Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session OH: Granular Flows V: Dense Flows |
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Chair: Jerry Gollub, Haverford College and the University of Pennsylvania Room: Tampa Marriott Waterside Hotel and Marina Florida Salon 6 |
Tuesday, November 21, 2006 12:15PM - 12:28PM |
OH.00001: Dense Inclined Flows of Spheres James Jenkins We operate in the context of a slightly modified hydrodynamic theory for frictionless spheres and consider deep, dense flows down a bumpy incline. The modification is the introduction of a length other than the diameter in the expression for the rate of collisional dissipation. The idea is that the first influence of the formation of particle chains is felt by the rate of dissipation. The chain length is determined by a simple algebraic balance between the creation and destruction of chains. The resulting theory is used together with the boundary conditions at a bumpy base and a free surface to determine the profiles of volume fraction, mean velocity, and fluctuation energy for steady, fully-developed flows of identical spheres. The profiles exhibit the features seen in numerical simulations. The integration of the energy balance through the depth of the flow result in an improvement of a velocity scaling often employed in the interpretation of physical experiments. [Preview Abstract] |
Tuesday, November 21, 2006 12:28PM - 12:41PM |
OH.00002: Phase diagram of granular flow on a rough inclined plane Robert Ecke, Tamas Borzsonyi We present an experimental characterization of the phase diagram of granular flow on an inclined plane for inclination angles up to twice the bulk angle of repose . The equation of state in terms of layer height, surface velocity and mean density is presented as a function of flow rate controlled by the opening aperture of the hopper. States of intermittent waves, uniform flow, and lateral stripes are found for dense granular flow. At low flow rates and high inclination angles a transition to a dilute gas phase is observed. The effects of air on the granular flow is also described with the conclusion that the equation of state of the dense granular flow is independent of entrained air although small air drag effects are observed for the dilute gas phase. [Preview Abstract] |
Tuesday, November 21, 2006 12:41PM - 12:54PM |
OH.00003: Comparing thresholds and dynamics for oscillating and inclined granular layers J.P. Gollub, S. Aumaitre, C. Puls The onset and dynamics of flow in horizontally oscillating granular layers are studied as a function of the depth of the layer. Measurements of the flow velocity made from the top and side are presented in the frame of reference of the container. The rheology of the material is found to vary in time during the cycle in surprising ways. If the inertial force (proportional to the container acceleration amplitude) is slightly higher than what is required to produce flow, then the flow velocity grows as soon as the inertial force exceeds zero in each cycle, but flow ceases long before the inertial force returns to zero. At higher accelerations, the motion is fluid-like over the entire cycle. As is also found for avalanches, the thresholds for starting and stopping are slightly different. The variation with depth of the starting acceleration for the oscillating layer matches (approximately) the corresponding variation of the tangent of the starting angle for avalanches in the same container. However, the mobile fraction of the cycle is typically far higher than what static considerations would predict. Finally, the flow profiles as a function of depth follow the simple Bagnold form when fully mobilized, even though the motion is oscillatory. [Preview Abstract] |
Tuesday, November 21, 2006 12:54PM - 1:07PM |
OH.00004: From quicksand to beach-sand: a phase transition in quasi-static granular media. Matthias Schr\"oter, Sibylle N\"agle, Charles Radin, Harry L. Swinney Granular media are often described as being in a gas, fluid, or solid phase. However, granular media are dissipative systems with macroscopic constituents, so it is not clear how to apply the precise definitions of phases used in thermodynamics and statistical mechanics. Here we present experimental measurements of the force needed to insert quasi-statically a circular rod into a granular bed at rest. The bed contains glass beads 0.265 mm in diameter. The rod diameter is 24 times the diameter of the beads. Varying the volume fraction of the initial granular bed from 0.57 to 0.63, we find two distinct phases of behavior [1]. The transition occurs at a volume fraction 0.59 . [1] cond-mat/0606459 [Preview Abstract] |
Tuesday, November 21, 2006 1:07PM - 1:20PM |
OH.00005: Role of Compressibility for Granular Flow Kevin Lu, Hossein Kavehpour, Emily Brodsky We study the transitional flow of a simple-sheared dry granular assembly. Between the familiar limiting regimes (grain-inertial and quasi-static) of granular flow, the physical description of the intermediate regime remains elusive. Our experiment utilizes a top-rotating torsional shear cell capable of micron gap accuracy and a velocity range. The results show that the shear and normal stresses exhibit an inverse rate-dependence under a controlled-gap environment in the transitional regime, while capturing the limiting regimes in agreement with previous work. The empirical data illustrates a previously unknown `dip' in the stress response to increasing shear rate. Under a controlled-force environment, however, the packing fraction is observed to be a positive function of strain rate within intermediate shear rates. We infer from our results that fluidized granular compressibility, as a function of rate, is a significant factor in granular dynamics. To account for the observations, a theoretical model is derived in an attempt to unify our findings. The formulation provides an equation of state for dynamic granular systems, with state variables of pressure, strain rate, and packing fraction. [Preview Abstract] |
Tuesday, November 21, 2006 1:20PM - 1:33PM |
OH.00006: Signal Propagation through Dense Granular Media Lou Kondic, Robert P. Behringer We consider propagation of signals through dense granular systems. The results are obtained by relatively large scale (up to 40,000 particles) discrete element simulations in two spatial dimensions. The properties of the signals are used to deduce the basic physical mechanisms of the force and energy transmission. In addition, we discuss the possibility of developing effective models for signal propagation which bridge the spatial scales between micro (grain scale) and meso (hundreds or thousands of grains) description of granular systems. We also discuss the influence of force anisotropy on the characteristics of the propagating signal. Finally, we will present preliminary results regarding signal propagation through dynamic (sheared) granular system. [Preview Abstract] |
Tuesday, November 21, 2006 1:33PM - 1:46PM |
OH.00007: ABSTRACT WITHDRAWN |
Tuesday, November 21, 2006 1:46PM - 1:59PM |
OH.00008: Velocity fluctuations in dense gravity driven granular flows obtained by internal imaging Ashish Orpe, Arshad Kudrolli We measure the structure and dynamics of a gravity driven granular flow inside a silo using a fluorescent refractive index matched interstitial fluid. The particle positions are identified and tracked over long durations to obtain flow characteristics in the plug flow region. The side walls induce significant structural order only on the granular layer adjacent to the front walls. The distributions of the horizontal and vertical displacements for short time scales show fat tails compared to a Gaussian indicating large fluctuations in particle displacements and possible cage breaking. No significant spatial velocity correlations are observed away from the sidewalls where shear is absent. However, velocity correlations are observed near the side walls where a weak shear is also observed. By varying the orifice width, we also show that the flow properties are observed to be flow-rate independent indicating that the grain interactions are dominated by enduring contacts. [Preview Abstract] |
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