  Direct numerical simulations of a temporally-developing, low-speed,
variable-density, turbulent, plane mixing layer are performed. The
Navier-Stokes equations in the low-Mach number approximation are solved using a
novel algorithm based on an extended version of the velocity-vorticity
formulation used by Kim et al. (1987) for incompressible flows. Four cases with
density ratios $s=1, 2, 4$ and 8 are considered. The simulations are run with a
Prandtl number of 0.7 and achieve a $Re_\lambda$ up to 150 during the
self-similar evolution of the mixing layer. It is found that the growth rate of
the mixing layer decreases with increasing density ratio, in agreement with
theoretical models of this phenomenon. Comparison with high-speed data shows
that the reduction of the growth rates with increasing the density ratio has a
weak dependence with the Mach number. In addition, the shifting of the mixing
layer to the low-density stream has been characterized by analyzing one point
statistics within the self-similar interval. This shifting has been quantified,
and related to the growth rate of the mixing layer under the assumption that
the shape of the mean velocity and density profiles do not change with the
density ratio. This leads to a predictive model for the reduction of the growth
rate of the momentum thickness, which agrees reasonably well with the available
data. Finally, the effect of the density ratio on the turbulent structure has
been analyzed using flow visualizations and spectra. It is found that with
increasing density ratio the longest scales in the high density side are
gradually inhibited. A gradual reduction of the energy in small scales with
increasing density ratio is also observed.
