Transport coefficients for an inelastic gas around uniform shear flow: Linear stability analysis

TL;DR

This paper derives transport coefficients for an inelastic granular gas near uniform shear flow using a Chapman-Enskog-like expansion, analyzing their dependence on shear rate and stability of the flow.

Contribution

It introduces a novel expansion around a shear flow distribution that includes all hydrodynamic orders, accounting for unsteady effects in inelastic gases.

Findings

Transport coefficients depend nonlinearly on shear rate.

Steady state conditions allow linear stability analysis.

Conditions for long-wavelength instabilities are identified.

Abstract

The inelastic Boltzmann equation for a granular gas is applied to spatially inhomogeneous states close to the uniform shear flow. A normal solution is obtained via a Chapman-Enskog-like expansion around a local shear flow distribution. The heat and momentum fluxes are determined to first order in the deviations of the hydrodynamic field gradients from their values in the reference state. The corresponding transport coefficients are determined from a set of coupled linear integral equations which are approximately solved by using a kinetic model of the Boltzmann equation. The main new ingredient in this expansion is that the reference state $f^{(0)}$ (zeroth-order approximation) retains all the hydrodynamic orders in the shear rate. In addition, since the collisional cooling cannot be compensated locally for viscous heating, the distribution $f^{(0)}$ depends on time through its dependence on temperature. This means that in general, for a given degree of inelasticity, the complete nonlinear dependence of the transport coefficients on the shear rate requires the analysis of the {\em unsteady} hydrodynamic behavior. To simplify the analysis, the steady state conditions have been considered here in order to perform a linear stability analysis of the hydrodynamic equations with respect to the uniform shear flow state. Conditions for instabilities at long wavelengths are identified and discussed.