CiteULike: dchen's Choi http://www.citeulike.org/user/dchen/author/Choi CiteULike.org en-gb Copyright © 2004-2008 citeulike.org http://www.citeulike.org/user/dchen/article/2281600 <i>Physical Review Letters, Vol. 92, No. 17. (27 April 2004), 174301.</i><br /><br />We study the transport properties of particles draining from a silo using imaging and direct particle tracking. The particle displacements show a universal transition from superdiffusion to normal diffusion; as a function of the distance fallen; independent of the flow speed. In the superdiffusive (but sub-ballistic) regime; which occurs before a particle falls through its diameter; the displacements have fat-tailed and anisotropic distributions. In the diffusive regime; we observe very slow cage breaking and Péclet numbers of order 100; contrary to the only previous microscopic model (based on diffusing voids). Overall; our experiments show that diffusion and mixing are dominated by geometry; consistent with long-lasting contacts but not thermal collisions; as in normal fluids. Diffusion and Mixing in Gravity-Driven Dense Granular Flows Jaehyuk Choi Arshad Kudrolli Rodolfo Rosales Martin Bazant doi:10.1103/PhysRevLett.92.174301 Physical Review Letters, Vol. 92, No. 17. (27 April 2004), 174301. 2008-01-23T19:08:39-00:00 Physical Review Letters 92 17 174301 American Physical Society flow grains gravity http://www.citeulike.org/user/dchen/article/2582925 <i>Journal of Physics: Condensed Matter, Vol. 17, No. 24. (2005), pp. S2533-S2548.</i><br /><br />We measure the flow of granular materials inside a quasi-two-dimensional silo as it drains and compare the data with some existing models. The particles inside the silo are imaged and tracked with unprecedented resolution in both space and time to obtain their velocity and diffusion properties. The data obtained by varying the orifice width and the hopper angle allow us to thoroughly test models of gravity driven flows inside these geometries. All of our measured velocity profiles are smooth and free of the shock-like discontinuities ('rupture zones') predicted by critical state soil mechanics. On the other hand, we find that the simple kinematic model accurately captures the mean velocity profile near the orifice, although it fails to describe the rapid transition to plug flow far away from the orifice. The measured diffusion length b, the only free parameter in the model, is not constant as usually assumed, but increases with both the height above the orifice and the angle of the hopper. We discuss improvements to the model to account for the differences. From our data, we also directly measure the diffusion of the particles and find it to be significantly less than predicted by the void model, which provides the classical microscopic derivation of the kinematic model in terms of diffusing voids in the packing. However, the experimental data are consistent with the recently proposed spot model, based on a simple mechanism for cooperative diffusion. Finally, we discuss the flow rate as a function of the orifice width and hopper angles. We find that the flow rate scales with the orifice size to the power of 1.5, consistent with dimensional analysis. Interestingly, the flow rate increases when the funnel angle is increased. Velocity profile of granular flows inside silos and hoppers Jaehyuk Choi Arshad Kudrolli Martin Bazant doi:10.1088/0953-8984/17/24/011 Journal of Physics: Condensed Matter, Vol. 17, No. 24. (2005), pp. S2533-S2548. 2008-03-24T22:32:04-00:00 Journal of Physics: Condensed Matter 17 24 S2533 S2548 experiment grains qualifer