Drag Reduction on a Transonic Wing

Flow control for reducing skin-friction drag in the turbulent regime is applied to a transonic airfoil to improve its global aerodynamic performance. The study relies on direct numerical simulations of the compressible turbulent flow around a supercritical airfoil at Reynolds and Mach numbers of Re∞ = 3 × 105 and M∞ = 0.7. The control is applied on a portion of the airfoil suction side only. However, besides locally reducing drag, the control modifies the position and the intensity of the shock wave and significantly improves the aerodynamic efficiency of the airfoil by increasing the lift and slightly decreasing the total drag. The increase of the aerodynamic efficiency implies that the airfoil can achieve the desired lift at a lower angle of attack and, therefore, with a much lower drag. Estimates of the benefits on a full aircraft are provided and indicate that substantial savings are possible, even when the energy cost of an active control is considered. These results suggest that skin-friction drag reduction should no be considered as a goal only, but also as tool to modify and control the global aerodynamics of complex flows.