The control of trailing edge separation on highly swept wings using vortex generators

Abstract

The results from a series of low speed wind tunnel tests on two half model highly swept wings (a symmetrical aerofoil section and a highly cambered aerofail section) are presented in order to examine the trailing edge flow separation mechanism and its development with wing sweep between 30' and 60'. The tests involved surface oil flow visualisation, smoke flow visualisation, surface static pressure and force balance measurements at streamwise chord Reynolds numbers from 1.5 x 105 to 5.2 x 106 and Mach number from 0.09 to 0.17. These results are used to assess two viscous-inviscid interaction CFD methods (BVGK and VFP) and two boundary layer methods (TAPERBL and WAKELAG) used to predict the flow over the highly cambered wing. A parametric study using cropped delta vane vortex generators in a co-rotating array was conducted on the 40' swept wing to investigate the effect of vane chordwise position, vane orientation, vane height relative to the boundary layer thickness and vane spacing on the prevention of the trailing edge separation. The performance of these flow control devices is assessed in terms of changes in; the wing surface flowfield, lift curve slope and the lift-dependant drag factor. In addition comparisons are made between the clean wing and flow control wing measured pressure distributions. The results and analysis show that the performance of the vortex generators is improved when the height of the vortex generator is approximately equal to that of the local boundary layer thickness and when the vane angular deflection to the local upstream flow direction is between 14' and 21'. The performance is also seen to depend on the vanes position ahead of separation and on the adverse pressure gradient to be restored and may also depend on a vane spacing made non-dimensional on the wing normal chord rather than the vane height. Similar performance improvements are observed with the wing swept to 50' using the positioning guidelines from this optimisation study. The performance of concave slats, canted cropped delta vanes, 'bent'wires and sub-boundary layer wires as vortex generating devices are seen to be not as effective as upright cropped delta vane vortex generators. To assist in the interpretation of the parametric vortex generator study a low speed wind tunnel technique is developed using shear stress sensitive liquid crystals to investigate the downstream development of vortices from cropped delta vane vortex generators. The results show that -- i) submerged vortices have less influence on the surface flow with downstream distance than vortices closer to the edge of the boundary layer, and ii) the primary increase in skin ffiction arises in the flow adjacent to the upflow side of the vortex. This area increases with vortex size. The results from this research programme are finally shown to be applicable in two market areas. The first is as a performance improvement on current highly swept winged military aircraft and the second is as flight controls on future aircraft from making the vortex generating devices active. The possible customers in these two areas are identified and marketing strategies developed for each case.