Fluid material
====




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Keyword factor FLUIDE
----

Definitions of constant fluid characteristics.


Operand RHO
~~~~

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    RHO = rho

Fluid density. No verification.




Operands CELE_R and CELE_C
~~~~

.. code-block:: text

    CELE_R = clear

Speed of propagation of acoustic waves in the fluid medium (real type).

No verification of the order of magnitude.


.. code-block:: text

    CELE_I = celi

Imaginary part of the speed of propagation of acoustic waves in the fluid medium (the speed becomes complex, especially for a porous medium). No verification of the order of magnitude.

Note: when using fluid modeling (3D_FLUIDE for example) and putting RHO =0. and CELE_R =0., we get matrices with mass and stiffness that are really zero by CALC_MATR_ELEM. (see [:ref:`R4.02.02 <R4.02.02>`]).




Operand PESA_Z
~~~~

.. code-block:: text

    PESA_z = pz,

Acceleration of gravity along axis :math:`z`, used only and mandatory if the modeling chosen in AFFE_MODELE is 2D_FLUI_PESA (gravity waves and sloshing modes in the fluid).


Operand COEF_AMOR
~~~~

     COEF_AMOR = alpha,

This parameter is defined as the reflection coefficient (amplitude ratio of a reflected P wave). This coefficient comes into play exclusively if an absorbent fluid boundary is defined. In fact, the impedance of an absorbing fluid boundary is defined as :math:`{Z}_{C}=\rho c{q}_{\alpha }`, where :math:`\rho` is the density of the fluid defined with the operand RHO, :math:`c` is the propagation speed of acoustic waves defined by the operands CELE_R and CELE_I and :math:`{q}_{\alpha }=(1+\alpha )/(1-\alpha )` where :math:`\alpha` is defined with the operand COEF_AMOR. By default this value is :math:`0`. When :math:`\alpha =0` we have :math:`{q}_{\alpha }=1`, that is to say that we have a perfectly absorbent border. This model is only relevant when considering compressible fluid. One can choose the commonly accepted value :math:`\alpha =0.2` for the effect of sediments at the bottom of a reservoir.


Operand LONG_CARA
~~~~

     LONG_CARA = alpha,

In order to take into account the first-order infinity radiation condition, we can define an impedance term :math:`{Z}_{R}=\rho R` where :math:`R` is defined with the operand LONG_CARA. To asymptotically approximate the behavior of a wave by a cylindrical (spherical) wave propagating to infinity, we generally truncate the fluid domain to a cylinder or half-cylinder (sphere or half-sphere) of radius :math:`R`. Note that the first-order radiation condition is equivalent to the exact radiation condition for the propagation of a spherical wave. By default this value is :math:`0` (the first order condition is not activated). This parameter has an impact exclusively if an absorbent fluid boundary is defined.




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