Turbulent Boundary Layer Development over Flexible Roughness Patches*

Flexible roughness patches, in the form of micropillar arrays inspired by hair-like sensors found in various arthropods and aquatic species, were used to perturb a zero-pressure gradient turbulent boundary layer. Particle image velocimetry (PIV) measurements over a streamwise-wall-normal plane were carried out at a nominal friction Reynolds number of approximately 2400. The measurement window was centered over each flexible roughness patch and encompassed the length of the patch (≈1.5δ₉₉) in the streamwise direction. The flow over several patches with varying micropillar stiffness and density was compared. The flow over a smooth wall was also used as a reference for comparison. Additionally, hot-wire measurements were carried out over the patches and downstream of the trailing edge of the patches. All arrays showed a shift in the near-wall peak away from the wall and above the micropillars. Generally, the near-wall turbulence intensity increased near the leading edge of the patch due to the abrupt introduction of the patch into the smooth wall flow. This near-wall turbulence intensity then decreased past the leading edge and gradually increased again towards the trailing edge. It was determined that the magnitude of these changes in the near-wall turbulence depended primarily on the spanwise and streamwise spacing. Downstream of the trailing edge, the increase in the near-wall peak observed over the patch shifted into the log region.