Mixing regimes in a spatially confined two-dimensional compressible mixing layer

Abstract

The evolution of a high-speed compressible confined temporally evolving supersonic mixing layer between hydrogen and oxygen gas streams is examined using time-dependent two-dimensional numerical simulations that include the effects of viscosity, molecular diffusion and thermal conduction. The flow shows three distinct mixing regimes: an apparently ordered, laminar stage in which the structures grow due to the initial perturbation; a convective-mixing regime in which vortices begin to interact and structures grow; and a diffusive-mixing regime in which vortical structures break down and diffusive mixing dominates. Varying the strength of the diffusion terms shows that diffusion is important in the laminar and diffusive-mixing stages, but not in the convective-mixing stage. Varying the convective Mach shows that compressiblity does not change the general structural features of the mixing process, although higher compressibility results in a slower transition between the various flow regimes. Increasing the size of the computational domain increases the absolute time of transition from convective to diffusive mixing, but does not affect the dimensionless time normalized to the system size. Comparisons between full Navier–Stokes computations at different levels of numerical resolution show that the measurements of scalar mixing converge for resolutions at an order of magnitude greater than the Kolmogorov scale, although measurements of turbulence intensity are more sensitive to grid size.