Strain dependent vorticity in sheared granular media

Abstract

Displacement fields in sheared particle packings often display vortex-like structures that reveal essential features about the mechanical state of the collection of particles. There are several metrics to quantify these flow field features, yet extracting such quantitative metrics from flow field or particle tracking data involves making numerous choices on the time and length scales over which to average. Here we employ a much used experimental data set on sheared disk packings to explore how such arbitrary data mining choices affect the obtained results. We focus on calculating the strain dependent vorticity, as this metric is a differential method hence potentially sensitive to the way it is computed. We find that the total surface area with an absolute vorticity above a certain threshold approaches a plateau value as shear progresses. This plateau value exhibits a non-monotonic dependence on packing fraction. We also show which range of choices yields results that can support an analysis method independent, physical interpretation of the flow field data.

How vorticity and agglomeration control shear strength in soft cohesive granular flows

“…The kinematics of third body flows must therefore be better understood, to grasp the interaction between the third body and stresses. Numerical [12] and experimental [13][14][15] approaches to calculate the vorticity of a granular flow have already been proposed, but they were limited to rigid granular materials. The terms agglomerate and vortex are complementary.…”