Structure of turbulent boundary layer on an oscillating flat plate

An oscillating turbulent boudnary layer was investigated experimentally in a blow-down wind tunnel in which a flat plate was driven sinusoidally parallel to the constant free stream. In the transition regime, two-dimensional turbulent patches appeared periodically in space and time, the velocity in the outer part of the patch being lower than that of the surrounding nonturbulent flow but significantly higher than the velocity near the wall. In the fully turbulent region, the mean velocity and turbulent profiles did not deviate much from the corresponding steady case. The phase-averaged profiles, however, depended weakly on the instantaneous flow conditions. The phase of the turbulence intensity variation coincided with that of the velocity change near the wall but delayed gradually outward to more than 180 deg at the outer edge of the boundary layer.