Mach Number is a ratio of inertia to compressibility. It is the non-dimensional factor governing resistance due to longitudinal (compressional) wave formation.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.The Mach number Ma allows us to define flight regimes in which compressibility effects vary.
if the mach number is < 1, the flow speed is lower than the speed of sound - and the speed is subsonic.
If the mach number is ~ 1, the flow speed is approximately like the speed of sound - and the speed is transonic.
If the mach number is > 1, the flow speed is higher than the speed of sound - and the speed is supersonic.
If the mach number is >> 1, the flow speed is much higher than the speed of sound - and the speed is hypersonic.
Ma = v/c
Where:
- Ma is the Mach Number
- V is the ratio of the speed of flow
- C is the speed of sound in a fluid
Notes
The Mach number M allows us to define flight regimes in which compressibility effects vary.
- Subsonic conditions occur for Mach numbers less than one, M < 1 . For the lowest subsonic conditions, compressibility can be ignored.
- As the speed of the object approaches the speed of sound, the flight Mach number is nearly equal to one, M = 1, and the flow is said to be transonic. At some places on the object, the local speed exceeds the speed of sound. Compressibility effects are most important in transonic flows and lead to the early belief in a sound barrier. Flight faster than sound was thought to be impossible. In fact, the sound barrier was only an increase in the drag near sonic conditions because of compressibility effects. Because of the high drag associated with compressibility effects, aircraft do not cruise near Mach 1.
- Supersonic conditions occur for Mach numbers greater than one, 1 < M < 3. Compressibility effects are important for supersonic aircraft, and shock waves are generated by the surface of the object. For high supersonic speeds, 3 < M < 5, aerodynamic heating also becomes very important for aircraft design.
- For speeds greater than five times the speed of sound, M > 5, the flow is said to be hypersonic. At these speeds, some of the energy of the object now goes into exciting the chemical bonds which hold together the nitrogen and oxygen molecules of the air. At hypersonic speeds, the chemistry of the air must be considered when determining forces on the object. The Space Shuttle re-enters the atmosphere at high hypersonic speeds, M ~ 25. Under these conditions, the heated air becomes an ionized plasma of gas and the spacecraft must be insulated from the high temperatures.
For supersonic and hypersonic flows, small disturbances are transmitted downstream within a cone. The trigonometric sine of the cone angle b is equal to the inverse of the Mach number M and the angle is therefore called the Mach angle.
sin(b) = 1 / M