An joint experimental and numerical investigation was performed to study the formation of secondary flow structures and interactions of a finite-span synthetic jet with a cross-flow over an airfoil at chord-based Reynolds numbers between 50,000 and 400,000 and angles of attack from 0 to 20 degrees. Six blowing ratios, in the range of 0.2 to 1.2 with an increment of 0.2 (where, the blowing ratio is defined based on the averaged outstroke jet velocity to the free-stream velocity) were tested.
Experiments and CFD were conducted on a finite wing (aspect ratio of 5.33) with a cross-sectional profile of NACA 4421. The experimental investigation included, both 2-D and stereoscopic PIV data at the center jet in the mid-span. The effect of the blowing ratio was analyzed based on 3-D time-averaged, phase-averaged and instantaneous flow fields at an angle of attack of 0o and a chord Reynolds number of 100,000.
For the low blowing ratio cases, spatial non-uniformities developed, due to the finite-span of the slit, which led to the formation of small and organized secondary structures or a streak-like pattern in the mean flow. On the other hand, for the high blowing ratio range turbulent vortical structures were dominant leading to larger spanwise structures, with a larger spanwise wavelength, in the mean flow. Moreover, the phase-locked flow fields exhibited a train of counter-rotating coherent vortices that lifted-off the surface as they advected downstream. In the mid-blowing ratio range, combined features of the low-range (near the slit) and high-range (in downstream locations) were found; where a pair of counter-rotating vortices issued in the same jet cycle collided with each other. In all cases the spanwise extent of the secondary coherent structures reduced with downstream distance with a larger decrease at higher blowing ratios. Similar observations were made in earlier studies on finite-span synthetic jets in quiescent conditions.