Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session LJ: Granular Media: General I |
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Chair: Jih-Chiang Tsai, Northwestern University Room: Hilton Chicago Williford C |
Tuesday, November 22, 2005 8:00AM - 8:13AM |
LJ.00001: A new rheology for dense granular flows Pierre Jop, Yoel Forterre, Olivier Pouliquen Recent experiments and numerical simulations of dry and dense granular flows suggest that a simple rheological description, in terms of a shear rate dependent friction coefficient, may be sufficient to capture the major flow properties [1,2]. In this work we generalize this approach by proposing a tensorial form of this rheology leading to 3D hydrodynamic equations for granular flows. We show that quantitative predictions can be obtained with this model by studying the flow of grains on a pile confined between two lateral walls. In this configuration we have experimentally measured the free surface velocity profile, the flowing thickness for different flow rates and channel widths. The results are compared with numerical simulations of the hydrodynamic model and quantitative agreement is observed. This study strongly supports the relevance of the proposed rheology. \\ \\ 1. F. da Cruz, S. Emam, M. Prochnow, J.-N. Roux and F. Chevoir, cond-mat/ 0503682 (2005)\\ 2. G.D.R. Midi, EPJE14 367-371 (2004) [Preview Abstract] |
Tuesday, November 22, 2005 8:13AM - 8:26AM |
LJ.00002: Disorder pressure in dense and flowing granular materials Christophe Josserand, Pierre-Yves Lagree, Daniel Lhuillier Dense granular materials with a volume fraction between the random loose and random close packings display several specific features that do not exist at smaller or higher concentrations. When flowing they act as weakly compressible liquids. Their compressibility stems from the many independent configurations the grains can explore when flowing, and is represented by a compaction-dependent ``disorder'' pressure which has a direct influence on many quasi-static flows. As an example, disorder pressure is a necessary ingredient to understand the layer thickness that remains on a rough incline just after flow has stopped. [Preview Abstract] |
Tuesday, November 22, 2005 8:26AM - 8:39AM |
LJ.00003: Investigation of structure and dynamics of gravity driven dense granular flows with internal imaging Ashish Orpe, Arshad Kudrolli We investigate the dynamics of dense granular flows during the drainage of a 3-dimensional silo with a rectangular cross-section. The silo is filled with glass particles, and a liquid with the same refractive index to visualize the grains away from the side walls. A plane of grains is illuminated using a laser, and dark particles against a fluorescent background are imaged using a digital camera. The particle positions are identified and tracked over long durations to obtain flow characteristics such as mean squared displacements, velocity correlations and cage correlation functions. A hexagonal close packing is obtained near the walls while a random packing is obtained inside the silo. The flow region is plug like high above the orifice and becomes parabolic as it approaches the orifice. In the region spanning 5 particle diameters from the orifice, the velocity profile at a constant height across the cell is non-Gaussian. The distributions of the horizontal and vertical displacements for very short time scales show fat tails compared to a Gaussian indicating large fluctuations in particle displacements and possible cage breaking. The preliminary results show absence of any spatial velocity correlations. The experimental results reveal a systematic effect of the side walls on the flow properties up to depths of 20 particle diameters. [Preview Abstract] |
Tuesday, November 22, 2005 8:39AM - 8:52AM |
LJ.00004: Thickness dependence of sheared granular layers on velocity profiles and effective friction Arshad Kudrolli, Ashish Orpe, Saloome Siavoshi We investigate the effective friction encountered by a mass sliding on a granular layer as a function of bed thickness and boundary roughness conditions. A rectangular slider of size 100 mm $\times$ 140 mm is pushed over a granular layer composed of 1~mm glass beads with a linear translating stage connected to a stepper motor. The friction is measured in the continuous sliding regime with the help of a spring and a displacement sensor. The observed friction has minima for a small number of layers before it increases and saturates to a value which depends on surface roughness. We use an index-matched interstitial liquid to probe the internal motion of the grains with fluorescence imaging in a regime where the liquid has no measurable effect on the friction. The shear profiles obtained as a function of depth show decrease in slip near the sliding surface as the layer thickness is increased. The gap between slider surface and the grains in the bed decreases on an average with layer thickness. From these measurements, we propose that the change in the friction with layer thickness is because of the increased confinement and locking of a grain relative to its neighbors. [Preview Abstract] |
Tuesday, November 22, 2005 8:52AM - 9:05AM |
LJ.00005: Velocity Correlations in Gravity-driven Dense Granular Flows Oleh Baran, Deniz Ertas, Gary S. Grest, Thomas C. Halsey We report numerical results on velocity correlations in dense granular flows on inclined planes. The velocity is obtained from the displacement of individual grains (spheres) over some averaging time $\delta$t. For the grains on the surface layer, our results are consistent with experimental measurements reported by Pouliquen [Phys. Rev. Lett. {\bf 93}, 248001 (2004)]. We show that the correlation structure within planes parallel to the surface shows similar behavior in the bulk of the flow. The two-point velocity correlation function exhibits exponential decay for small to intermediate values of the separation between spheres. The correlation lengths identified by exponential fits to the data show nontrivial dependence on the averaging time $\delta$t used to determine grain velocities. We discuss the correlation length dependence on averaging time, incline angle, pile height, depth of the layer, system size and grain stiffness, and we relate the results to other length scales associated with the rheology of the system. [Preview Abstract] |
Tuesday, November 22, 2005 9:05AM - 9:18AM |
LJ.00006: Application of a Particle-Substrate Model to a Two-Dimensional Driven Granular System Meenakshi Dutt, Robert Behringer We discuss a numerical model which accounts for collisional and surface frictional dissipation, and their influence on particle dynamics for a quasi 2-dimensional cooling granular material (Painter, et al. Physica D (2003); Dutt, et al. Phys. Rev. E (2004); Dutt, in preparation) confined to a substrate. This model has been further extended to study a horizontally vibrated particle-substrate system. We show that the ratio of the substrate acceleration to the particle-substrate static frictional force (Kondic, Phys Rev. E (1999)) dominates the individual particle dynamics and the collision dynamics. This, in turn, affects the time evolution of averaged dynamical variables, for example, the mean velocity and the center of mass position vector. We will present results from our numerical experiments which further higlight the critical role of static friction, relative to the driving acceleration. [Preview Abstract] |
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