DNS of Differential Thermal and Mass Diffusions in Free Turbulent Shear Flows

The diffusion of scalars in turbulent flows is the combined effect of the molecular diffusion and the turbulent diffusion, and the latter usually dominates. We investigated the diffusion of scalars with different diffusivities through two canonical free turbulent shear flows, i.e., the mixing layer and the homogeneous isotropic flow, by means of direct numerical simulation (DNS). In the mixing layer, the molecular diffusion has a big impact on the diffusion of scalars in the laminar region where the scalar gradient is high. In the turbulent region, the molecular diffusion has a negligible effect on the diffusion. In the homogeneous isotropic flow, the higher molecular diffusivity strengths the dispersion of scalars of relatively smaller values. However, the molecular diffusion exhibits a counter effect on the scalar diffusion for the relatively larger value. This work is intended to figure out the feasibility of the assumption that the thermal and mass diffusions of a contaminated content are equal, in other words, unit Lewis number assumption, which is often adopted.