When a fluid-structure interaction multiphysics interface is added using the Model Wizard, a Deforming Domain node can be added automatically in the Model Builder under the Definitions node. This is the case for the following interfaces:

The Deforming Domain node is however is not added for multiphysics interfaces denoted ‘Fixed Geometry’, which are intended for cases where the deformation of the fluid domains is small everywhere.

A deforming domain represents domains and boundaries where the mesh can deform. By default, the Deforming Domain node has an empty selection. You can the select any fluid domain. However, this is only needed if the geometry of such a domain experience significant changes due to the deformation or rotation of the adjacent solid domains. Otherwise, the moving mesh computations could introduce unnecessary overhead and nonlinearity in the model (see Deformed Mesh Fundamentals). Domains selected in the Solid Mechanics interface are not applicable. The deformations of the solid parts are handled in the formulation of the Solid Mechanics interface.

Under the Deforming Domain, you can also chose the Mesh smoothing type, by default set to Hyperelastic. More information of the smoothing type can be found in Deforming Domain chapter in the COMSOL Multiphysics Reference Manual

By default, the mesh is free at all external boundaries of the geometry, and it follows the solid boundaries at the solid-fluid interfaces. You can also add other types of boundary conditions for the mesh motion, for details see Deformed Geometry and Moving Mesh in the COMSOL Multiphysics Reference Manual.