Wall stagnating flow near the wall oscillated parallel to the flow

The velocity variation induced by the sinusoidal wall oscillation is measured along the stagnating streamline and its amplitude is estimated. The mechanisms of the flow velocity oscillation are discussed in terms of the amplitude of the flow velocity oscillation when the angular frequency of the wall oscillation w, the average distance between the nozzle and the wall ȳ. and the flow velocity at the nozzle exit U_o are varied. The characteristics of the flow velocity oscillation are classified into three regions by a Strouhal number S=w|ȳ_w/U_o. When S<2, the flow velocity variation follows the change of the wall location in a quasi-steady state. On the other hand, when S>6, it is dominated by the change of the wall velocity. In between, 2<S<6, the transition region is observed.