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3.3.8 Solver Parameters for Fluid and Fluid-FSI Analyses
A fluid analysis is defined by using the fluid type in Module section (see Section 3.2↑). In addition to the common parameters, the following parameters can be specified:
|vtol||convergence tolerance on velocity||0.001|
|ftol||convergence tolerance on dilatation||0.001|
|rhoi||Spectral radius parameter||0|
|reform_each_time_step||Flag for reforming stiffness matrix at the start of each time step||1|
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 determines the time integration scheme: Values in the range use the generalized method (see FEBio Theory Manual). With , 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 , no damping occurs, potentially leading to instability in the solution process. When solving transient flows that exhibit eddies or shed vorticies, a value of represents a good balance between too much and too little numerical damping.