Dear Jason,

I am trying to compare the ElastoDyn and BeamDyn in predicting blade deformations at different wind speeds with openFAST-v2.3.0. The bottom-fixed NREL-5MW turbine is used and only the GenDOF and blade DOFs are considered.

I find the BeamDyn results are very similar with ElastoDyn results for wind speeds below 11.4m/s, but the time-averaged In-plane deformations differ a lot for wind speeds above 11.4m/s, as shown below:

I know the modal approach for ElastoDyn is different with the geometrically exact beam model for BeamDyn, but I don not expect the In-plane deformations differ so much. So I have three questions:

(1) Does the **IpDefl1** in ElastoDyn represents the same quantity with **B1TipTDyr** in BeamDyn?

(2) If so, why the large differences are only happen at above-rated wind speeds for In-Plane deformations?

(3) I noticed the B1TipTDyr is defined in the floating reference coordinate system fixed to the root of the moving beam, and is defined relative to the undeflected position. I know this floating coordinate system moves with hub motions and blade azimuth rotations, but I am not sure whether it rotate with blade pitch angles? as the blade pitch only introduces rigid body rotation.

please help me,

Best regards,

Lin Yang.

Dear @Lin.Yang,

Actually, output `IPDefl1`

from ElastoDyn does not represent the same quantity as output `B1TipTDyr`

in BeamDyn. `IPDefl1`

is in a coordinate system that is fixed in the hub (doesn’t pitch with the blade), but `B1TipTDyr`

is in a coordinate system that is fixed in the root (does pitch with the blade). The more consistent comparison to `B1TipTDyr`

in BeamDyn would be ElastoDyn output `TipDyb1`

. Perhaps this explains the differences you are showing?

Best regards,

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Dear Jason,

Thanks for your reply and clarification! That is very helpful for me. Following your suggestions, I finally get reasonable results for the blade tip deformations as following:

As shown, the **TipDyrb1** in ElastoDyn indeed agree well with the **B1TipTDyr** in BeamDyn. To obtain the Inp and Oop results in BeamDyn, I performed a coordinate transformation for the results of **B1TipTDxr** , **B1TipTDyr** with blade pitch angle. The results are donated as BeamDyn_Oop and BeamDyn_Inp, which are calculated as:

BeanDyn_Oop = B1TipTDxr * cos( pitch_angle ) + B1TipTDyr * sin( pitch_angle )

BeanDyn_Inp = - B1TipTDxr * sin( pitch_angle ) + B1TipTDyr * cos( pitch_angle )

the resutls show consistent trends with the Oop and Inp results from ElastoDyn while certain discrapancies exist for Inp. By using the 3-rd Wiener-Milenkovic parameter **B1TipRDzr** as an approximation to blade torsion, as shown below,

I find the negative torsional deformations (nose-down) may reasonably explain the decrease of Inp results in magnitude.

To bring this topic to a conclude, I have another three questions:

(1) Is the calculation process of **BeamDyn_Inp** and **BeamDyn_Oop** acceptable?

(2) Dose the term **B1TipRDzr** can be used as an approximation for blade torsion?

(3) If so, is the previous explanation for Inp discrepancy correct or not？

Best regards,

Lin Yang.

Dear @Lin.Yang,

I agree with your equation for BeamDyn_Oop and BeamDyn_Inp.

Regarding `B1TipRDzr`

, see a similar discussion in the following forum topic: Extract blade tip torsional deformation from the Wiener-Milenković parameters in BeamDyn.

I agree that a small amount of blade torsion would influence in-plane deflection more than out-of-plane deflection.

Best regards,

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Dear Jason,

Thanks for your quick reply and confirmation. Now, I have a much clear understanding of the beamDyn and blade deformation features.

I hope this discussion would also be beneficial to other readers who are using beamDyn for blade aeroelasticity analysis or using beamDyn data for code validations.

Best regards,

Lin Yang.

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