Theory Manual Version 3.4
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Subsection 2.11.2: Energy Balance Up Chapter 2: Continuum Mechanics Subsection 2.12.1: FSI Governing Equations

## 2.12 Fluid-Structure Interactions

We model the fluid domain in an FSI analysis as a mixture of an isothermal compressible viscous fluid and a hyperelastic compressible solid, where the solid has zero apparent density, and negligible (but non-zero) elasticity [36]. In a mixture framework, all mixture constituents coexist at every point in the continuum. For our FSI implementation, the finite element mesh is defined on the solid material, and the negligible (but non-zero) elasticity of the solid is intended to regularize the mesh deformation. The fluid flows through this mesh, unimpeded by the solid. In particular, in this FSI model there is no frictional interaction between the fluid and solid materials inside the mixture domain (i.e., no Darcy-Brinkman type of friction), but the no-slip boundary condition may be prescribed on boundaries of the mixture domain, where applicable.
Subsection 2.11.2: Energy Balance Up Chapter 2: Continuum Mechanics Subsection 2.12.1: FSI Governing Equations