MIXING REGIMES IN A SPATIALLY CONFINED 2-DIMENSIONAL COMPRESSIBLE MIXING LAYER

The evolution of a high-speed compressible confined temporally evolvin g supersonic mixing layer between hydrogen and oxygen gas streams is e xamined using time-dependent two-dimensional numerical simulations tha t include the effects of viscosity, molecular diffusion and thermal co nduction. 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 i nteract and structures grow; and a diffusive-mixing regime in which vo rtical structures break down and diffusive mixing dominates. Varying t he strength of the diffusion terms shows that diffusion is important i n the laminar and diffusive-mixing stages, but not in the convective-m ixing stage. Varying the convective Mach shows that compressiblity doe s not change the general structural features of the mixing process, al though higher compressibility results in a slower transition between t he various flow regimes. Increasing the size of the computational doma in increases the absolute time of transition from convective to diffus ive mixing, but does not affect the dimensionless time normalized to t he system size. Comparisons between full Navier-Stokes computations at different levels of numerical resolution show that the measurements o f scalar mixing converge for resolutions at an order of magnitude grea ter than the Kolmogorov scale, although measurements of turbulence int ensity are more sensitive to grid size.