Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J8: Granular Flows |
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Sponsoring Units: DFD GSNP Chair: Wolfgang Losert, University of Maryland Room: Morial Convention Center RO6 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J8.00001: Washboard Road: The dynamics of granular ripples formed by rolling wheels Nicolas Taberlet, Anne-Florence Bitbol, Stephen Morris, Jim McElwaine We report laboratory experiments on rippled granular surfaces formed under rolling wheels. Ripples appear above a critical speed and drift slowly in the driving direction. Ripples coarsen as they saturate, and exhibit ripple creation and destruction events. All of these effects are captured qualitatively by 2D soft particle simulations in which a disk rolls over smaller disks in a periodic box. The simulations show that compaction and segregation are inessential to the ripple phenomenon. We describe a simplified scaling model which gives some insight into the mechanism of the instability. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J8.00002: Evolution of sand ripples in pulsed flow Jos\'e Eduardo Wesfreid, Joachim Kruithof We present high-resolution experiments showing the temporal evolution of sand ripples formed by oscillatory flow. We discuss the decompaction process observed during the formation of the ripples pattern. We have also studied the evolution of different parameters during the transition of rolling grain ripples to vortex ripples, as the slope of these ripples and we tested the validity of the Sleath criterion to discriminate the transition. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J8.00003: Granular Erosion of Pebbles Adam Roth, Douglas Durian Flowing grains are strongly abrasive, and cause erosion both of themselves and their surroundings. ~In a geophysical setting, the erosion of pebbles has traditionally been quantified by global measures such as aspect ratio. ~Recently we have focused on curvature, and its distribution around the contour, as a local measure more directly related to the microscopic action of erosion. ~Here we apply this method to linoleum shapes, eroded by rotation in an abrasive grit. Several shape parameters are measured at different stages in the erosion process, including the curvature distribution. ~A simple model of erosion is developed, and its predictions are compared to the data. ~The results are in reasonable agreement, and could be useful for understanding natural erosion processes. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J8.00004: Impact and Penetration of Granular Materials by Discrete Element Simulations Justin W. Garvin, Jeremy B. Lechman, J. Matthew D. Lane Granular material response to impact is important in a range of fields, from munitions delivery, to meteorite collision and crater formation. Recently a model for the force experienced on a penetrator has been proposed [L.S. Tsimring and D. Volfson, Powders and Grains 2005, 1215-1223] and shown to fit experimental data well [H. Katsuragi and D.J. Durian, Nature Physics, Vol. 3, June 2007]. This model describes two components of the force: i) a velocity dependent, depth independent term related to the inertial force required to mobilize a volume of grains in front of the penetrator; and ii) a velocity independent, depth dependent, Coulomb friction-like term. In the current study, massively parallel, discrete element simulations have been performed to study the penetration of a large spherical impactor into a multi-million particle bed of granular material. Results agree with previous work for slow impact speeds ($<$ 400cm/s). In addition, the current work extends the comparison with the proposed model to higher speeds ($\sim $1000cm/s). The physics of the phenomenon is discussed along with the challenges for modeling and simulation in the even higher velocity regime. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J8.00005: Gas-Mediated Impact Dynamics in Fine-Grained Granular Materials John Royer, Eric I. Corwin, Bryan Conyers, Mark L. Rivers, Peter J. Eng, Heinrich M. Jaeger Non-cohesive granular media exhibit complex responses to sudden impact that often differ from those of ordinary solids and liquids. We investigate how this response is mediated by the presence of interstitial gas between the grains. Using high-speed x-ray radiography we simultaneously track the motion of a steel sphere through the interior of a bed of fine-grained granular material and measure local changes in the bed packing density below the sphere. In an initially loosely packed bed, interstitial gas allows for nearly incompressible, fluid-like flow of the bed and aids the penetration of the sphere. In an initially densely packed bed the interstitial gas plays the opposite role, strengthening the bed and inhibiting the penetration of the sphere. These two seemingly incongruous effects are both due to the low permeability of the fine grained-bed, which traps the interstitial gas in the bed. This trapped gas resists changes in the bed packing density, inhibiting compaction in the loose bed and inhibiting dilation in the dense bed. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J8.00006: Impact cratering in fluidized granular matter Patrick Mayor, Hiroaki Katsuragi, Douglas J. Durian Impacts by projectiles dropped into granular media are an important example of how particulate materials respond to externally applied forces. Beyond the obvious geophysical case of planetary craters, understanding the details of impact mechanisms can provide valuable information on these systems, and the phenomenon has been actively investigated. In particular, recent experiments have studied the penetration depth of projectiles impacting granular materials at relatively low speeds, and measured the dynamics of the impact process, yielding force laws accounting for the observations. We have studied how the impact phenomenon is affected when the granular medium is submitted to a vertical upward (or downward) gas flow, in a range of flow rates below the bubbling regime. These fluidized granular systems yield, logically, deeper impacts, and dynamics measurements reveal that the stopping time is also longer, contrary to what is observed when deeper craters are obtained by increasing the impact velocity. We observe that the parameters involved in previously obtained force laws are modified in a simple way as a function of the flow rate and find a velocity-dependent inertial term and a depth-dependent friction force that vanishes as the flow rate approaches the fluidization threshold. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J8.00007: Drag Force in a Gas Fluidized Granular Bed T.A. Brzinski, D.J. Durian We use a rheometer to measure the torque acting on a rotating bar in a bed of gas-fluidized glass beads. We vary rotation rate from .001-10rps, vary depth from 1-10 cm, and increase the fluidizing gas flow from no flow well into the fluidized regime. We observe that at high rotation rates the drag is roughly proportional to velocity squared. At low rates we can rescale the measured torque by depth, and observe a collapse of the data. These results agree with the predictions of a granular drag force model which has proven effective in predicting granular impact dynamics. The model consists of an inertial drag term, which is depth-independent and scales as velocity squared, and a frictional drag term, which is independent of rate and varies linearly with depth. We find, as expected, that while the frictional term is airflow-dependent the inertial term is uncoupled from the fluidization. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J8.00008: Exploring penetration through granular media Daniel J. Costantino, Thomas J. Scheidemantel, Matthew B. Stone, Julia Cole, Casey Conger, Kit Klein, Matthew Lohr, William McConville, Zachary Modig, Krysten Scheidler, Peter Schiffer The motion of objects through granular media is an important physical problem involving local jamming of the grains. We report on an experiment dealing with the force needed to initiate upward motion through a granular pile, $F_{ini}$. As expected, this force scales monotonically with the depth of the intruder as well as its size, $D_{plate}$. However, unlike previous experiments this force also depends on the size of the particles making up the pile, $d_{grain}$. The force can be represented by the function $F_{ini}=A D_{plate} \quad d_{grain}+B D_{plate}^{2}$; which can be qualitatively explained within a simple model. Finally, preliminary results from a new experiment dealing with horizontal motion through a granular pile will be discussed. In this study, the effect of interstitial fluids on a granular material's resistance to an intruder will be investigated. Research supported by NASA grant NAG3-2384 and the NSF REU program. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J8.00009: Fluctuations in an agitated granular fluid Kiri Nichol, Martin van Hecke Granular media can be fluidized by a flow that occurs far away. Intruders placed in such a 'stationary granular fluid' sink until they reach a depth given by a granular analogue of Archimedes law. Once they float at this depth, these intruders effectively probe the microscopic agitations in the material that cause the fluidization. The spectrum of these fluctuations is anomalous. We present its dependence on experimental parameters such as driving rate, floating depth and probe size, and discuss the possibility of applying a non-equilibrium Fluctuation Dissipation relation to this system. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J8.00010: Jamming in Hopper Flow of Large Aspect Ratio Granular Materials Scott Franklin The clogging of granular materials at the exit of a silo or hopper is a matter of tremendous practical importance, as well as a canonical example of jamming. We investigate the effect of particle aspect ratio (length:width) on the jamming probability through experiments and discrete element simulations. Preliminary experimental results on particles with aspect ratios of 16 and 32 show that the probability $P(m)$ for $m$ grains to exit the hopper has an exponentially decaying tail that, when scaled by the mean number that exit, is independent of exit aperture size. This scaling of $P(m/\langle m \rangle)$ is also observed in hopper flow of ordinary round materials, but the proposed phenomenological explanation of uncorrelated behaviors seems unlikely in long, thin rods. Furthermore, while the mean exit number obviously increases with aperture size, it is not clear which length scale is most relevant: particle length, width, or some combination of the two. We are also writing new discrete element simulations that can be compared with the experiments, and I will discuss some of the computational nuances introduced by particle asymmetry and present initial results. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J8.00011: Granular Flow of Fluid-Submerged Particles: Effects of Fluid Viscosity H. King, D. Ertas, A. Kushnick, F. Zhou, P. Chaikin Gravity-driven flows of granular materials are often influenced by interstitial fluids. Using the rotating half-filled drum geometry, we investigated particle and fluid velocities for granular flows of nearly monodisperse spherical glass particles with interstitial fluids of varying dynamic viscosity (air to 4 cP). We utilize direct particle imaging and PIV methods. For dry flows the fundamental time scale is set by the gravitational constant and particle size. We observe two primary influences of the interstitial fluid on the granular rheology. First, density of the fluid changes both the driving force (due to buoyancy) and the inertial response (due to added mass), increasing the characteristic time scale. Second, the intrinsic time scale is influenced by the dynamic viscosity of the fluid. As a result, the changes associated with the 1 cP viscosity increase in going from air-to-water are considerably larger than those for subsequent viscosity increments. We also see that the surface drag associated with the fluid boundary layer progressively affects the grain velocity profile near the surface as the viscosity increases, giving a several-particle-deep zone of constant velocity. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J8.00012: Rheology and structure of granular flows in split-bottom geometries. Joshua Dijksman, Martin van Hecke Combining rheological methods with surface flow imaging, we probe the flow of slow dry granular media as function of driving rate and geometry. The flow rate affects the spatial structure of the flow much stronger than the stresses, while details of the boundary conditions significantly modify both stresses and flow. We discuss our results in the context of recent numerics on rapid flows in these split-bottom geometries, and various theories developed for slow flows. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J8.00013: Self-diffusion in bulk sheared granular materials Andreea Panaitescu, Ashish Orpe, Arshad Kudrolli We will discuss the diffusion and structural properties of granular particles in the bulk of a cyclically sheared three dimensional rectangular cell. The particles are visualized away from the side walls using a fluorescent refractive index matched interstitial fluid. Previous studies have shown that the diffusion is anisotropic with respect to the vorticity plane, but these results have been confined to either two dimensional systems or small three dimensional systems where the boundary effects could not be decoupled. In a cyclic shear cell, the packing fraction the particles and their orientational order vary smoothly over time. The particle positions are identified and tracked over long durations to obtain particle diffusivity, mean-squared displacements and probability distributions of particle displacements. An analysis of the effect of structural order on the motion of the particles will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J8.00014: Studies in a 2D granular pure shear experiment Jie Zhang, Peidong Yu, Trush Majmudar, Robert P. Behringer We have performed two dimensional granular experiments under pure shear using bidisperse photo-elastic disks. Starting from a stress free state, a squre box filled with granular particles is subject to shear. The forward shear involved thirty steps, leading to maximum strain of 0.1. The network of force chains gradually built up as the strain increased, leading to increased pressure and shear stress. Backward shear was then applied to return the system to zero strain in the next thirty steps. Following each change of the system, contact forces of individual disks were measured by applying an inverse algorithm. We also kept track of the displacement and angle of rotation of every particle from frame to frame. We present the results for the contact forces, particle displacement, particle rotations, fabric, etc. Work supported by NSF grant DMR0555431. [Preview Abstract] |
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