A fluid can be defined as a substance that conforms to the shape of the object that contains it. This definition can easily be demonstrated by a fluid that is at rest (i.e. the red fluid in the beaker). Another definition is that a fluid is a substance that will continually deform when it is exposed to a shearing stress. This definition is better suited for describing a fluid in motion.
The ability of a fluid to resist compressive stresses is generally much greater than its ability to resist shear stresses. The two most common classifications of fluids are liquids and gasses. Gasses have the characteristic that they can easily be compressed into smaller volumes. Many manufacturing processes and machines take advantage of this feature. On the other hand, a liquid doesn’t compress very easily. Hydraulic pistons and motors take advantage of the incompressibility of hydraulic oils.
The flow of a compressible fluid is generally very dependent on changes in pressure, temperature, and density. The interaction of these thermodynamic properties must be included in an analysis of the fluid. In an incompressible fluid, the density is constant and the coupling between pressure and temperature changes is less important. Because of the complexity associated with compressibility, the field of fluid dynamics is divided in the study of incompressible fluids (liquids) and compressible flows (gasses). An incompressible fluid is a special case of a compressible fluid.
- At the end of this page the reader will know the definition of a fluid and be able to describe a compressible and incompressible fluid.