Title: Geometric strengthening of fluid-sheared granular beds

Author (Talk): Abe Clark, Yale University

Abstract:

What determines the yield strength of a granular bed in response to a fluid shear force? This question is important in many geological processes (e.g., riverbeds) and industrial or commercial applications (e.g., slurry flows or erosion mitigation projects). It is well established that granular beds strengthen in response to weak fluid shear flows, in that the threshold stress required to transport grains downstream can increase over time. The microstructural origin of bed strengthening, however, is not well understood. Thus, to better understand how the particular arrangements of grains determine the strength of a granular bed, we perform numerical simulations of 2D and 3D systems of particles that are sheared by a model fluid force. Even in the case of beds composed of frictionless disks or spheres, we find that beds can strengthen over time. By varying the forcing strength, grain properties, and system size, we find that (1) grain motions are diffusive in their search for local stability, (2) local stability is highly correlated with the network of interparticle contacts, and (3) there are growing correlation lengths for stronger shear (i.e. fluid) forces that make global stability increasingly difficult for larger systems.