Shock wave

A shock wave is a rapidly propagating disturbance that produces abrupt changes in pressure, density, temperature, and flow velocity. It forms when a disturbance travels faster than information can spread through the medium.

Mach number: The ratio of an object's speed to the local speed of sound. It indicates whether a moving source can outrun sound and generate a shock.
Sonic boom: The sound heard when shock waves from a supersonic aircraft reach an observer. It is the familiar atmospheric consequence of a moving shock system.
Speed of sound: The speed at which small pressure disturbances propagate through a medium. A shock's defining motion is faster than this local signal speed.
Acoustic wave: A small-amplitude pressure disturbance that propagates through a medium, usually with nearly linear behavior. Unlike a shock, an ordinary acoustic wave has gradual, small changes in pressure.
Ernst Mach: An Austrian physicist who studied supersonic motion and gave his name to the Mach number. His experiments and analysis helped establish how projectiles behave beyond sound speed.
Rankine–Hugoniot conditions: Equations relating conserved quantities on either side of a discontinuity in a flowing medium. They determine how pressure, density, and velocity change across an ideal shock.
Bow shock: A curved shock that forms ahead of an object moving supersonically through a fluid. It forms around aircraft and planets as their motion compresses the surrounding medium.
Compressible flow: Fluid motion in which density changes significantly with pressure or velocity. Shock waves belong to flows where compression cannot be ignored.
Contact discontinuity: A boundary across which density or composition changes while pressure and normal velocity remain continuous. It can accompany a shock but does not itself produce a pressure jump.
William John Macquorn Rankine: A Scottish engineer and physicist who developed foundational work in thermodynamics and fluid mechanics. His equations describe conservation across shock discontinuities.

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