Title: Slow Granular Dynamics In Fluid-Driven Annular Couette Flow

Author (Poster): Carlos Ortiz, University of Pennsylvania

Abstract:

Field records and experimental studies show that the fluvial geophysical processes that shape the landscape, such as debris flows and river sediment transport, are extremely unpredictable in large part due to the nonlinear dependence of the transport rates on the structural properties of the sediment. There is a need for a more fundamental understanding of the physical processes that control sediment transfer rates, particularly the magnitude and frequency of geomorphic events, due to the need to estimate the effect of climate and human activity on these phenomena. We present experiments in a simple geometry, an annular couette cell, that allows us to study the free-surface flow at the interface between a flowing viscous fluid and a submerged jammed pack of spheres, a highly idealized river. We use the refractive-index-matched laser scanning to probe the dynamics of the grains on two-dimensional slices and three-dimensional regions, and over a range of timescales that includes six-orders of magnitude. Using Lagrangian particle tracking, we find that the grain dynamics are spatiotemporally heterogeneous, but the overall flow field reaches a well-developed steady-state. Below the granular surface, we find a wide flowing layer characterized by a fast, approximately exponential decay of the particle velocity versus depth, commonly referred to as creeping flow. We find that the thickness of the creeping flow layer increases with the applied shear stress. However, deep in the sample, the velocity profile does not indefinitely follow a exponential decay. Instead, the rate of decay of the velocity profile slows drastically, transitioning continuously to a quasistatic flow regime.