Skin Friction Reduction by Injection Through Combinations of Slots and Porous Sections

TT'LUID injection for thermal protection and skin -F friction reduction on flight vehicles has received considerably study. Energy conservation has now elevated the importance of skin friction reduction. The information available for supersonic speeds is limited, but it indicates drag reductions large enough to be interesting from a systems viewpoint. The competing configurations are the porous wall and the tangential slot, but there have been few studies when a direct comparison of the two schemes was made at the same nominal conditions. Further, combinations of the two schemes might be expected to be beneficial as a result of synergistic interactions. This report presents the results of a comparative study of slot injection, porous wall injection through a short strip and combinations of the two at freestream conditions of Mach 2.9, stagnation pressure of 6.9 N/m (100 psia) and total temperature of 290K. A "flat plate," solid wall configuration was also studied as a reference point. The principal data obtained were: 1) schlieren photographs; 2) wall pressure; 3) Mach number profiles; and 4) wall shear measured with a floating element balance. Wall shear was also inferred from Preston Tube measurements.