Flow Control over an Airfoil in Fully Reversed Condition Using Plasma Actuators

Flow control experiments were performed using nanosecond dielectric-barrier-discharge plasma actuators on a NACA 0015 airfoil with flow approaching from the geometric trailing-edge side, which is a condition anticipated to occur on the retreating blade side of advanced helicopters such as slowed-rotor compound rotorcraft. This symmetric airfoil, which is not typical of those used in rotorcraft blades, was used for simplification of an otherwise very complex problem. The Reynolds number based on the chord length was fixed at 0.50·106, corresponding to a freestream flow of approximately 38 m/s. An angle of attack of 15 deg was used. Fully separated flow on the suction side extended well beyond the airfoil with naturally shed vortices occurring at a Strouhal number of 0.19. Plasma actuation was evaluated at both the aerodynamic leading and trailing edges of the airfoil. Excitation at very low (impulse excitation) to moderate (∼0.4) Strouhal numbers at the aerodynamic leading edge generated organized coherent structures in the shear layer over the separated region with a shedding Strouhal number corresponding to that of the excitation, which caused changes in the size of the wake, the separation area, lift, and drag. Excitation at higher Strouhal numbers resulted in weaker naturally shed vortices (rather than generating new vortices) that diffused quickly in the wake. The excitation caused the wake to elongate slightly and skew toward the aerodynamic trailing edge, but it still reduced the separation area and significantly reduced drag. The primary mechanism of control at the aerodynamic leading edge is excitation of instabilities associated with the leading-edge vortices; the excitation generates 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. On the other hand, excitation over a broad range of frequencies at the aerodynamic trailing edge was found to significantly reduce organization of the naturally shed large-scale wake structures.