Session L1: Poster Session (3:15-5:00PM)

Particle transport near arterial stenosis

We will present work towards understanding particle transport near arterial stenoses. Prior studies have shown increased platelet aggregation downstream of stenosis, or analogous geometrical models that induce flow separation and recirculation via abrupt expansion. Stenosis leads to changes in fluid mechanical quantities such as shear stress, flow separation, recirculation and reattachment and there exists several hypotheses on how these conditions influence platelet activation and aggregation. In particular, it is thought that high shear at the stenotic throat ``activates'' platelets that subsequently aggregate in the low shear separation zone perpetuating thrombotic events. We aim to understand particle (e.g. platelet) transport downstream of a stenosis in close detail. Towards this objective, we have developed numerical models of pulsatile flow near arterial stenoses and methods for particle tracking, including quantification of mechanical stimuli thought to initiate platelet activation. We will discuss results of this effort, comparison with previous studies, and plans for continued numerical and experimental work.   

Toroidal Lagrangian Flow Structues in highly viscous fluids by moving bent rods

Motile cilia play a large role in fluid motion across the surface of ciliates. Flows caused by the cilia move debris and mucus through mass beat patterns controlled by the motor proteins while rotating about the basal body that attaches the cilium to the cell surface. This study approximates the cilium as a slender body rotating about a point of contact of one of its ends in a viscous fluid. The bent rod sweeps out a virtual cone with a chord connecting both ends. The bend of the rod, the cone angle, the angle between the central axis to the normal plane, and the angle of rotation of the bent rod about its chord affect the flow patterns in a Stokes fluid. The slender body theory allows for an asymptotic solution of the Lagrangian trajectories and flow patterns caused by the precessing rod, which can be directly compared to experimental data. Altering the above parameters produces different toroidal flow structures. Using 3D stereo calibration, accurate quantified comparisons of epicyclic particle trajectories in short and long time are made against the model predictions.   

Mechanical response of solutions of phospholipids to anisotropic compression

Solvated phospholipids in concentration higher than the critical micellar concentration self-assemble in micelles, vesicles and/or bilayer membranes. These structures are the building blocks of many biological systems, such as cell membranes and lining of lungs, and are well known for their ability to provide a biophysical barrier between two mediums. Another property that has not received as much attention is their macroscopic mechanical role based on nanoscale interactions, which is hypothesized to play a major role in the functions of articular joints. Using atomistic and coarse- grained molecular dynamics (MD), the existence of two universal linear elastic regimes of multilamellar bilayer membranes under anisotropic compression is identified for different level of hydration. Leveraging this property, the repulsive dynamics between self-assembled structures and the reduction of diffusion in supported membranes, a fluid nano ball bearing is constructed as an illustration for our hypothesis of synovial lubrication.   

Effect of the interaction on growing / condensation processes of vapor bubbles injected in subcooled pool

We carry out an experimental study with a special interest on a growing and collapsing processes of vapor bubbles injected into a subcooled pool. In the present system, we extract the liquid-vapor interaction in the boiling phenomenon consisting of complex three-phase interactions. Vapor of distilled water is generated in the vapor generator apart from the pool, and then is injected to the pool at a designated degree of subcooling. The degree of subcooling of the pool is controlled from 8 to 80 K. Bubble growth and condensation processes are detected by a high-speed camera at frame rate up to 140,000 fps with backlight illumination. We successfully detect an instability arisen on the vapor bubble interface in prior to the abrupt condensation to collapse. We figure out occurring condition of such instability by evaluating a condensing rate as functions of the degree of subcooling and the vapor injection rate. We then pay our special attention to the interaction of adjacent vapor bubbles injected through two neighboring orifices. Effect of interaction between vapor bubbles is discussed considering a distance between the orifices, and the degree of subcooling.   

Shock Turbulence Interaction using Observable Euler Equations

Accurate numerical simulations of turbulent flows requires minimizing numerical dissipation, while the usual shock-capturing schemes need numerical dissipation for the algorithm stabilization. To overcome this dilemma, the observable Euler equations (Mohseni, AIAA paper 2009-5695) were proposed as a technique for simultaneous regularization of shocks and turbulence. The effects of the observable Euler equations in 3D shock turbulence simulation have been tested in several problems including the Shu-Osher, Taylor-Green Vortex, Noh problem, and decaying compressible isotropic turbulence with eddy shocklets. The Taylor-Green Vortex problem tests the stability for severely under-resolved motions, as well as a measure of the preservation of kinetic energy and the growth of enstrophy. In the Noh problem, strong shock waves interact with interfaces separating different fluids and with the resulting turbulence. It tests the capability to handle a strong spherical shock. In the case of decaying compressible isotropic turbulence with eddy shocklets, the ability of the observable Euler equations to handle ``randomly'' distributed shocklets, as well as the accuracy for broadband motions in the presence of shocks was tested.   

Transport of an Active Scalar in a Chaotic Flow Field

We study the active transport of a scalar species in the flow field of Rayleigh-B\'enard convection that is exhibiting spatiotemporal chaos. Recent work has quantified the passive transport of a scalar species in a spiral defect chaos flow field to yield enhanced diffusion. In this work we are interested in allowing the scalar species to undergo active transport. For example, the combustion of premixed gases where the scalar quantity can react, or the motion of microorganisms in bioconvection where the scalar quantity can swim in some preferential direction. We use large-scale numerical simulations to solve a reaction-advection-diffusion equation for the scalar species simultaneously with the three dimensional time dependent Boussinesq equations. We use our numerical results to explore the transport of an active scalar species in a variety of chaotic flow fields and over a range of Lewis numbers.   

The safety of a cell in a droplet under high electric field

Electrically charged aqueous droplet can be transported by electrical field in a dielectric fluid without the flow of medium (Jung et al. J. Colloid Interface Sci. 2008). This phenomenon can be used to transport a single nanoliter droplet in a micro channel, which can serve as biochemical micro-reactor. Because an aqueous droplet is much conductive than the dielectric fluid, there is effectively no electric field inside the droplet suspended in dielectric fluid. Therefore bio-materials are protected from electricity even under high electric field. However, when the droplet is charged near an electrode by direct contact to the electrode, there is possibility that electric field can hurt bio-materials like DNA molecules, microorganisms, cells, protein in droplet. Because of this concern, we should confirm that bio-materials in droplet moving by direct charging are safe under strong external electric field especially to organism cells. Therefore we examine the effect of electric field on the cells such as yeast, E.coli., and sperm in droplet experimentally.   

Behavior of contact angles on rough solid surfaces

In this study, we consider 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. The surface energy of the drop have a wave like form affected by solid surface shapes and contact lines are pinned by the energy barriers. 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.   

Study of drop coalescence using the Lattice-Boltzmann Method

Drop coalescence in an emulsion is studied using the Lattice-Boltzmann Method (LBM) to simulate multicomponent fluid flow in 3D. Two cases are considered, drop interaction under shear flow and interaction during gravity induced sedimentation. Several experimental results on the topic have already been published and computational studies using the Boundary Element Method (BEM) are also available, however, it is of interest to investigate how well does the LBM behaves in comparison with other methods. For shear flow, drop size ratios, interfacial properties and flow regimes, which favor coalescence, are identified and discussed. Multiple drops interactions under shear flow are demonstrated and characterized. The interaction of drops under gravity-induced sedimentation is also considered. In this case, the effects of drop size ratio, surface tension, gravitational pull and viscosity ratio on drop interaction and coalescence are studied. The results are then compared to those obtained using the traditional BEM and experimental data from the literature. Discussion on code implementation, as well as advantages and disadvantages of each method are highlighted.   

Modern applications of shadowgraph imaging

Over the last hundred years the shadowgraph technique has been extensively used in the study of fluid dynamics, the visualization of objects in motion and in the optical inspection of transparent media. Shadowgraphy is often considered an inexpensive but powerful tool to visualize liquids and is generally used to obtain qualitative properties, such as shapes and motion behavior of gases and liquids. In this work, the shadowgraph technique is combined with digital image analysis generate quantitative data. Three experimental systems are described. The first example is a setup developed to visualize and quantify the droplet size distribution in sprays. The second is a shadowgraph system used to record and analyze the profile of modulated continuous jets in order to measure the dynamic surface tension via the Raleigh-Weber model. The final system uses shadowgraph images of moving droplets to identify and record their instantaneous position and direction of motion.   

PIV measurements of flow characteristics induced by mini plate-wing plasma actuators

The surface DBD plasma actuator is known to be effective for flow control process. Plasma is produced on actuator and gives a body force to the ambient air which is the mechanism for active flow control. Until now, the actuators have been mounted on the wall surface. The plasma actuator is thin and controllable electrically. If we combine the plasma actuator and the passive devices like a vortex generator and Large Eddy Break Up device, those passive devices would be activated. As the basis of the combination use, this paper investigated the wing-like plasma actuators, the width and chord length of which were 96mm and 19.6mm respectively. The electric wind was generated in the absence of external flow by the plasma actuator. Two electrodes were separated by a Kapton thin wing plate and were located at 5.75mm or 14mm from the leading edge. The induced flow was compared as a function of the distance from the leading edge to the actuator position. It was found that the increase in the distance shifted the point of maximum velocity downstream but the induce wake flow indicated the same momentum integral.   

Reduced-Order Models of a Natural Convection Loop for Known Heat Flux Conditions via Karhunen-Lo\`{e}ve Expansions

We build reduced-order dynamical models of a thermal convection loop using the Karhunen-Lo\`{e}ve decomposition (KL) methodology, in conjunction with the Galerkin projection technique. The convective loop has the form of a torus and is filled with a water. The loop receives heat in some parts and releases it in others through a known-heat-flow sinusoidal function, thus creating a natural circulation. Under suitable assumptions, the momentum and energy equations are reduced to a set of one-dimensional integro-differential equations, in which the tilt angle of the loop and the heat rate per unit length are the bifurcation parameters. The set of equations is first solved via finite differences to generate numerical solutions from which the KL model can be built. Then, the method of snapshots and the Galerkin projection are applied to find the KL basis functions, and the corresponding constants, that generate the most compact dynamical system. It is found that the number of KL modes required to build a model is a function of the linear stability of the steady states. As the system goes from stable to unstable regions, and finally to chaos, the number of required modes increases. However, for an accuracy level of, e.g., $10^{-4}$, these reduced-order models are at five orders of magnitude faster than the finite difference solver.   

Investigation of Internal Wave Spectra due to Observed Interactions

Fluids such as the ocean and atmosphere are stably stratified such that the density within the fluid increases with depth. Perturbations to the stratification for example by flow over topography, convective storms or turbulent mixing can lead to the generation of internal waves. As these waves are generated and propagate through their respective media, they interact with a multiplicity of other internal waves. Each wave-wave interaction outcome is governed by the parameters of each wave involved in the interaction. When one of the waves is significantly larger scale, such as an inertial wave, linear theory may be used to asses the interaction between it and a smaller scale wave. The result is three basic types of interactions: small wave vertical group velocity larger than the phase speed of the inertia wave, closely matches, or is much slower than the phase speed of the inertia wave. Using ray theory, these interactions are traced and analyzed to better understand the dynamics of each type of interaction. A set of waves, defined by observations, is tested and conclusions on their effect in the ocean are made. Although a single set of observational data is used, the behavior of small-scale internal waves during and after interacting with common large scale inertia waves will allow for an enhanced understanding the Earth's atmospheric and oceanic global circulation patterns.   

Numerical study of water flow in a system of two basins connected by a channel with periodic forcing

In oceanography the transport of particles is a frecuent phenomenon, for instance ocean currents carry the plankton from one place to another. In shallow waters drag and depositation of sand can affect positively or negatively certain human activities, such as the navigation near the coast; on the other hand, sand banks can help to mitigate the force with which a tsunami approaches a populated coastline. We study the flow of water in a system of two basins connected by a channel, generated by a periodic forcing that simulates the tidal force. The simulation is done by solving the system of equations in stream function ($\psi$ ) - vorticity ($\omega$ ) formulation, obtained from the Navier-Stokes and continuity in two dimensions. A pseudo-spectral method based on polynomials Chebyshev is used. The tidal forcing is reflected in the fact that Reynolds number becomes time dependent. We obtained results that are consistent with previous works (like: Wells, M. G. and Van Heijst, G.J.F., \textit{Dynamics of Atmospheres and Oceans}, \textbf{37} (2003) 223-244). For example the formation and displacement of a dipole at the exit of the channel is observed. The velocity field obtained numerically is used to study the transport of particles by the flow, where the dipole moves away from the channel output or return to it, depending on the geometry of the system and period occurrence of the phenomenon.   

Estimated Overturning of Internal Waves due to Time-Dependent Shear in the Ocean

The ocean and atmosphere have a particular characteristic that sustains propagation of internal gravity waves called a stable stratification. Internal waves are generated, with wavelengths which can vary from a few meters to kilometers. These waves propagate through the ocean and atmosphere exchanging energy and momentum as they interact with other fluid phenomena and break, which in turn affects circulation, heat transport, nutrient distribution and biological activity in the oceans and the atmosphere. However large scale circulation models lack the appropriate resolution to detect these motions, hence it is necessary to accurately parameterize internal wave breaking in order to establish a better relationship between wave energy dissipation and its effects on oceanic and atmospheric circulation patterns. In this research internal waves interact with a time dependent background in the form of a near-inertial wave, which are common in the ocean. Using a two dimensional, fully non-linear Navier-Stokes equation solver and ray theory, estimates of wave breaking parameters which predict breaking at the same location in both of these models are accomplished. A statistical analysis of waves observed during the Hawaiian Ocean Mixing Experiment will provide an estimate of the percentage of waves expected to break during propagation through an inertial wave.   

Nonlinear stability of granular shear flow: shear banding

We show that a Landau-type `order-parameter' equation describes the onset of shear-band formation in granular plane Couette flow wherein the flow undergoes an ordering transition into alternate layers of dense and dilute regions of low and high shear rates, respectively, parallel to the flow-direction. Even though the linear theory predicts the stability of the homogeneous shear solution in dilute flows, our analytical bifurcation theory suggests that there is a sub-critical finite-amplitude instability that is likely to lead to shearband formation in dilute flows which is in agreement with previous numerical simulations.   

Testing the continuum $mu(I)$ rheology for 2D granular flows on avalanches and collapse of columns

There is a large amount of experimental work dealing with dry granular flows (such as sand, glass beads, small rocks...) supporting the so called $\mu(I)$ rheology. This rheology states that the ratio of the tangential to the normal constraints behaves as a Coulomb like friction depending on the Inertial number (this number is the product of the grain size by the shear of the velocity divided by the square root of pressure divided by the grain density). Hence, we propose the implementation of this non newtonian rheology in a Navier Stokes Solver (the Gerris Flow Solver uses a finite-volume approach with the Volume-of-Fluid (VOF) method to describe variable-density two-phase flows). First we apply it on a steady infinite bi dimensional avalanching granular flow over a constant slope covered by a passive light fluid (it allows for a zero pressure boundary condition at the surface, bypassing an up to now difficulty which was to impose this condition on a unknown moving boundary). The classical analytical solution, known as Bagnold solution, is recovered numerically. Then the rheology is tested on the collapse of granular columns and quantitative comparisons with numerical simulations from Contact Dynamics are done.   

On influence of microstructure on granular impact

We use discrete element simulations to explore interaction of an intruder with a dense granular matter. Granular particles are modeled as soft, inelastic, frictional disks in two spatial dimensions, and the intruder is considered to be much larger than particle size. In this presentation we will concentrate in particular on the influence of granular microstructure on the impact, including the influence of system size, preparation, and material properties. The results will be compared to the existing ones, and new experiments will be proposed.   

Hydrodynamic Simulations of Density Inversion in Granular Layers

We model density inversion in vertically shaken granular layers using a proposed set of three-dimensional, time-dependent granular hydrodynamics equations. For a range of shaking amplitudes and frequencies, we numerically solve time-dependent equations derived to Navier-Stokes order for mono-disperse, frictionless, nearly elastic particles. For shaking at high frequency and accelerational amplitude, these simulations exhibit steady state behavior in which a high density layer is supported by a lower density granular gas. At lower shaking frequencies and accelerational amplitudes, density profiles display time-dependence in which density inversion is not maintained throughout the entire cycle. Results from these simulations are directly compared to molecular dynamics simulations to test the ability of these continuum simulations to accurately model both the time-dependent and steady-state phenomena found in experiments and molecular dynamics simulations.   

Continuum Simulation of Impact into Granular Beds

We investigate the dynamics of objects impacting into a granular medium using continuum simulations. Although a static bed with long-lasting contact between grains exhibits a solid-like configuration, the bed may become locally fluidized near an impact by an external object. Studies of shock propagation through granular beds suggest grains may flow freely near the impact site, yielding behaviors that could be analyzed using a granular hydrodynamics approach. We test the ability of a set of proposed granular hydrodynamics equations to describe the dynamics of a granular bed following impact using a numerical simulation. This system provides a test case to study the applicability and limitations of a hydrodynamics approach for modeling granular systems with coexisting static and fluidized states. Additionally, this system could provide a basic model to develop a better understanding of a range of phenomena such as meteor impact, biological locomotion over sand, and the performance of protective materials.   

RNS computations of the boundary layer flow over grooved plate

We use the Reduced Navier-Stokes equations (RNS) to compute the evolution of the 3d boundary layer flow over a plate with small depth streamwise grooves carved in it. The RNS are derived from the Navier-Stokes equations for flows with large Re number with one slow scale and two short scales. The resulting RNS are nonlinear and fully parabolic equations. In this work we comment the details of the numerical integration of the RNS, where we use a conformal mapping to include the effect of the grooved bottom. We present the resulting flow structures due to the geometry of the problem, also a parametric study to evaluate the effect of the spanwise wave number over the flow configuration. The RNS computations are much more less CPU costly than full 3D DNS, and does not exhibit the numerical instabilities present in previous PSE calculations.   

Direct route to turbulence in a rotating boundary layer

The transition to turbulence in a rotating boundary layer is analysed via DNS in an annular cavity made of two parallel co-rotating disks of finite radial extent, fed by a forced inflow at the hub. A former investigation [Viaud et al. JFM 2008] has established the existence of a primary subcritical bifurcation to nonlinear global mode with angular phase velocity and radial envelop coherent with the so-called elephant mode theory. When the Reynolds number based on the forced throughflow is increased above a threshold value for the existence of the nonlinear global mode, a large amplitude impulsive perturbation gives rise to a self-sustained saturated wave which is itself globally unstable. A second front appears in the lee of the primary where small-scale instability develops with characteristics indicating a Floquet mode of zero azimuthal wavenumber. This secondary instability leads to a very disorganized state, defining transition to turbulence. This transition, linked to the secondary instability of a global mode, confirms for the first time on a real flow the possibility of a direct transition to turbulence through an elephant cascade, a scenario up to now only observed on the Ginzburg--Landau model. Further work investigates the sensitivity of this scenario to environmental parameters, namely the streamwise extent of the flow, the incoming noise level, or the amplitude of the initial perturbation.   

Coating of a cylindrical fibre: Instability and drop formation

The instability of a liquid layer coating a thin cylindrical wire is studied experimentally and numerically with negligible gravity effects. The initial uniform film is obtained as the residual of a sliding drop, and the thickness measurements are performed with an anamorphic optical system. A primary mode grows in the early stages of the instability, and its wavelength $\lambda_1$ is not always in agreement with that predicted by the linear theory, $\lambda_m$. In later stages, a secondary mode appears, whose wavelength is half that of the primary mode. The behavior of the secondary mode allows us to classify the experiments into two cases, depending on whether it is linearly stable (case I) or unstable (case II). In case I, the amplitude of the secondary mode remains small compared with that of the primary one, while in case II both amplitudes may become very similar at the end. Thus, the distance between the final drops may be quite different from that seen between initial protuberances. The analysis of the experiments allows us to define a simple criterion based on the comparison between $\lambda_1$ and $\lambda_m$ (see Journal of Fluid Mechanics {\bf 651}, 117 (2010)).   

Modeling spreading of liquid crystal drops

A series of experiments involving spreading of nematic liquid crystal drops on solid substrates\footnote{Poulard and Cazabat, Langmuir, 6270, vol. 21 (2005)} have uncovered a surprisingly rich variety of behavior. The drops can either be arrested in their spreading, spread stably, or destabilize with or without spreading. We propose a relatively simple model which includes elastic contribution to the free energy as well as the finite anchoring energy due to the preferred orientation of the director field at the liquid/gas and liquid/solid interfaces. We find that the main features of the experiments, including spreading and instability regimes, can be in qualitative manner described by the proposed model.   

A low-speed corona jet for internal spot cooling of tubes

A high electric potential applied to a wire electrode at the centerline of a circular tube may cause gas ionization and corona discharge, leading to formation of secondary flows and a corona jet within the tube. A computational model was implemented to show that the corona jet appears only if the electrode is slightly off-center with respect to the tube. The computations indicate that the jet is oriented in the direction of electrode-tube offset, and it may be suitable for target-cooling of thermal components mounted on the inner surface of the tube. Because the direction of the coronal jet is adjusted by the orientation of the electrode, this arrangement may be used to focus the jet on specific areas for a more efficient and effective cooling of electronic components. In this study, the effectiveness of the aforementioned technique is investigated by reporting the local Nusselt number and rate of heat transfer. In addition, the corona-enhanced cooling is compared with the less efficient buoyancy-driven heat transfer in circular tubes. Variations of local dimensionless temperature, Nusselt number, and flow fields are among the presented results.   

The Enhancement of Streamwise Minus Modes during Evolution of a Subsonic Compressible Jet Flow

Direct numerical simulations in compressible jet flows within confined walls are studied. The results, excited by the random-broadband white noise in a sense of a natural experiment, will be further discussed in terms of jet instabilities from the beginning of noise level to the linear regime and transition to turbulence. An experimental study made of transition of a two-dimensional jet by Sato (1960) was chosen to run a similar direct numerical simulation in a temporal case and compared the results accordingly. Results of direct numerical simulations show a good agreement with experiment made by Sato in terms of linear growth rate and its eigenfunctions as well. In general the dominant spanwise modes (primary modes), usually the anti-symmetric modes, grow exponentially in the linear regime and saturated after the nonlinear interactions. However, there are streamwise minus modes developing, in the transitional to fully nonlinear regime, at rather bigger growth rates than the dominant spanwise modes. By examining the modal eigenfunction of the root-mean-square fluctuating velocity in downstream direction, the dominant spanwise mode behaves an anti-symmetric eigenfunction as measured by Sato's experiment. Interesting is it that the evolution of streamwise minus modes has a similarity to the symmetric eigenfunction as they develop into saturation.   

Fourier decomposition of periodic flow using DPIV

Digital particle image velocimetry (DPIV) is employed to measure the velocity fields of the flow past a circular cylinder in the 50$<$Re$<$340 range and a Fourier series based on the Strouhal period is employed to decompose the periodic time series up to the third harmonic. The series converges with decreasing coefficients and the final residual related to the DPIV initial data is very low. The first harmonic illustrates the von K\'arm\'an vortex street and its characteristic antisymmetric pattern with respect to the streamwise centerline. The second one is symmetric being responsible for the drag force main oscillations while the third is again antisymmetric. The harmonics respect the hierarchy suggested by the asymptotic series solution of the Hopf bifurcation and comparisons between the experimental results and FEM numerical simulations show good agreement in the considered Re range. Besides the validation of numerical simulations, the proposed decomposition can be used to reconstruct the periodic flow in order to minimize the effects of gappy data and measurement uncertainties. And for this case it is also very effective to decompose the flow in its most coherent structures.   

Penetration process and instabilities arisen on a liquid jet impinged to a liquid flowing in a channel

We conduct a series of experiments with a special interest on a penetration process and instabilities arisen on a liquid jet impinged to a liquid of the same kind flowing in a channel. The impinged jet penetrates into the flowing bath accompanying with entrainment of the ambient immiscible gas, which results in the impinged jet wrapped by the entrained gas as a ``sheath.'' This sheath formation enables the impinged jet to survive in the fluid in the channel without coalescing until the entrained-air sheath breaks down. Occasionally a ``cap'' of the entrained air is formed at the tip of the penetrated jet, and the jet elongates like a long balloon. Dynamic behaviors of the penetrated jet and the departure of the bubble of warring gas at the tip of the collapsing jet observed by use of a high-speed camera are discussed.   

Rayleigh Taylor Instability in the presence of Non-uniform Flow

We study the Rayleigh-Taylor Instability in the presence of an equilibrium flow which varies across the cross section. It is found when the flow varies linearly with the radial coordinate (flow shear) the growth rate of the instability increases whereas for the quadratic variation with the radial coordinate (flow curvature) the mode is stabilized. This might have important implications in inertial confinement fusion where the stabilization of Rayleigh Taylor mode is one of the biggest obstacles for energy generation by fusion. This result will also have important implications in identifying the origin of various space fluctuations.   

Richtymyer-Meshkov Instability: Effects of Mach Number and Initial Conditions on Turbulent Mixing

Effects of Mach number and initial conditions are studied on a thin Air-SF6-Air interface impulsively accelerated by a planar shock wave (Mach 1.2-1.8). A membraneless interface is formed using a nozzle design to create a stable gas curtain. Using particle image velocimetry and planar laser induced fluorescence, velocity and density fields are captured simultaneously to characterize the initial condition and the growth of the instability. To quantify and characterize the turbulent mixing, the evolving structure is reshocked at various times by varying the location of a moveable end wall. Turbulence statistics are compared between a single mode varicose curtain, and a multi-mode curtain. The results are compared with ongoing 3-D numerical simulations of the gas curtain experiment.   

The Effects of Varying Horizontal Boundary Conditions on the Momentum Distribution and Subcritical Turbulent Transition within Taylor-Couette Flow

We have experimentally investigated the momentum distribution and transition to turbulence within a high curvature (radii ratio of 0.55), low aspect ratio (height/gap of 6.3) Taylor-Couette flow using three different horizontal boundary conditions. End-caps between the two cylinders were wholly coupled to either the inner or outer cylinder, or otherwise split in half. By rotating only the outer cylinder we have obtained velocity data from fully cyclonic regimes using Laser Doppler Velocimetry (LDV). The subcritical transition to turbulence is clearly affected by the horizontal boundaries: end-caps that move with either cylinder yield a transition Reynolds number that is higher than when split. These results help clarify the role of secondary flows in the turbulent transition of this system, and also add to the early torque-based work of Wendt (1933) {\&} Taylor (1936).   

Multiphase microfluidics and Surface Enhanced Raman Spectroscopy

A two-phase microfluidic device is used to control the concentration and distribution of small numbers of silver nanoparticles in droplets, for experiments using Surface Enhanced Raman Spectroscopy (SERS). SERS is a widely used method that can allow detection and identification of trace quantities of chemicals, such as explosives or biological agents. Silver particles can be made to aggregate in the presence of an analyte, to create areas of intense electric field, called ``hot spots,'' and give out a SERS signal about 10 orders of magnitude stronger than traditional Raman spectroscopy. The mechanism of enhancement is theorized to depend on the number of particles forming the aggregate. By using a two-phase microfluidic system, small Poisson-distributed numbers of silver particles are confined in aqueous droplets and mixed with an analyte. In this way we explore the dependency of SERS intensity on the number of SERS-active particles in each droplet, thus enabling the detection and identification of single molecules in droplets.   

Development of ink transfer monitoring system for roll-to-plate gravure offset printing process

The gravure offset printing process is very cost-effective for printed electronics, such as printed solar cell, printed battery, printed TFT, printed RFID tag and so on. In gravure offset printing, there are two kinds of ink transfer processes -- off and set processes. At the off process, an elastic blanket cylinder picks up the ink from patterned plate or patterned cylinder. At the set process, ink on the elastic blanket cylinder is transferred onto the target substrate. These two ink transfer processes determine printing quality, therefore understanding of ink transfer mechanism during off and set processes are very important to control printing quality. In this study, we developed ink transfer monitoring system for roll-to-plate gravure printing. We visualized ink transfer from pattern plate to rolling blanket cylinder (off process) and from rolling blanket cylinder to plate substrate (set process) by using high-speed camera and long range microscope. We investigated the effects of pattern size, printing speed, rotational effect of blanket cylinder, contact angle and rheological property of ink to understanding gravure offset printing mechanism.   

Numerical Simulation of a Micropump with Step Electrodes Using the Steric Effects

Numerical simulations were made of a micro-pump with step electrodes. An AC voltage was applied to each pair of step electrodes. The existing numerical studies using linear assumptions have a limitation that the theory is limited to low external voltage, at most 25mV. However, the present study took into account the Steric effect which is recognized as a good candidate to overcome such a limitation. Geometrical optimization and the effects of an AC external voltage on pumping flow rate were studied.   

Role of bubble velocity fluctuations on the energy dissipation rates in bubbly Rayleigh-B\'enard convection

We report numerical results for thermal and kinetic energy dissipation rates in bubbly Rayleigh-B\'enard convection. The bubbles always homogenize the temperature field by absorbing heat from surrounding fluid and attenuate the thermal energy dissipation. A small number of non-growing bubbles can even halt convection by smoothing the temperature fluctuations which drive the convection. Growing bubbles imply additional forcing on the fluid and thus increase the fluctuations and hence the kinetic energy dissipation. This enhancement depends on the ratio of the sensible heat to the latent heat of the phase change, given by the Jakob number, which determines the dynamics of the bubble growth.   

Molecular dynamics simulations of Janus nanoparticle assembly at interfaces

We study the formation of clusters of nano-sized Janus particles (having surface regions with different interactions) at a liquid-vapor interface using molecular dynamics simulations. The individual particles are modeled as rigid spherical sections of an atomic lattice, with short ranged atom-atom interactions chosen to selectively attract subregions of the particle surface, and with a solid-liquid interaction favoring a 90 degree contact angle at the liquid interface. The simulations automatically incorporate the competition between Brownian motion, fluid convection and molecular attraction, as well as evaporation of the liquid if desired. We study the distribution and shape of the clusters found in equilibrium, and the structures resulting when the solvent evaporates.   

Atomistic simulation of water transport through graphene membrane

Graphene monolayer can be considered as the thinnest membrane reported so far as its thickness is only one carbon atom diameter. In this study, water transport through porous graphene membrane is investigated using molecular dynamics simulations. Water flux through graphene is compared to the water flux through thin (less than 10 nm in thickness/length) carbon nanotube (CNT) membranes at various diameters. For small diameter, where single-file structure is observed, water flux is lower through the graphene membrane compared to that of the CNT membrane. On the other hand, for larger diameter pores, where the single-file structure is no longer observed, water flux is higher through the graphene membrane, compared to that of the CNT membrane. We explain the results using hydrogen bonding dynamics, pressure distribution and potential of mean force.   

Experimental Evaluation of Simplified Theoretical Models for Fluid Mud Gravity Current Propagation

The propagation dynamics of fluid mud gravity currents were studied experimentally and theoretically. The experimental currents propagate under slumping, self-similar and viscous phases. The transition times from slumping to self-similar and from self-similar to viscous phases are parameterized. Predictive capabilities of the three existing theoretical modeling approaches (force-balance, box and shallow water) were evaluated based on our experimental observations. For the slumping and self-similar phases, both the force-balance and box model solutions showed a better predictive capability than the one-layer shallow water model solution. Having non-Newtonian rheology, the propagation dynamics of fluid mud gravity currents in the viscous phase vastly differ from the Newtonian currents. A force-balance expression for the viscous spreading of non-Newtonian power-law gravity current was derived. The predictions of this force-balance expression and a recent viscous shallow water model solution are observed to be in good agreement with the experimental data. The results of this study are expected to be useful in predicting the spreading of fluid mud gravity flows that occur in different natural and industrial situations.   

Permeability prediction of isotropic fibrous porous media

Fibrous porous media find application in several industrial engineering disciplines including filtration processes and fuel cells. In the present study a geometric pore-scale model is introduced and used to predict the permeability of isotropic fibrous porous media. The model is based on a unit cell approach in which the fibres of the actual porous medium are modelled based on rectangular geometry. At first the model is used to predict the permeability of cross-flow through an array of unidirectional fibres. The permeability is expressed as a function of the solid volume fraction and a pore-scale linear dimension. In addition a three-dimensional isotropic model is proposed by performing a weighted average on the model for cross-flow and a model from the literature for flow parallel to the fibre axes. The resulting model is compared to a comprehensive collection of experimental data from numerous authors, based on various types of fibrous porous media, including that of entangled polymer networks. The Kozeny ``constant'' is calculated for different solid volume fractions and it is illustrated that the pore-scale model introduced conserves the constancy of the Kozeny constant.   

Evolution of droplets of perfectly wetting liquid under the influence of thermocapillary forces

We consider evolution of sessile droplets of a nonvolatile perfectly wetting liquid on differentially radially heated solid substrates. The heating induces thermocapillary Marangoni forces which affect the contact line dynamics. In our experiments, we witness the opposing action of the thermocapillary Marangoni effect and capillary spreading. We record an interesting feature which develops during this phase -- while the bulk of the drop mass recedes toward the center, the contact line recedes at a much slower rate, leaving a stretched layer of liquid between the main body of the drop and the contact line. We find that this layer of liquid thins as evolution of the drop proceeds and that the thinning is more pronounced when the imposed temperature gradient in the contact line region is larger. Our theoretical model, based on the lubrication approximation and incorporating the Marangoni effect, recovers the main features observed in the experiments. The model also indicates a strong dependence of the drop shape on the imposed temperature gradient, and, for a particular class of temperature profiles, it predicts formation of a ridge between the thin liquid layer and the main body of the drop, which is still to be observed in experiments.   

Characterization of hydrophobic and hydrophilic coatings as deicing and anti-icing

Anti-icing is necessary in various fields, such as aeronautics, roads, power lines, ships, and architectures. Deicing fluids, and sometimes hot water, work to prevent from icing. Due to environmental issue, deicing fluids are not always welcome to use. We study hydrophobic and hydrophilic coatings for anti-icing. By coating these to a target surface, it prevents icing without damaging the environment. We present a characterization method of hydrophobic and hydrophilic coatings for deicing and anti-icing. We provide a temperature-control room to create an icing condition, such as -10 to 0 degrees C. Under the controlled room, the contact angle measurement as well as the force measurement is employed. Total 15 coatings are characterized. Based on the tests of all coatings, we propose a combined coating from some characterized ones.   

Lagrangian particle tracking in strained turbulence

We present initial results from experimental investigations of axis-symmetrically strained turbulent flow. Our focus is on the influence of the straining on the motions of passive and inertial particles. The results are compared with existing numerical and experimental data, and we seek to emphases the effects of the strain geometry and strain rate on the particle behaviour. Eulerian, PIV, flow field results are also presented. We furthermore present a new approach for the analysis and processing of particle tracks and discuss our experimental errors in detail.   

Viscous Flow past a Semi-Infinite Plate

Numerical investigations are made to simulate two dimensional viscous fluid flows past a semi-infinite plate at early short time. Details of the flow structures are visualized using a finite difference method. The shedding of the vortex sheet from the plate tip is studied.   

On the potential for transport via internal tides

Non-linear effects associated with internal waves lead to advection of fluid particles along with suspended mass such as sediment, nutrients, larvae, as well as contaminants. These factors contribute to the development of benthic communities, the geological shaping of the continental slope and, in some situations, play a role in the transport and fate of contaminants. We compute particle trajectories and resulting Stokes velocity profiles using a Navier-Stokes code with a Lagrangian particle tracking model. Results are compared to linear theory and a semi- nonlinear formulation using both uniform (idealized) and nonuniform (realistic) stratifications, namely what is found in offshore of Huntington Beach, CA. We conclude that Stoke's drift due to nonlinear internal waves is an important component to the overall transport budget, particularly in the nearshore region where nonlinear internal waves are ubiquitous and persistent.   

On the potential for transport due to internal tides in the coastal ocean

Non-linear effects associated with internal waves lead to advection of fluid particles along with suspended mass such as sediment, nutrients, larvae, as well as contaminants. These factors contribute to the development of benthic communities, the geological shaping of the continental slope and, in some situations, play a role in the transport and fate of contaminants. We compute particle trajectories and resulting Stokes velocity profiles using a Navier-Stokes code with a Lagrangian particle tracking model, both are second-order accurate in time and in space. Results are compared to linear theory and a semi-nonlinear formulation using a uniform stratification and stratification typically found at Huntington Beach, CA where there is recurring bacteriological contamination.   

Internal wave attractors in stratified fluids, robustness to perturbations

Previously, internal wave attractors have been studied in the laboratory in idealized situations. Here, we present a series of experiments in which these conditions are modified. Modifications are made by varying the forcing frequency, by using a non-uniform stratification, by introducing finite amplitude perturbations to the trapezoidal domain and by using a parabolic domain. All these new experiments reveal the persistence of internal wave attractors that remain reasonably well predictable by means of ray tracing. We conclude that the possibility of wave attractors has to be addressed whenever internal waves are found in stratified fluids.