Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session GS: Jamming |
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Chair: Lou Kondic, New Jersey Institute of Technology Room: 203B |
Monday, November 24, 2008 8:00AM - 8:13AM |
GS.00001: Forces and displacements near the granular jamming threshold Mahesh Bandi, Andras Libal, Michael Rivera, Robert Ecke We experimentally study the dynamics of jamming by dragging a probe disk in a two-dimensional bi-dispersed system of randomly packed photo-elastic disks. All measurements are made at packing fractions relative to the critical fraction at which jamming occurs. We measure the local force felt by the probe disk and compare it with the system's global response with sensors placed along the system boundaries. We also visually monitor the disk displacements in the system, which are expected to become increasingly constrained as a function of increasing packing fraction. [Preview Abstract] |
Monday, November 24, 2008 8:13AM - 8:26AM |
GS.00002: Thermostatistics of a Single Sphere in a Non-Gaussian Granular Bath Kristin Combs, Jeffrey Olafsen, Alexis Burdeau, Pascal Viot Velocity statistics of a single Delrin monomer on a shaken dimer lattice are examined to understand energy injection in a larger bilayer system of several thousand Delrin spheres driven atop a dimer lattice. In the larger cell, robust Gaussian statistics are found for the Delrin spheres, while in the smaller cell the upper single particle exhibits slightly non-Gaussian velocity statistics due to the presence of defects in the strongly non-Gaussian lower dimer layer. The smaller cell geometry makes these defects more prevalent, but it is their presence and not the dissipative effects of the sidewalls that leads to the non-Gaussian statistics. The defects allow the upper tracer particle to periodically become trapped in the valleys between the particles of the dimer layer. In addition, a simulation of a sphere receiving impulse kicks from a non-Gaussian bath demonstrates that the presence of a viscous drag, potentially associated with the defects, acts to increase the dissipation in the system and the associated kurtosis of the tracer particle. [Preview Abstract] |
Monday, November 24, 2008 8:26AM - 8:39AM |
GS.00003: A dynamic jamming point for shear thickening suspensions Eric Brown, Heinrich Jaeger Densely packed suspensions can shear thicken, in which the viscosity increases with shear rate. We performed rheometry measurements on two model systems: corn starch in water and glass spheres in oils. In both systems we observed shear thickening up to a critical packing fraction $\phi_c$ ($=0.55$ for spherical grains) above which the flow abruptly transitions to shear thinning. The viscosity and yield stress diverge as power laws at $\phi_c$. Extrapolating the dynamic ranges of shear rate and stress in the shear thickening regime up to $\phi_c$ suggests a finite change in shear stress with zero change in shear rate. This is a dynamic analog to the jamming point with a yield stress at zero shear rate. [Preview Abstract] |
Monday, November 24, 2008 8:39AM - 8:52AM |
GS.00004: The onset of yielding in a granular system Trey Suntrup, Matthias Schr\"oter, Charles Radin, Harry L. Swinney We investigate the mechanical stability of a granular system subjected to shear stress. We use flow pulses of water in a fluidized bed to prepare a granular sample with a specific volume fraction (0.57 $<\phi<$ 0.63) and then shear the grains using a small, embedded plate, observing the force required to unjam the system. An understanding of how the system responds to shear is crucial to constructing a phase diagram for granular materials. [Preview Abstract] |
Monday, November 24, 2008 8:52AM - 9:05AM |
GS.00005: Anatomy of a Jam Junyao Tang, Sepehr Sagdighpour, Robert Behringer Flow in a hopper is both a fertile testing ground for understanding models for granular flow and industrially highly relevant. However, the formation of arches in the hopper opening, which halts the hopper flow unpredictably, is still poorly understood. In this work, we conduct a two-dimension hopper experiments, using photoelastic particles, and characterize these experiments in terms of a statistical model that considers the probability of jamming. The distribution of the hopper flow times exhibits an exponential decay, which shows the existence of a characteristic ``mean flow time.'' We then conduct further experiments to examine the connection between the mean flow time, the hopper geometry, the local density, and geometric structures and forces at the particle scale. [Preview Abstract] |
Monday, November 24, 2008 9:05AM - 9:18AM |
GS.00006: The concept of temperature in driven granular suspensions Matthias Schr\"{o}ter, Sonia May, Alexander Buck, Harry Swinney Dense granular suspensions driven by a flow field are far-from equilibrium system. They exhibit some of the hallmarks of glasses like a strong increase of viscosity with density or dynamical heterogeneities. The possibility to describe such systems with a single effective temperature is still debated. Our measurements using a torsion pendulum and speckle spectroscopy test the applicability of this concept. [Preview Abstract] |
Monday, November 24, 2008 9:18AM - 9:31AM |
GS.00007: Experimental investigation of state variables in a dense granular layer Frederic Lechenault, Karen Daniels Stationary states in dense granular systems lack a predictive statistical description, as kinetic theory approaches fail when the interactions significantly deviate from binary collisions. In particular, because of the degeneracy of geometric states due to friction forces, it has been argued that a comprehensive theory of such dense granular systems must incorporate additional state variables associated with constraint fluctuations. We investigate the relevance of various ensembles in a dense mixture of disks laid on a horizontal air table and driven into steady states by random kicks at the boundaries. We study how microscopically defined intensive parameters affect the macroscopic response of the system, and clarify the equilibration properties of these parameters. [Preview Abstract] |
Monday, November 24, 2008 9:31AM - 9:44AM |
GS.00008: Discrete and Continuum Modeling of Energy Transport through Dense Granular Matter Lou Kondic, Robert Behringer The transport of stress and energy through dense granular matter (DGM) is of significant importance to a number of applications. Although a significant amount of work has been carried out in order to understand this process, there are still many unknowns, including lack of a generally accepted continuum model, or even lack of understanding of whether such a model should exist. With the goal of making a step towards answering these questions, we carry out large-scale discrete element simulations of DGM exposed to perturbations whose spatial and temporal characteristics are varied independently. The presentation will concentrate on the comparison between the results of the simulations, and of a relatively simple continuum model based on elastic response with damping. This comparison is expected to serve as a basis for further development of continuum models for energy transport through DGM. [Preview Abstract] |
Monday, November 24, 2008 9:44AM - 9:57AM |
GS.00009: Drag reduction mechanisms employed by burrowing razor clams (Ensis directus) Amos Winter, Anette Hosoi In this work we describe how razor clams use localized fluidization to reduce drag and efficiently burrow through a granular substrate. Razor clams require nearly two orders of magnitude less force to move through sand than a blunt body of the same size and shape. By visualizing substrate deformation during burrowing, we investigate the clamshell kinematics which fluidize a small pocket of substrate around the body of the organism. Through experimentation and scaling arguments, we show that moving through a fluidized substrate rather than a packed granular medium dramatically reduces the drag force on the clam's body to a point within the animal's strength capabilities. [Preview Abstract] |
Monday, November 24, 2008 9:57AM - 10:10AM |
GS.00010: Folding dynamics of a polymer-like granular chain in a thin layer of water Jeffrey Olafsen, Ben Bammes A stainless steel chain of loosely connected monomers shaken in a thin layer of water on a vertically oscillated, horizontal plate demonstrates dynamics that are visually similar to that of polymer collapse in a poor solvent. The Faraday waves on the surface of the shaken fluid layer act as a thermal bath of excitations, and the surface tension of the fluid plays the role of a long-ranged potential that is minimized in the folding process. While the system is only two-dimensional, there is a one-to-one correspondence in the dynamics of this system and that of polymer folding. The granular chain demonstrates both ``on-pathway'' and ``off-pathway'' intermediate states during the folding process, as well as crossovers to different characteristic folding times that correspond to the dominance of different terms in the folding potential. The ability to tune the excitations via the Faraday waves on the fluid surface may afford control of the thermal bath in a manner that is inaccessible in real polymer experiments. Altering the wetting properties of the monomers allows for the creation of both hydrophobic and hydrophilic portions of the same chain resulting in a model lipid. [Preview Abstract] |
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