Mach number (Ma) is defined as a ratio of speed to the speed of sound in the medium in case. The Mach number is commonly used both with objects travelling at high speed in a fluid, and with high-speed fluid flows inside channels such as nozzles, diffusers or wind tunnels. As it is defined as a ratio of two speeds, it is a dimensionless number. At standard sea level conditions, Mach 1 is 1,225 km/h (765.6 MPH) in the atmosphere. Speed (symbol: v) is the rate of motion, or equivalently the rate of change of position, expressed as distance d moved per unit of time t. ... The speed of sound c (from Latin celeritas, velocity) varies depending on the medium through which the sound waves pass. ... Rocket Nozzle A nozzle is a mechanical device designed to control the characteristics of a fluid flow as it exits from an enclosed chamber into some medium. ... Diffuser is a pop punk band from the Long Island area that formed in 1994. ... A wind tunnel is a research tool developed to assist with studying the effects of air moving over or around solid objects. ... In the physical sciences, a dimensionless number (or more precisely, a number with the dimensions of 1) is a quantity which describes a certain physical system and which is a pure number without any physical units; it does not change if one alters ones system of units of measurement... For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ...

Since the speed of sound increases as the temperature increases, the actual speed of an object travelling at Mach 1 will depend on the fluid temperature around it.

It can be shown that the Mach number is also the ratio of inertial forces (also referred to aerodynamic forces) to elastic forces.

The Mach number is named after Austrian physicist and philosopher Ernst Mach. Ernst Mach Ernst Mach (February 18, 1838 - February 19, 1916) was an Austrian-Czech physicist and philosopher. ...

High-speed flow around objects

High speed flight can be classified in five categories:

• Subsonic: M < 1
• Sonic: M = 1
• Supersonic: M > 1
• Transsonic: 0.8 < M < 1.3
• Hypersonic: M > 5

(For comparison: the required speed for low Earth orbit is ca. 7.5 km/s = Mach 22.06 in air at sea level) Subsonic has two possible meanings: A speed lower than the speed of sound is called subsonic. ... Any speed over the speed of sound, which is approximately 343 m/s, 1,087 ft/s, 761 mph or 1,225 km/h in air at sea level, is said to be supersonic. ... Transonic is an aeronautics term referring to a range of velocities just below and above the speed of sound. ... This article is about hypersonic speeds in aerodynamics. ... A low Earth orbit (LEO) is an orbit in which objects such as satellites are below intermediate circular orbit (ICO) and far below geostationary orbit, but typically around 350 - 1400 km above the Earths surface. ...

At transsonic speeds, the flow field around the object includes both sub- and supersonic parts. The transsonic regime begins when first zones of Ma>1 flow appear around the object. In case of an airfoil (such as an aircraft's wing), this typically happens above the wing. Supersonic flow can decelerate back to subsonic only in a normal shock; this typically happens before the trailing edge. (Fig.1a)

As the velocity increases, the zone of Ma>1 flow increases towards both leading and trailing edges. As Ma=1 is reached and passed, the normal shock reaches the trailing edge and becomes a weak oblique shock: the flow decelerates over the shock, but remains supersonic. A normal shock is created ahead of the object, and the only subsonic zone in the flow field is a small area around the object's leading edge. (Fig.1b)

(a)	(b)

Fig. 1. Mach number in transsonic airflow around an airfoil; Ma<1 (a) and Ma>1 (b). Drawing to show Mach number variation around an airfoil at transsonic speed, 0. ... Drawing to show Mach number variation around airfoil at transsonic speed, 1. ...

When an aircraft exceeds Mach 1 (i.e. the sound barrier) a large pressure difference is created just in front of the aircraft. This abrupt pressure difference, called a shock wave, spreads backward and outward from the aircraft in a cone shape (a so-called Mach cone). It is this shock wave that causes the sonic boom heard as fast moving aircraft travels overhead. A person inside the aircraft will not hear this. The higher the speed, the more narrow the cone; at just over Ma=1 it is hardly a cone at all, but closer to a wall only slight bending backwards. U.S. Navy F/A-18 at transonic speed. ... An aircraft is any machine capable of atmospheric flight. ... In fluid dynamics, a shock wave is a nonlinear pressure wave. ... When an aircraft breaks the sound barrier, an unusual cloud sometimes forms in its wake. ...

At fully supersonic velocity the shock wave starts to take its cone shape, and flow is either completely supersonic, or (in case of a blunt object), only a very small subsonic flow area remains between the object's nose and the shock wave it creates ahead of itself. (In the case of a sharp object, there is no air between the nose and the shock wave: the shock wave starts from the nose.)

As the Mach number increases, so does the strength of the shock wave and the Mach cone becomes increasingly narrow. As the fluid flow crosses the shock wave, its speed is reduced and temperature, pressure, and density increase. The stronger the shock, the greater the changes. At high enough Mach numbers the temperature increases so much over the shock that ionization and dissociation of gas molecules behind the shock wave begin. Such flows are called hypersonic. In fluid dynamics, a shock wave is a nonlinear pressure wave. ...

It is clear that any object travelling at hypersonic velocities will likewise be exposed to the same extreme temperatures as the gas behind the nose shock wave, and hence choice of heat-resistant materials becomes important.

High-speed flow in a channel

As a flow in a channel crosses Ma=1 becomes supersonic, one significant change takes place. Common sense would lead one to expect that contracting the flow channel would increase the flow speed and at subsonic speeds this holds true. However, once the flow becomes supersonic, the relationship of flow area and speed is reversed: expanding the channel actually increases the speed.

The obvious result is that in order to accelerate a flow to supersonic, one needs a convergent-divergent nozzle, where the converging section accelerates the flow to Ma=1, and the diverging section continues the acceleration to supersonic. Such nozzles are called De Laval nozzles. A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle ) is a tube that is pinched in the middle, making an hourglass-shape. ...