Roll-modes generated in turbulent boundary layers with passive surface modifications

Abstract

A unique class of directional surfaces arranged in a converging-diverging (herringbone) pattern are studied experimentally in a zero pressure gradient turbulent boundary layers. Hot-wire measurements using both single and cross-wire show that these small surfaces are able to generate large-scale counter rotating roll-modes/vortices within the turbulent boundary layer, resulting in dramatic spanwise variation in the boundary layer thickness δ (50% variation for the strongest case). The results reveal that above the converging region, the local mean velocity decreases while the turbulence intensity increases, resulting in locally thicker boundary layer. Over the diverging region, the opposite situation occurs, where the mean velocity increases and the turbulence intensity decreases, resulting in a locally thinner boundary layer. The strong perturbation effect from these surfaces to the overall flow dynamics seems unusual, considering that their peak-to-trough height is approximately only 1% of the boundary layer thickness. This study, also investigates the behavior of the large-scale counter-rotating roll-modes when the surface reverts from the herringbone pattern back to the smooth wall, to see how far they persist over the smooth wall. Our preliminary results show that the roll-modes above the smooth wall still persist even at 40δ downstream. The results of this study show that the herringbone surface roughness pattern can act as a novel method of generating counter rotating roll-modes (vortices) for flow control purposes in various engineering applications.