Turbulent Schmidt numbers for CFD simulations using the k-ε and k-ω models

Turbulent diffusion dominates mixing at high molecular Schmidt numbers. When solving the Reynolds averaged Navier-Stokes equations, one of the key parameters for the correct simulation of mixing is the turbulent Schmidt number (Sc-t). In the case of the k-epsilon turbulence model, many CFD packages suggest a value of 0.7 for Sc-t. However, it has been demonstrated that this value is not adequate for all cases. This paper reports simulations for three different cases of turbulent mixing and compares the results with experimental data. It has been found that, for jets in crossflow, Sc-t is a function of the Reynolds number of the jet, the ratio between the diameters of the jet and main flow and the turbulent intensity of the jet and the main flow. Numerical simulations using the k-omega model are also performed, showing that the required value of Sc-t is dependent on the turbulence model used.