Spatio-temporally periodic control for turbulent friction drag reduction

We evaluate the pre-determined control with temporally- and spatially-periodic spanwise velocity inputs at the wall in a fully developed two-dimensional turbulent channel flow. The spatially-periodic control generally achieves better performance than the temporally-periodic control, the latter of which is called conventionally the spanwise wall-oscillation control. To analyze the drag reducing mechanisms, we focus on the near-wall structures such as quasi-streamwise vortices and low-speed streaks. As a result, it is found that the drag is reduced through the modified spatial phase-relationship between the streamwise vortex and the low-speed streak.