Effect of low dorsal fin on the breakdown of vortices over a slender delta wing

Previous theoretical and experimental works show that the originally steady symmetric vortices over a flat-plate delta wing at zero side slip but high angles of attack are destabilized by the addition of a low dorsal fin, which renders the separation vortices asymmetric, unsteady, or non-conical. This paper examines the effect of the dorsal fin on the onset of vortex breakdown. A sharp-edged flat-plate delta wing with a 7.5 deg semi-apex angle is tested in a low-speed wind tunnel. The unsteady velocity and vorticity fields are mapped out at the 60% wing root chord location to quantify the vortex burst process caused by the addition of the low dorsal fins by using the laser Particle-Image-Visualization (PIV) technique. Two fin heights with the ratio of the local fin height to the local wing semi-span 0.3 and 0.6 are tested. The results demonstrate that the loss of global stability of the vortex configuration due to the addition of the low dorsal fin accelerates the onset of vortex breakdown and causes one of the vortices in the pair to burst periodically. The frequency and size of the burst are related to the height ratio of the fin.