Effect of Dorsal Fin on Symmetry of Vortices over Slender Delta Wing

Abstract

Three different sets of experiments are designed to reveal the behavior of the vortex flow over a delta wing with different fin heights. A sharp-edged flat-plate delta wing with an 82.6-deg-sweep angle is tested in low-speed wind tunnels at angles of attack up to 35 deg. The same tests are performed on an identical delta-wing model but with a flat-plate dorsal fin. The ratio of the local fin height to the local wing semispan varies from 0.3 to 1.5. The results of smoke laser-sheet visualization, particle image velocimetry, and force measurement are obtained and presented. The test results show that the vortices over the delta-wing-alone model at high angles of attack and zero sideslip are stable, symmetric, and steady before vortex breakdown, but a low dorsal fin renders the originally symmetric vortices asymmetric at the same angle of attack; the flow recovers symmetry and conicity when a higher fin is added. These results are in agreement with predictions by the stability theoryCaiet al., (“Stability of Symmetric Vortices in Two Dimensions and over Three-Dimensional Slender Conical Bodies,” Journal of Fluid Mechanics, Vol. null, 2003, pp. 65–94). This agreement demonstrates that the absolute type of instability is responsible for loss of symmetry of the vortices over a delta wing with a fin within the range of tested conditions.