3. Modeling A

3.1. Characteristics of modeling

The geometry of the structures and the characteristics of the material constituting the shells have been presented previously.

For the absolute wall roughness of structures, a value of 10—5 meters is taken.

The surrounding fluid is water. The values taken for density and kinematic viscosity are respectively

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{rho} _ {f} =1000mathrm {kg}/{m} ^ {3}

We consider the unconfined flow up and down stream of structures.

The fluid-structure coupling problem is solved by the analytical model MOCCA_COQUE [bib2] integrated in Code_Aster (operator CALC_FLUI_STRU), for an average flow speed of zero: the modal characteristics of the water system at rest are thus obtained, by taking into account the effects of added mass.

3.2. Characteristics of the mesh

Compared to the reference calculation, the characteristics of the mesh are similar, except that the fluid domain is no longer represented. In order to make the two shells integral to each other, a group of elements connecting the nodes is added at the level of the embedment.

We have:	17 knots on a vertical generator,
	60 knots on a crown,
	960 QUAD4 stitches on each shell,
	60 QUAD4 meshes to secure the two shells (embedded base).

3.3. Calculation steps

The definition of the characteristics of a hardware configuration consisting of two coaxial cylindrical shells for a fluid-structure coupling calculation is given using the operator DEFI_FLUI_STRU (keyword factor keyword COQUE_COAX).

The resolution of the fluid-structure coupling for a configuration of the « coaxial shells » type and the calculation of the modal parameters (reduced frequencies and damping) and modal deformations in water at rest are carried out with the operator CALC_FLUI_STRU.