A re-laminarization mechanism by sustainable thin air-films for turbulent drag reduction

Reduction of hydrodynamic frictional drag through introduction of air bubbles or films at the wall region has been tried by several groups in the past. However, this method requires energy input to pump air bubbles. Previously, we have demonstrated a novel technique that can stably maintain and acoustically modulate oscillating air-films in fully turbulent boundary layer. This technique generates large drag reduction effect. We will also present a re-laminarization mechanism in the near wall region due to the oscillation of air-films. This mechanism can explain the observed large drag reduction.