Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G37: General Fluid Dynamics IGeneral

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Room: 303 
Monday, November 20, 2017 10:35AM  10:48AM 
G37.00001: Colloidal band assembly from different suspended particles Andrew Yee, Minami Yoda Particle visualizations, mainly based on evanescentwave illumination, have shown that sulfate and carboxylateterminated polystyrene particles in a dilute suspension flowing through a microchannel assemble into nearwall bands when the flow is driven by a dc electric field and a pressure gradient along the channel axis applied in opposite directions. In these bands, the colloidal particles (of radius $a\approx 250500$ nm) are concentrated in a liquid state in regions with a crosssectional dimension of a few $\mu \mbox{m}$ and an axial extent comparable to the channel length of $O$(1 cm). In many cases, the particles first assemble into many closely spaced, fairly unstable bands before achieving a fairly stable ``steadystate'' configuration with fewer bands. Results at a given channel location for the timescales of particle assembly as well as the characteristics of the bands are presented for a range of particle and suspension properties including the particle volume fraction $\varphi_{\infty } ,\,\;a\mbox{,\thinspace }$ and particle zetapotential $\zeta_{\mbox{p}} $ and flow properties such as the electric field magnitude $\left E \right$ and nearwall shear rate $\dot{{\gamma }}$. The band characteristics appear to scale with the electric field ``offset,'' or $\left E \right\left {E_{\mbox{min}} } \right$ where $\left {E_{\mbox{min}} } \right$ is the minimum electric field magnitude at a given $\dot{{\gamma }}$ required for band formation. [Preview Abstract] 
Monday, November 20, 2017 10:48AM  11:01AM 
G37.00002: Colloidal band assembly in different microchannels Minami Yoda, Andrew Yee, Varun Lochab, Shaurya Prakash Over the last few years, we have shown that polystyrene colloidal particles in a dilute suspension flowing through a microchannel assemble into nearwall bands in combined steady Poiseuille and electroosmotic (EO) ``counterflow'' driven by a pressure gradient and dc electric field, respectively, in opposite directions. The bands of concentrated particles have a crosssectional dimension of a few $\mu \mbox{m}$ and an axial extent of $O$(1 cm) and a crossstream spacing of $O$(10 $\mu \mbox{m})$. This type of colloidal assembly has been demonstrated for a variety of particle radii, zetapotentials, and concentrations, and the flow characteristics required for band assembly, as well as the characteristics of the bands once formed, depend strongly upon these suspension and particle properties. More recently, we have started investigating how band assembly is affected by channel properties. Results are presented at different streamwise ($x)$ locations in different channel geometries for fusedsilica and polydimethyl siloxane(PDMS)fused silica channels with their different wall zetapotentials $\zeta_{\mbox{w}} $ . The effect of suspending liquid properties including electrolyte composition and ionic strength is also discussed. [Preview Abstract] 
Monday, November 20, 2017 11:01AM  11:14AM 
G37.00003: Observations of the initial stages of colloidal band formation Yanrong Li, Yoshiyuki Tagawa, Andrew Yee, Minami Yoda A number of studies have shown that particles suspended in a conducting fluid near a wall are subject to wallnormal repulsive ``lift'' forces, even in the absence of interparticle interactions, in a flowing suspension. Evanescentwave visualizations have shown that colloidal particles in a dilute (volume fractions \textless 0.4{\%}) suspension are instead \textit{attracted} to the wall when the suspension is driven through $\sim 30\mbox{\thinspace }\mu \mbox{m}$ deep channels by a pressure gradient and an electric field when the resulting combined Poiseuille and electroosmotic (EO) flow are in opposite direction, $i.e.$, ``counterflow,'' although the particles and channel walls both have negative zetapotentials. Above a minimum ``threshold'' electric field magnitude $\left {E_{\mbox{min}} } \right$, the particles assemble into dense ``bands'' with crosssectional dimensions of a few $\mu \mbox{m}$ and length comparable to that of the channel ($i.e.$, a few cm). The results suggest that the threshold field $\left {E_{\mbox{min}} } \right$ is large enough so that there is a region of ``reverse'' flow, along the direction of the EO flow, near the wall. Visualization of a large segment of the channel (\textgreater 300 hydraulic diameters) at frame rates as great as 1 kHz is used to determine banding maps for a variety of dilute colloidal suspensions and to investigate the initial stages of band formation over a wide range of flow conditions. [Preview Abstract] 
Monday, November 20, 2017 11:14AM  11:27AM 
G37.00004: Exploration of Piezoelectric Bimorph Deflection in Synthetic Jet Actuators Kevin Housley, Michael Amitay The design of piezoelectric bimorphs for synthetic jet actuators could be improved by greater understanding of the deflection of the bimorphs; both their mode shapes and the resulting volume change inside the actuator. The velocity performance of synthetic jet actuators is dependent on this volume change and the associated internal pressure changes. Knowledge of these could aid in refining the geometry of the cavity to improve efficiency. Phaselocked jet velocities and maps of displacement of the surface of the bimorph were compared between actuators of varying diameter. Results from a bimorph of alternate stiffness were also compared. Bimorphs with higher stiffness exhibited a more desirable (0,1) mode shape, which produced a high volume change inside of the actuator cavity. Those with lower stiffness allowed for greater displacement of the surface, initially increasing the volume change, but exhibited higher mode shapes at certain frequency ranges. These higher node shapes sharply reduced the volume change and negatively impacted the velocity of the jet at those frequencies. Adjustments to the distribution of stiffness along the radius of the bimorph could prevent this and allow for improved deflection without the risk of reaching higher modes. [Preview Abstract] 
Monday, November 20, 2017 11:27AM  11:40AM 
G37.00005: Direct numerical simulation of axisymmetric laminar lowdensity jets Daniel Gomez Lendinez, Wilfried Coenen, Alejandro Sevilla The stability of submerged laminar axisymmetric lowdensity jets has been investigated experimentally (Kyle \& Sreenivasan 1993, Hallberg \& Strykowski 2006) and with linear analysis (Jendoubi \& Strykowski 1994, Coenen \& Sevilla 2012, Coenen et al. 2017). These jets become globally unstable when the Reynolds number is larger than a certain critical value which depends on the density ratio and on the velocity profile at the injector outlet. In this work, Direct Numerical Simulations using FreeFEM$++$ (Hecht 2012) with P1 elements for pressure and P2 for velocity and density are performed to complement the above mentioned studies. Density and velocity fields are analyzed at long time showing the unforced spacetime evolution of nonlinear disturbances propagating along the jet. Using the StuartLandau model to fit the numerical results for the selfexcited oscillations we have computed a neutral stability curve that shows good agreement with experiments and stability theory. [Preview Abstract] 
Monday, November 20, 2017 11:40AM  11:53AM 
G37.00006: CellAveraged discretization for incompressible NavierStokes with embedded boundaries and locally refined Cartesian meshes: a highorder finite volume approach Amneet Pal Singh Bhalla, Hans Johansen, Dan Graves, Dan Martin, Phillip Colella We present a consistent cellaveraged discretization for incompressible NavierStokes equations on complex domains using embedded boundaries. The embedded boundary is allowed to freely cut the locallyrefined background Cartesian grid. Implicitfunction representation is used for the embedded boundary, which allows us to convert the required geometric moments in the Taylor series expansion (upto arbitrary order) of polynomials into an algebraic problem in lower dimensions. The computed geometric moments are then used to construct stencils for various operators like the Laplacian, divergence, gradient, etc., by solving a leastsquares system locally. We also construct the interlevel datatransfer operators like prolongation and restriction for multi grid solvers using the same leastsquares system approach. This allows us to retain highorder of accuracy near coarsefine interface and near embedded boundaries. Canonical problems like TaylorGreen vortex flow and flow past bluff bodies will be presented to demonstrate the proposed method. [Preview Abstract] 
Monday, November 20, 2017 11:53AM  12:06PM 
G37.00007: Solutions to the linearized NavierStokes equations for channel flow via the WKB approximation Anthony Leonard Progress on determining semianalytical solutions to the linearized NavierStokes equations for incompressible channel flow, laminar and turbulent, is reported. Use of the WKB approximation yields, e.g., solutions to initialvalue problem for the inviscid OrrSommerfeld equation in terms of the Bessel functions $J_{+1/3}, J_{1/3}, J_1,$ and $Y_1$ and their modified counterparts for any given wave speed $c = \omega/k_x$ and $k_{\perp}, (k_{\perp}^2 = k_x^2 + k_z^2)$. Of particular note to be discussed is a sequence $i = 1, 2,. . .$ of homogeneous inviscid solutions with complex $k_{\perp i}$ for each speed $c$, ($0 < c \leq U_{max}$), in the downstream direction. These solutions for the velocity component normal to the wall $v$ are localized in the plane parallel to the wall. In addition, for limited range of negative $c $, $(  c* \leq c \leq 0)$, we have found upstreamtraveling homogeneous solutions with real $k_{\perp}(c)$. In both cases the solutions for $v$ serve as a source for corresponding solutions to the inviscid Squire equation for the vorticity component normal to the wall $\omega_y$. [Preview Abstract] 
Monday, November 20, 2017 12:06PM  12:19PM 
G37.00008: Mixing in turbulent channel flow with Lagrangian computations Quoc Nguyen, Dimitrios Papavassiliou Turbulent mixing has been studied because of its importance in transport processes in both nature and engineering [1]. The effects of convection dominate over molecular diffusion, but effects of molecular diffusion are not negligible at molecular scales, and are significant in reacting flows. Several studies have been devoted to molecular mixing in homogeneous, isotropic turbulence. In this study, the effect of anisotropic turbulence, along with that of molecular diffusion, on passive scalar mixing is explored. Simulation of a turbulent channel flow is conducted by direct numerical simulation, followed by Lagrangian tracking of the motion of passive scalars with different Schmidt numbers (Sc) in the flow field. The friction Re is 300 and the Sc ranges from 0.7 to 2,400. Mass markers are released from instantaneous and continuous line sources, located at the center region of the channel to the wall. The combined effects of mean velocity difference, molecular diffusion and nearwall coherent structures lead to the observation of different concentrations of particles at different heights, downstream from the source. Mixing efficiency is quantified by measuring both the intensity and the area of the channel where mixing happens. While results with instantaneous sources demonstrate the physics of mixing, using continuous sources reveals how to control the timing and spatial distribution of the mixing. References 1. Dimotakis, P.E. Annu. Rev. Fluid Mech. 37, 329, 2005 2. Nguyen, Q., Papavassiliou, D.V. Phys. Fluids, 28(12), 125103, 2016 3. Nguyen, Q., Srinivasan, C., Papavassiliou, D.V. Phys Rev E, 91, 033019, 2015 [Preview Abstract] 
Monday, November 20, 2017 12:19PM  12:32PM 
G37.00009: Flow effects on the formation of nanoparticle aggregates in packed beds Dimitrios Papavassiliou, Ngoc Pham Aggregation of nanoparticles in porous media may give rise to pore clogging and sedimentation, which are undesirable phenomena in stable dispersing systems. In addition, deposited/settled aggregates might serve as secondary collectors for further attachment of suspended nanoparticles. In this work, the transient mean size of nanoparticle systems, propagating in packed beds is numerically explored. Primary nanoparticles can interact with each other to form aggregates with a known aggregation probability. The velocity fields in the packedsphere beds are generated by using the lattice Boltzmann method for single phase flow. In conjunction with that, a Lagrangian framework [1, 2] is employed to track the positions of the nanoparticles at every simulation step. We explore the change of the transient mean size under different hydrodynamic conditions (pore velocity, primary particle size, particle concentration, aggregation probability). An empirical correlation, predicting the final mean size of aggregates under given hydrodynamic conditions, is also obtained. References 1. R. S. Voronov, S. VanGordon, V. I. Sikavitsas, D. V. Papavassiliou, Int. J. Num. Methods Fluids, 67, 501517, 2011 2. N.H. Pham, D.P. Swatske, J.H. Harwell, BJ Shiau, D.V. Papavassiliou, Int. J. Heat {\&} Mass Transf. 72, 319328, 2014 [Preview Abstract] 
Monday, November 20, 2017 12:32PM  12:45PM 
G37.00010: Energy harvesting from the interaction of a Lamb dipole with a flexible cantilever Hui Tang, Chenglei Wang Energy harvesting from interactions of coherent flow structures with flexible solid structures can be used for powering miniature electronic devices. Although effective, the fundamental mechanism of such an energy extraction process has not been fully understood. Therefore, this study aims to provide more physical insights into this problem. The coherent flow structure is represented by a Lamb dipole, and the solid structure is assumed as a twodimensional flexible cantilever. The cantilever is placed along the propagation direction of the dipole, with its fixed end initially towards or away from the dipole and its lateral distance from the dipole center varied. As the dipole passes through the cantilever, the latter can extract energy from the former through effective interactions. Such a twodimensional fluidstructure interaction problem is numerically studied at a low Reynolds number of 200 using a lattice Boltzmann method (LBM) based numerical framework. The simulation results reveal that the flexible cantilever with a moderate stiffness is more beneficial to the energy harvesting, and it can scavenge more energy from the ambient vortices when its fixed end is initially away from the dipole with a relatively small lateral distance. [Preview Abstract] 
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