uclog2.gif (2613 bytes)AsE&EM On-Line Tutorial in Fluid Mechanics

Created by: Dr. K. N. Ghia
Scott E. Will
Modified by: Marshall C Galbraith
Last updated: July 17, 2002

 

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An Introduction to Shock Waves

When a body is moving through a fluid at a velocity exceeding the speed of sound, wave fronts in which the pressure, density, and temperature change abruptly within a very small distance are formed.  These wave fronts are called shock waves.   You can think of shock waves as surfaces in the flow field in which there is a sudden, discontinuous change in the flow properties.  If you were to be moving with the body at a constant supersonic velocity, the shock wave would be fixed in reference to the body but across which the fluid flows steadily, although it is experiencing a very rapid change in the flow conditions after crossing the wave front.   Shock waves form a pattern that is fixed in relation to the body when in steady flight.  These waves fronts are formed to the side and rear of the body.  img_shocks.jpg (8052 bytes)

A simple way of producing a shock wave is depicted in the figure below.   Basically, a hollow tube is filled with a uniform gas at rest and fitted with a piston at one end.  At time t = 0, the piston is suddenly put into motion with a constant speed, Vp.  The motion of the piston creates a shock wave that moves in the same direction as the piston, but a faster, constant speed, Vs (faster than the speed of sound).   Between the piston and the shock wave there is a certain amount of fluid that has a higher pressure and density than the undisturbed gas in front of the shock wave.   Because of these changes in pressure and density, the fluid which has passed through the shock wave occupies a smaller volume that it initially did. 

img_ShockPiston.gif (2702 bytes)

 

Did you know?

Supersonic aircraft are not the only things that produce shock waves and the resulting sonic boom... some of the more common occurrences of shock waves:

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Thunder clap - results from the shock wave created by supersonic expansion of the superheated air around a lightning bolt

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Firecracker's bang - caused by a small shock wave

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Crack of a whip - the tip of a whip exceeds the speed of sound and creates a shock wave when it is snapped very rapidly