Spatial Variability in the Motion of a Rolling Bead due to a Step in Bed Roughness

The impact of the sudden change in bed roughness due to a cluster of motionless, submerged bed-mounted posts on the dynamics of a coarse sphere transported by the turbulent flow as bed load was experimentally and analytically studied. The rolling motion of the bead upstream of the cluster, in almost straight paths at a virtually constant velocity, switched abruptly to a spatially varied, highly perturbed motion as soon as the particle moved through the cluster interstices, following tortuous path and performing cycles of strong decelerations (due to collisions) and accelerations (due to the drag force). Analysis of measurements in terms of a simplified initial value problem showed that the slowdown of the mean velocity of the sphere depends on the mean flow velocity in the rough layer, the geometrical constraint imposed by the mean interstitial length, the features of the collisional interaction, and the bead diameter and density.