Comparison between Localized Skin-Friction Reduction and Suction on a Supercritical Airfoil

In this work, direct numerical simulations are used to investigate which physical effects of active flow control are most relevant for improving the aerodynamic performance of airfoils. To this end, two active flow control strategies based on inherently different physi-cal mechanisms are systematically compared on the upper surface of the supercritical V2C airfoil in compressible transonic flow. The first strategy consists of streamwise-travelling waves of spanwise wall velocity (StTW), a skin-friction drag reduction technique that locally decreases wall shear stress in the region of application. The second strategy is uniform wall-normal suction, which locally increases skin-friction drag. One strength of the present study is that the control configurations are evaluated at matched lift coef-ficients to isolate the impact of the control mechanisms on drag components, boundary-layer development, shock position, and energetic cost. Despite their opposite local effects on skin-friction drag, both control strategies yield comparable and significant improve-ments in global aerodynamic efficiency. The analysis reveals that, for both approaches, the dominant mechanism responsible for the efficiency increase is a downstream displacement of the shock, accompanied by a reduction in boundary-layer momentum thickness. These results highlight the central role of shock-boundary-layer interaction control in enhanc-ing transonic aerodynamic performance and demonstrate that skin-friction drag reduc-tion alone is not a sufficient metric for assessing the effectiveness of active flow control strategies.