On the effects of lateral wall oscillations on a turbulent boundary layer

Abstract

A turbulent boundary layer modified by spanwise wall oscillations is experimentally studied in a water channel by means of hot-film anemometer and laser Doppler velocimeter. The primary goal is to confirm and extend previous experimental and numerical results concerning the modifications induced by the lateral cyclic wall motion on wall-bounded turbulent flows. A correction is applied to the hot-film data acquired in the very proximity of the wall because of the spanwise velocity component produced by the wall movement, which erroneously alters the longitudinal velocity measurements and leads to lower values of drag reduction. It is found that the excited boundary layer shows a character which is sustained in time with stationary time-averaged quantities. The mean streamwise friction at the wall and all the most relevant turbulence statistics are attenuated by the oscillation, thus confirming the oscillating wall as an effective vehicle for producing a drag reduction effect. Furthermore, the evolution of the skin-friction coefficient along the oscillating wall and its readjustment downstream of the moving section are also investigated. The length of the spatial transient from the beginning of the oscillating wall is at least double the distance downstream of the moving plate at which the turbulent flow relaxes back to its original unperturbed state. Experiments reveal that the drag reduction properties of the oscillating wall technique are not influenced by the variation of the Reynolds number, at least for the cases tested, i.e. for Re θ ⩽1400.