Shock diamonds (also known as Mach diamonds, Mach disks or dancing diamonds) are a formation of stationary wave patterns that appears in the exhaust plume of an aerospace propulsion system, such as a supersonic jet engine, rocket, ramjet, or scramjet when it is operated in an atmosphere.
Shock diamonds are formed when the supersonic exhaust from a nozzle is slightly over or under-expanded, meaning that the pressure of the gases exiting the nozzle is different from the ambient air pressure. A complex flow field results as the shock wave is reflected back and forth between the free fluid jet boundary and a visible repeating diamond-shaped pattern is formed which gives the shock diamonds their name.
The Daily Omnivore 
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