Depth resolved granular transport driven by shearing fluid flow

We investigate granular transport by a fluid flow under steady-state driving conditions, from the bed-load regime to the suspension regime, with an experimental system based on a conical rheometer. The mean granular volume fraction φg , the mean granular velocity ug , and the fluid velocity uf are obtained as a function of depth inside the bed using refractive index matching and particle-tracking techniques. A torque sensor is utilized to measure the applied shear stress to complement estimates obtained from measured strain rates high above the bed where φg ≈ 0. The flow is found to be transitional at the onset of transport and the shear stress required to transport grains rises sharply as grains are increasingly entrained by the fluid flow. A significant slip velocity between the fluid and the granular phases is observed at the bed surface before the onset of transport as well as in the bed-load transport regime. We show that ug decays exponentially deep into the bed for φg > 0.45 with a decay constant which is described by a nonlocal rheology model of granular flow that neglects fluid stress. Further, we show that uf and ug can be described using the applied shear stress and the Krieger-Dougherty model for the effective viscosity in the suspension regime, where 0 < φg < 0.45 and where ug ≈ uf .