On the stability of the $\mu(I)$-rheology for granular flow

TL;DR

This paper investigates the stability of the $(I)$-rheology in granular flow, showing that wave-vector stretching stabilizes certain instabilities and exploring enhanced models for steady shear bands, with implications for numerical simulations.

Contribution

It provides a comprehensive linear stability analysis of the $(I)$ model, including wave-vector effects and enhanced continuum models, advancing understanding of granular flow stability.

Findings

Wave-vector stretching stabilizes non-convective instabilities.

Enhanced continuum model predicts steady shear bands.

Connection between instability and loss of ellipticity in equations.

Abstract

This article deals with the Hadamard instability of the so-called $\mu(I)$ model of dense rapidly-sheared granular flow, as reported recently by Barker et al. (2015,this journal, ${\bf 779}$, 794-818). The present paper presents a more comprehensive study of the linear stability of planar simple shearing and pure shearing flows, with account taken of convective Kelvin wave-vector stretching by the base flow. We provide a closed form solution for the linear stability problem and show that wave-vector stretching leads to asymptotic stabilization of the non-convective instability found by Barker et al. We also explore the stabilizing effects of higher velocity gradients achieved by an enhanced-continuum model based on a dissipative analog of the van der Waals-Cahn-Hilliard equation of equilibrium thermodynamics. This model involves a dissipative hyper-stress, as the analog of a special Korteweg stress, with surface viscosity representing the counterpart of elastic surface tension. Based on the enhanced continuum model, we also present a model of steady shear bands and their non-linear stability against parallel shearing. Finally, we propose a theoretical connection between the non-convective instability of Barker et al. and the loss of generalized ellipticity in the quasi-static field equations. Apart from the theoretical interest, the present work may suggest stratagems for the numerical simulation of continuum field equations involving the $\mu(I)$ rheology and variants thereof.