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

TL;DR

This study uses high-resolution simulations to explore how density differences between fluids affect turbulence and mixing in shear-driven layers, revealing asymmetries and preferential turbulence in lighter fluids with implications for combustion.

Contribution

It provides new insights into variable-density effects in incompressible shear layers, highlighting asymmetries and turbulence distribution due to compositional density differences.

Findings

Turbulence migrates towards the lighter fluid side as density difference increases.

Interface thickness growth rates decrease with density variation.

Lighter fluids sustain more fine-scale turbulence and faster mixing.

Abstract

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.