What happens during an earthquake.Seismic waves belong to a form of propagation of turbulence.Like other common forms of waves, seismic waves can also transmit energy through media.In some special cases where there is no physical medium, seismic waves can transmit energy through fields, such as electromagnetic fields.
The main characteristic of the seismic wave is that the medium characteristics (such as pressure, temperature, or velocity) have a sudden change like a positive step function before and after the seismic wave.The corresponding negative step is expansion wave.The speed of acoustic shock wave is generally higher than the normal wave speed (i.e., sound speed in air).
Unlike solitary waves (another form of nonlinear waves), seismic waves dissipate quickly with increasing distance.Moreover, the expansion wave is always accompanied by the seismic wave, and eventually merged with the seismic wave.This partly offset the impact of seismic waves.Sonic boomSupersonic aircraftThe acoustic phenomenon generated when passing through is caused by the dissipation and annihilation of the shock expansion wave pair.
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Shock waves in supersonic flow
Shock wave is one of the compression phenomena in supersonic gas flow.The other two forms are isentropic flow and Prandtl Meyer flow.For a given pressure ratio, different compression methods will produce different temperatures and densities, and the results can be analytically calculated for gases that do not undergo chemical reactions.The shock wave will cause the loss of total pressure.This means that in some cases, such as the inlet of the supersonic ramjet, the shock wave is inefficient.Supersonic aircraftThe piezoresistance of is mainly caused by seismic waves.
When the object (or disturbance) moves faster than the surrounding fluid to transmit disturbance information, the fluid close to the disturbance cannot respond or "give way" in time before the disturbance arrives.In seismic waves, various properties of fluid (density, temperature, pressure, velocity, Mach number) always change instantaneously.The thickness of the shock wave is on the order of magnitude equivalent to the molecular free path of the gas.When the speed of gas movement is greater than its sound speed, the shock wave is formed.In some areas of the flow, the gas disturbance can no longer spread to the upstream, the pressure quickly accumulates, and the high-pressure shock wave quickly forms.
Shock waves are different from ordinary sound waves.Within the thickness of about several molecular free paths (about several microns in the atmosphere), the properties of the gas will change dramatically before and after the shock wave.In the air, the shock wave makes a loud popping sound or crackling noise.With the increase of distance, the seismic wave gradually changes from nonlinear wave to linear wave and degenerates into normal acoustic wave.This is due to the gradual loss of energy in the air in the shock wave.This sound wave sounds very similar to the usual thunder, that is, "sonic boom". It is generally made by supersonic aircraft.
Seismic waves generated by nonlinear kurtosis
Seismic waves can also be formed by sharpening ordinary waves.The most famous example is formed when deep-sea microwave approaches landtsunamiHas.In shallow water, the velocity of surface waves depends on the depth of water.For oncoming waves, since the wave height is much smaller than the water depth, the wave crest speed is slightly greater than the wave trough speed.In this way, the wave crest caught up with the wave trough until a huge water wall was formed, and then collapsed to form a tsunami, releasing the energy in the way of sound and heat.
The same phenomenon occurs in strong sound waves in gas and plasma, because sound speed depends on temperature and pressure.This phenomenon is rarely seen in the earth's atmosphere, but exists in the chromosphere and corona of the sun.
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The seismic wave can also be described as the farthest point upstream that can "sense" the movement of downstream objects.In this description, the location of seismic waves is defined as the boundary between the disturbance perceptible area and the disturbance blind area.This can be compared with the light cone in general relativity.
To get the shock wave, you must move faster than the speed of sound.Due to the amplification effect, the shock wave is very strong, especially like the explosion you have heard of (this is not unexpected, the explosion should produce shock waves).
Analogy has gone beyond the scope of fluid mechanics.For example, when an object in the medium moves faster than the speed of light in the medium (at this time, its speed is still lower than the speed of light in vacuum), refraction will produce a visible shock wave phenomenon, namely Cherenkov radiation.
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There are several types of seismic waves:
4.1 Seismic waves propagating in steady flow
This kind of shock wave usually occurs at the gas interface with pressure difference.At this time, the shock wave is transmitted to the low-pressure gas, and the expansion wave is transmitted to the high-pressure gas.
Examples: balloon explosion, shock tube, explosion shock wave, etc.
In this case, the gas before the shock wave is generally static, while the gas after the shock wave moves at supersonic speed.Seismic waves usually belong to incoming flow.The velocity of the shock wave depends on the pressure ratio of the two gases.
4.2 Shock wave in pipeline flow
This shock wave occurs when the supersonic flow in the pipe decelerates.
Examples: supersonic jet engine, supersonic ramjet, needle valve, etc.
In this case, the gas in front of the wave is supersonic, while the gas behind the wave is either supersonic (oblique shock wave) or subsonic (positive shock wave).This kind of shock wave is generated either when the gas decelerates in the converging pipe or due to the growth of the boundary layer in the straight pipe.
Recompression shock waves generated by transonic objects
This shock wave occurs when transonic flow decelerates to subsonic speed.
Examples: transonic wings, ducts.
The principle is omitted.
4.3 Body attached shock waves of supersonic objects
This shock wave appears at the top of a sharp object moving at supersonic speed in the form of "attachment".
Example: wedge or cone in supersonic motion
The principle is brief.
4.4 Out of body shock waves of supersonic objects
This shock wave occurs when the top of a supersonic moving object is very blunt.
Example: space return capsule (Apollo spacecraft, space shuttle), bullet, magnetosphere boundary layer
