ORGANIZED STRUCTURE IN A COMPRESSIBLE TURBULENT SHEAR-LAYER

An experimental study has been made of the large-scale structures in a two-dimensional, high Reynolds number, turbulent free shear layer bounded by Mach 5 and 3 airstreams. The convective Mach number was about 0.28. Mean pitot pressure surveys were made to determine the shear layer growth rate, the two-dimensionality, and the external flowfield. Two-point fluctuating pitot pressure measurements were made to investigate the large-scale structure at several stations from 600 to 2450 initial momentum thicknesses theta0 downstream of the shear layer origin. Power spectra show increasing organization with distance downstream, and at 2450 theta0, the large-scale structures are responsible for a strong concentration of energy between 16 and 25 kHz. In nondimensional terms, the power spectra are centered at a Strouhal number of about 0.35. The large-scale structures are inclined at about 42-48 deg to the now direction, which agrees well with schlieren photographs. Cross correlations of spanwise pitot pressure signals show that the spanwise extent of the large-scale structure is only about 1.0-1.5 shear layer thicknesses. Results from a preliminary study using fixed-spacing dual normal hot wires agree very well with results from fluctuating pitot pressures.