Granular Temperature and Competing Dilatational Effects in High Velocity Shear Flows of Angular Mineral Grains

Granular temperature may control high-speed granular flows, yet it is difficult to measure in laboratory experiments, particularly when using nonuniform, angular, polydisperse samples. We show that acoustic energy captures the anticipated behavior of granular temperature as a function of grain mass in aspherical mineral sand shear flows. We also find that granular temperature (through its proxy acoustic energy) is nearly linearly proportional to inertial number, and total dilation is proportional to acoustic energy raised to the power 0.6 ± 0.2, representing a combined effect of shear zone dilation, which image analysis shows is linearly proportional to granular temperature, and sub-shear compaction caused by vibration of non-moving grains. This demonstrates a dual role for acoustic vibrations in shear flows. Granular temperature can dilate a flow and simultaneously vibrate the bed, causing a competing compaction. We also find that despite the theoretical dependence of granular temperature on coefficient of restitution, granular temperature and dilation to be most dependent on grain mass. Angular grains of geologically relevant minerals with differing strength and elasticity exhibit effectively the same relationships between granular temperature, dilation and inertial number.