Composite drag control and energy flux analysis for wall turbulence

Drag reduction in wall flows is of both fundamental and engineering interest. Here, we review the recent developments in the drag reduction strategy and the underlying mechanism. First, the framework for energy flux analysis of drag control is reviewed, which builds up a quantitative relation between the skin-friction coefficient and the dissipation rate. The framework illustrates how the flux of pumping power and control power is distributed and dissipated by coherent, random, and mean fluid motions, applicable for complex control methods as well. Moreover, a specific large-scale control strategy by spanwise opposed wall-jet forcing is introduced, which reduces near-wall random turbulence intensities by merging velocity streaks together and hence suppressing the generation of streamwise vortices. By injecting control power from coherent motions, the spanwise forcing method achieves a maximum drag reduction rate of approximately 19% in the channel flow at a friction Reynolds number of 180. Furthermore, a composite control method combining large-scale control and opposed wall blowing/suction together, yields approximately 33% drag reduction and 32% net power saving at the same Reynolds number, both higher than that of the individual control method. Finally, we show that applying the large-scale control over riblets, the control efficacy is much higher than that of the riblets alone, hence demonstrating the robustness of the large-scale control method.