Editing Shock wave (section)

== In supersonic flows ==
[[Image:Pressure plot.png|200px|thumb|Pressure–time diagram at an external observation point for the case of a supersonic object propagating past the observer. The leading edge of the object causes a shock (left, in red) and the trailing edge of the object causes an expansion (right, in blue).]]
{{supersonic_shockwave_cone.svg}}

The abruptness of change in the features of the medium, that characterize shock waves, can be viewed as a [[phase transition]]: the pressure–time diagram of a supersonic object propagating shows how the transition induced by a shock wave is analogous to a ''dynamic phase transition''.

When an object (or disturbance) moves faster than information can propagate into the surrounding fluid, the fluid near the disturbance cannot react or "get out of the way" before the disturbance arrives. In a shock wave the properties of the fluid ([[density]], [[pressure]], [[temperature]], [[flow velocity]], [[Mach number]]) change almost instantaneously.<ref>[https://www.mdpi.com/2311-5521/7/1/16 Nikonov, V. A Semi-Lagrangian Godunov-Type Method without Numerical Viscosity for Shocks. Fluids 2022, 7, 16. https://doi.org/10.3390/fluids7010016]</ref><ref>[https://www.mdpi.com/2311-5521/10/5/133 Nikonov V. Modified Semi-Lagrangian Godunov-Type Method Without Numerical Viscosity for Shocks. Fluids. 2025; 10(5):133. https://doi.org/10.3390/fluids10050133]</ref> Measurements of the thickness of shock waves in air have resulted in values around 200&nbsp;nm (about 10<sup>−5</sup> in),<ref>{{cite book |title=Introduction To Fluid Mechanics |edition=Fourth |first1=Robert W. |last1=Fox |first2=Alan T. |last2=McDonald |date=20 January 1992 |publisher=Wiley |isbn=0-471-54852-9 }}</ref> which is on the same order of magnitude as the mean free path of gas molecules. In reference to the continuum, this implies the shock wave can be treated as either a line or a plane if the flow field is two-dimensional or three-dimensional, respectively.

Shock waves are formed when a pressure front moves at supersonic speeds and pushes on the surrounding air.<ref>{{Cite journal | title=High-speed Imaging of Shock Wave, Explosions and Gunshots | journal=American Scientist | volume=94 | issue=1 | year=2006 | pages= 22–31 | first1=Gary S. | last1=Settles| doi=10.1511/2006.57.22 }}</ref> At the region where this occurs, sound waves travelling against the flow reach a point where they cannot travel any further upstream and the pressure progressively builds in that region; a high-pressure shock wave rapidly forms.

Shock waves are not conventional sound waves; a shock wave takes the form of a very sharp change in the gas properties. Shock waves in air are heard as a loud "crack" or "snap" noise. Over longer distances, a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a [[sonic boom]], commonly created by the supersonic flight of aircraft.

The shock wave is one of several different ways in which a gas in a supersonic flow can be compressed. Some other methods are [[Isentropic process#Isentropic flow|isentropic]] compressions, including [[Ludwig Prandtl|Prandtl]]–Meyer compressions. The method of compression of a gas results in different temperatures and densities for a given pressure ratio which can be analytically calculated for a non-reacting gas. A shock wave compression results in a loss of total pressure, meaning that it is a less efficient method of compressing gases for some purposes, for instance in the intake of a [[scramjet]]. The appearance of pressure-drag on supersonic aircraft is mostly due to the effect of shock compression on the flow.