The role of gravity or pressure and contact stiffness in granular rheology

Abhinendra Singh,Kuniyasu Saitoh,Stefan Luding,Vanessa Magnanimo

doi:10.1088/1367-2630/17/4/043028

Abstract

The steady-state shear rheology of granular materials is investigated in slow quasistatic and inertial flows. The effect of gravity (thus the local pressure) and the often-neglected contact stiffness are the focus of this study. A series of particle simulations are performed on a weakly frictional granular assembly in a split-bottom geometry considering various magnitudes of gravity and contact stiffnesses. While traditionally the inertial number, i.e., the ratio of stress to strain-rate time scales, is used to describe the flow rheology, we report that a second dimensionless number, the ratio of softness and stress time scales, must also be included to characterize the bulk flow behavior. For slow, quasistatic flows, the density increases while the effective (macroscopic) friction decreases with increase in either particle softness or local pressure. This trend is added to the rheology and can be traced back to the anisotropy in the contact network, displaying a linear correlation between the effective friction coefficient and deviatoric fabric in the steady state. When the external rotation rate is increased towards the inertial regime, for a given gravity field and contact stiffness, the effective friction increases faster than linearly with the deviatoric fabric.

Highlights

Granular media are envisaged as a collection of macroscopic and athermal particles which interact through dissipative contact forces [1,2,3]
By increasing the external rotation rate, we study the dependence of effective friction and contact network anisotropy on the inertial number
Since we are interested in the flow behavior of the material, as default in the rest of the paper we focus only on the data well inside the shear band with local strain rate, Figure 2. (a) The local shear stress, τ (r, h), plotted against the local pressure, p (r, h), for different values of the local shear rate, γ (r, h) as given in the legend. (b) Pressure p, and (c) shear stress τ plotted against time t for the spatial position r = 0.08 m and h = 0.022 m

Summary

Introduction

Granular media are envisaged as a collection of macroscopic and athermal particles which interact through dissipative contact forces [1,2,3]. The recently proposed inertial number framework has been successful in describing the flow behavior in the liquid like regime when the particles undergo collisions and frictional interactions with other particles [3, 6,7,8]. Though it very well predicts the flow behavior in case of homogeneous shear, it fails in case of a non-homogeneous shear flow [9,10,11]

Objectives

Findings

Discussion

Conclusion