The default node attributed to The Transport of Diluted Species Interface assumes chemical species transport through diffusion and convection and implements the mass balance equation in Equation 8-7.

and each is treated slightly differently by the solver algorithms. In these equations D (SI unit: m2/s) is the diffusion coefficient, 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 non-conservative formulation above. This is in fact the default formulation in this physics interface and ensures that nonphysical source terms do not emerge from a solution for the flow field. To switch between the two formulations, click the Show button () and select Advanced Physics Options.