In fluids, aerodynamics, and thermodynamics, sometimes it is important to distinguish whether a gas stream needs to be treated as compressible flow or incompressible flow. In this context, these terms have a little different implication than they might have in common use. Gases are pretty much always compressible in the sense that their density changes significantly with changes in pressure. However, in the specialized context of “compressible flow” we are talking about situations where the gas velocity is high enough that the kinetic energy of the flow plays a significant part in determining the properties and changes in properties of the gas. A common rule of thumb is that a flow should be treated as compressible if the velocities involved exceed about 1/3 the speed of sound in the fluid. Of course, that is a general guideline, not a sharp limit. In this post we’ll explore that guideline a little bit.
