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Subsection 4.14.3: General Specification of Fluid-FSI Materials Up Section 4.14: Viscous Fluids Section 4.15: Prestrain material

### 4.14.4 General Specification of Biphasic-FSI Materials

The material type for a biphasic-FSI material is “biphasic-FSI”. This is a hybrid biphasic material used for modeling dynamic viscous fluid flow and frictional filtration through a porous-deformable solid. In contrast to the standard biphasic material described in Section 4.8↑, this material accommodates fluid viscosity and fluid dynamics; therefore it can be interfaced seamlessly with a fluid-FSI material to allow viscous fluid transport across that interface. The following parameters must be defined:
 solid volume fraction in the reference configuration ( [ ] Specification of the fluid material Specification of the porous solid material Specification of the hydraulic permeability
The parameter <phi0> must be greater than 0 (no solid) and less than 1 (no porosity). 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 matrix constitutive relation and associated material properties, such as those selected from the list provided in Sections 4.1↑-4.7↑. The <permeability> tag encloses a description of the permeability constitutive relation and associated material properties, as may be selected from the list presented in Section 4.8.2↑. To simulate the special case of zero friction between the fluid and solid (such as in the fluid-FSI material), it is permissible to set the permeability to zero.
Example:
<material id="2" name="Material2" type="biphasic-FSI">
<phi0>0.25</phi0>
<fluid type="fluid">
<density>1e-9</density>
<k>1e+09</k>
<viscous type="Newtonian fluid">
<mu>1</mu>
<kappa>0</kappa>
</viscous>
</fluid>
<solid type="neo-Hookean">
<density>2e-9</density>
<E>200</E>
<v>0</v>
</solid>
<permeability type="perm-const-iso">
<perm>0.005625</perm>
</permeability>
</material>

The value of <density> in the <fluid> and <solid> materials represents the true mass density of each constituent.
Subsection 4.14.3: General Specification of Fluid-FSI Materials Up Section 4.14: Viscous Fluids Section 4.15: Prestrain material