Effect of particle surface friction on nonlocal constitutive behavior of flowing granular media

A recently proposed nonlocal rheology for dense granular flow, based on the concept of nonlocal granular fluidity, has demonstrated predictive capabilities in multiple geometries. This work is concerned with determining how the parameters of this continuum model arise from the properties of the grains themselves. We perform a controlled study investigating how the surface friction of the grains influences the continuum parameters, with a focus on how the nonlocal amplitude, the model's one new parameter, is affected. This is achieved comparing two-dimensional discrete-element simulations of flowing disks to numerical solutions of the model in planar shear and several annular shear geometries. A multistep calibration scheme for the continuum parameter extraction is developed and implemented. Results indicate the nonlocal amplitude varies less than an order of magnitude over a wide range of surface frictions, with a slight tendency to increase as surface friction decreases, particularly in a regime of small surface friction. Our data also show that the stress and flow-rate variables deviate little from a local relationship as surface friction vanishes, which corroborates certain existing experimental findings.