The secondary vortex rings of a supersonic underexpanded circular jet with low pressure ratio

Study of the secondary vortex ring (SVR) is essential to understand the complicated flow structures of supersonic impulsive jets. In the present study, the main characteristics of compressible secondary vortex ring and the primary vortex ring (PVR) in the starting three-dimensional (3D) flow field of a supersonic underexpanded circular jet are investigated numerically for Ma=1.2, 1.4 and 1.8, at a low pressure ratio (jet flow pressure/ambient pressure) of 1.4. The governing equations of large eddy simulation (LES) for compressible flow have been employed and are solved numerically with the combination of high-order hybrid schemes. Our results illustrate the reason for generation of the SVRs by supersonic underexpanded jets, and it is the rolling up of the shear layer which is resulted from the combination of two slip lines when their two triple points on the embedded shock wave interact with each other. After formation, the SVR interacts with the PVR, and rolls over the periphery of PVR and moves upstream. For a higher Mach number of 1.8, multiple SVRs form during the evolution.