Blade Natural Frequencies using BeamDyn FE matrices

Wind & Water Computer-Aided Engineering Software Tools
1

Dear all,
I am using BeamDyn in OpenFAST in order to examine the response of the blade deflections to different rotor speeds. So, I was first trying to determine the blade natural frequencies from BeamDyn for the reference 5MW land-based wind turbine at zero rotor speed.
First, I used modal analysis techniques by exciting the blade in BeamDyn standalone by an impulsive force and I was successful to determine the blade natural frequencies and they agreed with those in the 5MW wind turbine definition report.
However, it was mentioned in the FASTv8_README that it is possible to determine the blade natural frequencies from the full FE mass and stiffness matrices output in the BeamDyn summary files. I tried this and the natural frequencies from the eigenvalue analysis using MATLAB do not match anything of the actual blade natural frequencies. Does this have anything to do with the definition of the blade rotational deflections as Wiener-Milenkovic parameters? Or is this technique invalid to check the blade natural frequencies since BeamDyn does not yet have the linearization capability implemented?

Thank you so much in advance!

Best Regards,
Shaimaa El Baklish

2

Dear Shaimaa,

Are you sure you calculated the natural frequencies from the mass and stiffness matrices included in the BeamDyn summary file correctly? Did you eliminate the first 6 rows and columns from the mass and stiffness matrices to force the blade-root cantilever connection?

Just FYI: We have implemented the capability of performing full-system linearization in OpenFAST with BeamDyn enabled and are currently testing it and documenting it before the public release later this year.

Best regards,