Mach wave
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In fluid dynamics, a slight change of pressure added to a compressible flow is transmitted as a pressure wave travelling with the speed of sound. These weak waves can combine in supersonic flow to become a shock wave if sufficient Mach waves are present at any location. Thus it is possible to have shockless compression or expansion in a supersonic flow by having the production of Mach waves sufficiently spaced (cf isentropic compression in supersonic flows). A Mach wave is a kind of weak shock wave caused by a small disturbance in the flow. They propagate across the flow at an angle=arcsin(1/M).
Mach waves can be used in schlieren or shadowgraph observations to determine the local Mach number of the flow. Early observations by Ernst Mach used grooves in the wall of a duct to produce Mach waves in a duct, which were then photographed by the schlieren method, to obtain data about the flow in nozzles and ducts.