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In fluid dynamics, the Mach number (M or Ma) (; ) is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound. : where : is the Mach number, : is the local flow velocity with respect to the boundaries (either internal, such as an object immersed in the flow, or external, like a channel), and : is the speed of sound in the medium. The local speed of sound, and thereby the Mach number, depends on the condition of the surrounding medium, in particular the temperature and pressure. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow. The medium can be a gas or a liquid. The boundary can be traveling in the medium, or it can be stationary while the medium flows along it, or they can both be moving, with different velocities: what matters is their relative velocity with respect to each other. The boundary can be the boundary of an object immersed in the medium, or of a channel such as a nozzle, diffusers or wind tunnels chaneling the medium. As the Mach number is defined as the ratio of two speeds, it is a dimensionless number. If < 0.2–0.3 and the flow is quasi-steady and isothermal, compressibility effects will be small and a simplified incompressible flow equations can be used.〔〔 The Mach number is named after Austrian physicist and philosopher Ernst Mach, a designation proposed by aeronautical engineer Jakob Ackeret. As the Mach number is a dimensionless quantity rather than a unit of measure, with Mach, the number comes ''after'' the unit; the second Mach number is "Mach 2" instead of "2 Mach" (or Machs). This is somewhat reminiscent of the early modern ocean sounding unit "mark" (a synonym for fathom), which was also unit-first, and may have influenced the use of the term Mach. In the decade preceding faster-than-sound human flight, aeronautical engineers referred to the speed of sound as ''Mach's number'', never "Mach 1."〔Bodie, Warren M., ''The Lockheed P-38 Lightning'', Widewing Publications ISBN 0-9629359-0-5.〕 == Overview == At standard sea level conditions (corresponding to a temperature of 15 degrees Celsius), the speed of sound is 340.3 m/s〔Clancy, L.J. (1975), Aerodynamics, Table 1, Pitman Publishing London, ISBN 0-273-01120-0〕 (1225 km/h, or 761.2 mph, or 661.5 knots, or 1116 ft/s) in the Earth's atmosphere. The speed represented by Mach 1 is not a constant; for example, it is mostly dependent on temperature. Since the speed of sound increases as the ambient temperature increases, the actual speed of an object traveling at Mach 1 will depend on the temperature of the fluid through which the object is passing. Mach number is useful because the fluid behaves in a similar manner at a given Mach number, regardless of other variables. So, an aircraft traveling at Mach 1 at 20°C (68°F) at sea level will experience shock waves just like an aircraft traveling at Mach 1 at 11,000 m (36,000 ft) altitude at −50°C (−58°F), even though the second aircraft is only traveling 86% as fast as the first.〔(()). National Aeronautics and Space Administration website page "Mach Number", NASA.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mach number」の詳細全文を読む スポンサード リンク
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