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).