Onset of erosion of a granular bed in a channel driven by fluid flow

We investigate the erosion threshold of a granular bed driven by a fluid flow as a function of grain size and grain roughness. Experiments are performed with a bed in an enclosed cylindrical channel under laminar flow conditions. The shear rate at threshold for a prescribed flow rate is obtained from the height of the fluid above the bed as it comes to rest, and used along with the grain size to determine the particle Reynolds number Re-p. We estimate that the shear lift force acting on the granular surface is negligible over the range of Re-p investigated. We calculate the critical Shields number theta(c) given by the ratio of the viscous shear stress and the normal gravitation and buoyancy stresses at the threshold of motion. We find that bed armoring leads to a systematic significant increase in theta(c) independent of the grain roughness. This observed increase is of the same order of magnitude as scatter reported in the literature when theta(c) is drawn from different data sets. While comparing similarly prepared beds with increasing particle size, we find that theta(c) decreases systematically with Re-p, in contrast with the Shields curve which is constant at low Re-p. In order to understand the condition at erosion threshold, we use the condition of torque balance at threshold to determine the critical torque needed to dislodge grains due to viscous drag. This torque is found to be significantly lower than the value needed to dislodge a spherical grain on the bed surface which is fully exposed to a linear shear flow. However, further studies of the surface packing and its evolution are needed to fully understand the observed systematic dependence on the grain size and bed preparation. (C) 2015 AIP Publishing LLC.