3. Use

The elements described above belong, for the fluid part, to the “3D_ FLUIDE “, “2D_”, “2D_ FLUIDE “or” AXIS_FLUIDE “modelling of the phenomenon MECANIQUE and, for the fluid-structure interface, to the” FLUI_STRU “or “2D_ FLUI_STRU” modeling. To define the fluid absorbent elements, which take into account condition BGT, it is necessary to assign the models “3D_ FLUI_ABSO “, “2D_ FLUI_ABSO” or “AXIS_FLUI_ABSO”.

To choose the wording (\(u-p\), \(u-p-\phi\), or \(u-\psi\)), we’ll use the keyword FORMULATION in AFFE_MODELE.

They lead to volume or surface elements, for the fluid part, and to surface or linear elements for the fluid-structure interface or for the absorbent elements.

3.1. Euler buckling, geometric stiffness

We consider the Euler buckling problem of an elastic solid in interaction with a fluid domain, modelled with the vibro-acoustic hypothesis. Fluctuating fields are modelled. Euler buckling is therefore treated by seeking the cancellation of the natural vibration frequencies of the coupled system, including the presence of the geometric stiffness associated with the prestressed static state in the solid, controlled by a critical loading parameter to be determined.

Initially, it is assumed that the geometric stiffness associated with the initial static state in the fluid is negligible. It is therefore proposed to define a zero geometric stiffness matrix in the fluid domain and on fluid/structure interfaces. However, this method of analysis neglects the fact that fluid pressure is a subsequent loading for the solid, which therefore introduces a non-linear term, which often has significant effects on critical buckling loads. It is therefore preferable to treat the problem by a non-linear dynamic analysis combined with a stability analysis, by controlling the mechanical load: consult document U2.06.11.