Direct numerical simulation of vortex structures during the late stage of the transition process in a compressible mixing layer

The paper describes the direct numerical simulation (DNS) of a spatially developing compressible mixing layer with a convective Mach number of 0.8. The work mainly studies on the evolution process and formation mechanism of vortex structures during the late stage of transition process. In flow visualizations, multiple ring-like vortices and multiple necklace-like vortices appear during the late stage of the transition process. These multiple necklace-like vortices have never previously been reported in a compressible mixing layer. The formation mechanism is presented that the Kelvin-Helmholtz instability (K-H instability) of shear layers is the root cause of multiple ring-like vortices and multiple necklace-like vortices. DNS results indicate that new shear layer regions form around the corresponding Lambda -vortices as the result of the first ejection and sweep events from each Lambda -vortex. As a consequence, ring structures form one by one because of the K-H instability of shear layers, which represents the formation of multiple ring-like vortices. As the flow proceeds, several new local shear layer regions are generated around the ring structures, induced by the second ejection and sweep events from the rings of the multiple ring-like vortices. Bridge structures are then induced one by one by the K-H instability of shear layers, representing the formation of multiple necklace-like vortices. In addition, DNS results show that, from the first multiple necklace-like vortex onward, dominant vortex structures develop separately in the upper and lower layers until flow runs into the fully developed turbulent state. In the process, vortex structures in the upper layer clearly lag behind those in the lower layer.