What distinguishes FEM simulations from CFD simulations—and when does multiphysics come into play?

The finite element method (FEM) is used to analyze the mechanical behavior of components and structures—such as stresses, deformations, or fracture behavior.
Computational fluid dynamics (CFD), on the other hand, describes flow and thermal processes such as air flows, cooling, or pressure distributions.
In many cases of damage, both disciplines overlap: a component can deform due to thermal stress, which in turn leads to mechanical damage. In such cases, a multiphysics simulation is useful because it combines the interactions of flow, heat, and structure, thereby providing a particularly realistic overall picture.