Mach number is the ratio between the speed of an object moving trough a fluid and the speed of the sound in the fluid.

It is very used in aerodynamic calculations for aircrafts or missiles.

Aerodynamic regimes can be classified using Mach Number in:

1)      Subsonic  [Ma < 1]; for example the flow around a F1 race car.

2)      Transonic [ 0.8 < Ma <1.2]; the cruise speed of every civil aircraft (e.g. Airbus or Boeing) is transonic.

3)      Sonic [Ma = 1]; flow in the throat of a supersonic nozzle (De Laval or convergent-divergent nozzle) is sonic.

4)      Supersonic [1.2 < Ma < 5]; flow around a fighter or a missile is supersonic.

5)      Hypersonic [Ma > 5]; flight of spacecrafts during the atmospheric reentry is hypersonic.

It is important to note that Mach number is related to sound’s speed, hence it is different in every fluids (gas or liquid) and solids. Furthermore the sound’s speed is influenced by temperature.

Higher temperatures are related with an higher sound’s speed.

Higher is the density of the material and higher is the sound’s speed; for example the sound’s speed is higher in the water than in the air and higher in the steel than in the water.