This section lists the predefined variables to evaluate fluxes used in the mass balance. All the variables names begin with the physics interface name (the prefix) and end with the species that you want to inspect. By default, the Transport in Solids interface prefix is ts and the default dependent variable name is c. As an example, you can access the variable named dflux for the dependent variable c using ts.dflux_c

The diffusive flux variable, dflux, is defined by Fick’s law:

The diffusive flux magnitude, dfluxMag, is the magnitude of the diffusive flux in material frame, defined as the square root of the sum of the diffusive components squared:

The variable domflux is used to calculate fluxes when the Compute boundary fluxes checkbox is cleared. The flux variables ndflux and ntflux are then computed from the dependent variables using extrapolation. The domain flux variable domflux is the defined as the diffusive flux times the out-of-plane geometry extension (out-of-plane thickness for 2D, or cross-sectional area for 1D or 1D axisymmetric):

For more information about the boundary fluxes settings, see Computing Accurate Fluxes in the COMSOL Multiphysics Reference Manual.

The external flux is a user input in the External Flux node.

On exterior boundaries, bndFlux is defined as

whether the adjacent domain is on the downside or on the upside. The up and downside is defined by the surface normal, which points from downside to the upside. On interior boundaries, bndFlux is the mean of the fluxes on both sides:

The variable bndFlux is defined using the Boundary Flux Operators: uflux and dflux. These operators are used when the Compute boundary fluxes checkbox is selected, see Computing Accurate Fluxes in the COMSOL Multiphysics Reference Manual.

The variable ndflux is defined as

The variable ntflux is defined as