Mach wave

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. 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 the Mach angle "μ" [ [http://www.grc.nasa.gov/WWW/K-12/airplane/machang.html Mach angle at NASA] ] :

:$mu\; =\; arcsinleft(frac\{1\}\{M\}\; ight),$

where "M" is the Mach number.

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.

See also

* Gas dynamics
* Prandtl-Meyer expansion fan
* Shadowgraph technique
* Schlieren photography

References