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Desde el punto de vista de la [[mecánica de fluidos]], la importancia del número de Mach reside en que compara la velocidad del móvil con la velocidad del sonido, la cual coincide con la velocidad máxima de las perturbaciones mecánicas en el fluido.
== Generalidades ==
El número Mach se usa comúnmente con objetos moviéndose a alta velocidad en un fluido, y en el estudio de fluidos fluyendo rápidamente dentro de [[tobera]]s, [[difusores]] o [[túnel de viento|túneles de viento]]. A una temperatura de 15º Celsius, Mach 1 es igual a 340,3 m·s<sup>−1</sup> (1.225 km/h<sup>−1</sup>) en la atmósfera. El número Mach no es una constante; depende de la temperatura. Por lo tanto, en la [[estratosfera]] no varía notablemente con la altura, incluso cuando la presión del aire cambia con la misma.
Este número es muy utilizado en [[aeronáutica]] para comparar el comportamiento de los fluidos alrededor de una aeronave en distintas condiciones. Esto es posible gracias a que el comportamiento de un fluido en el entorno de un objeto es igual siempre que su número de Mach sea el mismo. Por lo tanto, una [[aeronave]] viajando a Mach 1 experimentará las mismas [[Onda de choque|ondas de choque]], independientemente de que se encuentre al nivel del mar (340,3 m·s<sup>−1</sup>, 1.225,08 km/h) o a 11.000 metros de [[altitud]] (295 m·s<sup>−1</sup>), incluso cuando en el segundo caso su velocidad es tan sólo un 86% de la del primer caso.
La clasificación de los regímenes incluyendo el régimen hipersónico no es caprichosa: para M muy elevados (la frontera técnica depende de la forma del móvil, en general M>5), las ondas de choque son de tal magnitud que el aire se disocia tras ellas, y deja de ser aire, con las propiedades que en éste se aceptan, para convertirse en una mezcla de gases disociada, con capas eléctricamente cargadas aunque neutra en su conjunto, que deja de comportarse como lo hacía el aire.
Se demuestra que el número Mach es también el cociente de las fuerzas inerciales (también refiriéndose a las fuerzas aerodinámicas) y las fuerzas elásticas.
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== High-speed flow around objects ==
High speed flight can be classified in four categories:
*'''[[sonic]]:''' Ma=1
*'''[[Subsonic]]:''' Ma < 1
*'''[[Transonic]]:''' 0.8 < Ma < 1.2
*'''[[Supersonic]]:''' 1.2 < Ma < 5
*'''[[Hypersonic]]:''' Ma > 5
(For comparison: the required speed for [[low Earth orbit]] is ca. 7.5 km·s<sup>-1</sup> = Ma 25.4 in air at high altitudes)
At transonic speeds, the flow field around the object includes both sub- and supersonic parts. The transonic 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)
{| border="0"
|[[Archivo:Transsonic_flow_over_airfoil_1.gif]]
|[[Archivo:Transsonic_flow_over_airfoil_2.gif]]
|-
| (a)
| (b)
|}
'''Fig. 1.''' ''Mach number in transonic airflow around an airfoil; Ma<1 (a) and Ma>1 (b).''
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 a 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 slightly concave plane.
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.
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 ''M''=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 (i.e. making the channel narrower results in faster air flow) 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 ''M''=1, sonic speeds, and the diverging section continues the acceleration. Such nozzles are called [[de Laval nozzle]]s and in extreme cases they are able to reach incredible, [[hypersonic]] velocities (Mach 13 at sea level).
An aircraft Mach meter or electronic flight information system ([[EFIS]]) can display Mach number derived from stagnation pressure ([[pitot tube]]) and static pressure.
Assuming air to be an [[ideal gas]], the formula to computer Mach number in a subsonic compressible flow is derived from the [[Bernoulli]] equation for ''M''<1:<ref name=Olson>Olson, Wayne M. (2002). "AFFTC-TIH-99-02, ''Aircraft Performance Flight Testing''." (PDF). Air Force Flight Test Center, Edwards AFB, CA, United States Air Force.</ref>
:<math>{M}=\sqrt{5\Bigg[\bigg(\frac{q_c}{P}+1\bigg)^\frac{2}{7}-1\Bigg]}</math>
where
:<math>M</math> is Mach number
:<math>q_c</math> is impact pressure and
:<math>P</math> is static pressure.
The formula to compute Mach number in a supersonic compressible flow is derived from the [[Rayleigh]] Supersonic Pitot equation:
:<math>{M}=0.88128485\sqrt{\Bigg[\bigg(\frac{q_c}{P}+1\bigg)\bigg(1-\frac{1}{[7M^2]}\bigg)^{2.5}\Bigg]}</math>
where
:<math>M</math> is Mach number
:<math>q_c</math> is impact pressure measured behind a normal shock
:<math>P</math> is static pressure.
As can be seen, ''M'' apprears on both sides of the equation. The easiest method to solve the supersonic ''M'' calculation is to enter both the subsonic and supersonic equations into a computer spread sheet such as [[Microsoft Excel]] (or equivalent). First determine if ''M'' is indeed greater than 1.0 by calculating ''M'' from the subsonic equation. If ''M'' is greater than 1.0 at that point, then use the value of ''M'' from the subsonic equation as the initial condition in the supersonic equation. Then perform a simple iteration of the supersonic equation, each time using the last computed value of ''M'', until ''M'' converges to a value--usually in just a few iterations.<ref name=Olson />
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== Véase también ==
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