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the difference between BEM and CFD

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Old   August 24, 2014, 16:20
Default the difference between BEM and CFD
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Hi, i wanna calculate/simulate the power output of the wind turbine blade. When i use the BEM method, i will simulate the power of a 2D airfoil, then integrate all the airfoils to the the whole power of the 3D blade, which is introduced in the BEM theory.
but when it comes to CFD method, i don't know how to get the power in theory or in the software.
can anyone tell me the theory abt how to calculate the power in CFD? or any material abt that? or how to simulate the power in CFD software, like ansys CFX/fluent?

thank you in advance
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Old   August 25, 2014, 13:08
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You can determine the power from a CFD simulation by integrating the pressure and shear shear stresses over the surface of the wind turbine blade.
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Old   August 25, 2014, 14:28
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The major difference between Blade Element Momentum theory (BEM) and CFD is that BEM do not include 3D effects that might happen in the flow field.

These 3D effects are often negligible on a large section of a wind turbine blade, but are always significant around the tip and the hub region. There are several way to model these effects (i.e. Prandlt's Tip-Loss and Glauert's Hub-loss factors)

3D effects are also important to model turbulence. It is possible that some part of your wind turbine blade operates at high local angle of attack, resulting in flow separation and turbulent wake.

CFD has larger modelling possibilities but has a much larger computational cost than BEM.

One last thing to consider is that CFD, in my opinion, requires more skills than BEM. I would rather see a proper design based on BEM theory than some CFD using a bad mesh and the wrong sets of equations.
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Old   August 25, 2014, 17:01
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Quote:
Originally Posted by tas38 View Post
You can determine the power from a CFD simulation by integrating the pressure and shear shear stresses over the surface of the wind turbine blade.
thank you for your replying. but as far as i know, power = torque*rotational speed. How can i get power by integrating the pressure?
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Old   August 25, 2014, 17:07
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Quote:
Originally Posted by Alex C. View Post
The major difference between Blade Element Momentum theory (BEM) and CFD is that BEM do not include 3D effects that might happen in the flow field.

These 3D effects are often negligible on a large section of a wind turbine blade, but are always significant around the tip and the hub region. There are several way to model these effects (i.e. Prandlt's Tip-Loss and Glauert's Hub-loss factors)

3D effects are also important to model turbulence. It is possible that some part of your wind turbine blade operates at high local angle of attack, resulting in flow separation and turbulent wake.

CFD has larger modelling possibilities but has a much larger computational cost than BEM.

One last thing to consider is that CFD, in my opinion, requires more skills than BEM. I would rather see a proper design based on BEM theory than some CFD using a bad mesh and the wrong sets of equations.
thank you for your explain abt the difference between them. but could you please tell me some information abt how to calculate the power in CFD?
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Old   August 25, 2014, 22:02
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Originally Posted by csui View Post
thank you for your explain abt the difference between them. but could you please tell me some information abt how to calculate the power in CFD?
tas38 is right about the integration of pressure, it is what you are looking for.

Using CFD you will calculate a pressure field in the 3D domain. You then need to integrate the pressure times the lever-arm over the solid boundary of your blade (2D)

Pressure times area (you are integrating over an area) is a force, and this force times the lever-arm is a torque. By integrating, you're summing the individual contribution of each volume boundary located on the wind turbine blade.

I strongly suggest you read a good textbook on fluid mechanics, and another on CFD before anything else.
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