An Experimental Study of Compressible Turbulent Reattaching Free Shear Layers

Abstract

An experimental investigation was conducted to study compressible, two-dimensional, turbulent reattaching free shear layers formed by geometrical separation of a Mach 2.46 and a Mach 2.07 turbulent boundary layer flows over a backward-facing step. In the first set of experiments, the wind tunnel test section was specifically designed to obtain flow separation at the step without any flow expansion or compression. In the second set of experiments, the reattaching free shear layer in the classical backstep model was studied. A detailed survey of the flowfields was made utilizing a Schlieren system, static pressure probes, and a two-component coincident laser Doppler velocimeter. The expansion of the flow at the step produced stronger anisotropy and much higher turbulence level and shear stress. The compressibility effects and/or mean density change across the shear layers resulted in smaller growth rates of the shear layers and significant alteration in the turbulent kinetic energy diffusion in the transverse direction, transverse component turbulent intensity, and shear stress in the constant pressure region of the shear layers. In the reattachment region, a significant increase in turbulence level, shear stress, and turbulent triple products were observed which is in sharp contrast to incompressible reattaching shear layers and is believed to be a fundamental difference. Extremely large shear stress, turbulent triple products, mixing length scale, and extremely enhanced mixing were observed in the redeveloping boundary layer. The recirculating flows were noticeably directed toward the geometrical separation point which could be a cause for early flow separation on afterbodies with a centered propulsive jet.