STRUCTURE OF COHERENT INSTABILITIES IN A SUPERSONIC SHEAR-LAYER

Detailed measurements have been made of the instabilities present in s upersonic shear layers. A high-speed stream of Mach number 3 or 4 and a low-speed stream of Mach number 1.2 are produced and begin mixing at the trailing edge of the dividing centerbody. Glow discharge excitati on is used to excite either two-dimensional or oblique instability wav es. Mach-number profiles for the Mach 3 case show little effect of exc itation on the growth rate, whereas the higher Mach number case shows enhanced mixing with both excitation geometries. Four hot wires are us ed simultaneously to measure the axial and spanwise wavelengths for ea ch case. With these wavelengths, the propagation angles of the instabi lities are calculated. The instability waves in a two-dimensionally ex cited shear layer remain two dimensional. The three-dimensionally exci ted shear layer results in waves that travel at a nominal angle of app roximately 60 deg to the mean flow direction, even with three widely d ifferent excitation angles. Increasing the convective Mach number of t he shear layer results in a slightly larger oblique instability wave a ngle. The conclusions support the predictions of analytical and/or num erical studies.