that determine the turbulent burning velocity and identify two distinct regimes of flame evolution. Finally, we discuss the effects of nonequilibrium nonKolmogorov turbulence on the turbulent flame properties. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AM.00007: Transport by molecular diffusion in LES of a turbulent diffusion flame Konstantin Kemenov, Sharadha Viswanathan, Haifeng Wang, Stephen Pope Molecular diffusion effects in LES of a piloted methaneair (Sandia D) flame are investigated on a series of grids with progressively increased resolution towards the DNS limit. The role of molecular diffusivity in effecting spatial transport is studied by drawing a comparison with the turbulent diffusivity and analyzing their statistics conditioned on temperature. Statistical results demonstrate that the molecular diffusivity in the nearfield almost always exceeds the turbulent diffusivity, except at low temperatures (less than 500K). Thus, by altering the jet nearfield, molecular transport plays an important role in the further downstream jet development. Molecular diffusivity continues to dominate in the centerline region throughout the flow field. Overall, the results suggest the strong necessity to represent molecular transport accurately in LES studies of turbulent reacting flows. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AM.00008: A LES/FDF/PMCbased Detailed Model for Luminous Turbulent Flames Ankur Gupta, Daniel Haworth, Michael Modest A comprehensive model is presented for luminous turbulent flames with full consideration of turbulence/chemistry interactions (TCI), turbulence/radiation interactions (TRI), and detailed gas phase chemistry and soot. A largeeddy simulation/composition filtered mass density function (LES/CFDF) formulation is adopted that accounts exactly for the influence of subfilterscale turbulent fluctuations on chemical source terms and radiative emission. A consistent Lagrangian Monte Carlo particle/Eulerian mesh method is used to solve the modeled CFDF equation. A second Monte Carlo particle method (photon Monte Carlo  PMC) is used to solve the radiative transfer equation; the radiation model includes spectral radiation properties and absorption. Soot is modeled using a method of moments. The model is validated using experimental data for luminous turbulent nonpremixed jet flames. The model then is exercised to isolate and quantity different contributions to TRI. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AM.00009: Flow field structure near the reaction zone in turbulent nonpremixed jet flames Mirko Gamba, Noel T. Clemens, Ofodike A. Ezekoye Quasiinstantaneous pseudovolumes of the 3D velocity field in the far field of turbulent nonpremixed jet flames are constructed from cinematographic kilohertzrate stereoscopic PIV applying Taylor's hypothesis. Jet flames at jet exit Reynolds numbers of 8,00015,000 were considered. The approach enable computation of all nine velocity gradients and the 3D kinematic quantities. 10 Hz OH PLIF imaging was also included to mark the reaction zone. Threedimensional rendering of regions of intense vorticity and energy dissipation reveals their sheetlike nature and their tendency to exist near the OH layers. Contrary to nonreacting jets, this feature is believed to be a due to the stabilizing effect of heat release and the laminar shear caused by the flame. Singlepoint statistics of the velocity gradients indicate anisotropy in the flow with strong gradients predominantly in the radial direction. However, the 1D energy spectrum and singlepoint statistics of the principal strain and strainvorticity alignment follow the known trends from incompressible turbulence. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AM.00010: Effects of Swirl on StronglyPulsed Turbulent Diffusion Flames Y.H. Liao, J.C. Hermanson The dynamics of largescale structures in stronglypulsed, swirling, turbulent jet diffusion flames were examined experimentally. The combustor used a combination of axial and tangentiallyinjected air to produce a range of swirl numbers. Gaseous ethylene fuel was injected through a 2 mm diameter nozzle on the combustor centerline with a jeton Reynolds number of 5000. The flames were fullymodulated, with the fuel flow completely shut off between pulses. Highspeed imaging of the flame luminosity was employed to examine the flame dimensions and the celerity of the largescale flame structures. The flames were found to be approximately 1520{\%} shorter when swirl was imposed, depending on the injection time. The more compact flames in swirl appear to be due to the presence of recirculation inside the flames. For longer injection times, the celerity of the flame structures generally decreases as the swirl intensity increases. This is evidently due to the reversed velocity in the recirculation zone. For shorter injection times, the flame celerity has an increasing trend with increased swirl intensity due to flames being closer to the fuel nozzle at burnout. [Preview Abstract] 
Session AN: Experimental Techniques I
Chair: Lester Su, Johns Hopkins UniversityRoom: 200C
Sunday, November 22, 2009 8:00AM  8:13AM 
AN.00001: Evaluation of burstmode LDA spectra with implications Clara Velte, William George Burstmode LDA spectra, as described in [1], are compared to spectra obtained from corresponding HWA measurements using the FFT in a round jet and cylinder wake experiment. The phrase ``burstmode LDA'' refers to an LDA which operates with at most one particle present in the measuring volume at a time. Due to the random sampling and velocity bias of the LDA signal, the Direct Fourier Transform with accompanying weighting by the measured residence times was applied to obtain a correct interpretation of the spectral estimate. Further, the selfnoise was removed as described in [2]. In addition, resulting spectra from common interpolation and uniform resampling techniques are compared to the above mentioned estimates. The burstmode LDA spectra are seen to concur well with the HWA spectra up to the emergence of the noise floor, caused mainly by the intermittency of the LDA signal. The interpolated and resampled counterparts yield unphysical spectra, which are buried in frequency dependent noise and step noise, except at very high LDA data rates where they perform well up to a limited frequency.\\[4pt] [1] Buchhave, P. {\bf PhD Thesis}, SUNY/Buffalo, 1979.\\[0pt] [2] Velte, C.M. {\bf PhD Thesis}, DTU/Copenhagen, 2009. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AN.00002: Twodimensional velocity measurements using LaserCantileverAnemometry in comparison to xwire anemometry Michael H\"olling, Jaroslaw Puczylowski, Joachim Peinke We present an improved 2D LaserCantileverAnemometer (2D LCA) which allows for measurements in two dimensions. The two velocity components are resolved by detecting the bending and the torsion of a tiny cantilever using the laser pointer principle. Thereby a two dimensional position sensitive detector measures the movement of the reflected laser light coming from the cantilever. Measurements carried out with this 2D LCA in comparison to xwire data acquired in the wake of a cylinder are presented. [Preview Abstract] 

AN.00003: ABSTRACT WITHDRAWN 
Sunday, November 22, 2009 8:39AM  8:52AM 
AN.00004: Design and Characterization of an Optical FeedbackControlled Microphone for Aeroacoustics Research Eliott Radcliffe, Ahmed Naguib, William Humphreys, Jr. An optical feedbackcontrolled microphone was designed and tested for potential use in phased ``beamforming'' arrays used in aeroacoustics research. Optical sensing was employed as a means for measuring center displacement of a stretched thin membrane due to incident acoustic pressure. The membrane was constructed of PVDF which exhibits piezoelectric properties allowing actuation of the membrane in a feedback system. The latter was used to actively modify sensor parameters, most notably membrane stiffness, resonant frequency, and damping. Testing of a prototype microphone was performed using a plane wave tube calibrator. The results demonstrate that feedback control is an effective method for improving the microphone's transient response, as well as for ``selftuning'' and matching of microphone parameters in sensing arrays. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AN.00005: Measurements of instantaneous temperature in oscillating flows Philippe BlancBenon, Arganthael Berson, Gaelle Poignand, Genevieve ComteBellot Temperature fluctuations in turbulent flows are usually investigated using cold wires operated by a constantcurrent anemometer. However, the output voltage of such anemometers is not hardware compensated for the thermal inertia of the wire. A correction is applied only during the postprocessing of the data and requires the knowledge of the time lag of the wire, which depends both on the wire properties and on the instantaneous incident flow velocity. Here, a simple procedure for the instantaneous correction of the thermal inertia of cold wires is proposed. The method relies on the splitting of the time lag of cold wires operated in a constantcurrent mode into two factors: one depending on the wire properties and the other depending on flow velocity. These two factors are obtained from the operation of the wire by a constantvoltage anemometer in the heatedmode. The uncompensated signal delivered by the constantcurrent anemometer operating the cold wire is then processed to restore the signal that would be delivered by an ideal cold wire. Validation experiments are conducted in an acoustic standingwave resonator where largeamplitude oscillatory flows take place. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AN.00006: Use of grid generated turbulence to assess hotwire spatial resolution and Pitot probe turbulence corrections Anand Ashok, Mohammad Javed, Sean Bailey, Alexander Smits The objective of the present study is to use grid generated homogeneous isotropic turbulence as a benchmark flow to test the effect of turbulence on different measurement techniques. The grid turbulence is generated in a low speed 2 foot by 3 foot closed circuit wind tunnel using a 1 inch square mesh grid placed at the test section inlet. Measurements of the turbulence using a variety of hotwire sensor lengths, at a series of streamwise distances downstream of the grid, will be used to investigate the impact of spatial filtering on different turbulence statistics and fully characterize the grid turbulence. Points of comparison will include basic mean and turbulent kinetic energy profiles as well as higher order statistics, the turbulent dissipation rate and finally turbulent spectra. These statistics will then be combined with additional measurements to assess the performance of turbulence corrections for Pitot probe velocity measurements. Finally, it is hoped that this flow can be used to assess the performance of a new nanoscale hot wire probe currently under development. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AN.00007: The accuracy of crossstream velocity gradients measured by multisensor hotwire probes Milan \v{S}ekularac, Petar Vukoslav\v{c}evi\'{c}, James Wallace, Elias Balaras, Nikolaos Beratlis A highly resolved turbulent minimum channel flow DNS with $Re_{\tau} =180$ was used to investigate the effects on the accuracy of simultaneous measurements of velocity gradient components resulting from the spatial resolution and sensor arrangement of twelvesensor hotwire probes. The sensors were represented as points on the simulation grid, the effective velocity cooling each sensor was determined and sensor equations were then solved in response to the DNS field to obtain velocity and velocity gradient components. It has been found that all crossstream velocity gradients except $\partial v/\partial y$ and $\partial w/\partial z$ can be measured with reasonable accuracy. Depending on the arrangement of the sensors and the array and probe sizes, either one or the other of these two gradients is subject to high measurement error in the near wall region of this bounded flow. As a consequence, the estimation of $\partial u/\partial x$ from the direct measurement of $\partial v/\partial y$ and $\partial w/\partial z$ by applying the continuity equation for incompressible flow is questionable. It appears that this is the likely explanation for the weak correlation in the near wall region of $\partial u/\partial x$, estimated by applying Taylor's hypothesis, with its value estimated using the continuity equation as has been found in several investigations using twelvesensor probes. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AN.00008: Effect of Wind Tunnel Wall Adaptation on Flow over a Circular Cylinder Serhiy Yarusevych, Michael Bishop The presence of test section walls in many experimental facilities give rise to blockage effects, which detrimentally influence experimental data. A unique method to eliminate such blockage effects is to adapt the walls of a test section so as to mimic the conditions of an unbounded flow. The effect of such wall adaptation on flow development over a circular cylinder was the focus of this investigation. Velocity and surface pressure measurements were made in three test section wall configurations: geometrically straight walls (GSW), aerodynamically straight walls (ASW), and streamlined walls (SLW). In all the wall configurations investigated, tests were conducted for Re$_{d}$ = 58,000 and model blockage ratios of up to 17{\%}. The results show that, in GSW and ASW, blockage effects significantly alter flow development, affecting separated shear layer instability frequency, vortex shedding frequency, and limiting wake growth. Streamlining the walls successfully mitigates these adverse effects, with the relevant flow parameters shown to match those obtained in previous investigations conducted at low blockage ratios. Although the blockage effects produce an increase of both the separated shear layer instability and the wake vortex shedding frequency in GSW and ASW, the results suggest the ratio of these frequencies is invariant with the wall configuration. A comparative analysis of experimental data is performed to explain the observed trends. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AN.00009: Design of a High Viscosity Couette Flow Facility for Patterned Surface Drag Measurements Tyler Johnson, Amy Lang Direct drag measurements can be difficult to obtain with low viscosity fluids such as air or water. In this facility, mineral oil is used as the working fluid to increase the shear stress across the surface of experimental models. A mounted conveyor creates a flow within a plexiglass tank. The experimental model of a flat or patterned surface is suspended above a moving belt. Within the gap between the model and moving belt a Couette flow with a linear velocity profile is created. PIV measurements are used to determine the exact velocities and the Reynolds numbers for each experiment. The model is suspended by bars that connect to the pillow block housing of each bearing. Drag is measured by a force gauge connected to linear roller bearings that slide along steel rods. The patterned surfaces, initially consisting of 2D cavities, are embedded in a plexiglass plate so as to keep the total surface area constant for each experiment. First, the drag across a flat plate is measured and compared to theoretical values for laminar Couette flow. The drag for patterned surfaces is then measured and compared to a flat plate. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AN.00010: Redesign of contraction, test section and diffuser for a sixinch high speed water tunnel Ivaylo Nedyalkov, Martin Wosnik The sixinch high speed water tunnel was recently moved from St. Anthony Falls Laboratory to the University of New Hampshire, where it is being restored. This water tunnel was a 1:6 scale model for the 36inch Variable Pressure Cavitation Tunnel at David Taylor Model Basin and was used in many fundamental cavitation studies in the past, including the development of Schiebe bodies. It originally had a 6inch circular test section and was later retrofitted with a 7inch octagonal test section. In order to increase the maximum achievable velocity in the test section and improve the flow quality a new 6inch square test section with diminishing 1inch fillets was designed, which also required the design of a new contraction and diffuser. Contraction, test section and diffuser configurations were studied parametrically using CFD. The numerical predictions are compared to results in the literature and measurements in the tunnel. Further improvements include a new motor and control system. The renovated sixinch tunnel will be used for research on control of cavitating flows, hydrofoil development and general cavitation studies. [Preview Abstract] 
Session AP: Instability: Boundary Layers I
Chair: William Saric, Texas A&M UniversityRoom: 200D
Sunday, November 22, 2009 8:00AM  8:13AM 
AP.00001: Global optimal disturbances using timesteppers Antonios Monokrousos, Luca Brandt, Dan S. Henningson The global linear stability of boundarylayer flows subject to threedimensional disturbances is studied by means of Lagrange optimization. We consider the optimal initial condition leading to the largest growth at finite times and the optimal harmonic forcing leading to the largest asymptotic response (pseudospectrum). Both optimization problems are solved using a Lagrange multiplier technique, where the objective function is the kinetic energy of the flow perturbations and the constraints involve the linearised NavierStokes. Whereas the computation of optimal initial condition is known in the timestepper context, the formulation of the optimal forcing problem is novel. The approach proposed here is particularly suited to examine convectively unstable flows, where single global eigenmodes of the system do not capture the downstream growth of the disturbances. For spanwise wavelengths of the order of the boundary layer thickness finitelength streamwise vortices exploit the liftup mechanism to create streaks. For long spanwise wavelengths the Orr mechanism combined with the amplification of oblique wave packets are responsible for the disturbance growth. The latter mechanism is found to be dominant for the relatively long computational domain and high Reynolds number considered here. The use of matrixfree methods enables us to extend the present framework to any geometrical configuration. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AP.00002: Receptivity of G\"{o}rtler Flow LarsUve Schrader, Luca Brandt, Dan Henningson, Tamer Zaki The flow over a concave surface, e.g. the lower side of a turbine blade, is subject to centrifugal forces which may destabilize the boundary layer. The instabilities appear as streamwise aligned counterrotating vortices and may be steady or traveling, depending on the perturbation source. We consider the boundary layer on a concave wall with constant radius of curvature and expose the flow to two different disturbance sources: streamwiselocalized, spanwisesinusoidal roughness elements and freestream vortical disturbances modeled by continuousspectrum modes for the Blasius inflow. Results from numerical simulations using the Spectral Element Method (SEM) will be shown. The SEM provides spectral accuracy while allowing for geometries beyond the scope of global spectral methods based on Fourier modes. Owing to the nonparallel nature of the G\"{o}rtler vortices, threedimensional simulations are in particular appropriate to characterize the receptivity of the G\"{o}rtler boundary layer. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AP.00003: Boundarylayer transition in the wake of surface irregularities Jeffrey Crouch, Vladimir Kosorygin, Lian Ng Aerodynamic surfaces designed for laminar flow inevitably have geometric imperfections. These imperfections impact the unsteady processes in the boundary layer and may accelerate the laminarturbulent transition. An experimental study is conducted to investigate the steady and unsteady disturbances in the wake of protruding and recessed surface irregularities, and to link these disturbances to the initial movement of the transition location. The steady disturbance field on the centerline of the irregularity is characterized by a region of velocity deficit followed by a much longer region of velocity surplus. Unsteady disturbances in the wake of the irregularity, measured prior to transition, have increased magnitudes and display a shift toward higher frequency. Local stability analysis is shown to capture many of the features of the pretransitional flow. The transition Reynolds numbers collapse reasonably well when plotted in terms of the roughness height (nondimensionalized by the boundarylayer displacement thickness). The initial movement of the transition can be represented by a reduction in the critical Nfactor, consistent with a linearamplitude based transition criterion. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AP.00004: Calculating boundary layer receptivity to transiently growing roughnessinduced perturbations using experimental data Nicholas Denissen, Edward White The receptivity problem is of great interest in perturbations generated by surface roughness. To quantify nonmodal receptivity, continuous spectrum amplitude distributions are calculated for transiently growing roughnessinduced perturbations in a flatplate boundary layer. Complex, realistic, surface roughness is beyond the scope of direct numerical simulation (DNS) currently. This makes analyzing the receptivity of experimental results essential. A method using regularizing functionals is shown for calculating the distributions when only partial experimental data is available. These results are validated against DNS results. These amplitude distributions provide a way of rigorously characterizing the boundary layer receptivity to surface roughness. Extracting the continuous spectrum amplitudes using the partial data technique reveals the underlying vortex behavior that creates transient growth that is too difficult to measure experimentally. The method described is amenable to future work with realistic distributed roughness and complex surface geometries, and is applied to cases currently beyond the scope of DNS. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AP.00005: Control of Stationary Crossflow Modes in a Supersonic Boundary Layer using Distributed Roughness ChanYong Schuele, Eric Matlis, Thomas Corke, Stephen Wilkinson, P. Balakumar, Lewis Owens Passive methods like distributed micron sized roughness elements have proven to work efficiently as subsonic laminar flow control devices. Attempts to experimentally extend the principle of suppression of the most amplified stationary cross flow modes to supersonic boundary layers have not been successful until now. This study presents evidence for the receptivity of a supersonic boundary layer with transition dominated by stationary cross flow modes to patterned roughness with different wave numbers. Experiments have been performed at the Mach 3.5 NASA LaRC Supersonic Low Disturbance Tunnel on a $7\deg$ half angle sharp cone at $4.3\deg$ angle of attack and a unit Reynolds number of $2.5x10^5in^{1}$. Pitot tube pressure measurements as well as surface flow visualization were used to detect the occurence of stationary crossflow modes and transition. Based on these two measurement approaches we conclude that the stationary crossflow mode was receptive to the passive patterned roughness, indicating that control of transition to turbulence in crossflow dominated conditions should be possible. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AP.00006: Effect of Surface Thermal Perturbations on Compressible Boundary Layer Stability Christopher Alba, Datta Gaitonde Highspeed laminarturbulent boundary layer transition is a critical issue for reentry and sustained hypersonic cruise vehicles. Turbulent wall heating rates can increase several orders of magnitude compared to laminar rates and skin friction drag can become a major component of the overall drag. We analyze approaches to modulate transition by altering the stability features of the boundary layer through the use of thermal perturbations. To this end, highfidelity numerical simulations to generate basic states for Mach 1.5 and Mach 5.6 flat plate boundary layers with and without thermal bumps. Linear Parabolized Stability Equations (PSE) are solved using the STABL software suite to establish the flow stability characteristics under baseline (no excitation), constant and pulsed bump cases for each freestream Mach number. The effects are described in terms of neutral curves showing amplification for various frequencies versus Reynolds number. The threedimensional flow structure is also examined near the breakdown to turbulence flow region to gain insight into the final stages of transition. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AP.00007: Experiments and NPSE of roughness receptivity in sweptwing boundary layers William Saric, Matthew Woodruff, Helen Reed New data are presented on 3D boundarylayer receptivity to roughness in lowdisturbance environments. The measurements include infrared thermography with calibrated and temperaturecompensated hotfilms to study roughnessrelated issues of boundarylayer transition in flight. A sweptwing model is mounted on the wing of a Cessna O2 aircraft where nonlinear parabolized stability equations (NPSE) correlate the stability measurements and transition locations. The laminarization scheme of spanwiseperiodic discrete roughness elements (DRE) is investigated at chord Reynolds numbers of 7.5 million. Flight experiments were conducted where the surface roughness amplitude was varied from 6 to 50 microns while the disturbance shearstress was measured with calibrated hotfilm gauges in two locations: $x/c $= 15{\%} and 30{\%}; the former in the linear range and the later in the nonlinear range. In this way, the disturbance velocity amplitude was calculated as a function of roughness Reynolds number. These data were then used as initial conditions for the NPSE calculations to determine the efficacy of the DREs. The work was supported by: AFOSR Grant FA9550050044, AFRL, and NASA Langley Research Center. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AP.00008: Uncertainty quantification of the instability in a supersonic boundary layer with roughness Olaf Marxen, Gianluca Iaccarino, Eric Shaqfeh Knowledge of the location of laminarturbulent transition on the surface of vehicles (re)entering a planetary atmosphere is important for heatshield design. However, due to the heatshield material itself or as a result of ablation during flight, the surface of a heat shield is often not smooth. Instead, surface roughness occurs, but the height of this roughness may not be known beforehand. A numerical investigation of disturbance amplification in a laminar compressible flatplate boundary layer with a localized 2D roughness is carried out. Both linear and weakly nonlinear disturbance evolution are considered. The nonlinear case exhibits a secondary subharmonic resonance. In addition to deterministic simulations, a stochastic approach is applied to quantify uncertainties. The random parameter is chosen to be the height of the roughness in the linear case, while in the nonlinear case the amplitude of the primary disturbance is considered a random parameter. Deterministic simulations show that the 2D roughness acts as an amplifier for convective disturbances, and the resulting increased disturbance amplitude can enhance a secondary instability. The stochastic approach allows to quantify the probability for an increased or decreased amplification. [Preview Abstract] 

AP.00009: ABSTRACT WITHDRAWN 
Sunday, November 22, 2009 9:57AM  10:10AM 
AP.00010: Classification of the Flow Produced by an Oscillating Fence in a Laminar Boundary Layer Michael Hind, William Lindberg, Jonathan Naughton Flow visualization has revealed that an oscillating fence produces a range of vortical structures in a flat plate laminar boundary layer. The structure can be classified by the ratio $\phi_0$ of the fence oscillation frequency to the fundamental shedding frequency of the static fence. Particle image velocimetry was used to quantitatively investigate the flow structures of each classification regime. Fences operating in the subcritical flow regime ($\phi_0<0.1$) shed vortices due to vortex saturation behind the fence. The vortices of the critical flow regime ($\phi_0 \sim 1$) strengthen during the fence upstroke and are forced to shed once the fence begins to descend. The vortices of the supercritical flow regime ($\phi_0>1$) are shed once per fence oscillation cycle and coalesce to form larger vortices at the fundamental shedding frequency of the static fence. For the transitional flow regime ($\phi_0 \sim0.11$), the structures are twodimensional during the fence upstroke that become threedimensional once the fence begins to descend. Through this classification system, it is possible to determine the frequency required for a given flow to produce the desired type of structure. By varying the fence frequency, the structure can be made to change dramatically. [Preview Abstract] 
Session AQ: Instability: Interfacial and ThinFilm I
Chair: Andreas Acrivos, Stanford UniversityRoom: 200E
Sunday, November 22, 2009 8:00AM  8:13AM 
AQ.00001: Surfactant and elasticityinduced inertialess instabilities in vertically vibrated liquids Satish Kumar, Balram Suman We investigate instabilities that arise when the free surface of a liquid covered with an insoluble surfactant is vertically vibrated and inertial effects are negligible. In the absence of surfactants, the inertialess Newtonian system is found to be stable, in contrast to the case where inertia is present. Linear stability analysis and Floquet theory are applied to calculate the critical vibration amplitude needed to excite the instability, and the corresponding wavenumber. A previously reported longwavelength instability is found to persist to finite wavelengths, and the connection between the longwavelength and finitewavelength theories is explored in detail. The instability mechanism is also probed and requires the Marangoni flows to be sufficiently strong and in the proper phase with respect to the gravity modulation. For viscoelastic liquids, we find that instability can arise even in the absence of surfactants and inertia. Mathieu equations describing this are derived and these show that elasticity introduces an effective inertia into the system. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AQ.00002: Anatomy of a wave J\'er\^ome Hoepffner, Ralf Blumenthal, St\'ephane Zaleski A perturbation is induced at the sheared interface between a stream of liquid and a stream of gas. This initial perturbation then evolves as the response of inertia, viscosity and interfacial tension. We observe that the wave obtained by this procedure tends to a selfsimilar regime after a short transient. We describe the anatomy of this welldefined growing wave as the physical parameter are varied, in particular as the density ratio of the two phases is changed. This study is aimed at identifying a possible recurrent agent in atomization processes. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AQ.00003: Interfacial flow control in twophase systems with application to liquid bridges Ilya Ryzhkov, Valentina Shevtsova We perform a theoretical study of thermocapillary flows and their stability in a twophase system of infinite liquid column surrounded by the gas layer. This study is a complementary step in the JEREMI project (JapaneseEuropean Research Experiment on Marangoni Instability). It is devoted to the development of efficient means for controlling thermocapillary flows in liquid bridges (columns) and scheduled to fly on ISS in 2011. The flows are controlled by applying mechanical stresses to the interface and varying the interfacial heat exchange by blowing gas around the liquid. The analytical solution describing stationary velocity and temperature profiles in the liquid and gas is derived. It is shown that liquid motion can be completely suppressed by the gas flow. The linear stability analysis of stationary flows is performed. It is shown that when the gas flow is opposite to (codirected with) that of liquid on the interface, the system becomes more (less) stable. It occurs due to mechanical stresses applied to the interface and interfacial heat exchange. Consideration of liquid bridge with the surrounding gas provides better agreement with experimental results than previous calculations without gas phase. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AQ.00004: Stability and structure formation in films of binary mixtures Santiago Madruga, Uwe Thiele Films of polymer blends are used in technological applications such as coatings or structured functional layers. The evolution of those films is involved by the coupling of decomposition within the film and the dewetting of the film. We present a model for films of binary mixtures, such as polymer blends, with free evolving surfaces. The model is based on modelH describing the coupled transport of concentration and momentum fields supplemented by boundary conditions at the substrate and free surface. We analyze the linear stability of vertically stratified base states of free surface films with respect to lateral perturbations [1]. For purely diffusive transport, an increase in film thickness either exponentially decreases the lateral instability or entirely stabilizes the film. The inclusion of convective transport leads to further destabilization as compared to the purely diffusive case [2]. We study as well the dependence of the instability on parameters such as the Reynolds number, the surface tension number and the ratio of velocities of convective and diffusive transport. [1] U. Thiele, S. Madruga, and L. Frastia. Phys. of Fluids. 19, 122106, (2007). [2] S. Madruga and U. Thiele. To appear in Phys. of Fluids. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AQ.00005: Dynamics and stability of turbulent falling films Aliki Mavromoustaki, Lennon O'Naraigh, Omar Matar We study the dynamics of thin turbulent films falling under the action of gravity. A base state, corresponding to a waveless film, is obtained by balancing gravity against viscous drag. The latter includes turbulent viscosity contributions characterised by a simple mixing length model. A linear stability analysis of this base state is then carried out leading to the derivation of an OrrSommerfeldtype eigenvalue problem. Numerical solutions of this problem reveal a Reynolds numberdependent competition between destabilising contributions arising from the turbulent base state and stabilising ones from the turbulent stresses at the interface and in the bulk. An energy budget analysis demonstrates clearly that the destabilising mode corresponds to an interfacial one. Our results also reveal that the most dangerous mode is in the longwave regime. This provided motivation for the derivation of a longwave model for the nonlinear film dynamics, which represents an extension of the Shkadov equations for turbulent falling films. The results of a brief parametric study of this model are presented. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AQ.00006: Coherent wave structures on falling fluid films flowing down a flexible wall Grigori Sisoev, Richard Craster, Satish Kumar, Omar Matar The dynamics of a thin fluid film flowing down a flexible vertical wall at moderate flow rates is studied in order to identify the dominant wave structures that will be observed in experiments. An asymptotic reduction using boundarylayer theory, and the von K\'arm\'anPolhausen approximation, leads to coupled partial differential equations governing the nonlinear dynamics of the flow rate, and the gasliquid and liquidsolid interfaces; closure is provided by a semiparabolic fluid velocity profile. Fluid inertia, capillarity and viscous retardation effects are incorporated as are wall damping and tension. The validity of our approach is demonstrated using direct comparisons with predictions from the OrrSommerfeld equations. Nonlinear steadytravelling waves are identified from a nonlinear eigenvalue problem illustrating a multiplicity of solutions from which the dominating (attracting) solutions can be identified. Subsequent timedependent numerical simulations of the fullynonlinear partial differential equations demonstrate the selection of these dominant solutions, and, as such, they then constitute a point of direct comparison with physical experiments. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AQ.00007: The Effect of Dynamic Wetting on the Stability of a GasLiquid Interface Subjected to Vertical Oscillations Andrew M. Kraynik, Louis Romero, John R. Torczynski, Carlton F. Brooks, Timothy J. O'Hern, Richard A. Jepson, Gilbert L. Benavides The stability of an interface in a container partially filled with silicone oil and subjected to gravity and vertical oscillations has been examined theoretically and computationally. An exact theory for the onset of a parametric instability producing Faradaylike waves was developed for arbitrary liquid viscosity, stressfree walls, and deep twodimensional or axisymmetric containers. Finiteelement simulations for stressfree walls are in excellent agreement with the theory, which predicts instability in discrete frequency bands. These simpler calculations are a departure point for examining the more realistic problem, which involves noslip at the walls and dynamic wetting modeled with a Blake condition. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DEAC0494AL85000. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AQ.00008: On contact line induced instability in flow of hanging fluid films TeSheng Lin, Lou Kondic We consider free surface instabilities of films hanging on inverted substrates within the framework of lubrication approximation. Contrary to all the previous works, we include fluid fronts in formulation. It is found that the presence of contact lines leads to free surface instabilities of convective type without any additional natural or excited perturbations. A single parameter $D=(3Ca)^{1/3}\cot\alpha$ , where $Ca$ is the capillary number and $\alpha$ is the inclination angle, is identified as a governing parameter in the problem. This parameter may be interpreted to reflect the combined effect of inclination angle, film thickness, Reynolds number and the fluid flux. Variation of D leads to change of the wavelike properties of the instabilities, allowing to observe traveling wave behavior, mixed waves, and the waves resembling solitary ones, which were observed in many other unstable flows. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AQ.00009: Draw Resonance in Viscous Sheets Olus Boratav, Zheming Zheng, Alexey Amosov The instability known as the ``draw resonance'' in literature is studied for a viscous sheet considering the viscogravity balances (Stokes number) and the heating/cooling effects (Stanton number). The analysis considers lubrication approximation for continuity, momentum and energy equations and determines the critical draw ratio for a range of Stokes numbers and Stanton numbers. The critical draw ratio is~very sensitive to the~variation of Stokes and Stanton numbers. It is shown that the decrease in Stokes number and/or the increase in Stanton number results in a decrease in the critical draw ratio. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AQ.00010: Transience to Instabilty in a Liquid Sheet Nathaniel Barlow, S.P. Lin, Brian Helenbrook Series solutions are found which describe the evolution to absolute and convective instability in an inviscid liquid sheet flowing in an ambient gas and subject to a localized perturbation. These solutions are used to validate spatiotemporal stability predictions for sinuous and varicose modes. We show how recent disagreements in growth predictions stem from assumptions made when arriving at the Fourier integral response. Certain initial conditions eliminate (or reduce the order of) singularities in the Fourier integral. For the sinuous mode, deLuca and Costa (1997) predicted that an impulsive disturbance spreads both upstream and downstream and grows like $t^{1/3}$ when the Weber number is smaller than one. If a Gaussian perturbation is applied to both the position and velocity of the sheet, we observe this behavior in our series solution. However, when the initial disturbance velocity is taken to be zero, we find that the origin decays like $t^{2/3}$. This is the growth predicted by Luchini (2004). [Preview Abstract] 
Session AR: Buoyancy Driven Flows
Chair: Grae Worster, University of CambridgeRoom: 200F
Sunday, November 22, 2009 8:00AM  8:13AM 
AR.00001: Icicles: diffusive gravity currents and phase change M.G. Worster, J.A. Neufeld, R.E. Goldstein The growth and melting of icicles motivates this study of diffusive gravity currents. For example, the growth of an icicle vertically downwards is mediated by convective heat transfer in the surrounding air. The flow of the buoyant air is traditionally viewed as a nearvertical boundary layer and solved using the associated partial differential boundarylayer equations. However, near the tip of the icicle gravity acts in a direction orthogonal to the primary direction of flow. This situation is exemplified by the case of buoyancydriven flow above a cooled, finite, horizontal plate or below a heated, finite horizontal plate. We find solutions of the corresponding boundarylayer equations in the form of approximate, nonlinear, separable solutions, with the horizontal variation of the boundarylayer thickness being governed by equations similar to those for a gravity current. We exploit the structure of such solutions to compute the steadystate shape and rate of melting of an icicle. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AR.00002: Mean flow influence on a gravity current in a nearly horizontal confined geometry Thomas Seon, Mohammad Taghavi, Krista Thielmann, Mark Martinez, Ian Frigaard We study experimentally the effect of an imposed mean flow on a buoyancydriven exchange flow of two miscible fluids of the same viscosity in a long tube, oriented close to horizontal. Measuring the front velocity, $V_f$, as a function of the mean flow velocity, $V_0$, for different density contrasts, viscosity, and inclination angles has allowed us to identify two regimes. First, for low $V_0$ the flow is dominated by the buoyancydriven flow and the dynamics are similar to the exchange flow dynamics. Secondly, for high $V_0$ the flow is dominated by the imposed mean flow, the front velocity varies proportionally to the mean flow velocity and the ratio $V_f/V_0$ does not appear to be very sensitive to the density contrast or viscosity, but does increase as the tube gets closer to vertical. In this regime we find that the flow becomes more laminar and stable, as $V_0$ increases. This appears counter intuitive, since more energy is being injected into the system through the mean flow. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AR.00003: Modified thermal theory for gravity currents on sloping boundaries Albert Dai In this study we generalize the thermal theory adopted in Beghin, Hopfinger, and Britter (J. Fluid Mech. Vol. 107, 1981, p. 407) in order to account for both entrainment and detrainment effects occurring in the motion of gravity currents. We observe that although the model of Beghin et al. (1981) qualitatively captures the acceleration and deceleration phases of gravity current motion, their pure entrainment model consistently underpredicts the gravity current velocity and the distance before the maximum velocity is reached. Their model, therefore, could easily overestimate the arrival time of a gravity current generated by an instantaneous buoyancy release. We find that the effect of detrainment is to increase the predicted velocity of gravity current and extend the predicted distance before the maximum velocity is reached. The effect of detrainment is not immediately obvious, but it explains the differences between the experimental data and the model of Beghin et al. (1981). The idea presented here will lead to more investigations of gravity currents on sloping boundaries. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AR.00004: Study of the Front Structures of Gravity Current Using Simultaneous PIV/PLIF Measurement Jun Chen, Duo Xu The mixing and entrainment associated with the front development of gravity current have important implications in studying of many atmospheric and oceanic flow problems. A series of laboratory experiments are performed to investigate the development of the front structure of gravity current in an apparatus in which dense fluid is introduced into a less dense environment through a locking gate. A simultaneous PIV/PLIF system is developed to measure the velocity and density fields. The dynamics and structures around the current front are examined as well as the effect of bottom inclination. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AR.00005: Jets, plumes and particleladen jets in twodimensional environments J.R. Landel, C.P. Caulfield, A.W. Woods Results on the experimental investigation of liquid jets and plumes in a quasi Hele Shaw cell are presented. Few experimental studies have been conducted when jets are constrained in a narrow gap whose length is two orders of magnitude smaller than the length scales of the other two dimensions. In this configuration, the dynamics shown by the jets is very rich when parameters such as the initial flowrate and the buoyancy are changed. Furthermore, different behaviors have been observed for the front of the jet and the flow in steady state. In particular, the models for the rise of the jet and the expansion must be slightly modified between the two cases. PIV techniques have been used to measure accurately the flow field of the jets and to allow accurate comparison with the theoretical models. Finally, the results of this investigation on jets and plumes serve as a basis for more complex experiments involving particle laden jets. A better understanding of liquid jets with varying buoyancy proves to be useful to the study of this twophase flow. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AR.00006: Instability Phenomena in Stratified, Particleladen Flow Peter Burns, Lutz Lesshafft, Eckart Meiburg When a layer of particleladen water is placed above clear water of different temperature and salinity, various instabilities may arise. Depending on the specific density configuration, distinct convection patterns (``fingering'' vs. ``leaking'') have been reported from experiments (Parsons et al. 2001, Maxworthy 1999). We present linear stability results for such situations, with a focus on the role of particle settling. The effect of the settling velocity on the temporal instability growth rates is investigated in combination with various salinity distributions. The nonlinear evolution of the resulting instability structures is studied via DNS. Using linear analysis, DNS and experimental literature we hypothesize various mechanisms in an attempt to explain the occurrence of ``leaking'' and ``fingering.'' Current nonlinear results will be presented in an effort to fully explain the ``leaking'' mode. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AR.00007: Intrusiongenerated internal waves in a constantly stratified fluid Benjamin Maurer, Paul Linden Intrusive Gravity Currents (IGCs) occur when horizontal density gradients result in the intrusion of one fluid into another fluid at an intermediate depth. The vertical density stratification of the receiving fluid necessary for an IGC is also capable of supporting internal wave motion. Though many IGCs in the ocean and atmosphere propagate into a stratified fluid, traditional assessments of GC and IGC dynamics neglect energy losses to internal wave motion. We present an experimental study of the internal wave field propagating ahead of a wellmixed intrusion into a constantly stratified ambient fluid. Intrusions at various heights in the ambient fluid are examined, and synthetic schlieren imaging techniques are used to quantify wave motion ahead of the current. We note a strong influence of the level of IGC propagation on the forcing of particular supercritical wave modes, and estimate the associated energy fluxes. To construct a more balanced energy budget of intrusions into stratified environments, we compare these losses to the initial Available Potential Energy (APE) of the system and to estimates of the kinetic energy of the IGC. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AR.00008: Transient adjustment of UFAD systems in demand response operations Jong Keun Yu, Paul Linden Transient responses of a UFAD system due to Demand Response (DR) are investigated theoretically and experimentally. DR activities can be categorized by reducing thermal loads and increasing room setpoint temperature, which change the thermal environment in a room and can cause occupant thermal discomfort. By comparing the filling box time (Baines \& Turner 1968) and the replenishment time in which all the air in the enclosure is replaced by supply air, nondimensional models of UFAD systems are proposed and validated by laboratory experiments using a saltwater analogy. Twolayer stratification in the model allows us to estimate the temporal temperature change in the occupied zone and the interface height. Various DR activities, adjusting thermal loads and room setpoint temperature, are simulated to reveal the dynamic thermal responses. This study suggests that the interfacial height quickly converges to steady state compared to occupied zone temperature. The experiments show good agreement with the theoretical predictions of DR responses. [Preview Abstract] 

AR.00009: ABSTRACT WITHDRAWN 
Sunday, November 22, 2009 9:57AM  10:10AM 
AR.00010: The role of diffusion in natural displacement ventilation Nigel Kaye, Morris Flynn The classic natural displacement ventilation model of Linden {\it et al.} (1990) predicts the formation of a two layer stratification when a single thermal plume is introduced into a room with vents at floor and ceiling level. The model assumes that molecular diffusion plays no role in the development of the rooms ambient stratification as such diffusion is a slow process and the plume entrainment field will act to thin the interface between the warm upper layer and cool lower layer. The prediction of a sharp interface has been confirmed by small scale salt bath experiments. However, full scale measurements and CFD simulations at larger scale indicate that the interface between the two layers is not sharp but smeared out over a finite thickness. We present two simple models for predicting the thickness of the interface as a function of the room height, floor area and vent area as well as the plume buoyancy flux and the thermal diffusivity of the fluid. The interface increases in thickness with increasing room floor area and decreasing plume strength. Our model is compared to interface thickness measurements based on CFD simulations and salt bath models and is shown to agree both phenomenologically and numerically. \\[4pt] Linden, LaneSerf, \& Smeed, (1990) `Emptying filling boxes, the fluid mechanics of natural ventilation' J. Fluid Mech. {\textbf{212}} pp. 309335. [Preview Abstract] 
Session AS: Geophysical: Atmospheric I
Chair: Zellman Warhaft, Cornell UniversityRoom: 200G
Sunday, November 22, 2009 8:00AM  8:13AM 
AS.00001: Effects of Release Characteristics on Urban Contaminant Dispersal A.J. Wachtor, F.F. Grinstein, H.J. Catrakis The release of a chemical, biological, or radioactive contaminant in an urban environment is of particular interest due to the high population densities in urban areas. The wind flow that transports the contaminant through the urban setting is highly complex and exhibits a wide range of multiscale phenomena. Studies of urban flows can provide information that can be of critical importance to city, state, and federal officials for creating risk management plans. Classical field experiments measuring the dispersion of scalars in urban environments provide only rather limited results. Computational experiments have the advantage of being able to offer greater insight and knowledge about the threedimensional flow physics than field experiments are able to provide. Implicit Large Eddy Simulation (ILES) is currently a promising computational method to obtain reasonable results of urban flows. ILES resolves the large scale flow features and relies on inherent numerical dissipation to model energy transfer from the resolved scales to the subgrid scales. Since it is the large scale dispersion of the contaminant that is of key interest, ILES is particularly well suited for this application. NRL's FAST3DCT model based on ILES is used to simulate scalar contaminant transport in a complex urban setting. We present a study of the effects that location and associated potential temperature of the scalar contaminant release have on the subsequent dispersion of that scalar within the specified urban geometry. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AS.00002: Temporal behavior of topographic wavebreaking Olivier Eiff, Nicolas Boulanger, Karine Leroux, Alexandre Paci At low Froude numbers, the internal waves generated by flow over an obstacle or mountain will overturn and break. In the atmosphere, this results in high altitude clear air turbulence but also affects the flow field below, the most commonly known effect being the acceleration the downslope winds. Surprisingly litte is known, however, of the dynamics of the wave breaking itself. Afanasyev and Peltier (JAS, vol. 55, 1998) investigated the wave breaking region via LES and Eiff et al. (DAO, vol. 40, 2005) via PIV measurements, but both presumed a statistically stationary wavebreaking process after the initial wave overturning. Here, we propose to take a closer look at this assumption by closely analyzing the spatiotemporal structure of internal wave breaking region and the surrounding flow. The analysis is based on Hovm\"{o}ller diagrams and spatial correlations obtained from 2DPIV measurements of flows generated in uniform stratified flow over 2D and quasi2D obstacles in saltstratified hydraulic channels at different Reynolds numbers ranging from laminar to turbulent. The results reveal low frequency variations throughout the flow field, in and outside the wavebreaking region. This characteristic frequency can be related to be due to a sequence of growth and decay of wavebreaking. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AS.00003: Satellite observations of atmospheric water vapor distributions Kyle Pressel, William Collins The Intergovernmental Panel on Climate Change Fourth Assessment Report identified cloud feedback as the largest source of uncertainty in Global Climate Model (GCM) estimates of climate sensitivity. Cloud feedback is resultant from the sensitivity of clouds to the thermodynamic structure of the atmosphere which is in turn modified by the clouds themselves. Prognostic statistical cloud schemes have been developed to account for subgridscale cloud variability in a more physically consistent manner. Statistical cloud schemes assume a distributional form for some measure of water substance concentration and then determine cloud cover and properties based on a particular parameterization of that distribution. As the majority of atmospheric water substance exists in a vapor state, we will report preliminary results of a characterization of water vapor distributions based on retrievals from the Advanced Infrared Sounder (AIRS) onboard NASA's Aqua satellite. We will report on the vertical variation of distributional forms with height and comment on the physical mechanisms maintaining these distributions. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AS.00004: Experimental study of the effect of turbulence on the dynamics of sedimenting inertial particles Colin Bateson, Alberto Molina, Alberto Aliseda, Hossein Parishani, Lian Ping Wang, Wojciech Grabowski Understanding the dynamics and mutual interactions of droplets in turbulent flows is important to many engineering and environmental problems including fuel injector sprays, warm rain formation, and the mass and energy transfer between the ocean and the atmosphere. Specifically, the collision and coalescence in turbulent flows is considered a key element for the growth of condensation droplets into a size range where gravitational settling mechanism can take over to produce rain drops. We study experimentally the effect of turbulence on the collisioncoalescence of water droplets over a parameter range relevant to rain formation. Droplets in a size range between 1 and 40 microns are injected inside a low speed wind tunnel through an array of atomizers located at the nodes of a turbulenceinducing grid that covers the tunnel's cross section with uniform spacing. The evolution of the droplet size distribution, concentration and settling velocity is measured along the wind tunnel's test section. We will present a comparison between experimental measurements of the one and two dimensional droplet radial distribution functions and collision statistics against equivalent quantities computed from a three dimensional numerical simulation performed and presented here by Wang et al. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AS.00005: Statistics of SmallScale Velocity Fluctuations and Internal Intermittency in Stratocumulus Clouds Raymond Shaw, Holger Siebert, Zellman Warhaft Clouds are known to be turbulent but the details of their internal turbulent structure have been largely unexplored. Measurements of turbulent velocities in stratocumulus clouds reveal an intermittent structure consistent with that observed in classic homogeneous isotropic turbulence. The measurements were taken using a hotwire anemometer on the helicopterborne ACTOS measurement system. Hotwire signal artifacts resulting from droplet impacts are removed without significantly degrading the signal, such that highorder velocity structure functions can be evaluated. The structure function analysis for orders 2 through 8 show statistically significant departures from the Kolmogorov 1941 scaling, yielding scaling exponents consistent with the KolmogorovObukhov refined similarity hypothesis with an intermittency exponent of 0.25. We find no evidence of any departure from the large body of knowledge obtained from the laboratory on the fine scale turbulence structure. This suggests that processes depending on the finescale structure of turbulence that cannot presently be measured in clouds can be explored in the laboratory setting. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AS.00006: Remote flow sensing of complex systems: steps towards spatiotemporal~prediction of flow patterns Bruno Monnier, Paritosh Mokhasi, Dietmar Rempfer, Candace Wark Prediction of the spatial and temporal phenomena of wind flow patterns~through urban areas is investigated. Typically sparse measurements~are used in wind forecasting models for updating and prediction via a~method~called variational data assimilation. To improve upon this method, an~experimental investigation combining various measurement tools (Hot Wire~Anemometry,~Stereoscopic Particle Image Velocimetry SPIV), static pressure~measurements and Laser Doppler Velocimetry(LDV)) is carried out to study~the~airflow around wall mounted obstacles in a turbulent boundary layer.~The method of Proper Orthogonal Decomposition (POD) is used to~decompose the flow field into a finite set of POD coefficients which~vary only in time associated with a corresponding set of POD basis~functions which vary only in space. Direct measurement models utilizing~the~measurements from SPIV and LDV, along with indirect measurement models~using~sparse measurements from microphones are investigated and may ultimately~be~combined with statespace models to obtain more robust dynamical models. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AS.00007: Wind speed and direction measurements using the sphere anemometer Hendrik Heisselmann, Michael Hoelling, Joachim Peinke In times of growing energy demand, the importance of wind energy is rapidly increasing and so is the need for accurate wind speed and direction measurements. The widely spread cup anemometers show significant overspeeding under turbulent wind conditions as inherent in atmospherical flows while being solely capable of detecting the wind speed. Therefore, we propose the newly developed sphere anemometer as a simple an robust sensor for direction and velocity measurements. The sphere anemometer exploits the velocitydependent deflection of a tube, which is the order of $\mu$m and can be detected by means of a light pointer as used in atomic force microscopes. In comparative measurements under laboratory conditions the sphere anemometer showed a significantly higher temporal resolution then cup anemometers while it does not exhibit any overspeeding. Additionally, results of atmospherical wind measurements with the sphere anemometer and stateoftheart cup anemometry are presented. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AS.00008: Experimental study of \emph{starting plumes} simulating cumulus cloud flows in the atmosphere Duvvuri Subrahmanyam, K.R. Sreenivas, G.S. Bhat, S.S. Diwan, Roddam Narasimha Turbulent jets and plumes subjected to offsource volumetric heating have been studied experimentally and numerically by Narasimha and coworkers and others over the past two decades. The offsource heating attempts to simulate the latent heat release that occurs in cumulus clouds on condensation of water vapour. This heat release plays a crucial role in determining the overall cloud shape among other things. Previous studies investigated steady state jets and plumes that had attained similarity upstream of heat injection. A better understanding and appreciation of the fluid dynamics of cumulus clouds should be possible by study of \emph{starting plumes}. Experiments have been set up at JNCASR (Bangalore) using experimental techniques developed previously but incorporating various improvements. Till date, experiments have been performed on plumes at $Re$ of 1000 and 2250, with three different heating levels in each case. Axial sections of the flow have been studied using standard PLIF techniques. The flow visualization provides us with data on the temporal evolution of the \emph{starting plume}. It is observed that the broad nature of the effect of offsource heating on the \emph{starting plumes} is generally consistent with the results obtained previously on steady state flows. More complete results and a critical discussion will be presented at the upcoming meeting. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AS.00009: Attracting structures in volcanic ash transport Jifeng Peng Volcanic eruptions and ash clouds are a natural hazard that poses direct threats to aviation safety. They may also affect human and ecosystem health. Many transport and dispersion models have been developed to forecast trajectories of volcanic ash clouds, as well as to plan safety measures. Predictions based on these models are heavily dependent on initial parameters of ash clouds, e.g., location, height, particle size and density distribution, water vs. ash content, etc. However, these initial parameters are usually difficult to determine, leading to possible inaccurate predictions of ash clouds trajectories. In this study, a dynamical systems approach is combined with volcanic ash transport models to help improve prediction. A type of attracting structures in volcanic ash transport is identified. These structures act as attractors in volcanic ash transport, and they are independent of initial parameters of specific volcanic eruptions. The attracting structures are associated with hazard zones with high concentrations of volcanic ash. And the prediction in hazard maps can be used to plan flight route diversions and ground evacuations. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AS.00010: Mathematical Model for the Behavior of Wildfires Kevin DelBene, Donald Drew Wildfires have been a longstanding problem in today's society. In this paper, we derive and solve a fluid dynamics model to study a specific type of wildfire, namely, a two dimensional flow around a concentrated line of fire, resulting in a narrow plume of hot gas rising and entraining the surrounding air. The model assumes that the surrounding air is constant density and irrotational, and uses an unsteady plume model to describe the evolution of the mass, momentum and energy inside the plume, with sources derived to model mixing in the style of Morton, Taylor, and Turner (Proc. Roy. Soc. London, A 234, 123, 1956). The sources to the dynamical processes in the plume couple to the motion through the surrounding air through a BiotSavart integral formulation to solve the equations of motion with a line of singularities along the plume. The singularities model a vortex sheet in the same manner as Alben and Shelley (Phys. Rev. Letters, 100, 074301, 2008), except that we include a sink term in the BiotSavart integral to couple the entrainment. The results show that this model is capable of capturing a complicated interaction of the plume with the surrounding air. [Preview Abstract] 
Session AT: Vortex Dynamics and Vortex Flows I
Chair: Jeffrey Marshall, University of VermontRoom: 200H
Sunday, November 22, 2009 8:00AM  8:13AM 
AT.00001: Harvesting energy in the wake of a circular cylinder using piezoelectric materials Dogus H. Akaydin, Niell Elvin, Yiannis Andreopoulos The voltage generated by short, flexible piezoelectric cantilever beams placed inside turbulent wakes of circular cylinders at Reynolds numbers of 10,000 is investigated experimentally and computationally. The coherent vortical structures present in this flow generate a periodic forcing on the beam which when tuned to its resonant frequency produces maximum output voltage. There are two mechanisms which contribute to the driving forcing of the beam. The first mechanism is the impingement of induced flow by the passing vortices on one side of the beam and second is the low pressure core region of the vortices which is present at the opposite side of the beam. The sequence of these two mechanisms combined with the resonating conditions of the beam generated maximum energy output which was also found to vary with the location in the wake. The maximum power output was measured at about two diameters downstream of the cylinder. This power drops off the center line of the wake and decays with downstream distance as (x/D)$^{3/2}$. A threeway coupled interaction simulation that takes into account the aerodynamics, structural vibration and electrical response of the piezoelectric generator has been developed. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AT.00002: Integrability and Chaos in BodyVortex Interactions Johan Roenby, Hassan Aref We explore the class of dynamical systems consisting of a rigid body and $N$ point vortices in an ideal, unbounded, 2D fluid. The body is represented by a closed curve and is free to move in response to the fluid motion. It may have a prescribed circulation about it, which is conserved. The vortices have fixed strengths and are intended to model vortices that have been shed by the body or elsewhere in the flow field. The flow at any given time and position is determined by the instantaneous vortex and body positions together with the instantaneous linear and angular velocity of the body. The equations of motion may be cast in Hamiltonian form. We analyze the equations of motion using techniques from the theory of dynamical systems. The simplest such system, a single point vortex and a circular body, is integrable. As we add vortices, or change other features of the system such as the body shape, the motion may become chaotic. Numerical solutions are shown and analyzed with an emphasis on the transition to chaos and its physical meaning. This class of systems provides a rich family of fewdegreeoffreedom systems that capture essential fluidbody interaction physics. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AT.00003: Vortex suppression in the wake of counter rotating cylinders Peter Dewey, Alexander J. Smits Digital particle image velocimetry is used to study the flow past a pair of counter rotating cylinders placed sidebyside normal to the freestream flow direction. The Reynolds numbers based on cylinder diameter is varied from 100 to 200 and gaptodiameter ratios of 1, 3 and 5 are considered. An unsteady wake consisting of a pair of von K\'{a}rm\'{a}n vortex streets is present in the flow field when the cylinders are rotated below a critical value. Above this critical value, counter rotation of the cylinders suppresses vortex formation. The critical rotational speed varies only slightly with Reynolds number but exhibits a strong dependence on the gaptodiameter ratio. As the gaptodiameter ratio increases, the critical rotational speed approaches values expected to suppress vortex formation for a single rotating cylinder, indicating that the wakes of the cylinder pair have more interaction for small gaptodiameter ratios. At sufficiently high rotational speeds the streamlines around the cylinder pair resemble a doublet potential flow. The experiments were inspired by the computations performed by Andy Chan and Antony Jameson at Stanford University. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AT.00004: Flow Development over a Circular Cylinder with a Stepwise Discontinuity Chris Morton, Serhiy Yarusevych Crossflow around a step cylinder is common in various engineering applications, for example, heat exchangers and buildings, where understanding flow development is often of critical importance for engineering design. Moreover, the relatively simple geometry of a step cylinder allows modeling complex vortex interactions. For a step cylinder in uniform flow, the flow development is dependent on the Reynolds number (Re$_{D})$ and the ratio of the large cylinder diameter (D) to the small cylinder diameter (d). In this study, vortex shedding phenomena occurring in the wake of a step cylinder is investigated using an unsteady RANS based numerical approach for Re$_{D}$ = 300 and D/d = 2. Based on the numerical results, three distinct spanwise vortex cells were identified in the step cylinder wake: one vortex shedding cell in the wake of the small cylinder and two vortex shedding cells in the wake of the large cylinder. A comparative analysis with available experimental data showed that the numerical simulations adequately modeled wake vortex development and interactions in the near wake region. One of the vortex cells forming downstream of the step was found to have a cyclic appearance, with the periodicity being linked to downwash fluctuations near the step. In addition, the results suggest that streamwise vortices develop at the step and unsteady interactions between the streamwise and spanwise vortices occur in the near wake. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AT.00005: Three dimensional flow around a flexible circular cylinder in crossflow Francisco HueraHuarte, David Jeon, Morteza Gharib The three dimensional flow around a flexible cantilever model undergoing vortexinduced vibrations has been studied by using Defocusing Digital Particle Image Velocimetry (DDPIV). The DDPIV technique allowed the simultaneous measurement of the motion and the flow around a portion of the cylinder. Different circular cylinder models accounted for several high aspect (length over diameter) and low mass ratios (mass over mass of displaced fluid), leading to flowinduced vibrations with different dominant mode shapes and frequencies. The quantitative study of the wake structures of cylinders able to vibrate at different structural modes is of particular interest as there is a lack of published results. Moreover, the mechanisms yielding to mode lockedon behavior and the topology of the wake under this situation are poorly understood. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AT.00006: VortexBody Interaction using a LevelSet Based Overset Grid Method Simtha Renjitham, Jeffrey Marshall An overset grid method is presented for solution of the integral vorticityvelocity formulation of the NavierStokes equations. The method uses an inner bodyfitted grid and an outer Cartesian grid. The BiotSavart integral is solved using an adaptive, optimized multipole acceleration method. The integration is performed over all inner grid cells, over all ``active cells'' of the outer grid that lie entirely outside of the inner grid, and over subelements of a set of overhanging cells of the outer grid that overlap part of the inner grid. A levelset function is introduced in which the zero levelset curve coincides with the outer surface of the inner grid. This levelset function is used to rapidly subdivide the overhanging grid cells into triangular subcells which lie entirely outside of the inner grid, while omitting the parts of these cells that lie inside the inner grid, so as to avoid doublecounting the vorticity in these regions. The pressure is solved as a postprocessing variable using a boundaryelement formation that requires evaluation of an integral using a parallel method to that used for velocity calculation. The method is applied to twodimensional flow past stationary and moving bodies, and it is well suited to vortexbody interaction with complex, moving bodies. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AT.00007: On the frequency of high Reynolds number shedding in bluffbody wakes Fernando Ponta In this talk we shall explore the physical phenomenon of vortex shedding at high and extrahigh Reynolds number. We start from a previous work where Ponta and Aref introduced a rationale for the empirically observed StrouhalReynolds number relationship for vortex shedding at low Reynolds. Analyzing the turbulent transport of momentum, the rationale is extended to high Reynolds number regimes. Results compared satisfactorily with the existent experimental evidence, and their extension to extrahigh Reynolds geophysical flows will be discussed. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AT.00008: Using a hybrid cyberphysical system in the study of body motion due to vortex dynamics A. Mackowski, C.H.K. Williamson We are interested in the effect of vortex dynamics causing vibration of bodies in a flow. In these studies, one needs to select essential parameters for the body, such as mass, spring stiffness, and damping. Normally, these parameters are set physically by selecting mechanical elements. However, in our approach, which utilizes a computercontrolled XY$\Theta$ towing tank and a forcefeedback control system, we impose mass springdamping parameters in virtual space and in three degrees of freedom. [A similar concept, in one degree of freedom, was pioneered by a group at MIT (Miller 1996; Hover, Techet, Triantafyllou 1997), in studies of vortexinduced vibration of cables.] Although the use of a cyberphysical system has clear advantages over using a fixed, physical experiment, there are serious challenges to overcome in the design of the governing control system. The presence of noise in the dynamic force measurements and the effects of a finite time delay in controller response cause problems both for the implementation and physical accuracy of such a setup. In this presentation, we explore a new methodology for creating a controller suitable for systems with several degrees of freedom. Our controller, based on a discretization of Newton's laws, makes it straightforward to add and modify any kind of nonlinear, timevarying, or directional force, virtually. We shall present applications of this approach to problems in flowinduced vibration. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AT.00009: Vortexinduced vibrations of an elastically mounted sphere at Re = 300: Hysteresis and vortex shedding modes Suresh Behara, Iman Borazjani, Fotis Sotiropoulos We carry out fluidstructure interaction (FSI) simulations to investigate the excitation mechanisms and vortex shedding modes of an elastically mounted sphere that is free to oscillate in all three directions using the FSICURVIB method [Borazjani et al, J. Comp. Physics, 2008]. The simulations are performed for Re=300 over a range of reduced velocities. We report novel results showing hysteresis in the response curve depending on whether the reduced velocity is decreased or increased. Large amplitude oscillations are found to persist even for small reduced velocities when the reduced velocity is decreased from higher values. Increasing the reduced velocity from low values, on the other hand, causes the largeamplitude oscillations to be excited only at higher reduced velocities. Our simulations elucidate the 3D wake structures associated with each hysteresis branch and reveal a new vortex shedding mode. We show that the lower hysteresis branch exhibits the standard braidedhairpin wake mode while the upper branch exhibits a striking wake structure characterized by intertwined, longitudinal spiral vortices. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AT.00010: Vortex shedding interactions with an oscillating flat plate Arnold Song, Kenneth Breuer We present results from a model system designed to study the interactions between vortex shedding and structural compliance, as might be exhibited in systems as diverse as flying animals with compliant wings or traffic signs subject to hurricaneforce winds. A sharpedged plate is mounted at high angle of attack such that vortex shedding from the leading and trailing edges results in fluctuations of the aerodynamic forces. In its openloop mode of operation, the angle of the plate is oscillated in a controlled sinusoidal manner, and the aerodynamic forces and vortex characteristics are measured using a torque sensor at the root of the support rod and a hot wire located in the wake. The onset of hysteresis in the aerodynamic forces generated during the pitching cycle is documented as a function of mean and fluctuating angles. In its closedloop mode, the angle of the plate becomes a function of the aerodynamic forces such that an arbitrary virtual stiffness and damping can be proscribed. These different modes of operation, generated by the interactions between the fluid and structural forces are presented and discussed. [Preview Abstract] 
Session AU: Granular I: Jamming I
Chair: Robert Behringer, Duke UniversityRoom: 200I
Sunday, November 22, 2009 8:00AM  8:13AM 
AU.00001: Jamming of Granular Materials in Wedge Hoppers Summer Saraf, Scott Franklin We study the jamming of ordinary and rodlike granular materials in wedgeshaped hoppers and compare the probability distributions for exit mass with those obtained from cylindrical hoppers. While cylindrical hoppers show an exponential probability distribution, we find that the rectangular exit aperture of wedgeshaped hoppers exhibit a power law decay for both spheres and rods. This behavior can be explained with a model of the rectangular exit aperture as composed of a series of round, adjacent apertures each with a statistically independent jamming probability. We speculate that the spatially varying jamming probability results from inhomogeneities in the granular material, with regions of the material more tightly packed than others, and thus more likely to jam. [Preview Abstract] 
Sunday, November 22, 2009 8:13AM  8:26AM 
AU.00002: Jamming of Granular Flow in a TwoDimensional Hopper Junyao Tang, Sepehr Sadighpour, Robert Behringer We seek an understanding of the physics of jamming in flow from a hopper. Using spatiotemporal video data for photoelastic disks (mean diameter $d$) flowing through a twodimensional hopper (opening size $D$.), we have found experimental support for the hypothesis that the probability of flow surviving until time $t$ without jamming has the form $P_s(t) = \exp (t/\tau)$. The important physics is encapsulated in $\tau$, and how that depends on the ratio $D/d$. Estimates of $\tau$ vary as a powerlaw or an exponential in $D/d$ for a jamming model and an arch formation model. Through particle tracking we conclude that jamming requires both a high packing fraction and a stable force chain arch at the outlet. Work in progress is yielding data for $\tau$ vs. the hopper angle as well as $D/d$. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AU.00003: Twodimensional dense granular material subject to uniform simple shear Jie Ren, R. Behringer We have performed 2D shear experiments using photoelastic particles and a novel apparatus to investigate the role of shear strain on the jamming of a dense granular material. The goal of this work is to explore the shear stress axis in the jamming phase diagram proposed by Liu and Nagel. The experiments are carried out using a 2D simple shearing apparatus, whose base is specifically designed to perform nearly uniform shear at the particle scale across the shearing area. By using photoelastic particles, we can measure stresses, strains, contact forces, and particle displacements while shearing. We note two important observations from this work: 1) for densities that are below the value for isotropic jamming, the system jams when subject to shear strain; 2) starting from jammed isotropic states at higher densities, the application of quasistatic shear at constant density does not lead to failure, but rather a strengthening of the system. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AU.00004: Topology of force chains in dense granular materials Lou Kondic, Yiguang Yan, Miroslav Kramar, Konstantin Mischaikow Force chain structures are well known and well researched due to their importance in determining static and dynamic features of dense particulate systems. However, so far there is no well defined approach towards understanding properties of these structures and distinguishing them in different systems. In this talk, we will present novel approach based on algebraic topology techniques that will be used to analyze and quantify force chain structures. In particular, we will discuss how these properties differ for the systems exposed to shear versus compression, and correlate the topological measures to the phenomena such as jamming. While the present talk will concentrate on the results of discrete element simulations, we will see that this new approach has a significant potential in comparing experimental and theoretical results in a well defined and precise manner. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AU.00005: Frictioninduced hysteresis in quasistatic granular jamming Mahesh Bandi, Andras Libal, Michael Rivera, Robert Ecke Static granular packings are usually interrogated via quasistatic measurements where the packing fraction serves as the control parameter to study the pack evolution. In the absence of externally induced vibrations (effective granular temperature), quasistatic measurements are justified because the system in question is athermal. Whereas this is true for frictionless granular packings, we experimentally demonstrate the failure of quasistaticity for frictional packings in a quasi twodimensional system of disks. This failure is traced to hysteretic responses in the system which shifts the critical packing fraction at which the system jams to progressively higher values as the system is repeatedly jammed and unjammed. The shift in critical packing fraction marks the system's evolution from a Random Loose Packed (RLP) to a Random Close Packed (RCP) density. This rate of evolution is experimentally determined to depend upon the quasistatic step size and the static friction coefficient of the constituent disks in the system. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AU.00006: Is random close packing of beads well defined? Frank Rietz, Charles Radin, Harry L. Swinney, Matthias Schroeter The name random close packing refers to the experimental observation that some ways of packing of monodisperse beads (like vertical vibration or sedimentation) can't exceed a volume fraction of $\approx $64{\%}. There are several competing theories for this phenomenon [13]. However, it is possible to surpass the random close packing limit by cyclic shearing [4]. We investigate the threedimensional distribution of particles in such a shear cell. Index matching of the surrounding liquid provides access to the interior of the granular bed. A laser sheet is scanned through the sample and by adding a fluorescent dye to the liquid we can determine the particle positions. The experiment starts at packing fractions well below random close packing. After a few thousand cycles packing fractions above 64{\%} are achieved. By determination of Voronoi cells we characterize the local packing densities and measure order parameters around the onset of random close packing. This allows us to comment on the question if random close packing is well defined. [1] Torquato; Phys. Rev. Lett. 84, 2064 (2000). [2] Kamien; Phys. Rev. Lett. 99, 155501 (2007). [3] Radin; J. Stat. Phys. 131, 567 (2008). [4] Nicolas; Eur. Phys. J. E 3, 309 (2000). [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AU.00007: Random Packings of Rodlike Granular Materials Scott Franklin Piles of large aspect ratio granular materials are known to form solid plugs, significantly more rigid than piles of ordinary sand or rice. We create random, jammed packings of spherocylinders  cylinders with hemispherical endcaps  using energy minimization techniques. The packing fraction at high aspect ratios agrees with a meanfield model that scales as the inverse of excluded volume, implying that that contact number is constant even for very long, thin particles. This is confirmed by a direct analysis of the average contact number at large aspect ratios. The structure of the jammed state can be investigated through the dynamical matrix of elastic modes. In contrast with other work on ellipsoids, our packings show low energy translational modes and higher energy rotational modes, which we explain as resulting due to the absence of particle curvature. [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AU.00008: Jamming of granular materials under shear Jie Zhang, Robert Behringer We probe the transition of a disordered system between an unstable (e.g. fluidlike) state and a stable (e.g. solidlike or jammed) state. Examples of relevant systems include glasses, foams, colloids and granular materials. Liu and Nagle proposed a jamming diagram with axes of inverse density, temperature and shear stress, and a region near the origin was proposed to encompass the jammed states. Point J on the diagram, (isotropic jamming) was thought to be the lowest possible jammed density, and for denser systems, shear stress was thought to lead to unjamming. Recent work has focusd on isotropic jamming. Here, we explore the effect of shear on jamming. We have carried out experiments using quasi2D systems of photoelastic disks subject to pure shear. We obtain interparticle contact forces as well as other key information. From this data we compute stresses, densities, etc. Contrary to the above picture, we find that the application of shear to densities lower than that at point J can lead to jammed states. Shear applied to isotropic jammed states does not lead to unjamming, but rather to an increase in all stresses. These data, which obviously pertain to frictional particles, suggest a jamming diagram given by shear stress, pressure and inverse density. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AU.00009: A kinetic theory of plastic flows in jammed materials Lyderic Bocquet, Annie Colin, Armand Ajdari Amorphous jammed materials of diverse nature display complex flow properties intermediate between solid and liquid, as characterized by the existence of a yield stress. Furthermore flows in such systems usually exhibit spatial inhomogeneities, which cannot be reconciliated with classical rheological descriptions. We present a novel kinetic approach for the elastoplastic flow dynamics of jammed materials, describing the spatiotemporal collective dynamics of the localized plastic events occuring during the flow [1]. This description yields a nonlocal constitutive law for the flow, introducing as a key dynamic quantity the local rate of plastic events. This quantity, interpreted as a local fluidity, is spatially correlated with a correlation length diverging in the quasistatic limit, i.e., close to yielding. In line with recent experimental [2] and numerical observations, we predict finite size effects in the flow behavior, as well as the absence of an intrinsic local flow curve.\\[4pt] [1] L. Bocquet, A. Colin, A. Ajdari, {\it Phys. Rev. Lett.} (2009) in press\\[0pt] [2] J. Goyon, A. Colin, G. Ovarlez, A. Ajdari, L. Bocquet, {\it Nature} {\bf 454} 84 (2008) [Preview Abstract] 
Session AV: Flight I
Chair: Jane Wang, Cornell UniversityRoom: 205AD
Sunday, November 22, 2009 8:00AM  8:13AM 
AV.00001: Optimization Study for Hovering Flapping Flight Humberto Bocanegra Evans, James J. Allen, B.J. Balakumar A scaled robotic hummingbird model was used to perform a flow
analysis of hovering flight at a range of Reynolds numbers
(1,750$ 
Sunday, November 22, 2009 8:13AM  8:26AM 
AV.00002: Flapping counter torque (FCT) in animal flight: Experimental results and mathematical models Bo Cheng, Xinyan Deng From our previous studies on a range of insects from fruit flies to cockatoos during fast yaw turning maneuvers (body saccades), we found that body rotation causes a substantial aerodynamic counter torque, termed as flapping countertorque (FCT), which acts in the opposite direction of turning. In this study, we show that FCT exists in all roll, pitch and yaw axes and are linearly dependent on the flapping frequency and rotational velocity, respectively. We measured the FCTs systematically (by varying wing beat frequency and body turning velocity) on a pair of dynamically scaled robotic model wings. Furthermore, we developed mathematical FCT models based on quasisteady analysis for roll, pitch and yaw axes. The results show that the experimental data matches the prediction of the analytical models. FCT induced passive damping accounts for a large part of the deceleration in saccade of animal flight, and implies passive rotational stability of the angular velocity dynamics in flapping flight. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:39AM 
AV.00003: Controlling Pitching Instability in 3D Flapping Flight Song Chang, Jane Wang Flying insects actively control their wings to maintain the stability in steady flight as well as to execute maneuvers. The control strategies depend on the coupling of sensory feedback loops of insects and the underlying dynamics of the 3D flapping flight. In this talk, we first present a general method for efficiently simulating the 3D flapping flight of the coupled wingbody system in the quasisteady limit. We then quantify the stability of the periodic solutions that correspond to equilibrium flight. The analysis shows that the flapping system exhibits an inherent instability in pitching, and this instability can be further understood in a reducedorder model. We propose a simple control strategy for stabilizing the pitching by modulating wing motions. [Preview Abstract] 
Sunday, November 22, 2009 8:39AM  8:52AM 
AV.00004: Fruit flies use flight autostabilization to recover from aerial ``stumbles'' Leif Ristroph, Attila Bergou, Gunnar Ristroph, Katherine Coumes, Gordon Berman, John Guckenheimer, Z. Jane Wang, Itai Cohen Just as manned flight was made possible by control mechanisms, the flappingwing flight of animals also relies on strategies that ensure recovery from disturbances. Previous studies performed on tethered and dissected insects indicate that the sensory, neurological, and musculoskeletal systems play important roles in flight control. Such studies, however, have yet to produce an integrative model of flight stability since they do not incorporate the interaction of these systems with freeflight aerodynamics. Here, we directly investigate control and stability through the application of brief torques to freeflying fruit flies and measurement of their behavioral response. Highspeed video and a new motion tracking method capture the aerial ``stumble'', and we discover that flies respond to gentle disturbances by accurately returning to their original orientation. This accurate and fast recovery motivates a feedback control model that includes the insect's ability to sense body rotations, process this information, and actuate the wing motions that generate corrective aerodynamic torque. Thus, as with modern fighter jets, the common fruit fly employs an autostabilization scheme that maintains its flight course and allows for navigation through complex aerial environments. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:05AM 
AV.00005: Fruit flies modulate passive wing pitching to induce inflight turns Attila Bergou, Leif Ristroph, John Guckenheimer, Itai Cohen, Jane Wang To control their ?ight, insects must have mechanisms to modulate their wing kinematics. Exactly how insects control their wing motions to execute observed flight maneuvers is poorly understood. Here, we measure the wing and body kinematics of freely flying fruit flies performing turns and, in conjunction with numerical simulations and mathematical models, probe how they control their wing motion to ultimately alter their flight path. We find that these flies induce sharp turns by applying an overall bias to the passive pitching motion of their wings. We present a simple mechanical model for the wing actuation that quantitatively predicts the turning dynamics of the insect. [Preview Abstract] 
Sunday, November 22, 2009 9:05AM  9:18AM 
AV.00006: Flapping counter force  a unique flight stabilizing mechanism enabled by flapping wings Hu Dai, Haoxiang Luo, Xinyan Deng The flyers in nature are more sensitive to disturbances than the muchlarger airplanes, and meanwhile, many of them (e.g., insects) lack the geometrical features that airplanes typically have, e.g., the vertical/horizontal tails. Therefore, a passive flight stabilizing mechanism would be of particular importance to the biological flyers, who otherwise would have to spend a great deal of effort to actively control their flight. It was recently found that insects and other flying animals possess a unique passive stabilization mechanism that stems from the coupling between their body movement and the flappingwing motion (Hedrick, Cheng and Deng, Science, 2009). More specifically, the unsteady movement of the flyer's body in a disturbed flight modifies the effective kinematics of the wing, creating a resistant force that counteracts the body motion. In this work, we use direct numerical simulations to compute the flapping counter force associated with a twodimensional wing, and the transient process of the disturbed body motion is also computed via flowstructure interaction. The flyer's body is represented by a lumped mass, and the flow around the wing is resolved by the simulations to accurately account for the force production mechanism. The computed force and the body transition will be compared with a quasisteady analysis. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:31AM 
AV.00007: Aerodynamics of Dragonfly in Hover: Force measurements and PIV results Xinyan Deng, Zheng Hu We useda pair of dynamically scaled robotic dragonfly model wings to investigate the aerodynamic effects of wingwing interaction in dragonflies. We follow the wing kinematics of real dragonflies in hover, while systematically varied the phase difference between the forewing and hindwing. Instantaneous aerodynamic forces and torques were measured on both wings, while flow visualization and PIV results were obtained. The results show that, in hovering flight, wingwing interaction causes force reduction for both wings at most of the phase angle differences except around 0 degree (when the wings are beating inphase). [Preview Abstract] 
Sunday, November 22, 2009 9:31AM  9:44AM 
AV.00008: Aerodynamics of Dragonfly in Forward Flight: Force measurements and PIV results Zheng Hu, Xinyan Deng We used a pair of dynamically scaled robotic dragonfly model wings to investigate the aerodynamic effects of wingwing interaction in dragonflies. We follow the wing kinematics of real dragonflies in forward flight, while systematically varied the phase difference between the forewing and hindwing. Instantaneous aerodynamic forces and torques were measured on both wings, while flow visualization and PIV results were obtained. The results show that, in forward flight, wingwing interaction always enhances the aerodynamic forces on the forewing through an upwash brought by the hindwing, while reduces the forces on the hindwing through a downwash brought by the forewing. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  9:57AM 
AV.00009: Lift production of a hovering pyramid in an oscillatory airflow Annie Weathers, Brendan Folie, Bin Liu, Stephen Childress, Jun Zhang We investigate the dynamics of rigid, hollow ``pyramids'' placed within a background airflow, oscillating with zero mean. The asymmetry of the body introduces a net upward force. We find that when the amplitude of the airflow is above a threshold, the net lift exceeds the weight and the object hovers. Our results show that the objects hover at far smaller air amplitudes than would be required by a quasisteady theory. We find that paired vortices are generated during each period of the oscillatory flow, which provide the lift. We also observe that lighter objects do not necessarily hover more easily, because they tend to be entrained by the flow, reducing the relative motion and the resultant lift. In fact a finite flow amplitude is observed to be required for hovering in the limit of zero body mass. [Preview Abstract] 
Sunday, November 22, 2009 9:57AM  10:10AM 
AV.00010: Aerodynamic force variation in an inclined hovering motion by kinematic and geometric controls Hyungmin Park, Haecheon Choi Due to the excellent flight capability with a high maneuverability, dragonfly flight has been a great interest in various fields. In the present study, we construct a onepaired dynamically scaled dragonfly wing model, perform an inclined hovering motion by wing flapping in a whiteoil tank, and measure the normal and tangential forces on the wing. First, we investigate the effect of kinematic parameters of wing motion such as the attack angle ($\alpha$), pitching duration, pitching timing, etc. The Reynolds number is 1,900 or 2,430 depending on the wing shape. We find that the aerodynamic forces vary greatly with these kinematic parameters. On the other hand, the corrugation on the wing surface has been found to increase the lift force in gliding flight. In this study, we investigate the effect of surface corrugation on the force of the flapping wing. With the corrugation, the drag force slightly increases during a downstroke (high $\alpha$) and the lift force increases during an upstroke (small $\alpha$), respectively, resulting in the increase of the mean vertical force by $10 \sim 30\%$ depending on the wing trajectory. We further investigate the force variation by kinematic and geometric controls using flow visualization and the result will be shown in the presentation. [Preview Abstract] 
Session AW: MiniSymposium on Intracellular Fluid Dynamics
Chair: Juan Carlos del Alamo, University of California, San DiegoRoom: 208AD
Sunday, November 22, 2009 8:00AM  8:26AM 
AW.00001: Cellular hydraulics: physics and physiology Invited Speaker: L. Mahadevan Hydraulics (from the Greek word $\delta \rho \alpha \upsilon \lambda \iota \kappa \varsigma )$ is the study of fluid movements and fluid power. In this brief talk, I will discuss the implications of water movements through soft porous structures for the dynamics of nuclear swelling and the cytoskeleton in animal cells. If time permits, I will conclude with a quantitative description of the growth of pollen tubes treated as hydraulic machines. [Preview Abstract] 
Sunday, November 22, 2009 8:26AM  8:52AM 
AW.00002: Active Gels and Cell Quakes: Exploring the nonequilibrium rheology and fluctuation spectrum of motordriven polymer networks Invited Speaker: Alex Levine Recent experiments on molecular motor driven in vitro FActin networks have found anomalously large strain ?uctuations at low frequency. In addition, the shear modulus of these active networks becomes as much as one hundred times larger than that of the same system in equilibrium. In this talk we develop a theory of both these phenomena using a two?uid model of a lowdensity isotropic semi?exible network driven by molecular motors. Relying on only simple assumptions regarding the motor activity in the system, we find that we can quantitatively understand both the lowfrequency ?uctuation enhancement and the nonequilibrium stiffening of the network. We also show the results of new numerical studies of semiflexible networks driven by molecular motors that explore the effects of high motor density in isotropic networks and the effect of nematic order in the active filament network. These results have implications for the interpretation of microrheology in such active networks including the cytoskeleton of living cells. In addition, they may form the basis for theoretical studies of biomimetic nonequilibrium gels whose mechanical properties are tunable through the control of their nonequilibrium steadystate. [Preview Abstract] 
Sunday, November 22, 2009 8:52AM  9:18AM 
AW.00003: Resolving the Role of Actoymyosin Contractility in Cell Microrheology Invited Speaker: Denis Wirtz Einstein's original description of Brownian motion established a direct relationship between thermallyexcited random forces and the transport properties of a submicron particle in a viscous liquid. Recent work based on reconstituted actin filament networks suggests that nonthermal forces driven by the motor protein myosin II can induce large nonequilibrium fluctuations that dominate the motion of particles in cytoskeletal networks. Here, using highresolution particle tracking, we find that thermal forces, not myosininduced fluctuating forces, drive the motion of submicron particles embedded in the cytoskeleton of living cells. These results resolve the roles of myosin II and contractile actomyosin structures in the motion of nanoparticles lodged in the cytoplasm, reveal the biphasic mechanical architecture of adherent cellsstiff contractile stress fibers interdigitating in a network at the cell cortex and a soft actin meshwork in the body of the cell, validate the method of particle trackingmicrorheology, and reconcile seemingly disparate atomic force microscopy (AFM) and particletracking microrheology measurements of living cells. [Preview Abstract] 
Sunday, November 22, 2009 9:18AM  9:44AM 
AW.00004: Anisotropic viscoelastic properties and cytoskeletal structure of endothelial cells subject to shear flow Invited Speaker: Juan C. del Alamo Adherent cells remodel in response to mechanical stimuli leading to a redistribution of intracellular forces that depends on the viscoelastic properties of the cytoskeleton. We have analyzed the magnitude and anisotropy of these properties in confluent vascular endothelial cells subject to continuous flow. For this purpose we used Directional Particle Tracking Microrheology, which measures the secondorder tensor of intracellular marker displacements, allowing us to determine the principal directions of highest and lowest shear modulus at each position. We studied the orientation of these principal directions relative to those of the actin stress fibers. After the application of flow shear the cells' stress fibers gradually orient parallel to the flow and the principal directions of the shear modulus become parallel and perpendicular to the flow. The role of ATPdriven myosinII contractions in the observed anisotropy is analyzed by using cells treated with drugs inhibiting myosinII function. [Preview Abstract] 
Sunday, November 22, 2009 9:44AM  10:10AM 
AW.00005: Structural response and remodeling of red blood cells  a multiscale modeling approach Invited Speaker: Qiang Zhu A red blood cell contains cytosol enclosed inside a composite membrane consisting of a fluidic lipid bilayer reinforced by a single layer of protein skeleton. It has been demonstrated that mechanical loads can trigger dissociation of interprotein and proteintolipid linkages and cause structural remodeling and failure. To understand these effects, it is vital to quantitatively characterize the mechanical forces acting within the membrane. For this purpose we developed a multiscale model to study distributions of internal stress in response to external load. In this method, the cell is modeled at three length scales: in the completecell level it is depicted as two layers of continuum shells, one representing the lipid bilayer and the other the skeleton; a moleculardetailed model of the skeleton is developed to predict its constitutive properties; a nonlinear stainstretch model of Sp (a major protein in the skeleton) is applied to study the mechanical properties of the cell in large deformations. With this model we investigated mechanical responses of the system under canonical experiments such as micropipette aspirations and optical tweezer stretching. Model validations were conducted through comparisons with benchmark experiments. [Preview Abstract] 
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