Passive scalar mixing in variable-density, buoyant turbulent flows

The interplay between turbulence and buoyancy is not fully characterized despite its presence in a wide range of environmental phenomena and engineering problems. Although classical Kolmogorov theory states that the dissipative scales are purely isotropic, there is evidence that this no longer holds in the presence of buoyancy. In this a-priori analysis, we consider two incompressible, miscible fluids with different densities that are subject to external body forces (gravity). The simulation results are used to probe the effect of variable-density and buoyancy on turbulence generation, small-scale isotropy, kinetic energy evolution, and turbulent mixing. The presence of isotropic behavior at the Taylor micro- and dissipative scales is examined via the Favre Reynolds stress anisotropy tensor. Analysis is conducted on the alignment of vorticity with the direction of principle strains to verify observed directional preferences. The role of buoyancy in the generation of turbulence is isolated by examination of appropriate energy spectra. Finally, the efficacy of mixing at varying Atwood and Schmidt numbers is analyzed using the probability density function (PDF) of mixture-averaged specific volume, the PDF of the scalar dissipation rate, and the scalar energy spectra.