Shock wave

For the vector animation platform, see Macromedia Shockwave.

In fluid dynamics, a shock wave is a strong pressure wave. See Rankine-Hugoniot equation.

In compressible fluids such as air, disturbances such as pressure changes caused by a solid object moving through the medium propagate through the fluid as pressure waves traveling at the speed of sound. When the cause of the disturbance is moving slowly relative to the speed of sound, the pressure wave enables the fluid to redistribute itself to accommodate the disturbance, and the fluid behaves similarly to an incompressible fluid.

However, when a disturbance moves faster than the pressure waves it causes, fluid near the disturbance cannot react to it or "get out of the way" before it arrives. The properties of the fluid (density, pressure, temperature, velocity, etc.) thus change almost instantaneously as they adjust to the disturbance, creating thin disturbance waves called shock waves and shock heating. When meteors enter the earth's atmosphere, this phenomenon causes them to heat up and disintegrate; this is sometimes erroneously attributed to friction. Shock waves ultimately degenerate to normal pressure waves as their energy is absorbed by the medium.

See also: Mach wave.

Analogous phenomena are known outside fluid mechanics. For example, particles accelerated beyond the speed of light in a particular medium, such as water, where the speed of light is less than that in a vacuum, create shock effects, a phenomenon known as Cerenkov radiation.

There are two basic types of shock waves: blast waves and driven waves. A blast wave is produced by explosive phenomena. Blast waves travel out from their source at supersonic speeds. A driven wave is produced by a source that constantly ejects matter (for example, the solar wind). A driven wave can reach a static state where it bounds the wind.

An everyday example of a shock wave can be experienced in the form of a sonic boom, which is commonly created by the supersonic flight of aircraft.

Another example of a shock wave is the boundary of a magnetosphere. At the shock wave, particles from the solar wind will abruptly slow to subsonic speeds.

NASA Glenn Research Center information on:
Oblique Shocks http://www.grc.nasa.gov/WWW/K-12/airplane/oblique.html
Multiple Crossed Shocks http://www.grc.nasa.gov/WWW/K-12/airplane/crosshock.html
Expansion Fans http://www.grc.nasa.gov/WWW/K-12/airplane/expans.html

See also: magnetopause