Design and Scaling of Plasma Streamwise Vortex Generators for Flow Separation Control

The design and scaling of plasma streamwise vortex generators in a variable pressure gradient environment are presented. This involves measurements in the boundary layer on the suction surface of a V-22 wing model which provided a generic flowfield that undergoes trailing-edge flow separation as a precursor to lift stall. Experiments were performed at freestream Mach numbers from 0.1 to 0.2, which resulted in chord Reynolds numbers of 0.7 x 10(6) to 1.6 x 10(6). Different streamwise pressure gradients were examined by changing the airfoil angle of attack. Three components of the mean velocity in the boundary layer on the suction side of the wing model were measured using a five-hole pitot probe. These measurements were used to quantify the plasma streamwise vortex generator generated mean streamwise vorticity and the magnitude of the vorticity reorientation term in the vorticity transport equation that was used in the scaling analysis of the plasma streamwise vortex generator performance. The results validate the prediction of the streamwise vorticity production associated with the reorientation of the boundary-layer mean vorticity to scale as (L/delta*) (U-p/U-c) (Delta U/lambda), where L is the streamwise length of the plasma streamwise vortex generator array, U-p is the external velocity at the crest of the airfoil, U-c is the characteristic mean velocity of the flow over the plasma streamwise vortex generator array, Delta U is the change in the streamwise velocity produced by the plasma actuator, and lambda is the spanwise spacing between the plasma streamwise vortex generator electrodes. The criteria for L is that the convective timescale T-c = L/U-infinity be greater than the timescale for vorticity reorientation in the boundary layer, namely, T-c >= (partial derivative(U) over bar/partial derivative z)(-1). Both the plasma streamwise vortex generator generated. partial derivative(U) over bar/partial derivative z and. partial derivative(V) over bar/partial derivative z similar to omega(x) were found to scale with T-c. An optimally scaled plasma streamwise vortex generator array on the V-22 wing model produced as much as a 40 point drag count reduction at poststall angles of attack.