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Topic Title: Creating Energy??
Topic Summary: Kinetic Energy Formula, Betz law etc.
Created On: 18 January 2014 06:28 PM
Status: Read Only
Related E&T article: Vortex-induced vibrations: hydro energy of the future?
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 18 January 2014 06:28 PM
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Joined: 18 January 2014

It seems to me that some people out there believe they can create energy!

The kinetic energy in a flow is given by the schoolboy equation E=0.5 m.v**2, which evolves into E=0.5p.A.Cp.v**3 where p=density, A = x-section area,Cp is coef of performance and v=flow velocity.

If Cp is 1 then this is the energy in the flow. It doesn't matter if you use vibrations, vorticies, venturies etc. to increase the speed, the global energy in the flowstream is the same.

On a practical note, sure VIV's cause us problems on platform legs but as soon as you take power out of the vibrating column (usually by damper paddles) the vibration stops.

Maybe I am wrong, and I hope somebody can demonstrate to me that I am, because the ocean currents are there to be harnessed.

David Collard,
Tech. Director, Monobuoy SARL
 19 January 2014 09:46 PM
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Joined: 06 February 2013

Not really my field but IIRC this vibration of platform legs is probably a so called self excited vibration at or close to the resonant frequency of that part of the structure. The damping on the structure (broadly the electrical equivalent of Q factor) will limit the amplitude of the vibration and also shift the resonant frequency slightly. That's why dampers are used.

If you want to extract the energy you need to 'resist' the flow hence use a turbine or propellor, I think you would only be able to extract a very small amount of energy using self excited mode as the resonant freq would move and the system would prob no longer resonate in the same flow..... E.g. Due to eddy shedding.

....other views will prob shed more light... Might be better on mech eng forum if there is one.
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