Boundary conditions are a set of properties or conditions on surfaces of domains, and are required to fully define the flow simulation. The type of boundary condition that can be set depends upon the bounding surface.
A fluid boundary is an external surface of the fluid domain excluding surfaces where it meets other domains.
A solid boundary is an external surface of the solid domain excluding surfaces where it meets other domains.
A fluid-fluid interface is the interface between two fluid domains.
A fluid-solid interface is the interface between a solid and fluid domain.
A solid-solid interface is the interface between two solid domains.
