Expert: Paul Soderman - 5/30/2004

Question
Hi Paul,
My question is,at what point in the exhaust slipstram of a stationary fan or jet cease to be thrust and become wasted energy. For example using the exhaust slipstream from a stationary jet or fan to operate a propellor driven generator.I am sure there are many variables involved here, but basically I would like to know if thrust becomes thrust the very moment when high speed air contacts stagnant air and after that initial contact what happens. Thank you .
                  Ron

Answer
Hi Ron

That is a good question.  As far as the fan or jet engine is concerned, thrust occurs at the boundary, which would be the blade surface for a fan or the exhaust nozzle for a jet.  It doesn't care what happens downstream.  However, the jet itself is a fluid mechanic process whereby every region is connected to every other.  So, what happens downstream can have a direct effect on the initial conditions of the jet or wake.  I didn't quite understand your question about using an exhaust slipstream.  I think you are asking about the loss of energy in a jet as you move downstream.  The jet velocity decays exponentially downstream and the energy in the jet decays as velocity squared.  So, depending on what you want to do and how you want to capture the energy determines how far downstream you can be.  If I knew more about your problem I could be more definitive.

Paul