Linking bed forces to granular rheology in geophysical flows using DEM-CFD

Basal forces exerted by geophysical granular flows are responsible for the generation of observable seismic signals. These signals may provide a safe means for remotely analyzing internal dynamics in real-time. To realize the benefit of seismic measurements, basal granular forces must be linked to macroscopic flow dynamics across a range of flow conditions. We perform discrete element simulations of dry and submerged granular flows under plane-shear and inclined flow configurations, relating bulk rheology to basal forces. We find that the variance in basal forcing scales with inertial number (I). We differentiate four flow regimes tracked by this scaling: (1) unsteady particle rearrangement when I < 10^-3, where basal forces are dominated by low frequencies and granular temperature is anisotropic in the vertical direction; (2) an intermediate regime when 10^-3 < I < 10^-2, where granular temperature is isotropic, and basal force fluctuation scaling with I strengthens; (3) a transitional regime when 10^-2 < I < 10^-1, where increases in basal force fluctuations with I is interupted as the granular bed dilates, partially erasing the contact network and configurational memory; and (4) a collisional regime when I > 10^-1, granular temperature becomes anisotropic in the stream-wise direction, and the signal becomes white noise-like up to a cutoff frequency that is dependent on particle scale parameters. These data show that information about complex granular flow rheology maybe encoded in basal interactions.