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Compressibility of Fluids Review
Usually a fluid may be considered incompressible when the velocity of the fluid is greater than one-third of the speed of sound for the fluid, or if the fluid is a liquid. The treatment of a fluid that is considered incompressible is easy because the density is assumed to be constant, giving a simple relationship for the state of the substance. The variation of density of the fluid with changes in pressure is the primary factor considered in deciding whether a fluid is incompressible.
Fluids that are compressible have much more complex equations to deal with, due to density changes, and have property relationships that vary more rapidly than incompressible fluids. In addition, fixing the state of a liquid can be done easily by knowing its temperature and pressure. Once the substance becomes a gas, the process becomes more difficult.
In thermodynamics and fluid mechanics , compressibility is a measure of the relative volume change of a fluid or solid as a response to a pressure (or mean stress ) change.
where V is volume and p is pressure .