13. Fluid material

13.1. Keyword factor FLUIDE

Definitions of constant fluid characteristics.

13.1.1. Operand RHO

RHO = rho

Fluid density. No verification.

13.1.2. Operands CELE_R and CELE_C

CELE_R = clear

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

No verification of the order of magnitude.

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 [R4.02.02]).

13.1.3. Operand PESA_Z

PESA_z = pz,

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

13.1.4. 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 \({Z}_{C}=\rho c{q}_{\alpha }\), where \(\rho\) is the density of the fluid defined with the operand RHO, \(c\) is the propagation speed of acoustic waves defined by the operands CELE_R and CELE_I and \({q}_{\alpha }=(1+\alpha )/(1-\alpha )\) where \(\alpha\) is defined with the operand COEF_AMOR. By default this value is \(0\). When \(\alpha =0\) we have \({q}_{\alpha }=1\), 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 \(\alpha =0.2\) for the effect of sediments at the bottom of a reservoir.

13.1.5. Operand LONG_CARA

LONG_CARA = alpha,

In order to take into account the first-order infinity radiation condition, we can define an impedance term \({Z}_{R}=\rho R\) where \(R\) 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 \(R\). 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 \(0\) (the first order condition is not activated). This parameter has an impact exclusively if an absorbent fluid boundary is defined.