Intrinsic FSI is used to designate a simulation of fluid-structure interaction in which the same solver is used for solving simultaneously the flow governing equations and the solid deformations. In an intrinsic FSI context, fluid and solid domains share a common interface, whose discretization is conformal. The displacement of the solid domain will result from the continuous forces generated by the fluid; these forces are of course a priori unknown, and are part of the calculation.

If the expected displacements of the solid region are negligible, it will be sufficient to consider a one-way FSI simulation. A one-way FSI simulation can be steady or transient; this will be dictated by the settings selected for the fluid flow simulation.

A two-way FSI approach should be used when the displacements calculated on the solid domain affect the fluid domain, such as in the following examples: a high pressure flow around a flexible aluminum flap, or a cantilevered thin plate that is initially deformed and then allowed to freely oscillate in a fluid. For such cases, the fluid domain will deform in accordance with the displacements calculated on the solid domain; of course, these displacements are also a priori unknown and are part of the entire calculation. In a two-way FSI context, the physics and the numerics involved on both sides of the fluid-solid interface differ; however, it is necessary to ensure the continuity of the displacements and of the velocity, as well as the equilibrium of stresses. Two-way FSI is solved by means of a transient scheme, and explicit mesh updating is used by default, such that the dynamic mesh is only updated at the beginning of a time step; for cases with strong FSI coupling, you should enable the implicit update scheme instead, so that it is updated during a time step.

It is important to note that small displacements are considered within the context of linear elasticity. Therefore, although the solid body also deforms, the calculation of the displacement governing equations on the solid body will be performed on the basis of the initial coordinates.