Physics Interface Boundary Types

There are different types of boundaries for the physics interfaces, which all support different types of boundary conditions:

•	Exterior boundaries, where most boundary conditions are applicable — see below.

•	Interior boundaries, where special interface conditions can be applicable — see below.

•	Axial symmetry boundaries, which are artificial boundaries representing the symmetry axis in axisymmetric models.

If a selection for a boundary condition node, for example, contains boundaries of a type that is not applicable or supported, the Selection list has (not applicable) next to those boundary numbers.

Interior and Exterior Boundaries

When specifying boundary and interface conditions, COMSOL Multiphysics differentiates between exterior and interior boundaries:

•	An exterior boundary is an outer boundary of the modeling domain.

•	A interior boundary is a dividing interface between two domains in the geometry.

If an equation or physics interface is deactivated in one domain, the interior boundary between the active and inactive domain becomes an exterior boundary for its variables because it then borders on the outside of the active domain for those fields. The boundaries of the inactive domain are then void.

Figure 3-18: Examples of exterior and interior boundaries.