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 checkbox to activate migration is available) and implements the mass balance equation in Equation 3-30.

In these equations Ji is the diffusive flux vector, u is the solvent velocity field, and R is a production or consumption rate expression. The nonconservative is often more numerically robust, and therefore chosen by default for the interface. This formulation however disregards density changes in the fluid. This can be seen by expanding the conservative convection term using the chain rule. One of the resulting terms, c∇·u, equals zero for an incompressible fluid which results in the nonconservative form. To switch between the two formulations, click the Show button () and select Advanced Physics Options.