The streamwise drag-reduction response of a boundary layer subjected to a sudden imposition of transverse oscillatory wall motion

A direct numerical simulation study is presented, which examines the response of a spatially developing boundary layer to oscillatory spanwise wall motion imposed over a limited streamwise stretch. At the heart of the study is the dependence of the streamwise variations in skin friction and turbulence properties on the period of the oscillatory motion, with particular emphasis placed on the behaviour downstream of the start of the actuation. The friction Reynolds number just upstream of the actuation is Reτ = 520, and the wall-scaled actuation period, T+ = Tuτ2/ν, covers the range 80–200. In contrast to channel flow, the present configuration allows the processes during the transition stretch from the unactuated state to the low-drag state and the recovery from the low-drag state to be studied. Attention focuses primarily on the former. Results are included for the time-averaged turbulent stresses, their budgets and probability-density functions, as well as a range of phase-averaged properties. The study ...