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Shock Waves

 Shock Waves

  • When an airplane flies at subsonic speeds, the air ahead is “warned” of the airplane’s coming by a pressure change transmitted ahead of the airplane at the speed of sound.
  • Because of this warning, the air begins to move aside before the airplane arrives and is prepared to let it pass easily.
  •  When
  • the airplane’s speed reaches the speed of sound, the pressure change can no longer warn the air ahead because the the airplane is keeping up with its own pressure waves.
  •  Rather, the air particles pile up in front of the airplane causing a sharp decrease in the fl ow velocity directly in front of the airplane with a corresponding increase in air pressure and density.
  • As the airplane’s speed increases beyond the speed of sound, the pressure and density of the compressed air ahead of it increase, the area of compression extending some distance ahead of the airplane.
  •  At some point in the airstream, the air particles are completely undisturbed, having had no advanced warning of the airplane’s approach, and in the next instant the same air particles are forced to undergo sudden and drastic changes in temperature, pressure, density, and velocity.
  • The boundary between the undisturbed air and the region of compressed air is called a shock or “compression” wave.
  • This same type of wave is formed whenever a supersonic airstream is slowed to subsonic without a change in direction, such as when the airstream is accelerated to sonic speed over the cambered portion of a wing, and then decelerated to subsonic speed as the area of maximum camber is passed.
  • A shock wave forms as a boundary between the supersonic and subsonic ranges.

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