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 http://www.nrel.gov/docs/fy08osti/42250.pdf 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)