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The asymmetries that arise when a mixing layer involves two miscible fluidsof differing densities are investigated using incompressible (low-speed) directnumerical simulations. The simulations are performed in the temporalconfiguration with very large domain sizes, to allow the mixing layers to reachprolonged states of fully-turbulent self-similar growth. Imposing a meandensity variation breaks the mean symmetry relative to the classicalsingle-fluid temporal mixing layer problem. Unlike prior variable-densitymixing layer simulations in which the streams are composed of the same fluidswith dissimilar thermodynamic properties, the density variations are presentlydue to compositional differences between the fluid streams, leading todifferent mixing dynamics. Variable-density (non-Boussinesq) effects introducestrong asymmetries in the flow statistics that can be explained by thestrongest turbulence increasingly migrating to the lighter fluid side as freestream density difference increases. Interface thickness growth rates alsoreduce, with some thickness definitions particularly sensitive to thecorresponding changes in alignment between density and streamwise velocityprofiles. Additional asymmetries in the sense of statistical distributions ofdensities at a given position within the mixing layer reveal that fine scalesof turbulence are preferentially sustained in lighter fluid, which also iswhere fastest mixing occurs. These effects influence statistics involvingdensity fluctuations, which have important implications for mixing and morecomplicated phenomena that are sensitive to the mixing dynamics, such ascombustion.

Variable-density effects in incompressible non-buoyant shear-driven turbulent mixing layers