Modification of a wing tip vortex by vortex generators

The vortex wake hazard associated with transport aircraft is well known. One strategy for its alleviation is to alter the near field wake topology to distribute circulation over an increased area and reduce the consequent rolling moment on a following aircraft encountering the wake. The paper describes the results of an experimental investigation of the use of supplementary vortex generators to modify the tip vortex shed from a half-span airfoil. The experiments were conducted in a small low speed wind tunnel at a chord Reynolds number of 2.2×105 and the flow was characterised by measurement of the instantaneous and mean cross-stream flow field using particle image velocimetry. The results show that when the vortex generator is configured to produce a vortex that is co-rotating with respect to the wing-tip vortex, the two merge within five chord-lengths of the trailing edge resulting in a vortex with increased size relative to that of the baseline wake. Various vortex generator geometries were tested. A semi-circular generator produced an increase in core radius by a factor of five but the best shape was deemed triangular since generators of this shape produced substantial vortex enlargement with relatively low lift penalty. Low slenderness ratio generators were also deemed preferable since for a high slenderness generator it was shown that merging failed due to a negative vorticity buffer constrained to the region between the tip and generator vortex by the proximity of the generator vortex to the wing surface. Increasing the generator angle of attack was shown to increase the effectiveness of circulation redistribution. At the highest angles of attack increases occurred despite a decrease in VG vortex strength due to stalling. Unsteadiness in the stalled VG flow appears to enhance vorticity diffusion and it is hypothesised that it may also initiate cooperative instabilities in multiple vortex systems.