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 HU: Granular V: Particles in Fluids and Interparticle Forces |
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Chair: Martin Maxey, Brown University Room: 200I |
Monday, November 23, 2009 10:30AM - 10:43AM |
HU.00001: Controlling cohesive forces in granular media Christoph G\"ogelein, Matthias Schr\"oter, Martin Brinkmann, Stephan Herminghaus When adding a small amount of water to a pile of granular matter, e.g., sand heap, close-by grains can be connected by liquid bridges [1]. Thus, the material becomes plastically and can sustain a larger stress as compared to dry sand. Our general aim is to compare the mechanical properties of wet and dry granular media. For this purpose, we use a suspension of micrometer large glass or Latex spheres dispersed in a binary liquid mixture. The suspending water-lutidine(oil) mixture exhibits a lower critical solution temperature leading to a water-oil-like phase separation slightly above ambient temperature. Close to this demixing region, the oil-like phase undergoes a pre-wetting transition on the particle glass surface inducing liquid bridges [2]. Thus, by varying the temperature we can switch the liquid bridges on and off. We will report on our attempts to directly visualize the formation and control of liquid bridges using confocal and non- confocal microscopy. \\[4pt] [1] M. Scheel, {\it et al.}, Nature Materials 7, 174 (2008)\\[0pt] [2] D. Beysens, and D. Esteve, Phys. Rev. Lett. 54, 2123 (1985) [Preview Abstract] |
Monday, November 23, 2009 10:43AM - 10:56AM |
HU.00002: Energy dissipation and clustering in granular streams Scott Waitukaitis, John Royer, Helge Gruetjen, Heinrich Jaeger The presence of weak cohesive forces between macroscopic grains can lead to the break up of a free falling granular stream, similar to the surface-tension-driven break up of a liquid stream\footnote{ Royer, J. R. et al. Nature {\bf 459} 1110 - 1113 (2009).}. This sensitivity to minute forces suggests that these free falling streams could serve as a tool to probe the interactions between grains. In order to investigate the connection between the stream dynamics and the grain-grain interactions, we perform molecular dynamics simulations of a granular stream freely falling out of a hopper varying the cohesion and inelasticity of the grains. We find that in the absence of cohesive forces the stream breaks apart into isolated grains, in contrast to the clustering observed in simulations of inelastic granular gases. For sufficiently high cohesive forces we reproduce the break up of stream into droplets, while with lower cohesive forces the stream breaks up into smaller clusters consisting of only a few grains. Measuring the change in contact number and decay of velocity fluctuations with depth, we characterize the different regions of the force-inelasticity phase space. [Preview Abstract] |
Monday, November 23, 2009 10:56AM - 11:09AM |
HU.00003: A model for elastohydrodynamic collision of spheres in liquid John Wells, J.T. Jenkins We pursue a reduced-order model of normal ``elastohydrodynamic'' collision between two spheres in incompressible liquid (Wells 1993; \textit{Proc. Powders {\&} Grains '93}, ``W93''). W93 \textit{assumed }a ``quasi-Hertzian'' surface depression profile, yielding the first reduced-order model ODEs for this problem. Trajectories and coefficients of restitution agreed well with simulations ( Davis, Serayssol, {\&} Hinch , 1986; \textit{J. Fluid Mech }201; ``DSH86''). To avoid numerical integrations such as those in W93, Lian \textit{et al.} (1996, \textit{J. Fluid Mech} 311) assumed additionally that the gap inboard of a certain radius was constant, leaving only the centerline gap as an unknown. Then introducing an adjustable constant, they recovered coefficients of restitution close to DSH86. The present work extends W93 and Lian \textit{et al. }to achieve, without adjustable constants, the analytical character that until now was only afforded by perturbation approaches (DSH86) wherein centerline deformation is assumed to be much smaller than the gap. [Preview Abstract] |
Monday, November 23, 2009 11:09AM - 11:22AM |
HU.00004: The mechanism for shear thickening in suspensions Eric Brown, Heinrich Jaeger Densely packed suspensions can shear thicken, in which the viscosity increases with shear rate. Video microscopy along with rheology measurements show the shear thickening regime is a transition from negligible particle motion at low stresses to fully developed shear flow at higher stresses. The onset of shear thickening occurs when the shear stress is sufficient to pull particles apart; for example against gravity for large particles, and can be tuned by inducing electric or magnetic dipoles. Dilation can be observed as particles penetrate the fluid surface in the high stress regime. The maximum stress of the shear thickening regime is shown to match, for a wide range of suspensions, the ratio of surface tension divided by a radius of curvature comparable to the particle size. This suggests the large stress associated with shear thickening comes from capillary forces as a consequence of dilation. [Preview Abstract] |
Monday, November 23, 2009 11:22AM - 11:35AM |
HU.00005: Liquid effect on a granular avalanche: experiments and discrete element simulation using a liquid-modified contact model F.-L. Yang, W.T. Chang, C.S. Chen, S.H. Hsieh, Y.T. Huang This work extends a conventional Discrete-Element method to simulate the avalanche process and the subsequent bulk motion of a wet granular mixture, composed of identical solid spheres fully-immersed in a viscous liquid. A linear soft-sphere contact model is developed for interactions between dry surfaces to reproduce both the Hertzian contact time and the overall energy loss, characterized a measured coefficient of restitution. The contact model is modified for liquid dissipation effects according to previous experimental data. To assign parameters for tangential interactions, this works uses the measured bulk dynamic behavior, differing from most existing schemes. The simulated mixture motions are compared to experimental data, conducted on a lab-scale flume, at both flow and particle scales. General agreement is obtained on the whole granular avalanche process and the early stages of the subsequent downstream bulk motion. The validated simulation results are further employed to extrapolate bulk dynamics difficult to measure in the experiments, including 3D profiles of bulk velocity, strain, and granular temperature. The obtained results will be adopted to stress how liquid modifies the unsteady behavior of a granular mixture. [Preview Abstract] |
Monday, November 23, 2009 11:35AM - 11:48AM |
HU.00006: Liquid effects on the dynamics of granular avalanche Yung-Ta Huang, Fuling Yang This work examines experimentally how a viscous incompressible liquid modifies the behavior of a granular bulk in avalanche. A large and a small laboratory flume of identical length-to-width ratio have been constructed to initiate granular avalanche. Two types of monatomic spheres were used, preserving the ratio between the particle diameter and the flume width, with or without water to prepare the dry and the immersed mixtures. The bulk motion was monitored from the side by a high-speed digital camera. Quantitative measurements were conducted by integrating the techniques of particle-tracking velocimetry and coarse-grained spatial average. The examined bulk dynamics includes the two-dimensional profiles of bulk velocity and strain as well as the distribution of bulk solid volume fraction and granular temperature. The bulk slip velocity at the flume base and the volume discharge rate of a dry and an immersed mixture were also compared in space and in time throughout the avalanche. Further, a control volume analysis has been performed on the bulk mass, momentum, and energy (granular temperature) transport to extrapolate the stress fields. Finally, the scaling issue on granular flow dynamics will be addressed by comparing the bulk dynamics, on a non-dimensional reference frame, that was measured on the two flumes. [Preview Abstract] |
Monday, November 23, 2009 11:48AM - 12:01PM |
HU.00007: Flow of a dense granular model suspension Claire Bonnoit, Anke Lindner, Eric Clement We experimentally study the flow of dense granular suspensions on an inclined plane. The model suspensions are made of mono-disperse, spherical, non-Brownian polystyrene beads immersed in a density-matched silicon oil. The volume fraction $\phi$ varies from $30\%$ to $61\%$. During the flow on the inclined plane we measure simultaneously the surface velocity and the layer thickness. We identify two different scalings: one identical to flow of a viscous liquid at high thickness and one identical to flow of a dry granular media at low thickness. We show that particle migration can be neglected. In the viscous regime, this set-up thus allows for directly measuring the shear viscosity up to volume fractions of $61\%$, which is a challenge in a classical rheometer. We find that the inclined plane is a useful apparatus to explore the continuous transition from an effective viscous flow to a dense granular flow. As this geometry fixes the ratio of normal to tangential forces in the layer with respect to the angle, we are able to propose a rheological model based on a friction law. [Preview Abstract] |
Monday, November 23, 2009 12:01PM - 12:14PM |
HU.00008: Ballistic to centrifuging flow of granular materials in rotating tumblers Richard M. Lueptow, Gabriel Juarez, Pengfei Chen, Julio M. Ottino The critical rotation speed for centrifuging of granular materials in horizontal rotating tumblers, determined by the Froude number, was studied experimentally and computationally as a function of the particle diameter, tumbler fill fraction, interstitial fluid, and tumbler length. Particle size does not affect the critical speed for centrifugation provided that the fill fraction is below 50{\%}. The actual critical speed is typically 1.4 times the theoretical speed for centrifugation. Above 50{\%} fill, the critical speed increases with particle size. Simulations indicate that even for the centrifuging condition there is relative motion of particles due to gravity, especially near the free surface. The presence of an interstitial fluid alters the critical speed due to buoyancy. [Preview Abstract] |
Monday, November 23, 2009 12:14PM - 12:27PM |
HU.00009: Measurements of the Motion and Orientation of Rods in 2D Chaotic flow Shima Parsa Moghaddam, Greg A. Voth We study the dynamics of rod particles in a two dimensional time-periodic flow driven by Lorentz force. Video particle tracking is used to make accurate measurements of the motion and orientation of fluorescent rods along with the fluid velocity field. When the rods are very short, their rotation rate is dictated by the local velocity gradients. We study the deviations from this simple model as the rod length increases. The rods partially align with the stretching direction defined by the eigenvectors of the Cauchy-Green strain tensor. [Preview Abstract] |
Monday, November 23, 2009 12:27PM - 12:40PM |
HU.00010: Sedimenting spheres in bubbly fluid: a fluid Galton model Michael Higley, Andrew Belmonte A solid sphere sinking in a bubbly fluid and a solid sphere falling through a crowded bed of rigid obstacles (in air) share two common traits: the settling speed is slowed by the obstructions, and the sphere exhibits random lateral motion. In a previous study of sedimenting spheres in bubbly fluid we quantified both of these effects. Here we present a mathematical model which begins as an adaptation of Galton's board to the sedimenting sphere, which allows us to introduce various physical effects of the bubbly fluid, and test their importance, particularly bubble collisions. Comparison is made with experimental results. [Preview Abstract] |
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