Mach number

One of the basic criteria of aerodynamic similarity when the viscosity of the gas cannot be neglected. Mach's number $M=v/a$ is the ratio of the velocity of the gas flow and the velocity of sound at the same point of the flow (or the ratio of the velocity of a body in the gas and the velocity of sound in this medium). The number was called thus to honour E. Mach.

References

[a1]	L. Howarth (ed.) , Modern development in fluid dynamics. High speed flow , 1–2 , Oxford Univ. Press (1953)