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Mach wave

 

 
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Mach wave
 
 
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In fluid dynamics
Fluid dynamics

In physics, fluid dynamics is the sub-discipline of fluid mechanics dealing with fluid flow — the natural science of fluids in motion....
, a Mach wave is a pressure wave traveling with the speed of sound
Speed of sound

Sound is a vibration that travels through an elasticity medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time....
 caused by a slight change of pressure
Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
 added to a compressible flow
Compressible flow

In fluid dynamics, a flow is considered to be a compressible flow if the density of the fluid changes with respect to pressure. In general, this is the case where the Mach number of the flow exceeds 0.3....
. These weak waves can combine in supersonic flow to become a shock wave
Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field....
 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).






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In fluid dynamics
Fluid dynamics

In physics, fluid dynamics is the sub-discipline of fluid mechanics dealing with fluid flow — the natural science of fluids in motion....
, a Mach wave is a pressure wave traveling with the speed of sound
Speed of sound

Sound is a vibration that travels through an elasticity medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time....
 caused by a slight change of pressure
Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
 added to a compressible flow
Compressible flow

In fluid dynamics, a flow is considered to be a compressible flow if the density of the fluid changes with respect to pressure. In general, this is the case where the Mach number of the flow exceeds 0.3....
. These weak waves can combine in supersonic flow to become a shock wave
Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field....
 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 :

where M is the Mach number
Mach number

Mach number is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance. It is commonly used to represent an object's speed, when it is travelling at the speed of sound....
.

Mach waves can be used in schlieren
Schlieren photography

Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the Germany physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects....
 or shadowgraph observations to determine the local Mach number of the flow. Early observations by Ernst Mach
Ernst Mach

Ernst Mach was an Austrians physicist and philosopher and is the namesake for the Mach number and the optical illusion known as Mach bands....
 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
    Gas dynamics

    Gas dynamics is a branch of fluid dynamics concerned with studying the motion of gases....
  • Prandtl-Meyer expansion fan
    Prandtl-Meyer expansion fan

    A Prandtl-Meyer expansion fan is a centered expansion process, which turns a supersonic flow around a convex corner. The fan consists of infinite number of Mach waves, diverging from a sharp corner....
  • Shadowgraph technique
    Shadowgraph

    Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function....
  • Schlieren photography
    Schlieren photography

    Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the Germany physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects....