Session L1: Poster Session (15:15 - 17:00)

Visualization of airflow in the wake of a ship superstructure

Helicopter landings on naval surface ships, such as cruisers and destroyers, must take place in the presence of an air wake created by flow over the ship superstructure. Wake turbulence over the flight deck makes piloted landings dangerous and difficult, and poses significant problems for the use of unmanned rotorcraft. To address this problem, a comprehensive set of experimental and simulation data are being collected via concurrent field tests, wind tunnel measurements, and CFD simulations. These data will facilitate an understanding of the wake turbulence produced under a variety of weather conditions, and will allow assessment of the fidelity of lower order flowfield estimates.~A U.S. Navy Auxiliary Patrol (YP) Craft is used as a representative ship platform. The YP is over 100 ft long, has a similar shape to a modern destroyer, and has been modified to include a flight deck and hangar-like superstructure. Presented here are preliminary CFD results along with results from a large-scale flow visualization experiment. Qualitative information gleaned from the flow visualization is being used in the experimental design of upcoming quantitative air velocity measurements.   

Numerical simulations of bell contractions of upside down jellyfish using the immersed boundary method

Pulsatile jet propulsion is one of the simplest forms of locomotion utilized by macroscopic organisms. Jellyfish use contractions of their bells to form vortex rings that facilitate feeding and locomotion. Once they grow to about 2 cm in diameter, the upside-down jellyfish (genus Cassiopea) situates itself on the ocean substratum with its oral arms towards the sun. These organisms primarily utilize zooanthellae for photosynthetic feeding, while also pulsing their bells to generate feeding currents and to move short distances. Preliminary numerical simulations are presented here which model the motion of the jellyfish as they pulse on the ocean floor. The motion of the bell is measured and fit to a mathematical model using video. The bell motion is used as an input in numerical simulations. Ultimately, contraction of muscle fibers which induce bell contractions will be simulated across Reynolds numbers using the immersed boundary method.   

Experimental study of squeeze-film flow related to human voice production

Airway surface liquid (ASL) consists of a Newtonian/non-Newtonian bi-layer and lines the human airway, including the vocal folds. Several studies indicate that ASL properties affect vocal fold operation. In particular, adverse ASL properties may lead to voice discomfort and damage. This presentation will focus on research performed to elucidate the role of the ASL in voice production. Experimental measurements of squeeze-flow between parallel plates (similar to the ASL during vocalization) are presented. Simultaneous film thickness, force, and high-speed image data are reported and compared with computational solutions. The resulting squeeze force is compared for varying parameters including film thickness and fluid properties. This data will help develop an accurate ASL model to study its function in depth. Long-term results may help in treatment of voice disorders. Additionally this work provides experimental data to support the validation of existing thin-film squeeze-flow equations.   

Computational Analysis of Human Blood Flow

Fluid flow modeling with commercially available computational fluid dynamics (CFD) software is widely used to visualize and predict physical phenomena related to various biological systems. In this presentation, a typical human aorta model was analyzed assuming the blood flow as laminar with complaint cardiac muscle wall boundaries. FLUENT, a commercially available finite volume software, coupled with Solidworks, a modeling software, was employed for the preprocessing, simulation and postprocessing of all the models.The analysis mainly consists of a fluid-dynamics analysis including a calculation of the velocity field and pressure distribution in the blood and a mechanical analysis of the deformation of the tissue and artery in terms of wall shear stress. A number of other models e.g. T branches, angle shaped were previously analyzed and compared their results for consistency for similar boundary conditions. The velocities, pressures and wall shear stress distributions achieved in all models were as expected given the similar boundary conditions. The three dimensional time dependent analysis of blood flow accounting the effect of body forces with a complaint boundary was also performed.   

Analysis of Signal Propagation in an Elastic-Tube Flow Model

We combine linear and nonlinear signal analysis techniques to investigate the transmission of pressure signals along a one-dimensional model of fluid flow in an elastic tube. We derive a simple measure for the robustness of a simulated vessel against in vivo fluctuations in the pressure, based on quantifying the degree of synchronization between proximal and distal pressure pulses. The practical use of this measure will be in its application to simulated pulses generated in response to a stochastic forcing term mimicking biological variations of root pressure in arterial blood flow. Using spectral analysis methods based on synchronization theory, we introduce a novel nonlinear index for measuring the robustness of the model against fluctuations in the forcing signal, based on a general scheme for deriving low-dimensional measures of (biological) performance from higher-dimensional systems of equations.   

Shape oscillation of bubbles in the acoustic field

We focused on dynamics of multiple air bubbles exposed to ultrasonic wave while ascending in water. The bubbles were injected into the pool filled with water from a vertical capillary tube, and then the ultrasonic wave of 20 kHz was applied from above toward the bubbles. Volume and shape oscillations of the bubbles were captured by a high-speed camera at frame rates up to 40000 fps with a back-lighting system. We realized three major phenomena in this experiment; excitations of surface wave, volume oscillation, and shape oscillation. In the present study we paid our special attention to the shape oscillation and the transition from the volume to the shape oscillations of the bubbles in a row. Effects of the bubble volume and the distance between the bubbles were examined. We got relation among distinct mode number n, which is order of harmonic, bubble diameter, and distance of the bubbles.   

On the effects of Schmidt number and particle settling velocity on the calculated sediment concentration profile

The numerical simulation of natural stream flows is a powerful tool which is generally used to achieving the following goals: tracing the contaminants in natural waterways, computing the suspended sediment concentration, computing the bed evaluation of waterways, etc. In most natural rivers, the flow is turbulence and therefore the turbulent diffusion plays a critical rule in scalar term dispersion. To evaluate the effect of turbulent diffusion, the Schmidt number is widely used. In this paper, the effect of different Schmidt number on the calculated sediment concentration profiles is investigated. Also, the effect o sediment particle settling velocity of the computed sediment concentration profile has been investigated. Some standard test cases including net entrainment from a mobile bed, net deposition to the bed, dye material advection dispersion, have been chosen. For the net deposition, net entrainment and dye advection-dispersion the experimental data of Jobson and Sayre (1970), van Rijn (1981) and Jobson and Sayre (1970), respectively, has been used to comparing the model results by Schmidt numbers and settling velocity values. As the results, the most appropriate parameters for which a better prediction is achieved are presented for each case.   

A Numerical Model for Time-Dependent Gravity-Driven Flow in a Collapsible Tube

We present details of a Navier-Stokes solver to address fluid flows through a circular tube with elastic walls. This is the first implementation of a large structured-grid fluid dynamics code on this architecture. This class of problems, fluid flow through collapsible tubes, is very important to the study of biological systems (respiratory system, circulatory system, etc.) and physical systems (fluid dynamics, engineering, etc.). In contrast to other models, we focus on integrating wall elasticity and time dependance. We successfully model the flow of blood through the jugular vein of a giraffe over time by numerically evaluating a series of hyperbolic PDEs using Lax-Wendroff. Through careful error and stability analysis, we were able to create an accurate and stable simulation. We were able to examine the role that elasticity plays at various length scales and determine it has an impact on the flow velocity over large length scales (i.e. a giraffe) whereas it is negligible over small length scales (i.e. a human) as it is likely overwhelmed by factors such as lateral flow and viscosity. This work presents a strong framework for future CFD studies regarding various human blood flow physiologies including the abdominal aorta.   

Turbulence measurements in air near a heated vertical cylinder at high Raleigh number

Measurements are presented up to 4 m height of the temperature and velocity statistics along a heated vertical cylinder of diameter 0.15 m. The cylinder was heated by water flowing through it, and mounted inside a 1.2 m container through which the ambient flow could be controlled. The difference between the wall and ambient temperatures was approximately 40 degrees C resulting in a maximum Rayleigh number based on length, $Ra = g \beta \Delta T x^4/\alpha \nu \approx 2.3 \times 10^{11}$. The velocity was obtained using two-component burst-mode LDA, while the temperature was measured using 1-micron platinum wires. Arrays of thermocouples were used to monitor the ambient and wall conditions. Particular attention has been given to the buoyancy and moment integral equations in order to evaluate the residual effects of stratification and co-flow.   

Measurement of concentration filed in miscible displacement in a thin gap by means of microelectrode

When a miscible less-viscous liquid displaces a more-viscous one in a Hele-Shaw cell (a thin gap between two parallel plates, normally the gap width is less than 1 mm), it is known that a thickness of the less-viscous liquid's layer is expected to abruptly become small in a relatively long distance near the displacement front under a certain condition. We call this a sheet structure. The objective of the present study is to experimentally measure concentration profiles in the cell's gap direction to examine the expectation. However, because of the minuteness of the measured space, it is difficult to measure it by existing measurement techniques. In the present study, therefore, we attempt to measure it by means of a microelectrode which has been frequently used in the biological field. The obtained results suggest the presence of the sheet structure. The obtained concentration profile, however, shows that the sheet structure exists mainly in the upper half of the gap although we have been expected that the sheet structure is present in the middle of the gap.   

Using stabilized finite elements for understanding the performance of organic solar cells

Organic solar cells (OSCs) fabricated from polymer blends are a promising alternative to inorganic photovoltaics. Computational modeling of OSCs is of significant utility towards understanding the relationship between topology/ morphology and efficiency. Simulation of OSCs requires determination of electric potential and electron/ hole densities, which is described by Boltzmann Transport Equation (BTE). As the direct solution of BTE is computationally challenging, the drift-diffusion model is used for modeling traditional semiconductor devices. The drift component in the drift-diffusion equations causes large convection instabilities. This is mitigated by `Streamline Upwind Petrov Galerkin' (SUPG) stabilization traditionally used in convection dominated flow problems in computational fluid dynamics. Although this problem has been addressed for silicon semiconductors, simulation of organic bulk heterojunction (BHJ) solar cells presents new challenges. A comparative study is undertaken for solution of stabilized drift-diffusion equations with primitive, Slotboom and Quasi Fermi Level variables. A simplified drift-diffusion model (with analytical solution) is used to verify these implementations. The framework is utilized to investigate the effect of topology on photocurrent.   

Analysis of wake structure downstream of rough surface using volumetric 3-D measurement

Wake structures downstream of rough surface were analyzed by a 3-D volumetric image based measurement system. The wake structure was shown to have a direct correlation to the roughness of the surface, with the wake deviate from the boundary layer drastically for a very rough surface. A number of rough surfaces were tested to show the various wake formations due to various degrees of roughness. The 3-D volumetric system was employed for the measurement to allow the complete 3-D wake structure to be measured. Such 3-D results allowed instantaneous understanding of the roughness effect to the surrounding flows.   

Observations of the turbulence kinetic energy dissipation rate in the mid-water column of an estuary

A substantial number of oceanic observational studies show that beneath surface waves the turbulence kinetic energy dissipation rate is often much larger than in a comparable turbulent boundary layer over a rigid wall. However, the vertical extent of the region of enhanced dissipation rate is not well characterized primarily due to variability in atmospheric forcing and wave conditions. Here, observations of turbulent dissipation rate are examined together with simultaneous measurements of surface wind waves and tidal currents in Chesapeake Bay during the summer of 2009 under low to moderate wind conditions. The data were collected with two vertically separated acoustic Doppler velocimeters (ADVs) and a high-resolution pulse coherent profiler (Aquadopp) covering a range of depths from 2-4m below the surface in 5m of water. A bottom mounted acoustic wave and current meter (AWAC) provided wave measurements and mean velocity profiles. Estimates of dissipation rates are obtained from inertial subrange spectra derived from temporal (ADV's) and spatial (Aquadopp) data. Various dissipation scaling will be discussed to determine the relative importance of encountered wave and tidal conditions.   

Modeling Subsurface Flows Driven by Planetary Libration

Longitudinal libration is a non-zero mean, time-periodic variation in a planetary body's rotation rate. The outermost solid shells of numerous planetary bodies in the solar system ---including Mercury, Europa, and Earth's moon--- are currently librating. This libration is capable of driving flows in these planets' liquid metallic cores and subsurface oceans through viscous, topographic or electromagnetic couplings. We have carried out a suite of laboratory and numerical hydrodynamic libration experiments in sphere and spherical shell geometries. This set-up allows us to focus, at present, on the purely viscous coupling problem. Laboratory experiments demonstrate that longitudinal libration is capable of generating time-periodic centrifugal instabilities near the librating solid boundary, as well as inertial waves and zonal flows in the fluid interior. In an effort to apply these results to librating planets, we have carried out axisymmetric numerical simulations that access more extreme parameter values than can be reached in the laboratory experiment. These simulations show that the nonlinear interaction of inertial waves is the primary mechanism responsible for the zonal flow generation, whereas the Reynolds stresses generated from centrifugal instabilities only weakly influence zonal flow strength.   

Jamming ang energy propagation through dense granular matter

In previous work (Phys. Rev. E \textbf{79}, 041304 (2009)) we found using discrete element simulations that a reasonable description of energy propagation through dense (jammed) granular system in which volume fraction is keep fixed (CV protocol) can be reached by a linear wave equation with damping. In the present work we consider the systems where we either decrease the volume fraction, or a system under constant applied pressure (CP protocal). In both of the considered scenarios one may distinguish between the jammed and unjammed configurations, defined by the coordination number. We discuss how the nature of energy propagation changes as one goes through the jamming transition.   

Jamming in Microfluidic Channels

We experimentally investigate the flow of a colloid through a microfluidic device. The glass microfluidic device consists of a wide channel with spatially periodic funnels manufactured with photolithographic methods. The device was etched to a depth of about 1 micron that restricts the solid phase of the colloid, fluorescent polystyrene spheres with sub-micron radii, to quasi-2D motion. The liquid phase of the colloid is an aqueous solution with trace amounts of a non-ionic surfactant and with a pH about 2 units above the pKa of the surface groups on the polystyrene spheres to maintain a stable colloid at concentrations high enough to produce jamming. The flow rate of the colloid is controlled by a computer interfaced syringe pump with two controllable modes of operation: a continuous, steady mode that provides a plug-like velocity profile and a discrete, jerky mode that sends compressional waves of specifiable sizes through the colloid. Using fluorescence microscopy, we observe the interactions between the colloid and the glass funnels and investigate how the interaction depends on the funnel geometry. In particular, we investigate the jamming transition from a liquid-like flowing state to a solid-like stationary state.   

DEM simulation of granular flow in a Couette device

We study the shear motion of granular material in an annular shear cell operated in batch and continuous modes. In order to quantitatively simulate shear behavior of granular material composed of spherical shaped grains, a $3$D discrete element method (DEM) is used. The ultimate goal of the present work is to compare DEM results for the normal and shear stresses in stationary and moving granular beds confined in Couette device with experimental results. The DEM captures the experimental observation of transition behavior from quasi-- static (in batch mode operation) to rapid flow (in continuous mode operation) regime of granular flows. Although there are quantitative differences between DEM model predictions and experiments, the qualitative features are nicely reproduced. It is observed (both in experiments and in simulations) that the intermediate regime is broad enough to require a critical assessment of continuum models for granular flows.   

Large Reynolds number streak description using RNS

It has been recently shown [Choi, Nature, April 06 - Cossu et al., PRL, February 06] that the inclusion of 3D streaky structures in the boundary layer can make it remain laminar longer than the purely 2D Blasius flow. We compute the development of 3D streaks in the boundary layer over a flat plate using the so-called Reduced Navier Stokes equations (RNS). The RNS are a boundary layer like formulation, which is derived from the Navier-Stokes equations making use of the fact that in the large Re limit two very different spatial scales are present: one long (streamwise direction) and two short (spanwise and wall-normal direction). The resulting RNS are a nonlinear, parabolic, Re independent system that describes the streak structure in the large Re limit. The RNS streak computations are also much more less CPU costly than DNS for high Re, and they don't have the numerical problems that the PSE formulation exhibits (divergence of the results for small $\Delta x$, or blow-up of the solution when the amplitude of the deviation from Blasius is not small). In this work we comment the details of the numerical integration of the RNS, and we present some comparisons of the RNS results with the linear computations of streak development together with some fully nonlinear computations of streak evolution.   

Instability of Thin Viscous Sheets

Thin viscous fluid flows are characterized by a small aspect ratio, i.e., the characteristic thickness is much smaller than the characteristic tangential length scale. They have been used in a variety of manufacturing applications, including curtain coating, film blowing, film casting, extrusion and optical fiber drawing. An asymptotic theory for predicting the thickness distribution and the geometry boundaries of a thin nearly planar fluid sheet and analyzing the stability of its shape is developed, where the sheet thickness and out of plane displacement are additional distributed variables along with the continuum velocities. The sheet motion in the transverse direction is described by a transient second order PDE expressing the normal momentum balance: Its analysis showed that the existence of compressive stresses inevitably leads to viscous sheet shape instability, while the corresponding stationary equation becomes of mixed type. Two practical examples involving viscous sheets have been considered: an isoviscous two-dimensional viscous sheet, which shape is unstable in the case when the compressive stress is applied by setting an obstacle to the flow at the exit end, and a 3D problem of viscous sheet redraw (constant stretching), where existence of compressive stresses leads to development of hyperbolic zones in the sheet, and the sheet shape instability develops.   

Instability arisen on liquid jet penetrated in flowing liquid bath

We carry out an experimental study with a special interest on a penetration process and an instability on a liquid jet impinged to a flowing liquid pool. The impinged jet penetrates into the flowing bath accompanying with an entrainment of the ambient immiscible gas without coalescing with the liquid in the pool until the air wrap around the jet collapses. The wrapping air controls instabilities arisen on the jet. We observe the dynamic behaviors of the penetrated jet and the departure of the bubble of the wrapping gas at the tip of the collapsing jet by use of a high-speed camera in order to categorize the behaviors as functions of the velocities of the jet and flow in the pool. We also evaluate an averaged thickness of the wrapping gas through the observation.   

Wave-vortex interaction

We present an experimental study on the effect of a electromagneticaly generated vortex flow on parametrically amplified waves at the surface of a fluid. The underlying vortex flow, generated by a periodic Lorentz force, creates spatio-temporal fluctuations that interact nonlinearly with the standing surface waves. We characterize the bifurcation diagram and measure the power spectrum density (PSD) of the local surface wave amplitude. We show that the parametric instability threshold increases with increasing intensity of the vortex flow.   

Modeling of Flow and Water Quality Processes with Finite Volume Method due to Spreading and Dispersion of Petrochemical Pollution in the Hydro-Environments

Having two water frontiers, namely (everlasting) Persian Gulf and Oman Sea in the south and Caspian Sea in the north, intense dependence on extracting and exporting oil, especially via marine fleets and ever-increasing development of petrochemical industry, Iran is exposed to severe environmental damages caused by oil and petrochemical industries. This essay investigates how oil spill is diffused and its environmental pollution is spread. The movement of oil spill, and its diffusion in water and its effects on water and the environment has been simulated by developing a Depth-Averaged numerical model and using the Finite Volume method. The existing models are not efficient enough to fulfill current modeling needs. The developed model uses the parameters useful in the advection and diffusion of oil pollutions in a model appropriate for predicting the transport of oil spill. Since the Navier-Stokes Equations play an important role in the advection and diffusion of oil pollutions, it is highly important to choose an appropriate numerical method in the advection and diffusion section. In this essay, choosing the methods used in the advection and diffusion have been emphasized and highly-accurate algorithms has been used in the advection terms. These algorithms are not present in similar models. The resulting equations have been solved using the ADI method. This method solves the unknown parameters with solving a Penta-Diagonal matrix in each time step. It does so without sacrificing the desired precision.   

The refraction phenomena in the shock wave dispersion on plasma inhomogenities

Recently we introduced a new mechanism of the shock wave (SW) dispersion on a boundary [1] and then employed it to control the structure of a SW front as it interacts with plasma. Now we consider same effect but under specific conditions on the interface with weak gradients at the interface, but stronger in the bulk. These conditions appear more realistic for most applications, and more important, the SW dispersion in this case becomes stronger. We derive all relations using the approach similar to [1]. Then the model applied to calculate the structure of the SW front dispersed on a plasma sphere numerically and compared with the existing experiments. The strength of the effect is demonstrated by comparing results of calculation for different conditions on the interface. Comparative calculations show significant difference in the shock front shapes and they are in complete agreement with the experiments. More results are underway which can contribute to understanding of basic phenomena in weakly ionized gases, combustion, and shock dynamics on the interfaces.\\[4pt] [1] A. Markhotok, S. Popovic, L. Vuskovic, Phys. of Plasmas J. \textbf{15}, 3 (2008).   

On the origin of contact angle hysteresis

In this study, we consider a simple distribution of defects on a substrate, or a sinusoidal surface, and show explicitly how the pinning and depinning occur for a two dimensional liquid drop on such non-ideal surfaces as the volume of the drop is increased or decreased. We show that the contact angle hysteresis (CAH) emerges from this simple model even though we do not take any effect of viscous dissipation into account, which is in contrast with the conventional theory where the CAH originates from the viscous dissipation inside the liquid around the contact line just after depinning.   

Particle collision in Newtonian and viscoelastic fluids

Particle-particle and particle-wall collision occurs in many natural and industrial applications such as sedimentation, crystal growth, suspension rheology, and microfluidic devices such as those used in mechanical cell lysis. To accurately predict the behavior of particulate flows, fundamental knowledge of the mechanisms of single collision is required. In this work, particle-wall collision in Newtonian and viscoelastic fluids is numerically and experimentally studied. The effect of Stokes number, surface roughness, and Deborah number on the rebound velocity of a colliding spherical particle on a wall is considered. The experimental study of particle-wall collision in poly(ethylene-oxide) mixed with water shows that the results for the coefficient of restitution in polymeric liquids can be collapsed together with the Newtonian fluid behavior if one defines the Stokes number based on the local strain rate. In addition, the effects of particle interaction and collision on the droplet breakup in a particulate shear flow are studied. The presence of particles leads to larger droplet deformation and a perforation in the center of the droplet. It is found that the critical Stokes number above which a perforation occurs increases linearly with the inverse of the capillary number and viscosity ratio.   

Lagrangian Panel Method for Vortex Sheet Motion in 3D Flow

A Lagrangian panel method is presented for computing vortex sheet motion in 3D flow. The sheet is represented by a set of quadrilateral panels with a quadtree structure. The panels have active particles that carry circulation and passive particles used for adaptive panel refinement. The Biot-Savart kernel is regularized and the particle velocity is computed using a treecode. Results are presented for the azimuthal instability of a vortex ring and the oblique collision of two vortex rings.   

Mixing enhancement in a non-parallel microfluidic chip

Electrokinetic instability (EKI) flow can be use as an efficient tool for mixing in a lab-on-a chip system. In this report, we fabricated a quasi T channels with electrodes on the sidewall to enhance mixing with AC electrokinetics. A parametric study was conducted to explore the effectiveness of manipulating EKI waves to enhance fluid mixing inside the microchannel channel. Firstly, mixing results in two cases have been compared: electrodes are placed at the sidewall and electrodes are located at the ends of the channel. Secondly, the mixing results in the microchannel with different angle between two electrodes were assessed in terms of scalar concentration distributions. Thirdly, the effectiveness of the applied voltage phase variation between the two electrodes on the mixing process inside the quasi T channel were also explored for the further mixing enhancement. Fourthly, mixing result under high frequency was also achieved. Finally fluorescent particles in one of the two streams were used to obtain a more clear visualization of mixing process in the microchannel with 5$^{\circ}$ angle between the two sidewalls.