Compressible flow refers to the flow of fluids in which the density of the fluid changes significantly due to pressure or temperature variations. This is in contrast to incompressible flow, where the fluid density remains nearly constant throughout the flow field. Compressible flow is typically observed in gases, especially when the flow velocity approaches or exceeds the speed of sound (approximately 343 meters per second or 1,125 feet per second at sea level and at 20 degrees Celsius).
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In compressible flow, a fluid parcel's mass density can change as it moves. The continuity equation expresses the relation between density change and local expansion:A compressed gas parcel occupies less volume without losing mass, so its density rises. The resulting pressure response depends on an equation of state and on how the parcel exchanges heat.
A piston driven into a gas demonstrates why this matters: pressure disturbances propagate at the speed of sound, and sufficiently strong or rapid compression can produce a shock wave. The Mach number compares bulk speed with this propagation speed. Low Mach number often supports a nearly constant-density model for weakly heated gases, but heating or composition changes can still make density variations important at low flow speed. Compressibility is therefore a physical approximation to assess, rather than a label fixed by speed alone.