Mach number is the ratio of speed to sound speed. The sound speed (i.e. the speed of sound propagation) is differentheight、temperatureIt is different from the atmospheric density and other states, but it is a relative value. The specific speed of each "horse" is not fixed[1]。[2]Mach is actually Austriaphysical scientistErnst Mach (1838-1916) was named after him because he first quoted the unit.
Mach number 1 is doubleSonic speed(Sound waves can propagate in solid, liquid or gas media. The higher the density of the media, the faster the speed of sound, so the size of Mach is not fixed.): Mach numbers less than 1 are subsonic, and Mach numbers greater than 5 areHypersonic；Mach number is the ratio of flight speed to the speed of sound at that time. If it is greater than 1, it is faster than the speed of sound. Similarly, if it is less than 1, it is slower than the speed of sound.
Sonic speed
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Sonic speedIs the propagation speed of weak pressure disturbance in the medium[3]Its size varies with the nature and state of the medium.The sound speed in the air is about 340m/s at 1 standard atmospheric pressure and 15 ℃.
Ernst Mach
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Austrian scholar Ernst Mach
Mach was named in memory of Austrian scholarsErnst Mach（Ernst Mach, 1838-1916）[1]。
Ernst Mach (1838-02-18-1916-02-19), Austrian Czech physicist[4]Psychologist, philosopher[5], the founder of Machism.He was born on February 18, 1838 in Moravia (now Czech Republic) and died in Hal near Munich, Germany.
Mach devoted himself to the research of experimental physics in his life and published more than 100 research papers and reports.Mach is best known for his research on flying projectiles using schlieren technology. His most important achievement is that he discovered shock waves and took clear pictures of shock waves when studying the high-speed movement of objects in gas.In several papers since 1887, he pointed out that objects moving in the air emit sound at the speed of soundcThe propagating spherical disturbance wave, when the velocity of the objectvgreater thancThe wave front of the disturbance wave forms a conical envelope with the object as the vertex (see Mach cone), and the angle formed by the generatrix of the cone surface and the moving direction of the objectαAndv、cIs sinα=c/v。In 1907, L. Plunt first measured the angleαIs Mach angle.In 1929, J. Aklaite considered the ratiov/cIt is increasingly important in aerodynamic research, and it is suggested to use Mach number to express this ratio.Mach number has become an important parameter to characterize the state of fluid motion.the Second World WarAfter,Supersonic flightWith the appearance of atomic explosion, his research achievements have been paid more attention.Mach effect, Mach wave, Mach angle and other terms named after Mach are widely used in aerodynamics, which shows the historical contribution of Mach in mechanics.
mach number
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Mach number is widely used in aerodynamics and is generally used in aerospace vehicles such as aircraft and rockets[1]。
Mach number is the ratio of speed to sound speed. The sound speed (that is, the speed of sound propagation) has different values at different heights, temperatures and atmospheric densities. It is only a relative value. The specific speed of each "horse" is not fixed[1]。
If Mach number is to be used as speed unit, altitude and atmospheric conditions must be given at the same time (generally default toInternational standard atmosphereConditions), for example, Ma1.6 means that the flight speed is 1.6 times the local sound speed[1]。
Relatively speaking, it is easier to reach a higher Mach number at high altitude than at low altitude[1]。For example, the stratospheric air is thin, and there is only horizontal airflow, no weather change, small air resistance, and high visibility.At the sea level of 0 ℃, the sound speed is about 331.45m/s ≈ 1193km/h;The sound speed at 10000 m altitude is about 1062 km/h.Therefore, Ma cannot be converted into fixed units such as km/h or mplh (mile/hour, commonly known as "mile"). Mach number cannot be used as a speed unit. The "flying horse number" of an aircraft refers to the ratio of the speed (airspeed) of the aircraft relative to the stationary atmosphere to the local sound speed, where the stationary atmosphere has a certain height, temperature and atmospheric density.For example, the maximum speed of the US SR-71 "Blackbird" strategic reconnaissance aircraft is Ma3.2~Ma3.5, which is the record of the highest speed of air intake. The maximum flight speed of the D21 high-altitude high-speed UAV is Ma3.3~Ma3.5, which does not reach hypersonic speed.Hypersonic generally means that the speed of flow or flight reaches or exceeds 5 times the speed of sound, that is, Ma ≥ 5. At present, there are few aircraft in the world that can achieve hypersonic flight,Intercontinental ballistic missileOne of them is that the reentry velocity of its warhead is far greater than the sound velocity.It is worth mentioning that the US unmanned aircraft X-43A has broken through the Ma7, and the unmanned space aircraft X37B has reached the Ma25 in space[6]。
According to different Mach numbers, the fluid can be divided into several flow conditions:
Incompressible flow
Mach is used to indicate the speed of the aircraft
Mach is generally used in aircraft, rockets and other aerospace vehicles.Since the propagation speed of sound in the air varies with different conditions, Mach is only a relative unit, and the specific speed of each "Mach" is not fixed.At low temperatures, the propagation speed of sound is lower, and the specific speed corresponding to Mach is also lower.Therefore, relatively speaking, it is easier to reach a higher Mach number at high altitude than at low altitude (the sound speed at sea level at zero degrees Celsius is about 1193 km/hr; the sound speed at 10000 meters is about 1062 km/hr).
When Mach number Ma<0.3, the pressure on the fluid is not enough to compress the fluid, which will only cause fluid flow.Under this condition, the fluid density will not change with the pressure. This flow field is called subsonic flow, and the flow field can be regarded asIncompressible flow field。General water flow and air flow in the atmosphere, such as turbulent rivers, typhoon wind fields and the movement of cars, are all incompressible flow fields.However, when the fluid moves at a high speed (the velocity is close to or greater than the speed of sound), the density of the fluid will change with the pressure. At this time, the flow of the gas is called the compressible flow field.When Mach number Ma>1.0, it is called supersonic flow, and such flow conditions areAerodynamicsWill be encountered in.Now China has successfully developed and successfully test flown a supersonic aircraft, which can reach up to six times the speed of sound.
When any object flies at supersonic speed, it will generate ultrahigh temperature airflow behind the shock wave at its head, so it is necessary to select heat-resistant materials.
The approximate speed conversion of Mach is generally considered to be equivalent to 340.3 m/s, 1225 km/h, 761.2 mph, or 1116 ft/s.That is, the speed of sound propagation in air at 15 degrees Celsius (59 degrees Fahrenheit, 288.15 degrees Kelvin).
