Direct numerical simulation of turbulent structures and asymmetric properties in the supersonic non-isothermal mixing layer

Abstract

By direct numerical simulations, the non-isothermal effects on turbulent structures and asymmetric properties are investigated in the spatially developing supersonic mixing layers with high convective Mach numbers (Mc > 0.6). Hot air is blown in the high-speed stream, and cold air is added on the low-speed side. Two non-isothermal simulations with different temperature gradients are conducted and compared with the isothermal mixing layer. The self-similar model of the spatially developing supersonic turbulent mixing layer is analyzed to reveal the physical mechanisms for the asymmetry of non-isothermal mixing layers. The supersonic mixing layer is characterized by diverse vortices and unsteady shocklets, which increase in the initial shear layer and then decrease in the self-similar turbulent region. Also, the mixing layer is asymmetric between the high- and low-speed streams, and the shear layer center skews toward the low-speed side with more vortices and less shocklets, which is attributed to the streamwise momentum gradient. The effects of temperature gradients enhance the flow instability and accelerate the growth of vortices and shocklets in the initial mixing layer. Nevertheless, the turbulent structures are attenuated in the fully developed region, as the viscous dissipation is augmented and turbulence decays more strongly. In addition, the streamwise momentum gradient is reduced in the non-isothermal mixing layers. The vortical structures suffer from stronger attenuation on the cold side, while the shocklets are more significantly reduced on the hot side. Thus, the skewness of the shear layer center toward the low-speed side is reduced, and the mixing layer asymmetry is attenuated.