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| [[Image:Schlierenfoto Mach 1-2 Pfeilflügel - NASA.jpg|thumb|right|[[Schlieren photography|Schlieren photograph]] of an attached shock on a sharp-nosed supersonic body. The Mach angle is acute, showing that the body exceeds Mach 1.]]
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| In [[fluid dynamics]], a '''Mach wave''' is a pressure wave traveling with the [[speed of sound]] caused by a slight change of [[pressure]] added to a [[compressible flow]]. These weak waves can combine in supersonic flow to become a [[shock wave]] if sufficient Mach waves are present at any location. Such a shock wave is called a '''Mach stem''' or '''Mach front'''. 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 the weak limit of an oblique shock wave (a normal shock is the other limit). They propagate across the flow at the '''Mach angle''' ''μ'':<ref>[http://www.grc.nasa.gov/WWW/K-12/airplane/machang.html Mach angle at NASA]</ref>
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| :<math>\mu = \arcsin\left(\frac{1}{M}\right),</math>
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| where ''M'' is the [[Mach number]].
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| Mach waves can be used in [[Schlieren photography|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. Mach angles may also occasionally be visualized out of their condensation in air, as in the jet photograph below.
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| [[Image:FA-18 Hornet breaking sound barrier (7 July 1999) - filtered.jpg|right|thumb|U.S. Navy [[F/A-18 Hornet|F/A-18]] at transonic speed. The white halo is formed by condensed water droplets which are thought to result from an increase in air temperature and pressure behind the shockwave(see [[Prandtl-Glauert Singularity]]). The Mach angle of the weak attached shock made visible by the halo is seen to be close to arcsin (1) = 90 degrees.<ref>[http://antwrp.gsfc.nasa.gov/apod/ap070819.html APOD: 19 August 2007- A Sonic Boom<!-- Bot generated title -->]</ref><ref>http://www.eng.vt.edu/fluids/msc/gallery/conden/mpegf14.htm</ref>]]
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| == See also ==
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| * [[Gas dynamics]]
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| * [[Prandtl-Meyer expansion fan]]
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| * [[Shadowgraph|Shadowgraph technique]]
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| * [[Schlieren photography]]
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| * [[Shock wave]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| * [http://www.youtube.com/watch?v=iNBZBChS2YI Supersonic wind tunnel test demonstration (Mach 2.5) with flat plate and wedge creating an oblique shock along with numerous Mach waves(Video)]
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| [[Category:Fluid dynamics]]
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| [[Category:Waves]]
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| {{fluiddynamics-stub}}
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Revision as of 15:03, 5 February 2014
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