| Cai, Liu, and Luo~[1] developed a vortex stability theory for slender conical bodies. For the flat-plate delta wing at zero sideslip, the theory is that the vortices over the wing are conical, symmetric, and stable for all angles of attack, but adding a low dorsal fin to the wing would destabilize the vortices and the vortex pair is unstable and become non-conical, or non-stationary, or asymmetric, or all of them. The flow would recover symmetry only when the fin height is increased to a critical level.This paper is to study the vortex stability problem for the flow over a flat-plate delta wing with the dorsal fin. All tests are conducted in NF-3 wind tunnel which is located at the Center for Aerodynamic Design and Research, Northwestern Polytechnical University. For the flat-plate delta wing, a software which developed by Center for Aerodynamic Design and Research is used to compute the vortices flow over the flat-plate delta wing.A sharp-edged flat-plate delta wing of 82.5 sweep angle is tested at angles of attack of 28 and 29 and zero sideslip. A smoke-laser-sheet visualization technique is used to visualize and measure the positions of the vortex pair, which are found to be symmetric and conical over the wing, for this case, the computational results is the same as the experiment and the predict theory. Then, the same tests are performed on an identical delta wing model but with a flat-plate dorsal fin mounted vertically in the incidence plane of the wing. Two heights fin models are tested. The ratios of the local fin height to the local wing semi-span are 0.75 and 1.50 respectively. Flow visualization and measurement of the vortex positions clearly indicate that the vortex pair is unstable, asymmetric and non-conical over the model with the fin height of 0.75 and recover symmetric and conicity over the model with the fin height ratio of 1.5 for the same flow conditions, providing for the first time direct experimentalevidence of the theoretical predictions. |
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