Large Angle Of Attack Of A Slender Cone Detached Vortex Mechanism

Symmetric separation vortices over slender bodies may become asymmetric as the angle of attack is increased beyond a certain value, causing asymmetric forces even atsymmetric flight conditions. The purpose of the present paper is to investigate the characteristic feature of the aerodynamics of a nose cone at a high angle of attack and study the vortex stability problem for the flow over a flat-plate delta wing with the dorsal fin.Experiment one reports results from a pressure measurements of a circular cone-cylinder model in a low-speed wind tunnel. The semi-apex angle of the cone is 10°. The results consist of detailed pressure distributions over nine stations on the cone at 35 ° and 0 °～35 ° angle of attack. The Reynolds number based on the base diameter of the circular cone is 0.8 million and 0.32 million. The tests encompassed a complete coverage of roll angles in 9 ° interval. The local and overall forces and moments are calculated from the measure pressure. The results indicate that at angle of attack of 35°, the local side-force versus roll angle is nearly a square-wave curve. The period and phase of the wave is about the same for all measure stations. Pressure distributions and the sign change of the side-force indicates that the separation vortex pair is bi-stable and the instability is of absolute type. The direction is formed at the apex of the cone and maintained to the base of the cone. It is found for the first time that a characteristic pressure distribution is associated to local side force (not limited to the maximum local-side force),which is independent of roll angle. At angles of attack of 0°～35 °, as the angle of attack is increased, the local side force coefficient versus roll angle develops into three stages: zero curve, continuous-wave curve and square-wave curve. The period and phase of the wave curves are the same for all instrumented stations at all angles of attack. The state of the boundary layer is deduced from the measured pressure. The cause of the wave variation of the local side force versus roll angle is indentified as instability of the vortex flow separated from the cone surface. The results indicate that mechanism is inviscid in origin.The second experimental study was performed on a flat-plate delta wing of 82.5 degrees sweep angle and combinations of the identical delta wing with low dorsal fins. The test conducted at the low-speed wind tunnel using a six-component internal strain-gage balance at 12~32 degrees angles of attack. Two fins of different heights were tested, the ratios of the local fin height to the local wing semi-span were 0.3 and 0.6 respectively. There were two Reynolds numbers, 1.66 million and 2.33 million. The measurement of the aerodynamic forces and moments clearly indicates that no lateral force occurs over wing alone model at zero sideslip at lower angles; but a steady force-asymmetry occurs over the wing-fin models at certain angles, the vortex flow will become unsteady at higher angles of attack. The results provide for the first time force measurement evidence of the validity of the vortex stability theory of Cai, Liu, and Luo (J. Fluid Mech., 2003, 480:65-94) and depict the flow behavior after the onset of the vortex instability with increasing angles of attack.