A GUN TUNNEL INVESTIGATION OF HYPERSONIC FREE SHEAR LAYERS IN A PLANAR DUCT

An experimental investigation of high Mach number free shear layers ha s been undertaken. The experiments were performed using a Mach 7 gun t unnel facility and a planar duct with injection from the base of a cen tral strut producing a Mach 3 flow parallel to the gun tunnel stream. This configuration is relevant to the development of efficient scramje t propulsion, and the gun tunnel Mach number is significantly higher t han the majority of previous supersonic turbulent mixing layer investi gations reported in the open literature. Schlieren images and Pitot pr essure measurements were obtained at four different convective Mach nu mbers ranging from 0 to 1.8. Only small differences between the four c ases were detected, and the relatively large high-speed boundary layer s at the trailing edge of the struct injector appear to strongly influ ence the shear layer development in each case. The Pitot pressure meas urements indicated that, on average, the free shear layers all spread into the Mach 3 stream at an angle of approximately 1.4 degrees, while virtually no spreading into the Mach 7 stream was detected until all of the low-speed stream was entrained. The free shear layers were simu lated using a PNS code; however, the experimentally observed degree of spreading rate asymmetry could not be fully predicted with the k-epsi lon turbulence model, even when a recently proposed compressibility co rrection was applied.