A fluid-structure interaction analysis is defined by using the fluid-FSI type in Module section. It uses the parameters of solid and fluid analyses.

Transient fluid and fluid-FSI analyses may often run very efficiently using Broyden's method (qnmethod set to 1) with max_ups set to 50; efficiency may be further increased by setting reform_each_time_step to 0, which will postpone reforming the stiffness matrix at subsequent time steps until max_ups updates have been exhausted. When using Broyden's method with fluid and fluid-FSI analyses, set rtol to a non-zero value to ensure an accurate solution, for example rtol=0.001 (see Section 3.3.1↑).

The spectral radius parameter $\rho_{\infty}$ determines the time integration scheme: Values in the range $0\le\rho_{\infty}\le1$ use the generalized $\alpha-$ method (see FEBio Theory Manual). With $\rho_{\infty}=0$ , damping of higher frequency components (such as those produced by a step increase in velocity) occurs theoretically within a single time step; in contrast, with $\rho_{\infty}=1$ , no damping occurs, potentially leading to instability in the solution process. When solving transient flows that exhibit eddies or shed vorticies, a value of $\rho_{\infty}=0.5$ represents a good balance between too much and too little numerical damping.