Length Scales Governing Turbulence Transport and Mass Diffusion in Variable-Density Turbulent Shear-Driven Mixing Layers

Moment equations for quantities characterizing variable-density turbulence in shear-driven mixing layers include terms describing turbulence transport and mass diffusion behavior. Modeling these quantities with common closures (RANS-type modeling, e.g., Schwarzkopf et al., Flow, Turbulence and Combustion, 96, 2016) introduces length scales defining these processes for the moments evolving within the flow. Direct numerical simulations for low-speed mixing of two miscible fluids with Atwood numbers of up to 0.87 (Baltzer and Livescu, J. Fluid Mech, 900, 2020) allow the relevant length scales to be extracted using the actual budget terms. Besides Reynolds stresses and turbulent kinetic energy, which are relevant to all turbulent flows, this variable-density flow also requires that density correlations (turbulent mass flux and density-specific volume correlation) be considered. Analyzing the length scales for the various quantities reveals the degree to which the governing processes are similar between turbulent motions and variable-density effects.