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For the multimedia player platform, see Macromedia Shockwave. For the Transformers character, see Shockwave (Transformers). For the rollercoaster, see Shockwave Rollercoaster

In a supersonic flow the compression of a nonreacting gas can be most simply modelled as an isentropic or Prandtl-Meyer compression, or as a shock wave. When an object (or disturbance) moves faster than the information about it can be propagated into the surrounding fluid, fluid near the disturbance cannot react or "get out of the way" before the disturbance arrives. In a shock wave the properties of the fluid(density, pressure, temperature, velocity, Mach number) change almost instantaneously. Measurements of the thickness of shock waves have resulted in values approximately one order of magnitude greater than the mean free path of the gas investigated.

Shock waves are not sound waves; a shock wave takes the form of a very sharp change in the gas properties on the order of micro-meters in thickness. 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, commonly created by the supersonic flight of aircraft.

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

There are several types of shock wave:

1. Shock propagating into a stationary flow
   • This shock appears is generally generated by the interaction of two bodies of gas at different pressure, with a shock wave propagating into the lower pressure gas, and an expansion wave propagating into the higher pressure gas.
   • Examples: Balloon bursting, Shock tube, shock wave from explosion
   • In this case, the gas ahead of the shock is stationary (in the laboratory frame), and the gas behind the shock is supersonic in the laboratory frame. The shock propagates normal to the oncoming flow. The speed of the shock is a function of the original pressure ratio between the two bodies of gas.
2. Shock in a pipe flow
   • This shock appears when supersonic flow in a pipe is decelerated.
   • Examples: Supersonic Ramjet, Scramjet, needle valve
   • In this case the gas ahead of the shock is supersonic (in the laboratory frame), and the gas behind the shock system is either supersonic (oblique shock) or subsonic (normal shock). The shock is the result of the deceleration of the gas by a converging duct, or by the growth of the boundary layer on the wall of a parallel duct.
3. Recompression shock on a transonic body
   • These shocks appear when the flow over a transsonic body is decelerated to subsonic speeds.
   • Examples: Transonic wings, Turbines,shockwave at mach1
   • Where the flow over the suction side of a transonic wing is accelerated to a supersonic speed, the resulting recompression can be by either Prandtl-meyer compression or by the formation of a normal shock. This shock is of particular interest to makers of transonic devices because it can cause separation of the boundary layer at the point where it touches the transonic profile. This can then lead to full separation and stall on the profile, higher drag, or shock-buffet, a condition whereby the separation and the shock interact in a resonance condition, causing resonating loads on the underlying structure.
4. Attached shock on a supersonic body
   • These shocks appear as "attached" to the tip of a sharp body moving at supersonic speeds.
   • Examples: Supersonic wedges and cones at low angles.
   • The attached shock wave is a classic structure in aerodynamics because, for a perfect gas and inviscid flowfield, an analytic solution is available, whereby the pressure ratio, temperature ratio, angle of the wedge and the downstream Mach number can all be calculated knowing the upstream Mach number and the shock angle. Lower shock angles are associated with higher downstream Mach numbers, and the special case where the shock wave is at 90 degrees to the oncoming flow (Normal shock), is associated with a downstream Mach number of one.
5. Detached shock on a supersonic body
   • These shocks occur where the supersonic body is too blunt to allow the shock to attach to the tip.
   • Examples: Space return vehicles (Apollo, Space shuttle), bullets. The boundary of a magnetosphere. At the shock wave, particles from the solar wind will abruptly slow to subsonic speeds.
   • These shocks are curved, and form a small way in front of a supersonic body. Directly in front of the body, they are at 90 degrees to the oncoming flow, and then they curve around the body. Detached shocks allow the same type of analytic calculations as for the attached shock, for the flow near the shock. They are a topic of continuing interest, because the rules governing the distance between the blunt body and the shock are complicated, and are a function of the shape of the blunt body. Additionally, the distance of the shock standoff varies drastically with the temperature for a non-ideal gas, causing large differences in the heat transfer to the thermal protection system of the vehicle.
6. Detonation wave
   • This is not a shock wave, but is of a similar form. A detonation wave follows different rules to a shock wave since it is driven by chemical reactions occurring inside the wave itself. Detonation waves proceed at the Chapman-Jouguet velocity. It is also possible for a shock wave in a reactive mixture to induce combustion, but in this case the shock proceeds at the velocity indicated by the noncombusting mixture. This is shock-induced combustion.

External links

• Photo Gallery http://www.galleryoffluidmechanics.com/shocks/shock.htm
• eFluids gallery http://www.efluids.com/efluids/pages/gallery.htm
• Bomb Shock Wave Estimation

http://www.makeitlouder.com/document_bombshockwaveestimation.html

• 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

• Mach wave
• Magnetopause

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