Airfoil Aerodynamic Drag Reduction using Controlled Trapped Vorticity Concentrations

The aerodynamic performance of an airfoil at low angles of attack (with a fully attached base flow) is improved using fluidic modification of its “apparent” shape by superposition of controlled trapped vorticity concentrations near the surface. In the present wind tunnel investigations, a controlled trapped vorticity concentration is formed on the pressure surface of a NACA 4415 airfoil at x/c = 0.2 using a hybrid flow control actuator consisting of a passive obstruction of scale O(0.01c) with an integral synthetic jet actuator. The actuation frequency [St_act ~ O(10)] is selected to be at least an order of magnitude higher than the characteristic unstable frequencies of the airfoil wake, thereby decoupling the actuation from the global instabilities of the base flow. Jet actuation is used to regulate vorticity accumulation, shedding, and advection in the vicinity of the actuator with minimal changes in skin friction, as shown by detailed PIV measurements. In turn, actuation causes the local static pressure to be altered, leading to a significant reduction in drag with minimal lift penalty (owing to the sense of the trapped vorticity). For example, at α = 4° and Re = 6.1·10⁵, the drag is reduced by 30% of the airfoil profile drag while the lift is reduced by 2%.