Experimental study of compressibility effects on entrainment and mixing in supersonic planar turbulent bluff-body wakes

Understanding effects of compressibility on the entrainment and mixing properties of supersonic turbulent shear flows is a key to successful development of the next generation of high-speed airbreathing propulsion systems. Previous studies have focused largely on supersonic mixing layers, and have shown dramatic reductions in the entrainment and mixing rates with increasing compressibility which has been widely believed to be a generic effect of compressibility in supersonic turbulent shear flows.; The present dissertation reports results from an experimental investigation of entrainment and mixing in supersonic, planar, turbulent, bluff-body wakes to clarify the generic effects of compressibility in turbulent shear flows. The experimental techniques, including conventional pressure measurements, shadowgraph and planar laser Mie scattering (PLMS) visualizations, and particle image velocimetry (PIV), were used to study instantaneous and mean velocity fields, scaling properties, turbulence statistics, and large-scale structure in instantaneous and phase-averaged vorticity fields over a range of relative Mach numbers. These were compared with corresponding results from incompressible wakes and from supersonic mixing layers.; Results indicate that the classical vortex street-like large scale structure of incompressible planar turbulent wakes is recovered in supersonic wakes where the local relative Mach number M_r(x) has decreased to sufficiently small values, but no comparable large-scale organized structure is evident where the relative Mach number is large. Moreover, at downstream locations where M_r(x) is large, a reduction in the growth rate of the flow is observed due to compressibility, but this reduction is significantly smaller than that reported from studies of supersonic mixing layers. Results also show that the wake undergoes a self-induced forcing where it passes through reflected expansion waves produced by the wake generator. This local forcing alters the scaling constants for the wake, and affects the entrainment and mixing rate, only if the flow conditions produce a subsonic upstream path from the wave interaction point. However downstream of this point, the interaction leads to a dramatic increase in the growth rate and an attendant local increase in the entrainment rate, providing a means to increase the entrainment rate in supersonic turbulent shear flows.