About Turbulent Mixing
A flow field obtained using a turbulence model does not explicitly contain the small eddies. These unresolved eddies still have a profound effect on the species transport, an effect known as turbulent mixing.
The Transport of Diluted Species interface and the Transport of Concentrated Species interface support the inclusion of turbulent mixing via the gradient-diffusion hypothesis which adds the following contribution to the diffusion coefficient tensor:
(8-40)
where νT is the turbulent kinematic viscosity, ScT is the turbulent Schmidt number and I is the unit matrix. While having the form of a diffusive contribution, Equation 8-40 really models a convective phenomenon and it can therefore only be applied along with convection.
The Schmidt number is typically given a value between 0.7 and 0.72, but it can range between 0.3 and 1.3 (at least) depending on the application.