There are situations when you want to simulate transport of some specie in the flow. In general, you would have to employ species transport model. This model is rather complex and only applies to compressible flows, which means you would not be able to use it to for example simulate contamination in the water. In such cases, it might be appropriate to use the **Passive Scalars** model.

The passive scalar model can be used when changes in scalar concentrations do not influence (hence “passive”) the physical properties of the flow like density or viscosity. This condition is usually met when the scalar is used to model relatively small concentrations of some specie (e.g. CO_{2} in air).

The equation solved for each scalar

is a genera convection-diffusion equation of the form:

where

is velocity obtained from momentum equation, is diffusivity coefficient and

is optional source term.