OSCILLATORY LAMINAR FLOW ABOVE A ROUGH BED

Abstract

Measurements are reported of the velocity distribution in the laminar boundary layer above a rough bed oscillating in its own plane in still air. The measurements were made with a hot-wire anemometer and the beds consisted of spheres packed closely together on flat plates. Particular attention was paid to the jets of fluid associated with individual roughness elements reported by Keiller & Sleath (1976). The probe used by Keiller & Sleath could be traversed vertically but not parallel to the bed. Consequently it is not known whether or not the velocities which they measured represent the maximum jet velocity at a given height since the probe was not necessarily in the vicinity of the jet at the moment at which it was maximum. In these new tests the probe could be traversed horizontally as well as vertically. It is found that one jet is formed by each roughness element during each half cycle. This jet is directed upwards from the bed. Its intensity shows relatively little variation with Reynolds number for the range of conditions investigated but increases significantly with the relative roughness parameter BD. The moment in the half cycle at which the jet is formed and the location of its axis appear to vary with both gD and Reynolds number. Visual observations suggest that the jet is associated with incipient vortex formation. The variation in velocity with height in the jets of fluid is significantly different from that given by Stokes' theoretical solution for a flat bed. Except in the immediate vicinity of the bed, the jet velocity predominates over that during the rest of the cycle.