Title: Constitutive laws and transient jamming in vertical-channel granular flows

Author (Invited): Don Candela, UMass Amherst

Abstract:

A long-sought goal has been to find and justify quantitative laws describing granular flows over densities ranging from dilute to close-packed and flow rates ranging from quasistatic to highly inertial. Physical and numerical experiments delineate a number of "phases" or flow regimes in the pressure (or density) versus shear rate plane. However the constant-pressure and constant-density "phase diagrams" do not map onto each other, suggesting at least one of these ensembles is highly out of equilibrium. Constitutive equations have been developed that successfully describe granular flow experiments, mostly in the constant-pressure situation. Here we describe a new set of experiments using NMR to probe the structure and dynamics of a vertical-channel granular flow. In this common flow geometry, which is neither constant-pressure nor constant-volume, it appears that a fundamentally different physical mechanism (and set of dimensionless parameters) controls the flow than is the case for constant-pressure and open-chute flows. In particular the acceleration of gravity g emerges as a key physical parameter (not just a forcing term) and the flow appears to be controlled by a rapid series of transient jams that occur randomly in space and time. (Work supported by NSF CBET-0651397.)