Mixing enhancement in supersonic compressible shear layers

Experimental observations and numerical calculations have shown that mixing in supersonic shear layers is reduced greatly compared to that in their incompressible counterparts. This reduced mixing has created a technological challenge for efficient supersonic combustor development. In this thesis, two enhancement methods for ducted supersonic shear layers are investigated. One method is based on spatial forcing, and the other involves temporal excitation.;The spatial forcing is induced by a triadic resonant interaction between an instability wave and duct acoustic waves in the presence of Mach waves generated by wavy walls. If the wavelength of the wavy wall is chosen properly, this secular spatial excitation can lead to significant mixing enhancement. The required wavelength for the wavy wall to produce the maximum resonance, and the corresponding frequency of this resonance have been predicted numerically. A comparison with a parallel experimental study has shown reasonably good agreement. If the shear layer is excited temporally by a glow discharge, the waves become more organized spatially and temporally. This can lead to additional mixing enhancement.;Temporal excitation can also be induced by an absolute instability behind a thick splitter plate. One of the attractive features of the use of an absolute instability is that it behaves in the same manner as an externally driven excitation and provides continuous temporal excitation to the downstream flow field. The absolute instability is analyzed by the Green's function method. The effects of a wake are simulated by the superposition of a wake deficit onto a monotonic shear layer mean velocity profile. Since the onset of an absolute instability is of primary interest, a parametric study of the variation of absolute frequency with shear layer operating conditions has been performed. The required reversed flow, if any, needed to generate an absolute instability is also determined. The results of this parametric study provide guidelines that should aid in the design of scramjet operating conditions and geometries so that absolute instabilities will arise naturally.;These two methods are shown to be promising mixing enhancement techniques that could be implemented relatively easily in real engine designs.