Machnummer
Machnummer, or Mach number, is a dimensionless quantity used in fluid dynamics and aerodynamics to express the ratio of an object's speed to the local speed of sound in the surrounding medium. It is defined by M = v / a, where v is the flow velocity relative to the gas and a is the local speed of sound. For an ideal gas, a = sqrt(γ R T), with γ the heat capacity ratio, R the specific gas constant, and T the absolute temperature. The Mach number therefore depends on temperature and composition of the gas in addition to velocity.
The Mach number indicates the level of compressibility effects in the flow. For M much less than
Local versus free-stream: M can vary along a flow field. The free-stream Mach number M∞ is used
Typical regimes: subsonic roughly M < 0.8, transonic about 0.8–1.2, supersonic greater than 1, and hypersonic often
Applications: the concept is fundamental for aircraft design, propulsion, and aerodynamics, governing wave drag, lift, and
In German usage, Machzahl or Machnummer denotes the same quantity as the English Mach number.