Annular shear of cohesionless granular materials: from inertial to quasistatic regime

TL;DR

This study uses discrete simulations to analyze the transition from inertial to quasistatic regimes in annular shear of cohesionless granular materials, revealing the applicability of constitutive laws and microscopic behaviors.

Contribution

It introduces a detailed simulation-based analysis of granular shear behavior, clarifying the limits of existing constitutive laws and providing microscopic insights.

Findings

Inertial region develops near the inner wall at high velocities.

Slow creep occurs away from the wall in jammed conditions.

Microscopic contact network and velocity fluctuations are characterized.

Abstract

Using discrete simulations, we investigate the behavior of a model granular material within an annular shear cell. Specifically, two-dimensional assemblies of disks are placed between two circular walls, the inner one rotating with prescribed angular velocity, while the outer one may expand or shrink and maintains a constant radial pressure. Focusing on steady state flows, we delineate in parameter space the range of applicability of the recently introduced constitutive laws for sheared granular materials (based on the inertial number). We discuss the two origins of the stronger strain rates observed near the inner boundary, the vicinity of the wall and the heteregeneous stress field in a Couette cell. Above a certain velocity, an inertial region develops near the inner wall, to which the known constitutive laws apply, with suitable corrections due to wall slip, for small enough stress gradients. Away from the inner wall, slow, apparently unbounded creep takes place in the nominally solid material, although its density and shear to normal stress ratio are on the jammed side of the critical values. In addition to rheological characterizations, our simulations provide microscopic information on the contact network and velocity fluctuations that is potentially useful to assess theoretical approaches.