Eulerian and Lagrangian stagnation plane behavior of moderate Reynolds number round opposed-jets flow

Turbulent opposed jet (TOJ) flow is characterized by an unsteady stagnation plane formed by two counter-flowing jets. The complicated behavior of the stagnation plane significantly affects the turbulence characteristics in the impinging zone. In this study, direct numerical simulations (DNS) were performed to study the dynamic behavior of the stagnation plane in a three-dimensional round opposed-jet flow with a moderate Reynolds number (Re=4050). Proper orthogonal decomposition (POD) analysis was used to study the behavior of the Eulerian stagnation plane (ESP) and vortex structures. The results show that the first POD mode dominates the axial movement of the stagnation plane, and it contributes most of the axial velocity fluctuation. The stagnation plane is found to tilt relative to the normal direction of the jet flow by the second to fourth POD modes. The shape of the stagnation plane can be reconstructed to a large extent through the dominant POD modes (namely the first four modes). The Lagrangian property of the stagnation plane was investigated through calculation of the field of finite-time Lyapunov exponents (FTLE), in which the ridges can identify the Lagrangian coherent structures (LCSs) in the turbulent opposed jet flow. Throughout the domain three different types of LCSs were found, corresponding to the axial jet region, impinging zone and radial jet area. A Lagrangian stagnation plane (LSP) is defined as the LCS in the impinging zone, which indicates the boundary of the fluid parcels from the two nozzles. A discrepancy between the ESP and LSP was displayed, and this finding implies a backflow phenomenon, which means the moving impermeable LSP always forces the fluid parcels ahead of the LSP to move toward the ESP. As a result of this analysis, the unstable behavior of the stagnation plane is demonstrated.