Mach Number is defined as the ratio of speed to the speed of sound in the medium in study. Since the speed of sound increases with the increase of temperature, the actual speed of an object travelling on Mach 1 will depend primarily on the fluid temperature around it. The other values upon which the exact mach number depends are: the gas constant of the fluid (R), and the adiabatic index of the gas().

The Mach Number is named after Austrian physicist and philosopher Ernst Mach.


Unlike most units of measure, with Mach the number comes after the unit, so one says "Mach 2" instead of "2 Mach" (or Machs).


A Mach number of approximately 0.3 is representative of the threshold at which the effects of compressibility become important in aerodynamics and fluid mechanics. The basic assumption of bernoulli's equation is incompressible flow. This assumption becomes invalid at large Mach numbers.


The effects of shock waves and expansion waves also become apparent at high mach numbers. Shock waves are a sudden change in fluid properties across an infinitesimal distance. Shock waves produce instantaneous drops in pressure, density and velocity. Examples of shock wave formation include at the leading edges of super-sonic aircraft, and high speed gas flow nozzles.


See topics such as: compressible flow, variable area ducts/nozzles, adiabatic compressible flow, oblique shock waves, and normal shock waves.

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