Wave structure of coherent instabilities in a planar supersonic shear layer

Abstract

Introduction Detailed measurements have been made of the instabilities present in supersonic 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 hegin mixing at the trailing edge of the centerbody. Glow discharge excitation is used to excite either two-dimensional or oblique instability waves. Mach number profiles, for the Mach 3 case, show littlc effect of excitation on the thickness, while the higher Mach number case shows enhanced mixing with both forms of cxcitation, particularly three-dimensional cxcitation. Vorticity thickness growth rates, measured from hot-wire mean voltage profiles, also show an increascd growth rate due to excitation predominantly in the Mach 4 case. Four hot-wires are used to measure the axial and spanwise wavelengths for each case. With these wavelengths, the propagation angles are calculated. The instability waves in a twodimensionally excited shear layer remain twodimensional. The three-dimensionally excited shear layer results in waves that travel at an angle of about 60’ to the mean flow direction. Two oblique excitation angles result in the same propagation angle. Increasing thc speed of the shear layer results in a slightly larger oblique instability wave angle. 4 Research Assistant, Department of Aerospace Engineering, Student Member AIAA Professor and Head of Aerospace Engineering, Associate Fellow AIAA Copyright 8 1994, by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. * * With the resurgence of interest in hypersonic aerodynamics in the previous years, there is an acknowledged need for a more fundamental understanding of compressible shear layer mixing. This need is especially prevalent in the development of SCRAMET engines. Fuel is injected into a supersonic stream of air and then burned. It has long been documented that as the speed differential of the two streams exceeds supersonic values, the mixing rate decreases‘,2. A means of increasing the mixing of fuel and air in these engines, to increase efficiency, would he advantageous. This work is part of a fundamental study of the mixing process in supersonic shear layers. A unique aspect of this facility is that it produces flow in a low-to-moderate Reynolds number environment. This approach provides numerous advantages. First, the reduced Reynolds number results in a suppression of the small scale turbulence, due to viscous forces. This focuses attention on the remaining large scale turbulent StNctUreS, Or instabilities. These instabilities are very concentrated in frequency and coherent over relatively long streamwise distances. Also, these large scale instabilities or turbulence structures are relatively independent of Reynolds number. Second, glow discharge excitation may be used to introduce a perturbation at a chosen frequency in the shear layer. This technique only works at low pressures and densities. Finally, with the low density conditions and corresponding low dynamic pressure, standard hot-wire probes may be used with decreased problems due to wire breakage.