Skin-friction drag reduction using synergy of active controls

Synergy of two active controls, namely, streamwise traveling waves of spanwise velocity (STWSV) and opposition control (OC), is studied to maximize skin-friction drag reduction. Several cases of upstream- and downstream-traveling STWSV with OC are simulated for subsonic (at bulk Mach number Mab = 0.8) and supersonic (Mab = 1.5) turbulent channel flows. The synergy control implies that for certain parameters, the drag reduction capability over the solitary control can be strongly enhanced, and much higher skin-friction drag reduction can be achieved. The results suggest that the direction of the traveling waves strongly affects the overall drag reduction by the synergy. The combination of downstream-traveling STWSV with OC has a favorable interaction and enhances the performance of the individual control. A high skin-friction drag reduction of 44% with a net power saving of 14.2% is achieved for a particular set of phase speed and detection plane. This enhanced drag reduction is due to the suppression of the Reynolds stresses with weakened near-wall coherent structures in the flow field. The analysis of the Reynolds stresses τij profiles suggests a significant fall of all the stress components as well as the production Pk+ of the turbulent kinetic energy for the downstream-traveling synergy control case. The vortical structures and weakened velocity streaks corroborate the drag reduction and are suppressed significantly. The Fukagata, Iwamoto, and Kasagi decomposition of the skin-friction coefficient reveals the efficacy of the control in reducing the near-wall turbulence.