Boundary Conditions
In the interior of the selection where a physics feature is active, its behavior is governed by its model equations and material properties. Boundary conditions apply to the geometric entities separating this region from the unspecified outside and sometimes also to interior entities of the same dimension (interface conditions). Therefore, boundary conditions on a 3D solid object apply to the exterior surfaces of the solid and to interior surfaces embedded in the solid. On a shell geometry in 3D space, boundary conditions instead apply to the edges of the shell surface. In general, boundary conditions apply to geometric entities whose dimension is one less than the physics interface’s dimension.
All physics interface branches that contain a default model equation node also contain a default boundary condition node. This boundary condition is active on all exterior boundaries of the physics’ selection, except on the symmetry axis of axisymmetric 2D models. On interior boundaries, an implicit continuity condition applies, which makes the physics field (the temperature, for example) continuous across interior boundaries.
In physics interfaces defined on shells, for example, the domain equations are defined on 3D boundaries, and you define the interface conditions on edges as edge conditions rather than boundary conditions.