Tower first FA mode

Hello!

I faced a problem with tower first FA mode. When I compute it in BModes, it gives a good agreement with measurement (about 0.35 Hz); but the result of analysis the FAST output file is quite different (about 0.45 Hz).

The way I computed this factor in FAST, is to plot the power spectrum density of tower base load. It should be noted that I used the exact same sectional properties in FAST and BModes, and the mode shapes are calculated based on BModes results, but still the computed frequencies are different.

I need to know the reason for this difference, and if I can improve the FAST results without using the tuner (FAStTunr). It would be highly appreciated if you can help me.

Thank you so much in advance.

Dear Iman,

I would normally expect a closer agreement between the BModes- and FAST-computed natural frequencies if you are taking the mode shape calculated by BModes and using it as a shape function in FAST. The tower stiffness tuners (FAStTunr etc.) can be used for fine tuning of any minor differences.

Do the BModes and FAST tower models have the same distributed mass and stiffness properties, the same tower-base boundary condition (e.g. fixed), and the same tower-top mass and inertia (in BModes this is set directly, in FAST this is set through the nacelle, hub, and blade masses/inertias)?

Best regards,

Dear Jason,

Thank you so much for your reply.

Yes, all of the parameters you mentioned are exactly the same.

The model is quite accurate and is checked carefully. The single soft point of the model is actually the blade airfoils which are not accurate because I do not access the precise parameters.

Is there any other parameter that can affect the frequency?

Yours,

Dear Iman,

Have you followed the guidance for setting the distributed tower properties that are available in BModes, but not in the ElastoDyn module of FAST available from my post dated August 29, 2014 in the following forum topic: http://forums.nrel.gov/t/regarding-apparent-fixity-model/983/14?

Best regards,