# Something for nothing?

wolfgang59
Posers and Puzzles 07 Jul '07 10:34
1. wolfgang59
07 Jul '07 10:341 edit
If you had a aerofoil made of frictionless materiel (thought experiment .. ok?) in a moving fluid such that the Bernoulli effect raised the aerofoil. The aerofoil would gain energy (Potential Energy).

Now I'm not suggesting another wacky Perpetual Motion Machine or creating Energy from nowhere, but how does the fluid lose the energy? I presume it slows down but by what mechanism?
2. agryson
AGW Hitman
07 Jul '07 23:13
Originally posted by wolfgang59
If you had a aerofoil made of frictionless materiel (thought experiment .. ok?) in a moving fluid such that the Bernoulli effect raised the aerofoil. The aerofoil would gain energy (Potential Energy).

Now I'm not suggesting another wacky Perpetual Motion Machine or creating Energy from nowhere, but how does the fluid lose the energy? I presume it slows down but by what mechanism?
Assuming it's a closed pipe with some fluid flowing through it... (for an open system, like open air or water, certain things need to be changed buit I think the general principle stays the same.)

My understanding is that to raise the aerofoil, the fluid must build a pressure difference across the aerofoils cross section. So velocity changes along the surface of the aerofoil. Whatever energy the aerofoil gains is lost in the form of velocity, since the different flows above and below the aerofoil, when they mix and the vortices calm down and everything, the total velocity will be lower.
As flow in and out must also be kept constant for our entire system, the lower velocity downstream increases the entire systems total pressure meaning the actual energy is coming from your pump to maintain that flow rate. The system balances when the aerofoil reaches the top of the pipe.