Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session MT: Suspensions II |
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Chair: Thomas Ward, North Carolina State University Room: 200H |
Tuesday, November 24, 2009 8:00AM - 8:13AM |
MT.00001: Effects of inertia and viscoelasticity on the orientation dynamics of axisymmetric particles Vivekanand Dabade, Ganesh Subramanian The talk will focus on the analytical investigation of the effects of weak inertia and fluid viscoelasticity on the orientation dynamics of a spheroidal particle in two canonical flow situations: 1. A spheroid sedimenting in a quiescent fluid, and 2. A neutrally buoyant spheroid rotating in a simple shear flow. The spheroidal geometry is taken as representative of the general effects of particle anisotropy in disperse multiphase systems. The orientation distribution of a non-Brownian spheroidal particle remains indeterminate in both sedimentation and shear flow in the Stokes limit. Either of inertia or viscoelasticity remove this indeterminacy. The above problems are analyzed using a novel approach based on the formalism of vectorial spheroidal harmonics together with the generalized reciprocal theorem. We obtain closed-form expressions for the \textit{O(Re)} inertial and \textit{O(De)} viscoelastic torques in sedimentation, and the \textit{O(Re)} angular velocity in simple shear flow, valid for an arbitrary aspect ratio. The present results highlight errors in earlier theoretical and numerical calculations. [Preview Abstract] |
Tuesday, November 24, 2009 8:13AM - 8:26AM |
MT.00002: Particle acceleration model for gas--solid suspensions at moderate Reynolds numbers Sudheer Tenneti, Rahul Garg, Christine Hrenya, Rodney Fox, Shankar Subramaniam Particle granular temperature plays an important role in the prediction of core annular structure in riser flows. The covariance of fluctuating particle acceleration and fluctuating particle velocity governs the evolution of the granular temperature in homogeneous suspensions undergoing elastic collisions. Koch and co--workers (Phys. Fluid. 1990, JFM 1999) showed that the granular temperature has a source term due to hydrodynamic interactions in gas--solid suspensions in the Stokes flow regime. We performed direct numerical simulations (DNS) of freely evolving suspensions at moderate Reynolds numbers using the immersed boundary method (IBM). We found that simple extension of a class of mean particle acceleration models to their instantaneous counterparts does not predict the correct fluctuating particle acceleration--fluctuating velocity covariance that is obtained from DNS. The fluctuating particle velocity autocorrelation function decay and the Lagrangian structure function obtained from DNS motivate the use of a Langevin model for the instantaneous particle acceleration. [Preview Abstract] |
Tuesday, November 24, 2009 8:26AM - 8:39AM |
MT.00003: Batch Sedimentation in Impulsively Heated nearly mono-disperse Viscous Multiphase Fluids Ameya Joshi, Thomas Ward Suspensions of nearly mono-disperse spheres, subjected to impulsive-constant temperature heating from below, are studied in a batch sedimentation process. Experiments are performed on suspensions with a range of concentrations and different temperatures, to analyze the effects of varying temperatures on the rate of settling, shock formation and shock velocities. CCD imaging is used to observe and study the settling phenomenon. The Kynch theory of sedimentation (Trans. of Far. Soc., 1952) explains shock formation in low concentrations suspensions and states that particle speed is completely determined by the local density only. We extend this study to include both heating and high concentration suspensions. Higher concentration suspensions (greater than 20\%) are characterized by a single shock. Two distinct shocks are observed for all the experiments involving heating from bottom with an exception for the non-heated setup where only one shock is observed. Vortex patterns observed for the heated and the unheated case also show a substantial difference. The distinct vortex patterns produced in the suspension during settling are attributed to heating. The experiments suggest that other models may be more appropriate for high concentration studies. [Preview Abstract] |
Tuesday, November 24, 2009 8:39AM - 8:52AM |
MT.00004: Shear-induced fluid tracer diffusion in a semi-dilute suspension of spheres Takuji Ishikawa, Takami Yamaguchi We calculated tracer diffusion in a sheared suspension of non- Brownian rigid spheres and propose a new numerical method based on a boundary element method and Stokesian dynamics method. We present details of the numerical method and examine the accuracy of the method. The limitation of semi-diluteness is due to the accuracy of tracer velocity calculation. The results show that the diffusivity of fluid tracers is greater than that of suspended spheres in the semi-dilute regime. The diffusivity of velocity gradient direction is about threefold greater than that in the vorticity direction. Simple scaling demonstrates that the diffusivity of fluid tracers increases with the square of the volume fraction of spheres in the semi-dilute regime, which is confirmed numerically. [Preview Abstract] |
Tuesday, November 24, 2009 8:52AM - 9:05AM |
MT.00005: Yielding and vorticity banding in sheared nanotube suspensions Erik K. Hobbie We analyze the yielding and flow of nanotube suspensions in the parameter space of particle concentration, aspect ratio and applied shear stress. The density of mechanical entanglements increases with both the aspect ratio and number density of nanotubes, creating an elastic particle network at very low nanotube concentration. Under simple shear flow, these networks exhibit a well-defined yield stress and classic Bingham-fluid behavior. Unstable flows at moderate Peclet number are characterized by a periodic pattern of vorticity-banded domains, which transition to a stable and homogeneous flow-aligned state in the limit of very large Peclet number. [Preview Abstract] |
Tuesday, November 24, 2009 9:05AM - 9:18AM |
MT.00006: Numerical simulation of lateral segregation during Stokes bidisperse sedimentation and onset of irreversibility Xiaolong Yin It is known that in Stokes bidisperse sedimentation particles of different sizes or densities will segregate in the lateral direction and form column structures. Numerical simulations were used to study lateral segregation in periodic domains containing 1146 particles of same size but different densities (particle-fluid density ratios = 1.4 and 0.6). The volume fractions were 0.15 and 0.15. The presence of lateral segregation was confirmed using a combination of visual observation and measurements of sedimentation velocities, velocity variance, and structure factors. By changing the direction of gravity during sedimentation, the reversibility of the suspension was tested. It was found that the system demonstrated reversibility for only about 6 Stokes times, and then lost its memory. Simulations conducted in domains with top and bottom walls did not show significant lateral segregation. The hindered settling velocities of the suspension in wall-bounded domains were between the predictions of Davis and Gecol (AIChE J. 40:570 1994) and those of Revay and Higdon (J. Fluid Mech. 243:15 1992). [Preview Abstract] |
Tuesday, November 24, 2009 9:18AM - 9:31AM |
MT.00007: Self-organization of fiber suspension Alexandre Franceschini, Elizabeth Guazzelli, David Pine A single buoyant non-Brownian fiber in a low Reynolds shear flow has a fully determined motion, so-called Jeffery orbit. However, the behavior of a concentrated fiber suspension remains unclear; even slight interactions between objects can disturb the orbits and collective behavior cannot be neglected [1]. By observing the system under a periodic shear, we are able to specifically observe the irreversible motions due to fiber-fiber interactions. collective motion is followed by viscosity measurements and quantitative image analysis [2, 3]. The angle and center-of-mass of marked fibers are also tracked. The system self-organizes and always becomes more reversible with time until a steady state, fluctuating or not, is reached. A dynamical phase transition between a quasi-reversible and a fluctuating state is observed, with a transient time that exhibit a power law divergence at the critical points. [1] Okagawa A and Mason SG, Science, Volume 181, Issue 4095, p159 (1973) [2] Pine DJ and al., Nature, vol 438, Issue 7070, p997 (2005) [3] Corte L and al., Nature physics, vol 4, Issue 5, p420 (2008) [Preview Abstract] |
Tuesday, November 24, 2009 9:31AM - 9:44AM |
MT.00008: Guided colloidal crystallization in a galvanic micro reactor Christian Punckt, Linda Jan, Boris Khusid, Ilhan A. Aksay We present a novel method for assembling colloidal particles into an ordered coplanar array of two-dimensional crystals. This technique utilizes an autonomous galvanic micro reactor to control the location and morphology of colloidal crystals. Coplanar arrays of copper and gold microelectrodes are placed into a dilute water solution of hydrochloric acid to form a galvanic couple between the copper acting as an anode and the gold as a cathode. Under appropriate conditions, colloidal particles suspended in the solution assemble into two-dimensional colloidal crystals adherent to the anodic copper. Polystyrene and silica particles having similar sizes and zeta potentials but different densities were employed to study the fluid flow in the galvanic reactor. Spatially resolved, optical analysis of the reaction rate was used to estimate the magnitude and distribution of the electric current over the copper electrodes. Physical mechanisms governing the particle motion and aggregation will be discussed. [Preview Abstract] |
Tuesday, November 24, 2009 9:44AM - 9:57AM |
MT.00009: Order transition in non-colloidal Couette suspension flows:effects of external torques Kyongmin Yeo, Martin Maxey Suspensions of non-colloidal particles in Couette flows are investigated by using the force-coupling method. It is observed that a hexagonal order begins to develop near the wall at a volume fraction as low as $\phi = 0.48$, while the suspensions in the center of the channel remains disordered. The ordering transition depends on the ratio of the channel width to the particle radius. It is shown that the order state can be modified by applying external torques on the particles. The hexagonal order of the particles is weakened by the negative torque, leading to the increase of the shear viscosity. The positive torque has a favorable effect on the ordered state. However, if the magnitude of the positive torque exceeds a certain threshold, the hexagonal order begins to be weakened. On the other hand, at a moderate volume fraction ($\phi = 0.40 $), the external torques do not have a significant effect on the suspension rheology. The nonlinear responses of rheological parameters, such as the shear and vortex viscosities, to the external torques are investigated. [Preview Abstract] |
Tuesday, November 24, 2009 9:57AM - 10:10AM |
MT.00010: Flow Irreversibility in Particle Suspensions with Non-Uniform Strain J.S. Guasto, A.S. Ross, J.P. Gollub Sheared particle suspensions are irreversible even in Stokes flows.\footnote{D.J. Pine, \textit{et al.}, Nature \textbf{438}, 997 (2005).}$^,$\footnote{L. Cort\'{e}, \textit{et al.}, Nature Physics \textbf{4}, 420 (2008).} Here, we extend this previous work to a system with spatially varying strain, an oscillatory, rectangular channel flow. The particle volume fraction is 40\% and the Reynolds number is $\sim 10^{-3}$. Deviations from reversible behavior are parameterized by the mean square particle displacements sampled once per cycle, and are compared to the local strain obtained from measured velocity profiles. Strikingly, the particle motions across the entire channel become irreversible simultaneously, despite the non-uniform local strain, and the irreversibility increases roughly exponentially with the wall strain. Measured velocity profiles are plug-like near the channel center, which may indicate shear-induced particle migration. We examine the possibility that irreversibility is a mechanism for shear-induced migration. [Preview Abstract] |
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