Editing Mach wave (section)

{{Short description|Pressure wave}}
[[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. The angle of the Mach wave (~59 degrees) indicates a velocity of about Mach 1.17.]]
In [[fluid dynamics]], a '''Mach wave''', also known as a '''weak discontinuity''',<ref>Landau, Lev Davidovich, and Evgenii Mikhailovich Lifshitz. Fluid mechanics: Landau And Lifshitz: course of theoretical physics, Volume 6. Vol. 6. Elsevier, 2013.</ref><ref>Zelʹdovich, I︠A︡kov Borisovich, Yurii Petrovich Raizer, and Wallace D. Hayes. Physics of shock waves and high-temperature hydrodynamic phenomena. Vol. 1. New York: Academic Press, 1966.</ref> 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 process|isentropic]] compression in supersonic flows). A Mach wave is the weak limit of an oblique shock wave where time averages of flow quantities don't change (a normal shock is the other limit). If the size of the object moving at the speed of sound is near 0, then this domain of influence of the wave is called a '''Mach cone'''.<ref name="sasoh" /><ref>{{cite book |last1=E. Carscallen |first1=William |last2=Patrick |first2=H. Oosthuizen |title=Introduction to Compressible Fluid Flow |date=2013-07-12 |publisher=[[CRC Press]] |isbn=978-1-4398-7792-0 |edition=2 |language=en}}</ref>