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

摘要

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 theory Cai et al., ("Stability of Symmetric Vortices in Two Dimensions and over Three-Dimensional Slender Conical Bodies," Journal of Fluid Mechanics, Vol. 480, 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.