Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X24: Granular Flow II |
Hide Abstracts |
Sponsoring Units: GSNP DCMP DFD Chair: Jeremay Lechman, Sandia National Laboratory Room: LACC 411 |
Friday, March 25, 2005 8:00AM - 8:12AM |
X24.00001: Instabilities of granular flows down an inclined plane - the role of air entrainment Tamas Borzsonyi, Robert Ecke The rheology of a granular flow down a rough inclined plane was investigated experimentally. It is known that at steeper inclinations the grain velocity is not negligible compared to the terminal velocity (the velocity by which a grain would free fall in air), giving rise to the question whether the role of air entrainment is an important factor in the development of experimentally observed pattern instabilities of the flowing granular system. In the present experiments the rheology of granular flows has been measured on a 2 m long rough plane in vacuum and in ambient air. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X24.00002: Temporally heterogeneous dynamics in granular flows Leonardo Silbert Granular simulations are used to probe the particle scale dynamics at short, intermediate, and long time scales for gravity driven, dense granular flows down an inclined plane. On approach to the angle of repose - the jamming point - the dynamics become intermittent over intermediate times, with strong temporal correlations between particle motions - a signature of {\it temporally heterogeneous} dynamics. This intermittency is characterised through large scale structural events whereby the contact network periodically spans the system. A characteristic time scale associated with these processes increases as the stopped state is approached. These features are discussed in the context of the dynamics of supercooled liquids near the glass transition. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X24.00003: Two mechanisms for avalanche dynamics in inclined granular layers Robert Ecke, Tamas Borzsonyi We report on our recent experimental results on avalanche dynamics on a rough inclined plane. Studies on a set of materials with different grain shapes (sand, salt, glass beads and copper particles with four different shapes ranging from spherical beads to very anisotropic dendritic forms) confirm that the properties of avalanches depend dramatically on the shape of the grains. At one end of the spectrum (for spherical beads) we find smaller avalanches with less kinetic energy and material failure ahead of the front, while on the other end (for highly anisotropic particles) bigger ``overturning'' avalanches with large kinetic energy and dynamic grain motion. Comparisons are made with avalanche flow in narrow channels where the subsurface flow can be measured. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X24.00004: Crossover from collisional to frictional regime in a 3-dimensional granular flow Effrosyni Seitaridou, Ellen Keene, Nalini Easwar, Narayanan Menon We have made measurements of the fluctuations in the pressure at several points on the boundary of a 3-dimensional, gravity- driven flow of slightly-polydisperse, smooth glass spheres. The flow is contained in a cylinder, with the flow rate being controlled over a factor of 30 by an aperture far downstream of the pressure measurement. As the flow velocity is decreased, we observe a crossover from a situation where momentum is transferred to the walls almost entirely by collisions to a situation where balls are almost always sliding against the walls. We parametrize this transition by measuring the fraction of time that a ball is in contact with the wall, a number that tends to unity in very slow flows prior to jamming. We present measures of the statistics and temporal fluctuations of the force delivered to the wall in both regimes. We find that in the collisional regime distributions of force are similar to those previously found in 2-dimensional flows(1), however, the distributions in the frictional regime are fundamentally new. Supported by NSF DMR 0305396 and NSF MRSEC DMR 0213695 (1). E. Longhi, N. Easwar and N. Menon, Phys.Rev.Lett, 89 (2002) [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X24.00005: Experimental Measurements on a Three-Dimensional Vertical-Channel Granular Flow Kevin Facto, Chao Huan, Donald Candela, Ronald Walsworth, Ross Mair Experimental measurements using NMR techniques are presented for a dense, three-dimensional granular flow through a cylindrical vertical channel. The flow is measured far from the channel inlet and outlet, where it assumes an asymptotic form in which gravitational stress is completely supported by the side walls. Theoretical descriptions of this system have ranged from transitory force chains transmitting long-range stresses on the one hand, to viscous-fluid or other local constitutive equations on the other hand. Using NMR, we are able to measure both the mean flow profile and the spectrum of fluctuating deviations from the mean flow on millisecond time scales. The mean flow profile can reveal deviations from the parabolic profile of an ordinary liquid, while the fluctuations probe the diffusivity of grains. More complex NMR experiments probe time correlations in the grain motion, which may give information on caging effects similar to those proposed for glasses. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X24.00006: Critical evaluation of continuum models for granular drainage Ken Kamrin, Jaehyuk Choi, Martin Z. Bazant, R. R. Rosales, Arshad Kudrolli Which dense flow model is the best for which problems? Currently, there exist several known models for predicting steady-state velocity distributions in hopper or silo flow. This work offers a detailed comparison of the common dense flow models. Hourglass Theory, the Kinematic Model, and Saint-Venant's Plastic Model are applied within a narrow wedge hopper geometry, and the predicted flow is closely compared with experimental velocity profiles for glass beads in quasi-two-dimensional flows. Those models which support alternate boundary configurations are tested against experiment in spout and silo geometries. Due consideration is also made for the ease-of-use and restrictions of each model. Our goal is to develop general criteria for selecting the most suitable model under different circumstances. We also seek to evaluate the microscopic physical justifications for each model. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X24.00007: Plug formation in the flow of cohesive granular media down an incline Robert Brewster, Alex Levine, Gary Grest, James Landry The study of cohesive granular media is fundamental to the exploration of sand in a geophysical context where small quantities of a wetting fluid generate cohesive stresses within the granular aggregate. We have performed large-scale, three-dimensional molecular dynamics simulations of the flow of cohesive and non-cohesive granular media down an incline. We find that cohesive granular media generically separates into a plug flow regime near the free surface of the pile and a flowing regime whose rheology does not fit the standard Bagnold scaling. We analyze both the structure of the coexisting plug and flowing states and determine the velocity, density, and stress profiles throughout the material. Based on this numerical data, we propose a generalization of the Bagnold constitutive relation to describe the relation between shear stress and rate of strain in the flowing cohesive state. We also discuss the relationship between the thickness of the steady-state plug and the magnitude of the internal cohesive forces. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X24.00008: Fluctuations and Response in a Dense Granular Flow Kevin Facto, Thomas Schicker, Narayanan Menon We have performed experiments to determine the spectrum of fluctuations of various dynamical degrees of freedom in a dense granular flow. The measurements are made at the boundary of a 3-dimensional flow of smooth, slightly-polydisperse glass beads (d approx 1 mm) contained in a vertical channel of rectangular cross section (approx 210d x 40d x 760d). The flow velocity is controlled by a sieve of continuously variable mesh at the bottom of the channel. In the steady-state of the flow, we measure temporal fluctuations of all three components of force and torque, averaged over an area of 5 d $^{2}$. We are thus able to measure all 6 components of the stress tensor over a frequency range of 10Hz to 10kHz. We also present measurements of the response to sinusoidal forcing normal to the flow direction in order to determine whether the spectrum of the fluctuations determine the frequency dependence of the response, via a fluctuation-dissipation theorem. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X24.00009: Dynamics of Random Packings in Granular Flow Chris H. Rycroft, Martin Z. Bazant, James W. Landry, Gary S. Grest How do random packings flow? Dilute ``packings'' (gases) flow by the accumulated effect of independent, random collisions. Dense, ordered packings (crystals) flow collectively via the motion of defects, such as vacancies, interstitials, and dislocations. Similarly, existing theories of the dense, disordered packings in granular drainage are based on either gas-like inelastic collisions or crystal-like void diffusion, but experiments show that a fundamentally different approach is needed. Here, we propose that dense random packings flow co-operatively in response to diffusing ``spots'' of free volume. The Spot Model is very simple to simulate and may be analyzed in the continuum limit (via a non-local stochastic differential equation). With only a few fitting parameters, it predicts the mean flow, spatial velocity correlations, cage breaking, diffusion, and packing structure, in good agreement with experiments and molecular dynamics simulations. The results suggest that flowing random packings have universal structural features. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X24.00010: Discrete Element Simulations of Granular Flow in a Pebble Bed Nuclear Reactor Gary S. Grest, Chris H. Rycroft, Martin Z. Bazant, James W. Landry Pebble-bed reactor technology, which is currently being revived around the world, raises fundamental questions about granular flow in silos. The reactor core is composed of spherical billiard-ball sized (6cm diameter) graphite fuel pebbles containing sand-sized uranium fuel particles. The fuel pebbles drain very slowly through the core as a continuous refueling process. In some designs, a dynamical central column is formed from graphite moderator pebbles, physically identical to the fuel pebbles without any fuel. The total number of pebbles is of order 440,000 in a cell approximately 3.5m in diameter and 8.5m tall. Using discrete element (molecular dynamics) simulations we have studied a full scale model of the system. We find that the interface between the fuel and moderator particles remains sharp, as there is very little horizontal motion of the pebbles as they flow through the reactor. We measure mean velocity profiles and compare to various continuum models. We also investigated the feasibility of a bi-disperse core, containing smaller moderator pebbles, with the same size fuel pebbles, which could improve performance by focusing helium gas flow on the hotter fuel region. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04- 94AL85000. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X24.00011: Slow flows of yield stress fluids: complex spatio-temporal behaviour within a simple elasto-plastic model Lyderic Bocquet, Guillemette Picard, Armand Ajdari, Francois Lequeux A minimal athermal model for the flow of dense disordered materials is proposed, based on two generic ingredients: local plastic events occuring above a microscopic yield stress, and the non-local elastic release of the stress these events induce in the material. A complex spatio-temporal rheological behaviour results, with features in line with recent experimental and numerical observations. At low shear rates, macroscopic flow actually originates from collective correlated bursts of plastic events, taking place in dynamically generated fragile zones. The related correlation length diverges algebraically at small shear rates. In confined geometries bursts occur preferentially close to the walls yielding an intermittent form of flow localization. [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X24.00012: Shear viscosity of suspensions and the glass: Turning power-law divergence into an essential singularity Narendra Kumar Extreme slow dynamics defines approach to the glassy state. At the macroscopic scale, it manifests as a rise of shear viscosity as that state is reached through supercooling. The Vogel-Fulcher (VF) law describes that growth of viscosity. This work derives the VF law. Starting with an expression, due originally to Einstein, for the shear viscosity $\eta(\delta\phi)$ of a liquid having a small fraction $\delta\phi$ by volume of solid particulate matter suspended in it at random, we derive an effective-medium viscosity $\eta(\phi)$ for arbitrary $\phi$ which is precisely of the Vogel-Fulcher form. An essential point of the derivation is the incorporation of the excluded-volume effect at each turn of the iteration $\phi_{n+1} = \phi_{n} +\delta\phi$. The model is frankly mechanical, but applicable directly to soft matter like a dense suspension of microspheres in a liquid as function of the number density. Extension to a glass forming supercooled liquid is plausible. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700