Kinematic flow patterns in slow deformation of a dense granular material

TL;DR

This study investigates the detailed flow patterns in slow deformation of dense granular materials using high-resolution imaging and simulations, revealing stagnation zones, vortices, and shear bands, with implications for understanding granular flow mechanics.

Contribution

It introduces a combined experimental and numerical approach to analyze flow features in dense granular deformation, providing detailed flow characterizations and validating them with simulations.

Findings

Identification of stagnation zones and vortices in granular flow

Quantitative analysis of shear band formation and strain rates

Validation of experimental results with exact-scale NSCD simulations

Abstract

The kinematic flow pattern in slow deformation of a model dense granular medium is studied at high resolution using \emph{in situ} imaging, coupled with particle tracking. The deformation configuration is indentation by a flat punch under macroscopic plane-strain conditions. Using a general analysis method, velocity gradients and deformation fields are obtained from the disordered grain arrangement, enabling flow characteristics to be quantified. The key observations are the formation of a stagnation zone, as in dilute granular flow past obstacles; occurrence of vortices in the flow immediately underneath the punch; and formation of distinct shear bands adjoining the stagnation zone. The transient and steady state stagnation zone geometry, as well as the strength of the vortices and strain rates in the shear bands, are obtained from the experimental data. All of these results are well-reproduced in exact-scale Non-Smooth Contact Dynamics (NSCD) simulations. Full 3D numerical particle positions from the simulations allow extraction of flow features that are extremely difficult to obtain from experiments. Three examples of these, namely material free surface evolution, deformation of a grain column below the punch and resolution of velocities inside the primary shear band, are highlighted. The variety of flow features observed in this model problem also illustrates the difficulty involved in formulating a complete micromechanical analytical description of the deformation.