Mid-span PIV Measurements of a Low Aspect Ratio Circulation Control Wing

Abstract

The present work was undertaken to measure and analyze the external flowfield around an existing rectangular circulation control wing having a geometric aspect ratio of 0.5 and dual upper/lower surface trailing edge slots adjoining a Coanda surface. A stereoscopic particle image velocimetry system was configured in “dual-mono” mode to image a large interrogation area at the mid-span location; tests were performed in a low-speed wind tunnel at Reynolds numbers of 710,000-1,060,000. The circulation control wing was operated at momentum coefficients between 0 and 0.11 at a zero incidence angle. For pre- and postcirculation stall cases, the effects of a 5% “opposite slot assist” from the lower surface jet were studied. For zero-lift, zero-drag cases, the Reynolds number effect was studied. The results showed how the trailing edge jet augmented the velocity in the wake through upper surface entrainment and shifted the aft separation point onto the lower surface. The measured leeward flow patterns showed velocities from 25-175% of the freestream velocity. At high pre-circulation-stall momentum coefficients, both the wake and the jet were split: one jet was the primary jet effect accelerating around the Coanda surface while the other was entrained momentum from the external upper surface flow. The region of velocity deficit was also split: one between the split jets and one upstream of the stagnation streamline. At momentum coefficients above 0.07, circulation stall shifted the stagnation streamline forward from 95% to 80% of the chord length and resulted in an 8% decrease in lift and a 16% increase in drag. Stall effects were suppressed with the opposite slot assist technique. Even with the non-stalled flow conditions, the flow unsteadiness was high, with variations as high as 50% of the freestream velocity. Operating the wing at a zero-lift, zerodrag condition using the dual slots with equal jet momentum produced a well-defined wake, but the distinction was more due to differences in velocity components than overall velocity magnitude.