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Yesterday I was at the airport planespotting. I observed the exhaust of a couple of turbofan engines, and found it curious how the fluid seemed to behave as it was leaving the engine and gaining distance to the aircraft.

From observation, this is what it appeared like:

  • The exhaust left the engine in a high speed, violent manner and quickly slowed down as it got farther from the engine.
  • The exhaust appeared to move in the other direction, aka going back towards the airplane before it met with new high-velocity exhaust gas and was violently mixed and pushed again further from the plane.

I remember the Saturn V's exhaust being pulled back into the flame duct right after engine start as the flame gained speed and the surrounding pressure dropped, and I was curious if something similar was happening with the turbofan?

enter image description here

Source of image: https://space.stackexchange.com/questions/26183/why-does-air-get-sucked-in-for-a-moment-before-engine-start

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What you are describing is a characteristic of fluid dynamics known as eddies. As different portions of the fluid flow faster than others, the friction and surface adhesion between the two layers cause swirling of the fluid and reversal of the current, creating a turbulent flow region. It is this same principle that holds a ping pong ball stationary in a moving column of air.

For a simple overview, you can check out this wikipedia page: https://en.wikipedia.org/wiki/Eddy_(fluid_dynamics)

For a more scholarly exposition, try this: https://arc.aiaa.org/doi/abs/10.2514/2.1794?journalCode=aiaaj

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