NREL 5MW Blade Eigenfrequencies with Bmodes

Wind & Water Computer-Aided Engineering Software Tools
1

Hi to all, I’m Ivano Quarta and i’m new to this forum. I’m using 5MW NREL model to compare two different aeroelastic codes. For this reason i would like to get the natural frequencies of blade and tower using Bmodes.
Reading the document [url]http://www.nrel.gov/docs/fy08osti/42250.pdf[/url] I understand that there is two way to obtain uncoupled modes of Blade: decoupling the flexural problem from the torsional one, or considering the modal participation of the single mode.

With the fist way i got the following results obtained eliminating the inertia and cg offset and increasing the axial and torsional stiffness (see the attachment):

Mode Bmodes Calc err % 1 0.69584 0.66696 4.15% 2 1.17550 1.08420 7.77% 3 1.95960 1.88500 3.81% 4 4.04710 3.95923 2.17% 5 4.56570 4.23554 7.23% 6 8.14450 7.03750 13.59%

I think the difference is due to a partial coupling of modes (for example, the first flap mode have a little partecipation of the edge).
How i can risolve this mismatch?
Which method has been used in the document to derive the frequency of the mode with greater participation?

Thank you very much. I am appreciated your help.
Bmodes.xlsx (177 KB)

2

Solved! Reading another post i set the twist angle to zero to uncouple flap and edge modes. I also change some parameters to my model and results match.

Mode Bmodes Calc err % 1 0.67639 0.67289 0.52% 2 1.08970 1.08429 0.50% 3 1.94890 1.95460 -0.29% 4 4.04300 4.00517 0.94% 5 4.51440 4.54865 -0.76% 6 8.10160 8.22479 -1.52% 7 9.3866 9.35616 0.32% 8 12.747 12.50460 1.90% 9 17.157 16.88141 1.61% 10 18.54 18.22292 1.71%