Spatial evolution of the wake of discrete roughness elements in a turbulent boundary layer at a moderate Reynolds number

In this study, the effect of cylindrical discrete roughness elements on multiscale turbulent motions in a canonical turbulent boundary layer was explored. Flow fields with a considerably large field of view in streamwise—wall-normal planes—were measured by two-dimensional particle image velocimetry. It was found that large-scale motions and very-large-scale motions in the outer layer of the middle-to-far-wake regime are enhanced due to the bottom-up process of roughness-introduced disturbance, where vortical structures shedding from the roughness elements lift up to higher flow layers as they convect downstream, thus promoting the cross-layer interaction. The modulation effect was found to extend to the lower bound of the outer region and persist longer than traditionally believed. An interesting finding is that in the near-wall region of the far-wake regime, small-to-moderate-scale components of Reynolds shear stress is suppressed, which may be associated with the inhibition of the generation of near-wall prograde vortical structures. This study offers a new perspective for turbulent control by passive discrete elements.