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Subsubsection 4.14.2.5: Cross Model Up Section 4.14: Viscous Fluids Subsection 4.14.4: General Specification of Biphasic-FSI Materials

### 4.14.3 General Specification of Fluid-FSI Materials

The material type for a fluid-FSI material is “fluid-FSI”. This type of material is used for fluid flow through a deforming mesh. The material is treated as a special case of fluid-solid mixture, with the solid material used to regularize the mesh deformation. The following parameters must be defined:
 Specification of fluid model Specification of elastic solid model for mesh deformation
The <fluid> tag encloses a description of the fluid, as given in Section 4.14.1↑. The <solid> tag encloses a description of the solid, which should be modeled as an unconstrained elastic solid, as given in Section 4.1.3↑. The recommended choice is the unconstrained neo-Hookean solid defined in Section 4.1.3.16↑, with and set to a very small (but non-zero) value.
Example:
<material id="1" name="Fluid-FSI domain" type="fluid-FSI">
<fluid type="fluid">
<density>1000</density>
<k>2.2e9</k>
<viscous type="Newtonian fluid">
<mu>0.001</mu>
<kappa>0</kappa>
</viscous>
</fluid>
<solid type="neo-Hookean">
<density>0</density>
<E>1e-9</E>
<v>0</v>
</solid>
</material>
The value of <density> in the <solid> material is internally set to zero regardless of the user-defined value, since the deforming mesh should not be subjected to inertial forces.
Subsubsection 4.14.2.5: Cross Model Up Section 4.14: Viscous Fluids Subsection 4.14.4: General Specification of Biphasic-FSI Materials