An Experimental Investigation of Perturbations on Vortex Breakdown over Delta Wings

摘要

An experimental investigation on vortex breakdown on delta wings at high angles of attack is presented. As suggested by previous works, perturbations are used to change the platform of the delta wings to reduce the detrimental effects of vortex breakdown brought about by the Self-Induction Theory. Different patterns of ‘round’ perturbations are tested to obtain the favourable lift and drag characteristics for each wing. With the best pattern identified later, optimization of the shape of perturbation is explored to further improve the results. ‘Teardrop’ and ‘diamond’ perturbations are introduced as basis of comparison. Force measurements were conducted over a range of α = 0 to 40° to justify the concept of surface shaping and evaluate its effectiveness. Dye flow visualization were used to obtain sectional views of the leading-edge vortices as they break down for a series of delta wings having sweep angles of 60°, 65° and 70°. The wings are tested constantly at a low speed of U∞ = 0.05 m/s in a water tunnel facility. A combination of side and plan views provides information on the three-dimensional nature of the vortex structure before, during and after breakdown. Details of the flow at α =15° for every wing are identified in still photographs while the dynamic characteristics of the breakdown process are examined from recorded high-speed movies. The force measurement supported by the flow visualization shows that certain combinations of perturbations indeed provide encouraging results. For wings with perturbations, generally, the vortex structure transforms from a linear structure to a wavy or “kink” structure which effectively delay or even suppress vortex breakdown. Various results have shown an increase of approximately 10% in lift characteristics and delay of stall angle for certain scenarios. The best results have been for the 60° wing where the ‘teardrop’ bulge in a mild perturbation pattern managed to improve lift characteristics by about 15% over the whole range of angle of attack for the tests. Results for the 65° wing and 70° wing are generally positive with the ‘teardrop’ perturbation again providing the best results, however with existence of discrepancies over certain angles of attack.