Numerical Simulations of Transitional and Turbulent Flows in Plane Mixing Layers

In this paper, three types of 3-D spatial evolving compressible plane shear flows, namely, supersonic/supersonic mixing layers, supersonic/subsonic mixing layers and subsonic/subsonic mixing layers, are investigated by the large eddy simulation(LES) method using three-dimensional compressible Favre-filtered Navier-Stokes equations. Based on the linear stability theory, a pair of the most unstable oblique first mode disturbances and spontaneous perturbation is imposed on the inflow boundary respectively. The transition process and fully developed turbulent flows are simulated at different convective Mach numbers with focus on the unsteady characteristics of turbulence, including the mechanism of destabilization, inherent structures and statistic parameters. The computational results show that in low convective Mach number (Mc 0.6), the 3-D oblique wave perturbation and Λ vortices dominate the transition process. The consistency of LES results with natural transition and the relevant literature data indicates that it is reasonable to investigate free shear layer development by linear disturbance. At the same time, flow parametric analysis indicates that momentum thickness saturation position can be viewed as the index of transition completion. Also in this paper, shocklets with symmetrical distribution have been captured by the LES method at the convective Mach number of 1.0. Their shapes are consistent with experimental observation. In the fully developed turbulent region, the velocity fluctuation tends to be isotropic and the compressibility effect increases with increasing convective Mach number.