In fluid mechanics, **Mach number** ( or ) (generally /ˈmɑːk/, sometimes /ˈmɑːx/ or /ˈmæk/) is a dimensionless quantity representing the ratio of speed of an object moving through a fluid and the local speed of sound.

where

- is the Mach number,
- is the velocity of the source relative to the medium and
- is the speed of sound in the medium.

Mach number varies by the composition of the surrounding medium and also by local conditions, especially temperature and pressure. The Mach number can be used to determine if a flow can be treated as an incompressible flow. If M < 0.2–0.3 and the flow is (quasi) steady and isothermal, compressibility effects will be small and a simplified incompressible flow model can be used.

The Mach number is named after Austrian physicist and philosopher Ernst Mach, a designation proposed by aeronautical engineer Jakob Ackeret. Because the Mach number is often viewed as 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."

In French, the Mach number is sometimes called the "nombre de Sarrau" ("Sarrau number") after Émile Sarrau who researched into explosions in the 1870s and 1880s.

Read more about Mach Number: Overview, Classification of Mach Regimes, High-speed Flow Around Objects, High-speed Flow in A Channel, Calculation

### Other articles related to "mach number, mach numbers, mach":

**Mach Number**

... The Coefficient of pressure varies with

**Mach number**by the relation given below where Cp is the compressible pressure coefficient Cp0 is the incompressible pressure coefficient M∞ is the freestream ...

... describes the angle through which a flow can turn isentropically for the given initial and final

**Mach number**... expressed as follows, where, is the Prandtl–Meyer function, is the

**Mach number**of the flow and is the ratio of the specific heat capacities ... As

**Mach number**varies from 1 to, takes values from 0 to, where For isentropic expansion, For isentropic compression, where, is the absolute value of the angle through which the flow turns, is the flow ...

... the pressure ratio, temperature ratio, angle of the wedge and the downstream

**Mach number**can all be calculated knowing the upstream

**Mach number**and the shock angle ... Smaller shock angles are associated with higher upstream

**Mach numbers**, and the special case where the shock wave is at 90° to the oncoming flow (Normal shock), is associated with a

**Mach**...

**Mach Number**From Pitot Tube Pressure

... At altitude, for reasons explained,

**Mach number**is a function of temperature ... operate using pressure differential to compute

**Mach number**, not temperature ... Assuming air to be an ideal gas, the formula to compute

**Mach number**in a subsonic compressible flow is found from Bernoulli's equation for M ...

... the speed of sound, it first reaches its critical

**mach number**, where air flowing over low-pressure areas of its surface locally reaches the speed of sound, forming shock waves ...

**Mach number**is more useful, and most high-speed aircraft are limited to a maximum operating

**Mach number**, also known as MMO ... For example, if the MMO is

**Mach**0.83, then at 30,000 feet (9,144 m) where the speed of sound under standard conditions is 590 knots (1,093 km/h 679 mph), the true airspeed at MMO is 489 knots (9 ...

### Famous quotes containing the words number and/or mach:

“As equality increases, so does the *number* of people struggling for predominance.”

—Mason Cooley (b. 1927)

“Physics is experience, arranged in economical order.”

—Ernst *Mach* (1838–1916)