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

摘要

The results from a series of low speed wind tunnel tests on two half model highly sweptwings (a symmetrical aerofoil section and a highly cambered aerofail section) arepresented in order to examine the trailing edge flow separation mechanism and itsdevelopment with wing sweep between 30' and 60'. The tests involved surface oil flowvisualisation, smoke flow visualisation, surface static pressure and force balancemeasurements at streamwise chord Reynolds numbers from 1.5 x 105 to 5.2 x 106 andMach number from 0.09 to 0.17. These results are used to assess two viscous-inviscidinteraction CFD methods (BVGK and VFP) and two boundary layer methods (TAPERBLand 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 wasconducted 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 onthe prevention of the trailing edge separation. The performance of these flow controldevices is assessed in terms of changes in; the wing surface flowfield, lift curve slope andthe lift-dependant drag factor. In addition comparisons are made between the clean wingand flow control wing measured pressure distributions.The results and analysis show that the performance of the vortex generators is improvedwhen the height of the vortex generator is approximately equal to that of the localboundary layer thickness and when the vane angular deflection to the local upstream flowdirection is between 14' and 21'. The performance is also seen to depend on the vanesposition ahead of separation and on the adverse pressure gradient to be restored and mayalso depend on a vane spacing made non-dimensional on the wing normal chord ratherthan the vane height.Similar performance improvements are observed with the wing swept to 50' using thepositioning guidelines from this optimisation study. The performance of concave slats,canted cropped delta vanes, 'bent'wires and sub-boundary layer wires as vortex generatingdevices 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 windtunnel technique is developed using shear stress sensitive liquid crystals to investigate thedownstream development of vortices from cropped delta vane vortex generators. Theresults show that --i) submerged vortices have less influence on the surface flow with downstreamdistance than vortices closer to the edge of the boundary layer, andii) the primary increase in skin ffiction arises in the flow adjacent to the upflowside of the vortex. This area increases with vortex size.The results from this research programme are finally shown to be applicable in two marketareas. The first is as a performance improvement on current highly swept winged militaryaircraft and the second is as flight controls on future aircraft from making the vortexgenerating devices active. The possible customers in these two areas are identified andmarketing strategies developed for each case.