Direct numerical simulation of fine flow structures of subsonic-supersonic mixing layer

Abstract

Subsonic-supersonic mixing flow is one of the important research fields in turbulence research. For example, in the combustion chamber of the rocket ramjet combination engine, the rocket jet flow and the air inflow is a typical shear mixing flow, which has the characteristics of high convection Mach number and large flow parameter gradient. It is of great significance to explore the development law and flow structure of subsonic-supersonic mixing layer to enhance the mixing and thus improve the performance of rocket ramjet engine.In this paper, in order to further study the evolution process and obtain the evolution mechanism and fine flow structure of subsonic-supersonic mixing layer, three groups of high-order format direct numerical simulations of typical working conditions are carried out. The convective Mach numbers are 0.69, 0.92 and 1.27, respectively. The direct numerical simulation results show that: (1) the large-scale structure in the subsonic-supersonic shear mixing flow formed the small-scale structure earlier, and the flow was dominated by the small-scale and broken eddy structure; (2) the development process of the subsonic-supersonic shear mixing flow was mainly divided into three stages: laminar flow zone, transition zone and development zone. The transition zone was a transition zone from two-dimensional to three-dimensional, which began to show three-dimensional flow characteristics. The vorticity shows a twisted-winding structure. The large-scale Coherent structure, the Λ eddy and hairpin eddy, can be observed, and the Λ eddy structure is gradually elongated with the increase of convective Mach number; (3) the shocklets structure mainly occurs in the stage when the mixing layer has not fully developed. The starting point of the shocklets structure is near the vortex core. With the increase of convective Mach number, the position of the shocklets generation moves to the front of the vortex core; (4) the dynamic mode decomposition results show that the eigenvalues of each mode are basically located in the unit circle, which indicates that the calculated modes are stable; in addition, it is found that there is a certain dominant frequency in the flow structure, which can provide a reference for the active mixing enhancement method.