Multiscale modeling of crystal plasticity is based on the dynamics of defects (dislocations and grain boundaries), but are there any analogous mesoscopic entities controlling the flow of amorphous materials? Here, we propose a multiscale model for random-packing dynamics based on coarse-grained "spots" of free volume causing localized, cooperative rearrangements of particles. Motivated by an application to pebble-bed nuclear reactors, we apply the spot model to slow granular drainage from a silo, calibrated to and compared with brute-force discrete-element simulations of frictional, viscoelastic spheres. The spot simulations run over 100 times faster and produce realistic flowing packings of 100,000 spheres with only 5 fitting parameters. A mechanical basis for the coarse-grained dynamics may come from a stochastic reformulation of (Mohr-Coulomb) 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). These ideas suggest a general framework for the multiscale modeling of amorphous plasticity.
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