  Flow control for turbulent skin-friction drag reduction is applied to a
transonic airfoil to improve its aerodynamic performance. The study is based on
direct numerical simulations (with up to 1.8 billions cells) of the
compressible turbulent flow around a supercritical airfoil, at Reynolds and
Mach numbers of $Re_\infty= 3 \times 10^5$ and $M_\infty =0.7$. Control via
spanwise forcing is applied over a fraction of the suction side of the airfoil.
Besides locally reducing friction, the control modifies the shock wave and
significantly improves the aerodynamic efficiency of the airfoil by increasing
lift and decreasing drag. Hence, the airfoil can achieve the required lift at a
lower angle of attack and with a lower drag. Estimates at the aircraft level
indicate that substantial savings are possible; when control is active, its
energy cost becomes negligible thanks to the small application area. We suggest
that skin-friction drag reduction should be considered not only as a goal, but
also as a tool to improve the global aerodynamics of complex flows.
