Turbulent drag reduction by traveling wave of flexible wall

A direct numerical simulation of channel turbulent flow and its control by spanwise traveling waves of flexible wall shows that such an on-wall open-loop vorticity creation control can effectively reduce the friction drag for any incompressible Newtonian fluid. The control effect is similar to that of existing transverse-wave controls by spanwise near-wall oscillating flow/wall oscillation and traveling body-force wave. The main physical mechanism is the change of boundary vorticity flux due to wall acceleration, which forces the streamwise vorticity generated at the wall to be confined in a thin generalized Stokes layer and hence alleviates the low-speed streaks, suppresses the streamwise vortices and hairpin vortices in viscous sublayer, associated with a regularized wall skin-friction pattern. A new potential is revealed toward reducing the power input in open-loop control by waves, which is worth further exploring: the optimization of imposed wave pattern.