[mrswordf1sh]

Hi,

I am trying to design a wind turbine blade.
As for I know there are two methods to calculate the output power and Power coefficient (Cp): Blade I can't say the same about panel method (I just know that the blade is divided to panels)

The programs like XFOIL and RFOIL are using panel method. There are many people using these, but there are also many people using BEM.

What is the difference between these two?

Thanks

[robo]

A blade element momentum (BEMT) code uses relations that have been derived based on global momentum conservation along the stream tube which flows through the turbine. There are a number of simplifying assumptions used in this derivation, such as averaging the flow along annular sections. A variety of empirical formulae are applied to correct for things which have been neglected in the derivation (eg dynamic wake response, finite blade number, etc). They will always require information from either experiment or some other simulation in order to account for the blade loading, as they cannot natively predict the lift/drag performance of the blade.

Panel methods are based on a potential flow solution, which is synthesized from a variety of simple flows (eg source/sink, vortex, dipole, etc) in order to approximate the turbine blade. The assume that the flow is inviscid (which it isn't) but otherwise make less simplifications then BEMT, and can natively deal with dynamic wakes, finite blades, etc. They may not require info from experiments or other simulations, as SOME potential flow methods can natively predict the lift performance of the blade. Induced drag will be calculated natively, but viscous drag will not, and they made still need corrections/experimental data/other simulations to deal with viscous drag and stall effects.

Comparing the two, panel methods are more strictly correct in an analytical, first principles sort of way, however BEMT codes run much faster, and their answers are still good for many cases. Panel codes will give you a higher fidelity data set at the end, which you may or may not need. Panel codes can potentially run independently, which may be an advantage if you lack the foil performance data. Which one you want to use will depend on what you are actually investigating. For early-stage design work, I would recommend BEMT.

[mrswordf1sh]

Originally Posted by robo

A blade element momentum (BEMT) code uses relations that have been derived based on global momentum conservation along the stream tube which flows through the turbine. There are a number of simplifying assumptions used in this derivation, such as averaging the flow along annular sections. A variety of empirical formulae are applied to correct for things which have been neglected in the derivation (eg dynamic wake response, finite blade number, etc). They will always require information from either experiment or some other simulation in order to account for the blade loading, as they cannot natively predict the lift/drag performance of the blade.

Panel methods are based on a potential flow solution, which is synthesized from a variety of simple flows (eg source/sink, vortex, dipole, etc) in order to approximate the turbine blade. The assume that the flow is inviscid (which it isn't) but otherwise make less simplifications then BEMT, and can natively deal with dynamic wakes, finite blades, etc. They may not require info from experiments or other simulations, as SOME potential flow methods can natively predict the lift performance of the blade. Induced drag will be calculated natively, but viscous drag will not, and they made still need corrections/experimental data/other simulations to deal with viscous drag and stall effects.

Comparing the two, panel methods are more strictly correct in an analytical, first principles sort of way, however BEMT codes run much faster, and their answers are still good for many cases. Panel codes will give you a higher fidelity data set at the end, which you may or may not need. Panel codes can potentially run independently, which may be an advantage if you lack the foil performance data. Which one you want to use will depend on what you are actually investigating. For early-stage design work, I would recommend BEMT.

Thank you for the great explanation robo, I appreciate it..