Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session M25: Suspensions III |
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Chair: James Gilchrist, Lehigh University Room: 328 |
Tuesday, November 22, 2011 8:00AM - 8:13AM |
M25.00001: Unifying granular and suspension rheology Francois Boyer, Elisabeth Guazzelli, Olivier Pouliquen Using an original pressure-imposed shear cell, we study the rheology of very dense suspensions. We show that they dexhibit a visco-plastic behavior similarly to granular media successfully described by a frictional rheology and fully characterized by the evolution of the friction coefficient $\mu$ and the volume fraction $\phi$ with a dimensionless viscous number $I_v$. Dense suspension and granular media are thus unified under a common framework. These results are shown to be compatible with classical empirical models of suspension rheology and provide a clear determination of constitutive laws close to the jamming transition. [Preview Abstract] |
Tuesday, November 22, 2011 8:13AM - 8:26AM |
M25.00002: Suspensions in a tilted trough: second normal stress difference Elisabeth Guazzelli, Etienne Couturier, Francois Boyer, Olivier Pouliquen We measure the second normal-stress difference in suspensions of non-Brownian neutrally-buoyant rigid spheres dispersed in a Newtonian fluid. The second normal-stress-difference is found to be negative and linear in shear stress. The ratio of second normal-stress difference to shear stress increases with increasing volume fraction. A clear behavioural change exhibiting a strong (approximately linear) growth in the magnitude of this ratio with volume fraction is seen above a volume fraction of 0.22. By comparing with previous data obtained for the same batch of spheres in a rotating rod geometry, the ratio of first normal-stress difference to shear stress is estimated and its magnitude is found to be very small. [Preview Abstract] |
Tuesday, November 22, 2011 8:26AM - 8:39AM |
M25.00003: Roles of Particle-Wall and Particle-Particle Interactions in Highly Confined Suspensions of Spherical Particles being Sheared at Low Reynolds Numbers Ashok Sangani, Andreas Acrivos, Philippe Peyla The roles of particle-wall and particle-particle interactions are examined for suspensions of spherical particles in a viscous fluid being confined and sheared at low Reynolds numbers by two parallel walls moving with equal but opposite velocities. It is shown that the channel-width scale interactions between the spheres tend to decrease the overall viscous dissipation in highly confined suspensions. In other words, the increase in the viscous dissipation caused by the particle-wall interactions is partially compensated by the particle-particle interactions. As a consequence, the total dissipation as a function of particle volume fraction in random suspensions may go through a maximum for a fixed ratio of sphere radius to spacing between the walls. [Preview Abstract] |
Tuesday, November 22, 2011 8:39AM - 8:52AM |
M25.00004: Direct experimental investigation of suspension microstructure Bu Xu, Tharanga Perera, James Gilchrist Microstructure is key to understanding rheological behaviors of a flowing particulate suspension. Previous computational results reveal an anisotropic structure forming under high P\'{e}clet number conditions. An experimental technique based on confocal microscopy is developed to study microstructure of a colloidal suspension in a microchannel. Results of simulation-like quality are produced and the effect of \textit{Pe} and interparticle force is investigated. Microstructure is shown to be consequential from competitions between hydrodynamic, thermal and electrostatic forces. The stress of suspension, calculated based on the microstructural information, is compared with previous computations. [Preview Abstract] |
Tuesday, November 22, 2011 8:52AM - 9:05AM |
M25.00005: Sedimentation and suspension of a cylinder confined in a Hele Shaw cell Harold Auradou, Veronica D'Angelo, Jean-Pierre Hulin We present experiences of settling or suspension of a rigid horizontal cylinder moving between two vertical parallel plates. In this study, the ratio between the cylinder diameter and the cell aperture ranges between 0.4 and 0.9 and the Reynolds number is always below the value for vortex shedding at the rear of a fixed cylinder. For ratio below 0.5, the cylinder stays horizontal and translates vertically at a constant velocity. For ratio between 0.5 and 0.7, the cylinder is observed to oscillate in the gap of the cell. This induces a variation of the drag which in turn results in a vertical velocity which oscillates around a mean value. For confinement above 0.7, the cylinder flutters in the Hele Shaw plane. The periodic lateral motion of the cylinder is studied as function of the flow conditions. The dynamic of the system is studied as function of the Reynolds number, confinement and cylinder of various density are also considered. Our observations are compared to a recent study concerning confinement induced vibration for a tethered cylinder. [Preview Abstract] |
Tuesday, November 22, 2011 9:05AM - 9:18AM |
M25.00006: Are falling plumes of particles unstable in Stokes flow? Andrew Crosby, John Lister A falling plume of heavy particles in viscous flow is observed experimentally to undergo a varicose instability (Pignatel et al., 2009), even though a continuum plume in Stokes flow is known to be stable. We explore this problem through numerical simulations of non-Brownian particle plumes in Stokes flow, varying the average particle number density. An initially cylindrical, axially periodic, particle plume does develop varicose perturbations to its boundary. We demonstrate that this apparent instability is caused by fluctuations in mean particle density, which provide the mechanism for radial perturbations to grow, and determine the growth rate. A competing non-linear wave-breaking mechanism and hydrodynamic diffusion cause the perturbations to saturate and destroy individual bulges. Density fluctuations, wave-breaking and diffusion combine to give a statistically quasi-steady state of bulges on the plume, with much slower diffusive growth of the mean radius. This provides an example of how a background shear flow can greatly reduce hydrodynamic diffusion due to the reversibility of the Stokes equations. [Preview Abstract] |
Tuesday, November 22, 2011 9:18AM - 9:31AM |
M25.00007: Non-monotonic settling of a sphere in a cornstarch suspension Devaraj van der Meer, Stefan von Kann, Jacco Snoeijer, Detlef Lohse Cornstarch suspensions exhibit remarkable behavior. Here, we present two surprising observations for a sphere settling in such a suspension: In the bulk of the liquid the velocity of the sphere oscillates around a terminal value, without damping. Near the bottom the sphere comes to a full stop, but then accelerates again towards a second stop. This stop-go cycle is repeated several times before the object reaches the bottom. We show that common shear thickening or linear viscoelastic models cannot account for the observed phenomena, and propose a minimal jamming model to describe the behavior at the bottom. [Preview Abstract] |
Tuesday, November 22, 2011 9:31AM - 9:44AM |
M25.00008: The instability of sedimenting spheres in a second-order fluid David Saintillan, Ramanathan Vishnampet The slow sedimentation of a dilute suspension of spherical particles in a second-order fluid is investigated using theory and numerical simulations. A linear stability analysis is performed in the limit of small Deborah number, and shows that such a suspension is unstable to density fluctuations as a result of the nonlinear coupling of the settling motion of the particles under gravity with the local flow field driven by a perturbation in density. Based on this linear theory, an initially homogeneous suspension is expected to develop inhomogeneities, a prediction supported by recent experiments on sedimentation in polymeric liquids. We further confirm this prediction using weakly nonlinear large-scale numerical simulations, which indeed demonstrate the formation of large clusters in the suspensions, resulting in a strong enhancement of the mean settling speed and velocity fluctuations. [Preview Abstract] |
Tuesday, November 22, 2011 9:44AM - 9:57AM |
M25.00009: Accelerated drop detachment in granular suspensions Thibault Bertrand, Claire Bonnoit, Eric Clement, Anke Lindner We experimentally study the detachment of drops of granular suspensions using a density matched model suspension with varying volume fraction ($\phi = 15\%$ to $55\%$) and grain diameter ($d = 20 \mu m$ to $140 \mu m$). We show that at the beginning of the detachment process, the suspensions behave as an effective fluid. The detachment dynamics in this regime can be entirely described by the shear viscosity of the suspension. At later stages of the detachment the dynamics become independent of the volume fraction and are found to be identical to the dynamics of the interstitial fluid. Surprisingly, visual observation reveals that at this stage particles are still present in the neck. We suspect rearrangements of particles to locally free the neck of grains, causing the observed dynamics. Close to the final pinch off, the detachment of the suspensions is further accelerated, compared to the dynamics of pure interstitial fluid. This acceleration might be due to the fact that the neck diameter gets of the order of magnitude of the size of the grains and a continuous thinning of the liquid thread is not possible any more. The crossover between the different detachment regimes is function of the grain size and the initial volume fraction. We characterize the overall acceleration as a function of the grain size and volume fraction. [Preview Abstract] |
Tuesday, November 22, 2011 9:57AM - 10:10AM |
M25.00010: Correlation between particle diffusion and effective heat transfer in a sheared suspension Xiaolong Yin, Bloen Metzger We show that enhanced solute or heat transport observed in sheared suspensions of neutrally buoyant particles with low Reynolds numbers are well correlated to the particle diffusion. Experiments were conducted in a Couette cell where the effective heat transfer coefficients were extracted from the decay of a heat pulse applied to a bounding wall. The particle diffusion coefficients were measured by particle tracking enabled by laser-induced fluorescence and refractive index matching. A numerical method based on a combination of the lattice Boltzmann method and Brownian tracers was developed to simulate the convective transport process. Effective heat transfer coefficients were obtained from matching the tracer profile with a transient analytical solution of the heat transfer equation. Heat transfer barriers were identified near the walls and the particle surfaces due to lack of convective motion in the direction normal to the surfaces. Both experimental and numerical data show that the effective heat transfer coefficient increases substantially with increasing shear rate, and the dependence on the volume fraction shows a peak at about 35\% volume fraction that coincides with the peak in the particle diffusion coefficients. [Preview Abstract] |
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