Shock waves

[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 5-46]

What’s shock waves?

Shock waves, also known as “compression” wave, is the boundary between the undisturbed air and the region of compressed air.

Source: FAA-H-8083-25B Page 5-47

Shock wave is formed whenever a supersonic airstream is slowed to subsonic without a change in direction – such as when 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.

When shock waves occur most?

As the airplane’s speed increases beyond the speed of sound or any part of the aircraft above “force divergence Mach number”  (5-10% Mcrit), the pressure and density of the compressed air ahead of it increase, the area of compression extending some distance ahead of the airplane.

What will happen in a “normal shock wave”?

“normal shock wave” means it forms perpendicular to the airflow and experience few changes:

  1. airstream is slowed to subsonic
  2. airflow immediately behind the shock wave does not change direction
  3. Static pressure and density of the airstream behind the wave is greatly increased
  4. energy of the airstream (indicated by total pressure—dynamic plus static) is greatly reduced

 

Impact(s) of shock waves?

  1. Formation of shock wave causes part of the velocity energy of the airstream is converted to heat
  2. a drag known as “wave drag” is induced. It is because a transonic region is formed immediately behind the wave, and the boundary layer may not have sufficient kinetic energy, the air becomes separated, the would further cause Mach buffet.

How to delay shock waves?

Ans: using vortex generators and sweepback design

 

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