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 Concentrated Species Interface supports the inclusion of turbulent mixing via the gradient-diffusion hypothesis. In the case of mixture-averaged diffusion, it adds the following contribution to the diffusion coefficient for each species:

(8-24)

while for Fick’s law diffusion, it adds the following contribution to the diffusion coefficient tensor:

(8-25)

Here, νT is the turbulent kinematic viscosity, ScT is the turbulent Schmidt number and I is the unit matrix. While having the form of diffusive contributions, Equation 8-24 and Equation 8-25 really model a convective phenomenon and they 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.