Experimental Investigations on Stability of Vortex Flow over Slender Delta Wing with Dorsal Fin

Abstract

A vortex stability theory for slender conical bodies developed by Cai, Liu, and Luo predicted that vortices over a flat-plate delta wing at high angles of attack with zero sideslip are conical, symmetric, and stable but adding a low dorsal fin to the wing would destabilize the vortices and therefore render the originally symmetric vortices asymmetric and/or nonconical. To verify the validity of the theoretical predictions and show the behaviors of the vortices over the slender delta wing and its combinations when the flowfield-asymmetry onset occurs, a 2-D Particle Image Velocimetry (PIV) study is presented in this paper. A sharp-edged flat-plate delta wing of 82.5 sweep angle is tested in a low-speed wind tunnel at angle of attack of 35 and zero sideslip. The PIV technique is used to visualize and measure the distributions of the vorticity component normal to the cross-flow plane at 60% wing root chord. The same test is performed on an identical delta wing model but with a flat plate dorsal fin mounted vertically in the incidence plane of the wing. Two fin heights are tested， the ratios of the local fin height to the local wing semi-span are 0.3 and 0.6 respectively. The PIV results indicate that the vortex pair is symmetric and steady over the wing, but vortex breakdown occurs periodically when adding the low dorsal fin. The results provide direct experimental evidence of the theoretical predictions.