In physics, gases and liquids are summarized under the term . Many physical laws apply equally to gases and liquids, because these substances differ in some properties quantitatively (in the order of magnitude of the effect) instead of qualitatively.
The fluid mechanics means any substance as the fluid that a sufficiently slow shear no resistance opposes (finite viscosity ). In this sense, the term includes not only matter in the liquid and gaseous state of aggregation but also plasma , suspensions and aerosols .
Real fluids are divided into
- Newtonian fluids : they can be described by the Navier-Stokes equations ;
- Non-Newtonian fluids : their behavior is more complex and is treated in rheology .