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Shock wave - s in Supersonic Flows, Analogies

waves sound pressure gas

A shock wave is one way that gas in a supersonic flow can be compressed. Other methods include Prandtl-Meyer compressions and isentropic compressions. How a gas is compressed determines the results in different temperatures for a pressure ratio. The different temperatures can be calculated, for a non-reacting gas, analytically. A shock wave compression culminates in the loss of total pressure which means it is much less efficient in compressing gases for purposes such as in the intake of a scramjet. When a disturbance moves faster than can be accomodated by surrounding fluid, this fluid can’t react prior to the arrival of the disturbance. In a shock wave, fluid properties are altered instantly. Shock waves are formed when the speed of gas is altered by more than the speed of sound. Sound waves traveling against the flow reach a point at the region where they can’t go any farther. This causes the pressure to build up in that region which, in turn, causes high-pressure shock waves to form. Keep in mind that shock waves are not your everyday sound waves. A shock wave forms a super-sharp change in the properties of a gas at the behest of a number of mean free paths. Shock waves can be heard as a cracking and snapping noise. If it covers a long enough distance, a shock wave can change from a nonlinear wave to a linear one. The shock wave degenerates into an everyday sound wave after it heats up the air, losing energy. . This sound wave is heard as a thudding or thumping of a sonic boom sound. This is created mostly by an aircraft’s supersonic flight.

Analogies

The furthest point upstream of a moving object that knows the object is approaching is described as a shock wave. The shock wave is defined as a boundary between a zone that has no information about the shock-drving event and the zone that is aware of the shock-driving event. This is similar to the light cone as described in the theory of general relativity. In order to get a shock wave, the object must be traveling faster than the speed of sound is, locally. Some areas of the air surrounding an aircraft might be travelling at the speed of sound precisely, which makes the sound waves leaving the aircraft pile up on top of one another. This formation of a sound wave is like a tailback on a road. As a shock wave forms, pressure is increased and then blooms outwards. Because of this effect, shock waves are intense and comparable more to an explosion than anything else. This is not coincidental because explosions can create shock waves.

Shockley, William Bradford [next]

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