Active wall motions for skin-friction drag reduction

In the present study we investigate a possibility of reducing skin-friction drag in a turbulent channel flow with active wall motions. The wall is locally deformed according to two successful control strategies [J. Fluid Mech. 262, 75 (1994); J. Fluid Mech. 358, 245 (1998)]. Results show that overall 13-17% drag reductions are obtained with the active wall motions, and turbulence intensities and near-wall streamwise vortices are significantly weakened. It is remarkable that instantaneous wall shapes are elongated in the streamwise direction and resemble riblets in appearance. However, the mechanism of the present drag reduction is essentially different from that of riblets. (C) 2000 American Institute of Physics. [S1070-6631(00)51412-3].