Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N8: Granular Flows |
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Sponsoring Units: DFD Chair: Erin Rericha, University of Maryland Room: Baltimore Convention Center 314 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N8.00001: Assessing a Continuum Description of Wide Shear Zones in Slow Granular Flow by Discrete Element Simulations Jeremy B. Lechman, Gary S. Grest, Martin Depken, Martin van Hecke While the rheology of rapid granular flows is becoming well established, slow, dense flows are not well characterized in part because the strain localization (i.e., shear bands) they often exhibit is not easily amenable to continuum descriptions. Recently, a novel experimental system (split-bottom Couette Cell) was developed with promising potential to give new insight into these flows due to its wide, smooth shear zones (Fenistein et al. PRL \textbf{92}, 94301). Subsequent experimental and numerical studies have lead to a good understanding of the nature of the flow in this device, which has lead Depken et al. (cond-mat/0510524) to propose a set of testable constitutive relations between the internal stresses and flow field. In particular, they suggest that the bulk, effective friction coefficient between sliding layers of particles is not constant, but has a subtle dependence on the orientation of the layers with respect to the bulk force. Here we present large-scale Discrete Element Simulations to analyze the bulk flow in both circular, above and below the critical height, and linear, where no critical height for slip at the base is found, split-bottom geometries. We check the proposed form of the stress tensor and assess the validity of the claim that the effective friction coefficient depends on the shape of the shear zone with respect to gravity. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N8.00002: Dynamical heterogeneities in dense granular flow: timescales and large-scale particle rearrangements Allison Ferguson, Bulbul Chakraborty Recent interest in understanding the dynamical arrest leading to a fluid $\rightarrow$ solid transition in both thermal and athermal systems has led to questions about the nature of these jamming transitions (PRL {\bf 86}, 111 (2001), Nature {\bf 411}, 772 (2001)). It is believed that these jamming transitions are dependent on the influence of extended structures on the dynamics of the system (Science {\bf 287}, 627 (2000)). Simulations of steady-state gravity-driven flows of inelastically colliding hard disks show the formation of large-scale linear chains of particles with a high collision frequency even at flow velocities well above the jamming transition (EPL {\bf 66}, 277 (2004)). These chains can be shown to carry much of the collisional stress in the system due to a dynamical correlation that develops between the momentum transfer and time between collisions in these ``frequently-colliding'' particles. While measurements of slowly decaying stress correlations yield an average lifetime for these structures which scales inversely with the flow velocity (cond-mat/0505496), distributions of time scales associated with the stress chains may provide more information about their effect on the dynamics of the flowing granular medium. These distributions may be obtained by considering time scales related to large-scale rearrangements of neighbouring particles in analogy with measurements done on supercooled fluids. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N8.00003: Dense granular flows down an inclined plane Robert Ecke, Tamas Borzsonyi Granular flow on a rough inclined plane is an important model system in which to study the basic rules of the dynamics of granular materials. Despite intensive study, many features of such flows are still incompletely understood. For uniformly flowing layers at relatively shallow inclination, we consider experimentally the the basic flow rheology of the granular media and propose new scalings to collapse our data for glass beads and rough sand as a function of inclination angle and particle diameter. At steep inclinations above some angle $\theta_s$ ($\tan\theta_s/\tan\theta_r \approx 1.3-1.5$, where $\theta_r$ stands for the angle of repose) for flowing grains, numerics and theory predict that the surface roughness is inadequate to dissipate energy gained in the gravitational field, and the flow should continue to accelerate. We report on our experimental results on the properties of granular flows on a steeply inclined plane and define the domains of steady flows. We also discuss the instabilities of such flows leading to spatial patterns. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N8.00004: Transverse Instability of Avalanches in Granular Flows down Incline Igor Aranson, Florent Malloggi, Eric Clement Avalanche experiments on an erodible substrate are treated in the framework of ``partial fluidization'' model of dense granular flows. The model identifies a family of propagating soliton-like avalanches with shape and velocity controlled by the inclination angle and the depth of substrate. At high inclination angles the solitons display a transverse instability, followed by coarsening and fingering similar to recent experimental observation. A primary cause for the transverse instability is directly related to the dependence of soliton velocity on the granular mass trapped in the avalanche. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N8.00005: Swirling in a Vibrated Monolayer of Rods Vijay Narayan, Sriram Ramaswamy, Narayanan Menon We report observations of the spatiotemporal behaviour of a vertically vibrated horizontal monolayer of copper rods (aspect ratio $\approx $ 5) etched to a rolling-pin-like shape. The spatial organization of the rods resembles a highly-defected nematic state with large, coherently moving swirls. We measure spatiotemporal correlations of the single-particle and collective velocities, and study the structure and dynamics of the system as a function of density and vibration amplitude. We analyze the observed patterns in the light of theories\footnote{ J. Toner, Y. Tu and S. Ramaswamy, Ann. Phys. 318 (2005) 170.} of orientational ordering, dynamics, and topological defects in systems of driven particles. We make comparisons to related but different experiments\footnote{ D.L. Blair, T. Neicu, and A. Kudrolli, Phys. Rev. E 67, 031303 (2003).}, as well as to our earlier measurements\footnote{ V. Narayan, N. Menon and S. Ramaswamy, J. Stat. Mech. (2005) in press; cond-mat/0510082.} on similar particles with higher aspect ratio. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N8.00006: Simple Power Law for Transport Ratio with Bimodal Distribution of Coarse Sediment Christopher Thaxton, Joseph Calantoni Using a discrete particle model, we have simulated sheet flow transport of coarse bimodal sediment distributions in the bottom boundary layer over a range of oscillatory waves and steady currents. The ratio of large grain to small grain diameter was varied as 5:4, 3:2, and 2:1. For each bimodal distribution, the mass ratio $M_{L}/M_{S}$ ($M_{L}$ and $M_{S}$ are the masses of large and small grains respectively -- the total mass was fixed for all runs) was varied from 1/9 up to 9/1. We find that, independent of wave and current forcing for the range of conditions considered, the ratio of large to small grain time-average transport rate obeys the power law $Q_{L}/Q_{S}=C(M_{L}/M_{S})^{k}$, where $Q_{L}$ and $Q_{S}$ are the time-average transport rates of the large grains and small grains respectively and $C$ and $k$ are regression constants. A linear regression in log space (including 81 different simulations per diameter ratio) suggests that \textit{k$\approx $D}$_{L}/D_{S}$ with R$^{2}>$0.9. The robust nature of the results suggests that the new power law may have a broad range of applications for shear flows of bimodal granular mixtures. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N8.00007: Large scale surface flow generation in driven suspensions of magnetic microparticles. Maxim Belkin, Alexey Snezhko, Igor Aranson Nontrivially ordered dynamic self-assembled snake-like structures are formed in an ensemble of magnetic microparticles suspended over a fluid surface and energized by an external alternating magnetic field. These self-assembled multi-segment structures emerge as a result of the collective interaction between the particles oscillations induced by an external magnetic field and the standing waves on the surface of fluid. Surprising large-scale vortex flows are generated by these snake-like structures. The flows can be as fast as 2 cm/sec and strongly depend on the driving magnetic field parameters. We report on systematical experimental study of the vortex flow properties and generation mechanisms. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N8.00008: Green-Kubo expressions for transport coefficients of a granular fluid. Aparna Baskaran, James Dufty A formal derivation of linear hydrodynamics for a granular fluid is given. The linear response to small spatial perturbations of the homogeneous state is studied in detail using methods of nonequilibrium statistical mechanics. A transport matrix for macroscopic excitations in the fluid is defined in terms of the response functions. An expansion in the wavevector to second order allows identification of all phenomenological susceptibilities and transport coefficients through Navier - Stokes order [1]. The transport coefficients in this representation are the generalization of Helfand and Green-Kubo relations to granular fluids. The analysis applies to a wide range of collision rules. Several differences from the corresponding expressions in the elastic limit are noted. Then, the particular case of inelastic hard spheres is considered and some approximate analytical evaluations illustrated. [1] A preliminary report of these results can be found in J. W. Dufty, A. Baskaran and J. J. Brey cond-matt/0507609 [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N8.00009: Dynamics angle and surface flow properties of wet and cohesive granular matter Qing Xu, Arshad Kudrolli We will discuss an experimental study of the flow of grains mixed with a small amount of liquid using a horizontally rotated drum apparatus, extending on our previous work on the maximum angle of stability of wet granular materials [1]. We focus on the continuous avalanching regime observed at high rotation rates, and examine the shape of the granular surface and depth of flow with imaging techniques as a function of amount, viscosity and surface tension of the liquid. Glass beads with 1mm diameter, and a drum with a diameter 295 mm and a width of 145mm is used to minimize the effect of the boundary. We find that the shape of the surface may be approximated by two linear segments in the upper and lower halves. The slope of the upper segment corresponding to the dynamical angle of repose $\theta_d$ is observed to initially increase with rotation rate and volume fraction of liquid as expected, while the lower segment has an approximately constant slope. Interestingly, $\theta_d$ is observed to peak before decreasing to an approximately constant value as the volume fraction is increased. The rate of increase of $\theta_d$ is observed to decrease with rotation rate and viscosity. The role of the time scale over which wet grains shear past each other and the time over which grains actually come into contact due to lubrication forces on the observed change in scaling will be discussed.\newline [1]: S. Nowak, A. Samadani, and A. Kudrolli, Nature Physics {\bf 1}, 50 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N8.00010: Characterizing the banded state of granular material in a rotating drum. Michael Newey, Kenneth Desmond, Wolfgang Losert Why do particles of different size segregate axially in a horizontal rotating tumbler? We aim to understand the microscopic mechanisms for axial segregation through direct measurements of the motion of individual particles. Imaging the surface of the flowing layer, we extract flow angles, velocities, drift and diffusion for different particle types and mixtures of particles. Surprisingly, the direction of surface drift and steepest flow angle do not coincide and that surface drift cannot explain the axial segregation in our mixtures. On the other hand, particles in small particle bands flow significantly faster then particles in large particle bands, and this can be observed before visible band formation. We discuss the possible role of velocity differences in the axial segregation process. We characterize the fluidity of the flowing layer from its response to gentle sideways forcing. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N8.00011: Mechanisms in size segregation of binary granular mixtures Stephan Ulrich, Jennifer Kreft, Matthias Schr\"oter, Jack Swift, Harry Swinney Shaking of a mixture of large and small particles can lead to segregation. One distinguishes between the Brazil-nut effect (large particles go to the top) and its opposite, the reverse Brazil-nut effect. In this talk, experiments of vertically shaken binary mixtures are presented. Using image analysis, the number of large particles visible at the top and bottom of the granulate are counted to determine the state of segregation. By complementing these results with molecular dynamics simulations, we are able to identify different segregation mechanisms discussed in recent theoretical approaches: a geometrical mechanism called void filling, transport of particles in sidewall-driven convection rolls, and thermal diffusion, a mechanism predicted by kinetic theory. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N8.00012: Upward penetration through a granular medium D. Costantino, T.J. Sheidemantel, M.B. Stone, J. Cole, C. Conger, K. Klein, M. Lohr, W. McConville, Z. Modig, P. Schiffer We measure the force needed to push a flat plunger upwards through a granular medium. The plunger begins flush with the base of the grains' container, and we focus upon the force necessary to initiate motion. The data show that this break-out force increases monotonically with plunger diameter and pile height as expected. In contrast to previous measurements of the force needed for vertical penetration from above and of the horizontal drag force, this break-out force has a strong dependence on bead diameter. Research supported by NASA grant NAG3-2384 and the NSF REU program. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N8.00013: Correlation in granular shear flows Gregg Lois, Jean Carlson We investigate the effects of long-range correlation in simulations of sheared granular materials and develop theories to model force propagation in the dense regime. Measurements of spatial force correlations determine the size of force networks that emerge as the density is increased. The magnitude of the correlation length separates the dilute regime, where kinetic theory holds, from a dense regime where its assumptions break down. In the dense regime we introduce theories that successfully predict constitutive relations for the stress tensor, using geometrical properties of the force networks. Additionally, we observe that the behavior of the contact force distribution at small forces is highly dependent on the size of the force networks. [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N8.00014: Shear reversal in granular flows Masahiro Toiya, Wolfgang Losert The reversal of the shear direction in flow of monodisperse and bidisperse granular matter in a shear cell of Taylor-Couette type is characterized experimentally. By changing the boundary conditions we tune the location and width of the shear band in steady state flow. When the shear direction is reversed, the system compacts over a characteristic length of half a particle diameter, and shear forces reach a steady state over a chacteristic length of 1-3 particle diameter. A linear strain is found at the onset of shear reversal before a steady state shear band develops. We associate this extra strain during shear reversal with the displacement needed to jam particles in regions away from the shearband. We find that the strain decreases with increasing particle size for a fixed system size. We also find radial components in average particle velocities at the top surface, suggesting a convection current in the bulk. [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N8.00015: Flux from a vibrated granular medium Ke Chen, Matthew Stone, Rachel Barry, Matthew Lohr, William McConville, Kit Klein, Ben-li Sheu, Andrew Morss, Thomas Scheidemantel, Peter Schiffer We have studied vertically vibrated granular media by measuring the flux through a hole in the container's bottom surface. We find that when fully fluidized, the flux is controlled by the peak velocity of the vibration, $v_{max}$, i.e., the flux is nearly independent of the frequency and acceleration amplitude for a given value of $v_{max}$. The flux decreases with increasing peak velocity and then becomes constant for the largest values of $v_{max}$. We demonstrate that the data at low peak velocity can be quantitatively described by a hydrodynamic model. By contrast, the nearly constant flux at larger peak velocity signals a crossover to a state in which the granular density near the bottom is insensitive to the energy input to the system. This research was supported by the NASA through grant NAG3-2384 and the NSF REU program through grant DMR 0305238. [Preview Abstract] |
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