Modeling A
==============


Characteristics of modeling
-----------------------------------

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

Concerning the absolute wall roughness of the structures, a value of :math:`{10}^{–5}m` is taken.

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


.. math::
: label: EQ-None

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

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

The resolution model used is the analytical model MOCCA_COQUE absorbed in the *Code_Aster* (operator CALC_FLUI_STRU). The fifth air mode of the structure is selected and then the coupling problem is solved for this mode. Average flow velocities vary from :math:`0` to :math:`40m/s` in steps of :math:`5m/s`. For this mode, the changes in the natural frequency and in the reduced damping are thus obtained as a function of the flow speed.


Characteristics of the mesh
----------------------------

The two shells are meshed in the same way. In order to make them integral to each other, a group of elements connecting the nodes is added at the embedment level.


.. csv-table::

    "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)"



Features tested
-----------------------

Operator DEFI_FLUI_STRU: definition of the characteristics of a hardware configuration consisting of two coaxial cylindrical shells for a fluid-structure coupling calculation; keyword factor COQUE_COAX.

Operator CALC_FLUI_STRU: resolution of fluid-structure coupling for a configuration of the "coaxial shells" type; calculation of changes in modal parameters (reduced frequencies and damping) as a function of the fluid flow speed.