Control of transonic buffet through deployable flaps

We carry out a series of numerical simulations of transonic flow over a supercritical airfoil at M=0.7, incidence angle alpha=7°, and chord Reynolds number Rec=300000, with the aim of suppressing the large-scale shock oscillations (buffet) which are expected to occur under these conditions. For that purpose we use a deployable flap, which is placed in the aft part of the airfoil suction side. The main rationale is to mimic the behavior of shock holders, which are frequently used in experiments to stabilize normal shock waves. For preliminary simulations, the flap has been shaped as a bi-convex airfoil. Two sample configurations have been considered, namely a HIGH configuration, with flap chord cf=0.2c, relative thickness of 6%, and distance of about 0.15c from the airfoil, and a LOW configuration, with flap chord cf=0.1c, relative thickness of 12%, and distance of about 0.075c from the airfoil. The main result is that both the HIGH and the LOW configuration are capable of effectively suppressing buffet, after an initial transient. Both types of control have the main effect of pushing the main shock upstream than its time-average uncontrolled location. However, whereas LOW actuation has the effect of slightly extending the region of flow reversal past the airfoil trailing edge, HIGH actuation is effective in reducing its size. Accompanied with HIGH actuation also comes some small flow separation past the flap, whereas in the LOW case the flap is embedded in a low-speed region, and the flow does not separate. The practical implications of control are observed in the time history of the aerodynamic efficiency. While the efficiency levels off in both cases, LOW actuation retains the (time-average) efficiency of the uncontrolled case, whereas HIGH actuation is responsible for additional losses. A series of numerical simulations covering a wide range of control parameters is currently ongoing, a full account of which will be given in the final paper.