Flow control experiments were performed on a NACA 0015 airfoil in fully-reversed condition, which is anticipated to occur on the retreating blade side of advanced helicopters such as slowed-rotor compound rotorcraft. Control was achieved using nanosecond dielectric barrier discharge (NS-DBD) plasma actuators. The Reynolds number based on a chord length of 203 mm was fixed at 5.0 · 105, corresponding to a freestream velocity of ~38 m/s. Two angles of attack were considered: α = 0° and 15°, each of which is relevant to a particular implementation of slowed-rotor technology.
At α = 0°, the flow resembles that of a flow behind a cylinder. A von Karman vortex street formed in the wake where alternating vortex shedding occurred at a Strouhal number of 0.12. Excitation was performed using an NS-DBD on one side of the airfoil, with plasma formation just upstream of the separation line. However, there was no discernible influence upon the baseline behavior.
At α = 15°, fully separated flow on the suction side extended well beyond the airfoil with naturally shed vortices at a Strouhal number of 0.19. Plasma actuation was evaluated at both the aerodynamic leading-edge (ALE) and aerodynamic trailing-edge (ATE) of the airfoil. The flow responded to the plasma actuation at the ALE by generating organized coherent structures in the shear layer over the separated region. Moderate excitation around the natural shedding Strouhal number had the most significant effects: synchronizing the shedding from the ALE and ATE, creating moderately sized structures that convected far downstream, greatly reducing the separation area, increasing lift, and decreasing drag. Excitation at much higher Strouhal numbers resulted in the flow returning to its natural shedding state, but with less coherent structures that diffused in the wake. This reduced the separation area and significantly reduced drag. Plasma actuation at the ATE caused a reduction in the magnitude of the fundamental and harmonic peaks in pressure spectra over a broad range of excitation Strouhal numbers. Excitation at the ATE altered the structures over the separated region, suggesting an upstream communication. At excitation frequencies higher than the natural shedding frequency, the natural shedding process was disrupted, weakening the naturally shed structures in the wake. Synchronous excitation at the ALE and ATE was predominantly characterized by the associated ALE excitation. Two cases were found where ATE excitation in addition to ALE excitation had a significant effect, but in those two cases, the flow shared characteristics of individual excitation at the ALE and ATE. The resultant flow was somewhere between the two independent excitations. With asynchronous excitation, the addition of ATE excitation counteracted the lift benefits of ALE excitation. As the ATE excitation increased, the amount of lift decreased. The effect on drag was minimal, suggesting that ALE excitation has a much more significant influence on drag than ATE excitation, even at high ATE frequencies.