Science Fair Project Encyclopedia
- For the multimedia player platform, see Macromedia Shockwave.
In compressible fluids such as air, disturbances such as the pressure changes caused by a solid object moving through the medium will 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 takes the form of conventional sound waves. The pressure waves enable 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.
Shock waves are not sound waves, a shock wave takes the form of a thin membrane (on the order of centimeters in thickness,) and the pressure excursion within the shock wave is so extreme that it causes the speed of sound within the wave to change. Shock waves in air are heard as a loud "crack" or "snap" noise. Over time a shock wave can change from a nonlinear wave into a linear wave; degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom.
There are two types of shock waves: normal shocks, and oblique shocks. A normal shock extends across perpendicular to the flow of fluid, and the flow goes from supersonic upstream of the shock wave to subsonic downstream. An oblique shock is formed at an angle to the flow, and although the component of flow perpendicular to the oblique shock goes from supersonic to subsonic in crossing the wave, the tangent component of flow is not affected, so the net flow may remain supersonic downstream of an oblique shock wave.
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 can 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.
When meteors enter the earth's atmosphere, this phenomenon causes them to heat up and disintegrate; this is sometimes erroneously attributed to friction.
Photo Gallery http://www.galleryoffluidmechanics.com/shocks/shock.htm
eFluids gallery http://www.efluids.com/efluids/pages/gallery.htm
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
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