Influence of discharge energy on the lift and drag forces induced by a nanosecond-pulse-driven plasma actuator

Wind tunnel experiments at a flow velocity of 40 m s^-1 with a nanosecond-pulse-driven plasma actuator (ns-DBDPA) on an airfoil have been performed (i) to study discharge parameters inducing the ns-DBDPA flow control effect and (ii) to investigate discharge-mediating flow parameters representing the induced discharge-flow interactions. The lift and drag forces' measurements demonstrate that, in addition to the well-known frequency effect, the discharge energy per pulse can be the key discharge parameter representing the ns-DBDPA effect on the forces rather than the discharge power under various discharge energy per pulse raised up to 80 mJ m^-1 and discharge frequencies ranged from 10 to 1600 Hz. In a single pulse operation free from the discharge frequency effect, Schlieren imaging and particle image velocimetry show that the dynamic of two heated zones generated by ns-DBDPA is identical to those of the induced two vortices. This discharge-flow interaction observed under the frequency-free condition implies that the key discharge mediating flow parameter can lie in the identical dynamics of the heated zones. This study suggests that the discharge-mediating flow parameters for the discharge-flow interaction leading to the flow control effect on the forces can be a statistical variation in the Schlieren image intensity or the angles of the heated zones' trajectories.