Decrease of streamwise mean velocity of single spheres due to presence of a sparse network of immobile bed-mounted particles

Abstract

The transport of spheres as a bed-load through a sparse network of immobile, bed-mounted submerged cylinders was experimentally studied to quantify how the mean velocity of the spheres in the streamwise direction is altered, with respect to transport in a bare bed. Two regimes of transport were observed: (1) a tracer-type motion regime in which the spheres are able to circumvent the cylinders with few collisions between them and (2) a collisional regime in which impacts play a key role in predicting the experimental results. It was quantified as the critical value of a dimensionless parameter, below which the first regime takes place, whereas the second one triggers above this value. This parameter plays a central role in a one-dimensional (1D) model, which includes a dissipative force based on the average contact work between moving and motionless particles. At a critical flow velocity, all the spheres stopped, remaining motionless within the network, suggesting the appearance of a threshold for the onset of the spheres motion due to the network.