Mode shapes of the blade used in ElastoDyn

[Riad.Elhamoud]

Dear @Jason.Jonkman,

I have a question regarding the mode shapes of the blade used in ElastoDyn.

Indeed, the mode shapes are polynomials that are used to fit points given by B_modes. In the input file of B_modes, one should specify the rotor speed. This means that the mode shape depends on the rotor speed which seems reasonable for me since the more the rotor speed is, the more the stiffening effect is pronounced and therefore, the mode shapes are affected.

However, in OpenFAST, ElastoDyn does not have the capability to change the mode shape automatically according to rotor speed.

Is it a limitation of ElastoDyn and one should use BeamDyn instead ?

What do you think ? If there is any NLR publications about this, i will be glad whether you can share them.

Thanks in advance,

Best Regards,

Riad

[Jason.Jonkman]

Dear @Riad.Elhamoud,

I agree that the blade mode shapes would depend on rotor speed, blade pitch, etc., and the fact that they don’t change in ElastoDyn is a key limitation of modal-based methods. However, I have not noticed this itself to be a big source of error. This topic has been discussed in other forum posts, e.g., see:

The consideration of interactive effect between blade and tower/rotor when the BModes mode shapes are entered to FAST.

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Rather from my experience, the bigger limitation in ElastoDyn is the lack of offsets in the mass and stiffness from the pitch axis and the lack of a torsional degree of freedom.

Best regards,