The nature of rough-wall steady, oscillatory and combined boundary layers

Oscillatory (turbulent) boundary layers over a rough wall were studied experimentally by oscillating a flat bottom 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 wall. The flow visualization revealed that the boundary layer is replete 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 at phases conducive for vorticity generation, that is in the proximity of maximum flow velocities. At high Reynolds numbers, the integral length-scales agreed well with the existing theoretical predictions. Eddy diffusivities based on the Reynolds and total stresses, however, did not agree with available models and showed wide variability over an oscillating cycle.; The turbulent wave-current boundary layers were studied using the long tank in the test section with the sinusoidally oscillating bottom. The turbulent steady current was achieved by water circulation throughout the long tank and the wave component was produced by oscillating bottom. The studies included determination of the boundary layer thickness, velocity profiles, turbulent kinetic energy, shear stresses, eddy viscosity and integral length-scales. It has been found that the assumption about the existence of the wave sublayer within a wave-current layer has a strong physical foundation based on the consistent behavior of various turbulent flow characteristics. The fit of mean velocity to the logarithmic profile also confirmed the existence of the wave sublayer and was used to obtain the “apparent roughness.” The friction velocity changes very consistently for all experiments and its value strongly depends on the ratio of mean and amplitude velocity of oscillation. The friction factor was determined to increase linearly with $Um/Aw$. Eddy viscosities based on mean and phase-averaged equations of motion, change linearly in the wave sublayer, but eddy viscosity based on phase-averaged equations of motion takes negative values. The integral length-scales were determined to be much larger than in the case of wave boundary layer, due to the strong influence of current motion that imposes large scales. The ratio of integral length-scales based on horizontal and vertical velocity components was determined to be constant with respect to Reynolds number of oscillations.