Coherent structures in wall turbulence and mechanism for drag reduction control

Herein is introduced the mechanism for active control influencing the generation of the near-wall streamwise vortices, which are closely related to the production of high skin friction in wall-bounded turbulent flows. A new opposition control scheme with adjusting control amplitude is proposed and evaluated in turbulent channel flow by direct numerical simulations. The maximum drag reduction rate can be greatly enhanced by the strengthened control. Finally the effectiveness of the control to the coherent structures at high Reynolds numbers is investigated by using a linear transient growth model.