Advanced Topics and Future Trends - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 318

9.2.2 Fluid Structure Interaction

In all the examples presented thus far, the boundary walls have been treated as
fixed rigid walls. However, respiratory functions are dynamic in nature with moving
anatomical parts. As its name suggests, a fluid structure interaction (FSI) involves
the influence of the moving fluid on a flexible structure which deforms and in turn
influences the fluid flow. This differs from the deforming/moving mesh presented in
Sect. 9.2.1 in that for FSI, the moving structures are determined through structural
dynamics equations coupled with the fluid dynamics equations, whereas for a de-
forming mesh, the moving structure is predefined by an independent equation and
is not influenced by the fluid dynamics. FSI involves multiphysics, and the different
coupling methods to describe the transfer of the boundary conditions at the fluid-
structure interface are a critically important feature of FSI. Generally there are two
approaches to solve an FSI problem: monolithic and partition.

Fig. 9.3 a Monolithic
approach: the equations
governing the flow and the
displacement of the structure
are solved simultaneously.
b Partition approach: the
equations governing the flow
and the displacement of the
structure are solved separately

In the monolithic approach (Fig. 9.3 a), the governing equations are reformulated
by combining the fluid and structural equations of motion that are solved and inte-
grated in time simultaneously. This kind of method poses severe limitations because