A shock wave ( engl. Shockwave ), also bang wave is in the fluid mechanics a strong pressure wave caused by detonation is produced, or other phenomena that parts of the medium are moved faster than the speed of sound in the medium. In contrast to normal sound waves , the state variables at the wave front change almost suddenly.
Origin and basic properties
In compressible, i.e. H. Compressible media, for example gases, disturbances, for example pressure changes caused by solid bodies moving through, propagate through the medium as pressure waves that move at the speed of sound of the medium. If the interferer moves only slowly compared to the speed of sound, the pressure wave allows the medium to redistribute itself in order to compensate for the disturbance. The medium behaves exactly like an incompressible medium.
However, if the interferer moves faster than the speed of sound in the undisturbed medium, a thin shock front forms within which the state variables of the medium, including the speed, change almost suddenly. The front moves more slowly through the medium than the disturber, creating conical shapes. Since the energy density behind the front is higher than in front of the front, shock waves gradually decay to normal pressure waves, their speed reducing to the speed of sound.
There are two basic types of shock waves, which are physically equivalent and only differ in the choice of the reference system :
- Progressive shock waves in the stationary medium: They are caused by sudden disturbances in the medium, such as an explosion or a projectile , and they move at supersonic speed. See also: shock wave .
- Standing shock waves in a flowing medium: They are caused by the fact that matter is continuously ejected from a source, e.g. B. the solar wind of the sun or the propulsion gases in a rocket engine . Such waves can reach a state of equilibrium in which they limit the wind output.
Progressive shock waves
- Built around a lightning channel strongly heated air plasma caused by him (a few meters) outgoing shock wave the Thunderclap in thunderstorms . So it's a kind of explosion along the lightning canal. As a result of the irregular, often heavily branched shape of the lightning canal, the different distance of the lightning canal sections from the observer, as well as atmospheric temperature fluctuations and strong winds, which influence the course of the waves, not a single blow is heard, but usually a whole series of more or less strong blows, which at a greater distance - where the shock waves have subsided into acoustic waves - form the typical rumble or rumble of thunder.
- The bow wave of ships that travel faster than the propagation speed of the water waves is a shock wave. Instead of the speed of sound in water, however, the phase speed of disturbances on the water surface is decisive here.
- Detonations of bombs , for example, are able to throw objects through the air with their shock waves and destroy them. Exact empirical and theoretical calculation models exist for such detonation waves . A pyrotechnic charge does not generate a shock wave, so it does not detonate either, one speaks of (rapid) burn-up or deflagration .
- Airplanes or projectiles that fly at supersonic speed cause an approximately conical shock wave, the Mach cone . A sonic boom or projectile bang can occur with the observer . Meteoroids penetrating into the atmosphere also cause shock waves, which are partly responsible for the light appearance of the meteors .
- The impact or impact of meteorites triggers the impact metamorphosis or shock wave metamorphosis of rocks .
- In the interstellar medium , shock waves can be caused by supernovae or by the penetration of gas and dust clouds (e.g. a dwarf galaxy ) into a gas-rich galaxy like the Milky Way and lead to star formation through the compression of gas and dust clouds . They can also be detected using the X-rays emitted by the impact-heated medium .
- Correspondingly, electrically charged particles that move faster in optical media than the speed of light ( phase speed ) applicable there ( e.g. in water only about 230,000 km / s) generate an electromagnetic bow wave, the Cherenkov radiation .
Standing shock waves
- The limit of the earth's magnetosphere is marked by a shock wave, at the front of which the particles of the solar wind are abruptly decelerated ( bow shock wave ). Since the average speed of the particles in the undisturbed flow relative to the earth is greater than the speed of sound within the moving medium, shock wave formation occurs. The same applies to the magnetosphere of Jupiter .
- At a distance of about 80–100 AU , the solar wind is slowed down by the interstellar medium to below the speed of sound. The shock wave at the border is called termination shock .
- Shock waves occur in the nozzle flow of rockets if the pressure at the nozzle outlet differs from the ambient pressure. With some nozzle shapes this can lead to destruction of the nozzle in the worst case. In order to avoid damage, the minimum outlet pressure when the engine is ignited is therefore limited. As a result, the nozzle does not have optimal efficiency at a greater height because of the lower ambient pressure.
Use in medicine and physiotherapy
In the medicine by means of the "be Extracorporeal shockwave lithotripsy high-energy" (ESWL) focused shock waves for disintegrating the urinary , biliary , kidney and salivary stones used in the orthopedic the extracorporeal shock wave therapy for the treatment of non-healing fractures ( nonunion ) and for erosion of Calcifications on tendons and deposits in joints . Since around 2010, extracorporeal shock wave therapy has also been used in urology to treat erectile dysfunction : the application of shock waves stimulates neo-angiogenesis, i.e. the formation of new blood vessels, which leads to better blood flow to the penis and thus a higher quality erection should lead.
In physiotherapy , due to its relatively simple handling (no imaging procedures necessary, hardly any risks, little pain for the patient), low-energy radial shock wave therapy in particular has recently been increasingly used.
Low-energy shock waves enable an effective treatment of pain at tendon attachments such as tennis elbows , golfers' elbows or the painful heel spur and pain in arthrosis (wear and tear) as well as in chronic pain in soft tissues near the bones.
The shock wave is also becoming increasingly popular as a painless procedure in " trigger point therapy " of muscle hardening . The trigger points (= myogeloses) of the muscle z. B. Treated neck pain and lower back pain. Not only can the treatment be carried out with the devices, but they also enable targeted diagnosis of the hardened and shortened muscle areas, which are often very painful. These muscle areas are provided with local or remote triggers, depending on the clinical picture, which must be observed in the course of further treatment.
These shock waves are generated hydraulically , electromagnetically , piezoelectrically or pneumatically - ballistically outside the human body, ie "extracorporeally". A distinction is made between radial and focused shock waves of different energy strength according to the form of their propagation.
- Exploding Asteroids Pose Greater Threat Than Direct Hits nationalgeographic.com, accessed May 19, 2016
- Zhihua Lu, Guiting Lin, Amanda Reed-Maldonado, Chunxi Wang, Yung-Chin Lee: Low-intensity Extracorporeal Shock Wave Treatment Improves Erectile Function: A Systematic Review and Meta-analysis . In: European Urology . tape 71 , no. 2 , February 2017, ISSN 1873-7560 , p. 223–233 , doi : 10.1016 / j.eururo.2016.05.050 , PMID 27321373 .
- Anne B. Olsen, Marie Persiani, Sidsel Boie, Milad Hanna, Lars Lund: Can low-intensity extracorporeal shockwave therapy improve erectile dysfunction? A prospective, randomized, double-blind, placebo-controlled study . In: Scandinavian Journal of Urology . tape 49 , no. 4 , 2015, ISSN 2168-1813 , p. 329-333 , doi : 10.3109 / 21681805.2014.984326 , PMID 25470423 .