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Old   May 4, 2012, 08:19
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Glenn Horrocks
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How can I know? You know your experimental results and simulation best, it is up to you to find it.
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Old   May 4, 2012, 08:23
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is your "A" represent full area of inlet or just for one periodic part? If it is for one periodic part, you should not use multiplication by 3 for P calculation.

Another thing, why do you consider only dynamic pressure?
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Old   May 4, 2012, 10:38
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Hi ghorrocks,

"When you are converging on the wrong answer it tells you that you have missed an important piece of physics in your simulation."
This is what I was referring to when I asked about the simulation tells you that you missed something? (sorry if I misunderstood you).

Altano,
The method you outlined gives the same result as I get using the method I outlined...

Thanks again for your help............

FMOR
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Old   May 4, 2012, 12:02
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Hi Altano,

I tried the method you outlined again, it seems to make a difference. Further work needs to be done to be sure....

I'll report back with progress.

Once again thanks again for the valuable advice.

Regards,

FMOR

P.S. What is the difference in two methods, is it basically the inclusion of static and dynamic pressures. Is this the standard method to calculate the power available in the wind?
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Old   May 4, 2012, 12:25
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Hi FMOR,

Of course there should be a differance.

Actually fluid power is always related with total pressure not only dynamic pressure.
For the wind turbine in open air, static pressure can be neglected as a part of fluid power.
But for a water turbine, or any fluid machine in a closed channel like you have simulated, static pressure will be important part of fluid power.

Another view, ask yourself; let be your inlet and outlet have same area, so dynamic pressure will be same if the fluid is not compressible and no leakage. By your calculation method, you will find that, same energy is available both inlet and outlet, where the energy on the shaft is coming from?
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Old   May 4, 2012, 12:48
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Altano,

So for a tidal turbine in open flow this method is ok?

FMOR
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Old   May 4, 2012, 13:35
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Quote:
Originally Posted by FMOR View Post
Altano,

So for a tidal turbine in open flow this method is ok?

FMOR
It' s ok if you can model the surrounding large enough
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Old   May 5, 2012, 07:28
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Quote:
"When you are converging on the wrong answer it tells you that you have missed an important piece of physics in your simulation."
This is what I was referring to when I asked about the simulation tells you that you missed something? (sorry if I misunderstood you)
I do not know the precise physics of what your experiment or your simulation. So I cannot tell you what you missed - you are the one who has to figure that out as you know what you are modelling best.
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Old   May 9, 2012, 05:26
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Hi Altano and ghorrocks,

Thank you both for your replies. I have always calculated the power in the fluid flow, for example if I was modeling a wind turbine, P = 1/2 ρAU^3 where U is the freestream velocity specified at the inlet.

ghorrocks - The model is for a tidal turbine using periodicity and frozen rotor model. The model consists of a one third cylindrical domain.

Altano - the use of the method you outlined to calculate the power in the fluid has made a significant difference to the results!! I am currently doing a domain convergence study. Is the approach you outlined suitable for a wind turbine/tidal turbine?


Many thanks,

FMOR
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Old   May 9, 2012, 06:10
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Hi FMOR,
Your Cp is not completely non dimensional. Isnt the general term for Cp represented by Cp = [Delta (P)]/(1.5*rho*U*U).
Have you cross checked your Cp definition with the literature you are comparing it with.
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Old   May 9, 2012, 07:23
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Quote:
The model is for a tidal turbine using periodicity and frozen rotor model. The model consists of a one third cylindrical domain.
OK, but details are important - is the free surface important? How have you modelled it? Does it have waves on it? Does the water have sediment in it? How smooth are the impellor blade surface finish? How turbulent is the water? What is the nature of the turbulence - intensity, length/time scales?

And these are just a few details I can think of right now.
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