Using the Fluid-Structure Interaction (fsi) interface (), found under the Fluid Flow branch () when adding a physics interface, you can model phenomena where a fluid and a deformable solid affect each other. The physics interface models both the fluid domain and the solid domain (structure) and includes a predefined condition for the interaction at the fluid-solid boundaries. An ALE formulation is used for incorporating the geometrical changes of the fluid domain.

When the Fluid-Structure Interaction interface is added, the following default nodes are added to the Model Builder—Fluid Properties, Linear Elastic Material, and Free Deformation (for the mesh movement and default boundary conditions) in the domains; Wall (for the fluid), Prescribed Mesh Displacement (for the mesh movement), and Free (for the solid mechanics, which initially is not applicable to any boundary because the default settings assume a fluid domain) as default boundary conditions; and Initial Values.

From the Physics toolbar, add other nodes that implement, for example, loads, constraints, and other nonlinear materials for the solid domain. You can also right-click Fluid-Structure Interaction to select physics features from the context menu.

The Fluid-Structure Interaction interface default is to treat all domains as fluid. The Linear Elastic Material node, which is the default node for the solid domain, initially has an empty selection. When a solid mechanics material is added to the solid domains, the physics interface automatically identifies the fluid-solid interaction boundaries and assigns the Fluid-Solid Interface Boundary condition to those boundaries. Two materials are typically defined in an FSI model: one for the fluid and one for the solid.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is fsi.

Select a Mesh smoothing type — Winslow (the default), Hyperelastic, Laplace, or Yeoh. For the Yeoh mesh smoothing type, also specify a Stiffening factor (default: 100). See Smoothing Methods in the COMSOL Multiphysics Reference Manual for more information.

By default the physics interface uses the Compressible flow (Ma<0.3) formulation of the Navier-Stokes equations. Select Incompressible flow to use the incompressible (constant density) formulation.

Enter a Reference pressure level pref (SI unit: Pa). The default value is 1[atm].

Enter the coordinates for the Reference point for moment computation, xref (SI unit: m). All moments are then computed relative to this reference point.

Select a Structural transient behavior—Include inertial terms (the default) or Quasi-static.

To display this section, click the Show button () and select Advanced Physics Options. Normally these settings do not need to be changed.

The Use pseudo time stepping for stationary equation form check box is active per default. It adds pseudo time derivatives to the momentum equations when the Stationary equation form is used. When selected, also choose a CFL number expression—Automatic (the default) or Manual. Automatic sets the local CFL number (from the Courant–Friedrichs–Lewy condition) to the built-in variable CFLCMP which in turns trigger a PID regulator for the CFL number. For Manual enter a Local CFL number CFLloc.