Kinetic theory applied to pressure-controlled shear flows of frictionless spheres between rigid, bumpy planes

TL;DR

This study uses discrete element simulations and kinetic theory to analyze pressure-controlled shear flows of frictionless spheres between bumpy planes, validating theoretical predictions and proposing a criterion for crystallization onset.

Contribution

It provides a detailed numerical validation of kinetic theory for granular flows and introduces a criterion to predict crystallization based on flow parameters.

Findings

Validation of kinetic theory equations of state and viscosity for granular flows.

Successful prediction of flow profiles and stress ratios using integrated differential equations.

Identification of a critical load threshold for crystallization in shear flows.

Abstract

We numerically investigate, through discrete element simulations, the steady flow of identical, frictionless spheres sheared between two parallel, bumpy planes in the absence of gravity and under a fixed normal load. We measure the spatial distributions of solid volume fraction, mean velocity, intensity of agitation and stresses, and confirm previous results on the validity of the equation of state and the viscosity predicted by the kinetic theory of granular gases. In a first, we also directly measure the spatial distributions of the diffusivity and the rate of collisional dissipation of the fluctuation kinetic energy, and successfully test the associated constitutive relations of the kinetic theory. We then phrase and numerically integrate a system of differential equations governing the flow, with suitable boundary conditions, and show a remarkable qualitative and quantitative agreement with the results of the discrete simulations in terms of the dependence of the profiles of the hydrodynamic fields, the ratio of shear stress-to-pressure and the gap between the bumpy planes on the coefficient of collisional restitution, the imposed load and the bumpiness of the planes. Finally, we propose a criterion to predict, on the basis of the solution of the boundary-valued problem, the critical value of the imposed load above which crystallization may occur. This notably reproduces what we observe in the discrete simulations.