The nature of rough-wall oscillatory boundary layers

Abstract

The nature of an oscillatory (turbulent) boundary layer over a rough wall was studied experimentally by oscillating a flat plate in an otherwise quiescent fluid. The velocity field was measured with respect to a fixed (laboratory) coordinate system and was converted to that relative to a coordinate system fixed to the plate, thus allowing to make inferences on oscillatory flows over rough surfaces such as the coastal ocean boundary layer. The flow visualization revealed that the boundary layer is fed with dipole-like vortex structures generated due to flow separation at roughness elements. The boundary-layer thickness was found to scale with the extent to which these vortex structures travel away from the wall. Enhanced turbulent intensities as well as vertical fluxes of horizontal momentum were observed in the boundary layer at phases conducive for vorticity generation (i.e. in (he proximity of maximum flow velocities). At high Reynolds numbers, the turbulent length-scale measurements agreed well with a parameterization proposed by Grant and Madsen (1979). Eddy diffusivities based on Reynolds and total stresses, however, did not agree with available models and showed wide variability over an oscillating cycle.