In most cases you model FSI problems so that the geometry sequence is set up to form a union, and the same multiphysics coupling, Fluid–Structure Interaction, is used irrespective of the type of structural mechanics interface. This coupling will automatically find all boundaries that are shared between the structure and the fluid.

There are, however, cases where the assembly mode must be chosen, particularly when having mechanisms, as is common in the Multibody Dynamics interface. In that case, the interface between the solid and the fluid is no longer formed by a common boundary. Rather, it consists of two boundaries, located at the same place in space. These boundaries will in general slide with respect to each other. To model this, you use the Fluid–Structure Interaction, Pair multiphysics coupling. You must create appropriate pairs containing the boundaries from both types of physics under Definitions, and manually select them in the Fluid–Structure Interaction, Pair node.

When using Fluid–Structure Interaction, the spatial frame also deforms with a mesh deformation that is equal to the displacements u_solid of the solid within the solid domains. The mesh is free to move inside the fluid domains, and it adjusts to the motion of the solid walls. This geometric change of the fluid domain is automatically accounted for in COMSOL Multiphysics by the ALE method.

The Fluid–Structure Interaction, Pair coupling does not automatically transfer the mesh deformation at the interface from structural displacements.

For the moving mesh, you must specify the deformation of the mesh manually. Add a Prescribed Mesh Displacement node under the Moving Mesh node, in which you give the structural displacement as the mesh displacement expression. The variables for the displacement in the structure is provided by the multiphysics coupling.