Onset of motion of a partly hidden cylinder in a laminar shear flow

In this paper, the onset of motion of an isolated cylinder partially exposed to a shear flow is experimentally investigated. The experiments are performed in a small narrow channel which provides a vertical shear layer flow whose sizes correspond with the channel width. The bottom of the channel is smooth except in the test zone, at long distance from the inlet, where the cylinder is placed with its principal axis perpendicularly directed to the main flow. The geometry of the channel bottom at the test zone is such that the cylinder is partially buried and presents different expositions to the incident flow. In this way, the geometrical constraints imposed by the sediment bed on a single particle in a natural sediment transport situation are reproduced in an idealized context. The results are interpreted in terms of the relation between the particle mobility parameter at the critical condition and the here defined particle burial degree with respect to the bed geometrical constraints beta. We emphasize the role played by this burial degree that is dependent on the particle exposure to the incident flow E and the resistance to the motion by mechanical contacts with its surroundings given by the so-called static pivot angle phi.