Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session GK: Granular Flows I 
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Chair: Ivan Christov, Northwestern University Room: Long Beach Convention Center 201B 
Monday, November 22, 2010 8:00AM  8:13AM 
GK.00001: Transient rheological behavior of suspensions near the jamming transition Eric Brown, Heinrich Jaeger We performed transient rheological measurements on suspensions at several packing fractions near the jamming transition. A slow shear displacement was applied, then the shear stress was abruptly set to zero to observe the relaxation behavior. A harmonic oscillator model can be used to obtain the elastic part of the yield stress from oscillations and a transient viscosity from the relaxation time. For displacements smaller than a particle size elastic behavior is found if there is shear thinning at packing fractions below the jamming transition. For larger displacements, there is a relaxation but no oscillation. Remarkably, the transient viscosity differs from the steadystate viscosity at packing fractions near the jamming transition; the transient viscosity remains finite while the steadystate viscosity diverges at jamming. [Preview Abstract] 
Monday, November 22, 2010 8:13AM  8:26AM 
GK.00002: Granular collapse in a fluid: Role of the initial volume fraction Lo\"Ic Rondon, Olivier Pouliquen, Pascale Aussillous The collapse of a granular column suddenly released on a plane has been intensively studied the last ten years in the case of a granular medium with no interaction with the interstitial fluid. However, in many geophysical events like submarine avalanches or landslides, the interaction between the grains and the surrounding fluid plays a crucial role. In this work, we experimentally studied the collapse of a granular mass in a viscous liquid. We found that contrary to the dry case, the aspect ratio of the initial mass is no longer the only relevant parameter controlling the deposit morphology. In the viscous regime, the dynamics is controlled by the initial volume fraction of the mass. Two different regimes are identified. For initially loose packing the deposits are thin and long, the dynamics is fast and a positive liquid pressure is measured below the column. For dense packing, the run out distance is twice less, the flow is slow and a negative pore pressure is measured during the flow. These results suggest that the dynamics of the granular collapse in a fluid is strongly affected by the dilatancy or contractancy behaviour of the granular media. [Preview Abstract] 
Monday, November 22, 2010 8:26AM  8:39AM 
GK.00003: The effect of volume fraction on granular slope stability Nick Gravish, Nick Ward, Daniel I. Goldman We study the stability of granular slopes as a function of the prepared volume fraction 0.58$<\phi<$0.62. A bed of 250$\mu m$ diameter glass beads with an initial slope angle $\theta$=0$^\circ$ and initial $\phi$ is slowly rotated at constant angular velocity to a final angle of $50^\circ$. We monitor the motion of grains at the top surface and observe that the angle at which continuous surface flow occurs is sensitive to $\phi$ and increases from $\theta_0 \approx 26^\circ$ at low $\phi$ (loosely packed) to $\theta_0 \approx 32^\circ$ at high $\phi$ (closely packed). Prior to the uniform failure at $\theta_0(\phi)$ the grain motion during tilting differs between the loosely packed to the closely packed regimes. Tilting loosely packed beds results in rapid intermittent grain rearrangement at the surface; the angle at which these begin is $\theta \approx 15^\circ$s. In the closely packed beds grain motion at the surface is not observed until $\theta \approx 31^\circ$, prior to continuous failure. [Preview Abstract] 
Monday, November 22, 2010 8:39AM  8:52AM 
GK.00004: Terminal velocity of a heavy object in a superlight granular medium Gabriel A. CaballeroRobledo, Felipe PachecoVazquez, J. Carlos RuizSuarez A granular material is a system composed of many solid particles interacting mainly through contact forces. Therefore, the dissipation of energy usually plays a dominant role in the dynamics of these materials. For this reason, in experiments done so far, when an object impacts on a granular bed it eventually dissipates all its energy and comes to rest. In contrast, when a dense enough object is placed inside a fluid it keeps falling, asymptotically approaching a terminal velocity. Here we present experiments of a heavy object falling into a silo full of expanded polystyrene spherical particles. The density of the granular medium is so low that it can not bear the weight of an intruder whose mass is beyond a threshold value, even if the object is very deep in the silo. We systematically vary the mass of an object impacting in such a granular bed and we find a transition between the commonly observed behavior where the object stops at a given depth, and a situation where the object keeps falling and reaches a terminal velocity, just as in a fluid. [Preview Abstract] 

GK.00005: ABSTRACT WITHDRAWN 
Monday, November 22, 2010 9:05AM  9:18AM 
GK.00006: Stokes' Cradle: Oblique Collisions between Wetted Particles Carly Donahue, Christine Hrenya, Robert Davis, William Brewer Granular particles can be made more cohesive by applying a viscous liquid to the surface of the particle. Such wetted particles are naturally involved in pollen capture and avalanches and can be found in industrial processes such as granulation and filtration. The focus here is on collisions between wetted particles in which lubrication forces dominate over capillary forces (i.e., high capillary number). Previous experiments with such systems have been limited to normal (headon) collisions of spheres and collisions between a sphere and an immobile wall. In these cases, rebound (deagglomeration) was found to depend upon the surface roughness of the solids, the elastohydrodynamic interaction, or the pressuredependent viscosity. In this effort, we experimentally investigate collisions between two wetted particles impacting at an oblique angle. Now, in addition to the above interactions, the presence of a centrifugal force also contributes to the mechanism for reboundedagglomeration. A theoretical analysis of the associated regime maps provides useful insight to unravel the relevant physical processes that occur in oblique collisions. [Preview Abstract] 
Monday, November 22, 2010 9:18AM  9:31AM 
GK.00007: Macroscopic Characteristics of Unsteady Granular Flows in Rotating Tumblers Daniel Paprocki, Nicholas Pohlman Flow of silicate beads in rotating tumblers of triangular crosssections are explored with respect to transient response of macroscopic properties. Highspeed digital images are synchronized to tumbler orientation through an inline rotary encoder. Image processing toolboxes are utilized to generate quantitative data for analysis. Timedependent properties of free surface length, flowing layer curvature, and dynamic angle of repose are reported. The correlation of these properties with the orientation exhibits a phase difference that is a function of tumbler dimensions and fill fraction. Concurrent measurements of input energy to the system may lead to control algorithms to generate steady flow in inherently unsteady systems that would improve efficiency of granular transport methods. [Preview Abstract] 
Monday, November 22, 2010 9:31AM  9:44AM 
GK.00008: A comparison of the granular flow of glass spheres and sands Steven W. Meier, Katherine P. Barteau, Deniz Ertas, Hubert E. King We investigate the effect of particle shape irregularity on granular flow on an erodible bed 2 cm wide confined by sidewalls 50 cm long and 20 cm tall. Three types of particles were studied: 0.65 mm spherical glass particles, rounded sand particles with a mean size of 0.87 mm, and angular sand particles with a mean size of 0.83 mm. The dynamics of the flows were measured with a high speed camera. For all particle types, the angle between the flowing free surface and horizontal increases linearly with mass flow rate. While the surface angle is greater for flows of sand particles than spherical glass particles at all mass flow rates, the change in surface angle with respect to mass flow rate is smaller for flows of sand than spherical glass particles. For all particle types, the velocity profile increases exponentially into the flowing layer from the erodible bed. For fast flows, the velocity profile is linear in the upper portion of the flowing layer. However, spherical glass particles flow in thicker flowing layers with lower surface velocities than sand particles for comparable mass flow rates. [Preview Abstract] 
Monday, November 22, 2010 9:44AM  9:57AM 
GK.00009: Cutting and shufflingMixing in Spherical Tumblers Richard M. Lueptow, Ivan C. Christov, Gabriel Juarez, Julio M. Ottino, Rob Sturman, Stephen Wiggins Cutting and shuffling and the underlying mathematical formalism of piecewise isometries (PWIs) offer means to predict mixing of granular material in 3d tumblers. We have studied various mixing protocols for twoaxis rotation of a spherical tumbler using a continuum model with a vanishinglythin flowing layer (PWIs) and a realisticthickness flowing layer. The PWIs describe the skeleton of the mixing emerging from the container shape, fill fraction, and rotation protocol. Mixing measures based on the center of mass of an ensemble of seed tracer particles and based on concentration variance provide similar results. Poor mixing occurs when the center of mass of the seed particles does not converge to the center of mass of the domain, but instead evolves toward a ``limit cycle'' due to symmetries in the rotation protocol. Good mixing occurs when the ``limit cycle'' is avoided. Comparison with simulations having a realistic flowing layer discerns the role of the flowing layer's thickness on mixing. The quality of mixing predicted by the center of mass measure based on the model with a vanishinglythin flowing layer (PWIs) correlates with the quality of mixing predicted by the decay of concentration variance, allowing for fast optimization of mixing protocols using PWIs. [Preview Abstract] 
Monday, November 22, 2010 9:57AM  10:10AM 
GK.00010: Particle Tracking Velocimetry and Granular Flow Correlations in Triangular Tumblers Joseph Szalko, Nicholas Pohlman Granular flow has traditionally been examined at steady state with time averaged results. Circular shaped tumblers with constant rotation rates eliminate most transient effects in dynamic flow. This research examines the transient flow induced by triangular shaped tumblers. High speed imaging and custom particle tracking velocimetry (PTV) are used to analyze several aspects of the flow: shear layer thickness along the variable angle of repose, transverse flow within the shear layer, and velocity profiles at different tumbler orientations and dimensions. Correlations of these properties with one another and the time/orientation dependence of the nonuniform tumbler are reported. Results indicate transient flow may not be equivalent to instantaneous conditions of steady flow. For example, highest velocities exist where the shear layer is thinnest. [Preview Abstract] 
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