The default node attributed to The Transport of Diluted Species Interface assumes chemical species transport through diffusion and convection (depending on the modules licensed, a check box to activate migration is available) and implements the mass balance equation in Equation 6-15.

and each is treated slightly differently by the solver algorithms. In these equations Ji (SI unit: mol/(m2·s)) is the diffusive flux vector, R (SI unit: mol/(m3·s)) is a production or consumption rate expression, and u (SI unit: m/s) is the solvent velocity field. The diffusion process can be anisotropic, in which case D is a tensor.

If the conservative formulation is expanded using the chain rule, then one of the terms from the convection part, c∇·u, would equal zero for an incompressible fluid and would result in the nonconservative formulation above. This is in fact the default formulation in this physics interface. To switch between the two formulations, click the Show button () and select Advanced Physics Options.