  The asymmetries that arise when a mixing layer involves two miscible fluids
of differing densities are investigated using incompressible (low-speed) direct
numerical simulations. The simulations are performed in the temporal
configuration with very large domain sizes, to allow the mixing layers to reach
prolonged states of fully-turbulent self-similar growth. Imposing a mean
density variation breaks the mean symmetry relative to the classical
single-fluid temporal mixing layer problem. Unlike prior variable-density
mixing layer simulations in which the streams are composed of the same fluids
with dissimilar thermodynamic properties, the density variations are presently
due to compositional differences between the fluid streams, leading to
different mixing dynamics. Variable-density (non-Boussinesq) effects introduce
strong asymmetries in the flow statistics that can be explained by the
strongest turbulence increasingly migrating to the lighter fluid side as free
stream density difference increases. Interface thickness growth rates also
reduce, with some thickness definitions particularly sensitive to the
corresponding changes in alignment between density and streamwise velocity
profiles. Additional asymmetries in the sense of statistical distributions of
densities at a given position within the mixing layer reveal that fine scales
of turbulence are preferentially sustained in lighter fluid, which also is
where fastest mixing occurs. These effects influence statistics involving
density fluctuations, which have important implications for mixing and more
complicated phenomena that are sensitive to the mixing dynamics, such as
combustion.
