Physics Colloquium
University of Pennsylvania
October 5, 2005

Title:   Random-packing dynamics in granular flow

Speaker: Martin Z. Bazant
 	 Department of Mathematics & Dry Fluids Laboratory
 	 MIT

Abstract:

The jamming transition of disordered hard spheres has attracted much 
recent attention, but how do slightly less dense random packings flow, 
e.g. in granular materials? Beyond its fundamental interest, this open 
question is critical to the design of pebble-bed nuclear reactors, which 
we study via simulations and scaled-down experiments on slow silo 
drainage.  Classical statistical mechanics and hydrodynamics do not apply, 
and Mohr-Coulomb plasticity, routinely used to calculate stresses in 
silos, also fails to describe flows. Here, we propose a cooperative flow 
mechanism based on diffusing "spots" of free volume, which produces 
realistic flowing packings and yet is simple enough for mathematical 
analysis. Spot simulations run over 100 times faster than discrete-element 
simulations with frictional, visco-elastic spheres, which demonstrates the 
possibility of multiscale modeling for amorphous materials. The 
coarse-grained dynamics may derive from a new, stochastic formulation of 
plasticity, where spots move randomly along slip planes, driven by body 
forces (e.g. gravity) and local fluidization (e.g. switching from static 
to dynamic friction).