Direct numerical simulation of subsonic-supersonic mixing layer

Rocket jet flow and airflow represent a typical mixing layer flow in a combustion chamber of a combined air-breathing propulsion system. Previous studies indicate that highly and weakly compressible mixing layers are similar in growth rate, turbulent intensity, and Reynolds stress. Flow Reynolds number was set to a range that a direct numerical simulation (DNS) can address. A high-order discretization DNS of a 3D spatially developing supersonic mixing layer flow was conducted, and detailed mixing layer flow structures were acquired. Results showed that the flow structure of a subsonic-supersonic mixing layer is similar to a supersonic-supersonic mixing layer. A shocklet structure can be observed in the mixing layer with a convective Mach number (Mc) of 0.69. However, the flow structure in the development region of the mixing layer becomes minimally ordered and lacks main frequency. Proper orthogonal decomposition (POD) analysis shows that a high POD mode has a relatively large proportion. In addition, the transition region of the subsonic-supersonic mixing layer has strong 3D flow characteristics.