Conditionally Sampled Vorticities and Near-Wall Turbulence Structure

Vorticity maps of the turbulent boundary layer over the riblet surface during the near-wall burst, an event associated with a downwash of high-momentum fluid giving rise to a high wall-skin friction, were constructed using experimental data obtained in a wind tunnel at a constant adverse pressure gradient condition. They seem to support the conceptual model of near-wall turbulence activities proposed by Choi, depicting a downwash sequence close to the wall (y+ ≃ 15) in an early part of the near-wall burst, followed by an up-wash sequence away from the wall (y+ ≃ 30). The following downwash sequence was found to be shifted in spanwise direction by a half spacing of the near-wall structure, suggesting that the near-wall bursts take place in a staggered fashion.