Compressible flow

Fluid motion in which density changes significantly, typically because pressure variations produce appreciable changes in the fluid’s volume.

Mach number: The ratio of an object's speed to the local speed of sound. It indicates whether disturbances can travel upstream through the flow.
Equation of state: A relation connecting thermodynamic properties such as pressure, temperature, and density. It links pressure changes to density changes in a compressible fluid.
Supersonic flight: Flight at a speed greater than the local speed of sound. Aircraft at these speeds generate strong compressibility effects and often shocks.
Incompressible flow: Fluid motion modeled with constant density, often because density variations are negligible. It is the limiting approximation that compressible-flow analysis replaces when density changes matter.
Speed of sound: The speed at which small pressure disturbances propagate through a medium. It sets the reference speed for judging compressibility effects.
Ideal gas law: The equation relating pressure, volume, temperature, and amount of an ideal gas. It provides a common pressure–density–temperature relation for gas flows.
Converging-diverging nozzle: A nozzle with a narrow throat and widening downstream section, capable of accelerating gas to supersonic speed. Its area change can accelerate a compressible gas through Mach one.
Low-Mach-number flow: Flow whose speeds are much lower than the local speed of sound. Pressure-driven density changes are usually small in this regime.
Shock wave: A thin region across which pressure, temperature, and density change abruptly. Supersonic compression can concentrate into a shock wave.
Continuity equation: A mathematical statement that mass is conserved in a fluid flow. Changing density alters how mass conservation relates area and velocity.

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