The Flux/Source boundary condition adds a flux or source g on the boundary:

The first equation describes this boundary condition for a Coefficient Form PDE, and the second term describes it for a General Form PDE. The g term may contain a general expression of the dependent variables. The q coefficient simplifies the implementation of a Robin boundary condition by including a term on the form qu, where u is the dependent variable.

By default, all check boxes are selected for the Diffusive flux, Conservative convective flux, and Nonconservative convective flux to include all flux components. You can use the Flux/Source boundary condition in the Stabilized Convection-Diffusion Equation interface as an outflow condition by clearing the Nonconservative convective flux and Conservative convective flux check boxes and as a supersonic outflow condition by clearing all check boxes so that the flux is computed from inside the domain.

Enter a value or expression for the value of the boundary flux or source g in the corresponding field or fields. The default value is 0.

Enter a value or expression for the value of the coefficient q in the corresponding field or fields. The default value is 0. It adds a term qu to the boundary flux or source, which can represent absorption or impedance at the boundary.

For the Stabilized Convection-Diffusion Equation, there is an additional flux term, (k · n)u, so, in addition, you can enter the components of the k vector in the corresponding text fields.