# Mach Number (subsonic compressible flow)

## Description

In fluid mechanics, Mach number (M or Ma) is a dimensionless quantity representing the ratio of speed of an object moving through a fluid and the local speed of sound.

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.

Assuming air to be an ideal gas, the formula to compute Mach number in a subsonic compressible flow is derived from Bernoulli’s equation for M < 1 and is shown here.

Related formulas## Variables

M | mach number (dimensionless) |

γ | ratio of specific heat of a gas at a constant pressure to heat at a constant volume (1.4 for air) (dimensionless) |

q_{C} | impact pressure (dynamic pressure) (pascal) |

p | static pressure (pascal) |