Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session BJ: Granular Media: Shear Flow |
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Chair: Stephan Koehler, Emory University Room: Hilton Chicago Williford C |
Sunday, November 20, 2005 10:56AM - 11:09AM |
BJ.00001: Granular flows and Reynolds Dilatancy Alexandre Kabla, Lakshminarayanan Mahadevan, Tomaso Aste, Tim Senden The quasi-static rheology of granular matter is a stubborn problem. In this regime, the structure of the pack controls the mechanical response but also depends on the whole plastic history, leading to singular behaviours such as flow localization or memory effects. The coupling between structure and flow is often associated with the Reynolds dilatancy principle, which states that the density of a random pack decreases with increasing strain. In order to obtain a quantitative description of grain packing dilatancy, we use real time X-Ray digital imaging, associated with a specific image processing algorithm. Our technique allows local and time resolved measures of grain displacement fields and density variations. We observe clear correlations between the shear history and the local density, leading to a phenomenological relationship between them. [Preview Abstract] |
Sunday, November 20, 2005 11:09AM - 11:22AM |
BJ.00002: Granular Flow Fields Around Plunging and Withdrawing Intruders Tristan Dennen, Stephan Koehler, Robert Matson The drag resistance of a vertically moving intruder in dense granular media follows a supra-linear power law with depth. We investigate this behavior by optically tracking the flow fields in a Hele-Shaw cell and correlating them with the drag resistance on the intruder. In particular, the vertical extraction of an intruder causes a wedge of material to be raised, and the volume of this wedge is closely correlated to the extraction force. [Preview Abstract] |
Sunday, November 20, 2005 11:22AM - 11:35AM |
BJ.00003: A Minimalistic Approach to Swimming Through Sand Matt Bzdega, Tristen Dennen, Stephan Koehler Inspired by microorganisms swimming at low Reynolds, we are interested in understanding how self-propelled robots can swim through sand. We find that a two-hinged swimmer can propel itself forwards and backwards through a simple sequence of cyclically repeated stroking motions. A range of parameters including paddle size, shape, and stroking angles, along with variations of the swimming strategies were investigated and the results show similarities to Purcell's two-hinged swimmer. [Preview Abstract] |
Sunday, November 20, 2005 11:35AM - 11:48AM |
BJ.00004: Evolution of the local packing density in a sheared granular material Paul Umbanhowar, Martin van Hecke, Ken Sakaie The local particle packing density, $\eta$, in a bed of poppy seeds sheared in a vertical, split-bottom cylindrical container is measured using magnetic resonance imaging. Wide shear zones away from the wall are generated when a thin concentric disk at the bottom of the cylinder, and with radius less than the cylinder radius, is rotated slowly. Experiments reveal that, initially, a shear band forms at the outer edge of the disk which decreases in radius with decreasing depth. Material near the shear zone dilates with a corresponding decrease in $\eta$ of about $15\%$. The radial extent of the dilated region increases approximately logarithmically with increasing rotation in shallow layers. In relatively deep layers, the shear zone is at first entirely below the free surface, but with further rotation grows vertically creating a low $\eta$ core. We discuss the relation between the previously reported universal velocity profiles across the shear zone and the spatial and temporal evolution of the packing density. [Preview Abstract] |
Sunday, November 20, 2005 11:48AM - 12:01PM |
BJ.00005: Wide granular shear zones: effect of vibrations Martin van Hecke, Alexey Zanin In recent years, we have explored wide shear zones in granular media that occur in `split-bottom' Couette cells. Here we report on novel experiments that probe the effect of weak vibrations on these shear zones. [Preview Abstract] |
Sunday, November 20, 2005 12:01PM - 12:14PM |
BJ.00006: Statistics of particle velocity in slowly sheared granular materials Prabhu R. Nott, Ananda K. S. Our understanding of the hydrodynamics of molecular fluids has benefited greatly from knowledge of their statistical properties at the microscopic level. While knowledge of the statistics of dry granular materials is desirable for the same reason, it is lacking because of the complex nature of grain interactions. The difference between the deformation of dense granular materials and conventional fluids prompts the question of whether there is a fundamental difference in their statistical nature. Recently, we had reported the statistical distribution of particle velocity in gravity-driven flow of a granular material through a vertical channel. Our study had raised some important questions, namely whether our velocity distribution is universal or is limited to gravity-driven flows, and what the role of grain rotation is in determining macroscopic deformation. In this paper, we present measurements, made by video imaging and particle tracking, of the mean velocity field and the statistical distribution of velocity fluctuations of glass beads sheared in a cylindical Couette cell. Our results show important similarities with the velocity distribution in vertical channels, and hence indications of universality, but also some differences. In addition, we present measurements of the mean angular velocity field and the distribution of angular velocity fluctuations for a two-dimensional layer of disks in a vertical channel. We compare these observations with the predictions of the frictional Cosserat model we have recently proposed. [Preview Abstract] |
Sunday, November 20, 2005 12:14PM - 12:27PM |
BJ.00007: Shear flow in a vertically vibrated granular layer David Egolf, Francisco Vega Reyes, J. Cameron Booth, Jeffrey Urbach Couette flow has been an important testing ground for hydrodynamic descriptions of granular fluids. Typically the shearing of the medium is the only fluidizing force, so that the energy of the grains can not be changed independently from the shear rate. We present a series of experiments and molecular dynamics simulations of a horizontally sheared granular monolayer which is also heated by vertical vibration. We find that the experimental velocity profile is approximately exponential over a wide range of conditions. This behavior is reproduced in the simulation if friction with the vibrating plate is included. With frictionless plates, the velocity profile is approximately linear. We also present measurements of the profiles of other fundamental hydrodynamic variables. [Preview Abstract] |
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