Relaxation processes after instantaneous shear-rate reversal in a dense granular flow

Abstract

A numerical model of granular material at different packing fractions and under steady shear is submitted to a sudden shear reversal. We monitor consequences of the strong density and shear rate spatio-temporal heterogeneities, on the constitutive relations. We show that the dynamics can be decomposed into two subsequent regimes spanning a time scale inversely proportional to the shear rate. In the first regime, the non-local constitutive relation proposed by Bouzid et al. (Phys. Rev. Lett., 111 (2013) 238301) is satisfied hence accounting for the spatial heterogeneity of the fluidity parameter. However at later time, we find that the local μ(I) constitutive relation can be applied, in spite of the fact that the fluidity parameter remains heterogeneous. This result opens a discussion on the significance of fluidity and hidden variables accounting for non-locality in a frictional packing of grains.