Leading Edge Separation Control on an Airfoil in Fully-Reversed Condition

Abstract

Leading edge separation control was investigated using nanosecond pulsed DBD plasma actuators on a NACA 0015 airfoil installed in a recirculating wind tunnel such that the flow was fully-reversed over the airfoil. A 15° angle of attack at a Reynolds number of 0.50·10 was selected for detailed investigation. Fully separated flow on the suction side extended well beyond the airfoil with highly asymmetric velocity and vorticity fields and a shedding Strouhal number of 0.19 with a harmonic at 0.38. Excitation at very low (impulse excitation) to moderate (~0.4) Strouhal numbers using plasma actuators at the aerodynamic leading edge generated organized coherent structures in the shear layer over the separated region with a frequency corresponding to the excitation frequency, which caused changes in the size of the wake, the separation area, lift, and drag. Excitation near = . had the most significant effects: creating moderately sized structures that convected far downstream, reducing the separation area by 39%, increasing lift by 6%, and decreasing drag by 8%. Excitation at high Strouhal numbers modified the baseline vortices (rather than generating new vortices) and resulted in weaker vortices that diffused quickly in the wake, causing the wake to elongate slightly and skew toward the aerodynamic trailing edge, but still reducing the separation area and also reducing lift and drag. The primary mechanism of control is excitation of instabilities in the shear layer over the separated zone that generate more coherent large-scale structures over a range of excitation frequencies increasing their entrainment abilities to bring high-momentum fluid into the separation region to reduce the separation size and increase the lift.