Interaction of tip vortex and laminar separation bubble over wings with different aspect ratios under low Reynolds numbers

In present study, aerodynamics of a NACA4412 wings with aspect ratio of 1 and 3 was considered experimentally at Reynolds numbers of 2.5x10(4), 5x10(4) and 7.5x10(4). Studies for AR=1 wing showed that stall was delayed and extra (vortex) lift was obtained, because separation bubble got smaller in both chordwise and spanwise axes with effect of wing-tip vortices. Oil-flow experiments at higher angles of attack clarified the reason for vortex lift obtained from AR=1 wing. However, there was an increase in drag coefficient as well as vortex lift, and stall delayed due to tip vortex. Turbulence intensity distributions pointed out location of the transition to turbulence; Reynolds stress and turbulence kinetic energy distributions indicated shear layer. Furthermore, in experiments of AR=3 wing, the viscous forces and leading edge vortices were effective at Re=2.5x10(4) and Re=5x10(4), but flow over the wing at Re=7.5x10(4) acted as a 2D flow. After =12 degrees, bubble burst and stall consisted abruptly because effectiveness of 3D flow decreased over wing. Strouhal (St) numbers of vortex shedding frequencies in wake of AR=3 wing had a certain difference from St=0.17/sincurve at lower angle of attack (=0 degrees-10 degrees) due to separation bubble, but AR=1 wings showed that St numbers were near St=0.17/sincurve.