The Tower Natural Frequency of the Tower of the NREL 5 MW-OC4-DeepCwind SemiSubmersible wind turbine. about openfast HOT 3 OPEN

Dear @126qoq,

Here are my responses:

1. The tower-top boundary conditions are quite different in the fore-aft and side-side directions, e.g, different rotor-nacelle assembly (RNA) inertias, different blade flexibility flapwise and edgewise, as well as the impact of the free generator rotation on the side-side mode.
2. Computing FFTs of time series from simulations with stochastic excitation is one way to identify full-system natural frequencies. Full system linearization of the OpenFAST model, followed by eigenanalysis is another way.
3. I'm not sure. Can you clarify the approach you used to compute the tower natural frequencies through linearization? Where the same degrees of freedom in the RNA enabled in both models (FFT versus linearization)?

Best regards,

from openfast.

Dear Dr. Jonkman,

Thank you for your answer. I didn't actually do the linearization.The tower natural frequencies through linearization are my quotes from other people's calculations: https://forums.nrel.gov/t/the-frequency-component-of-flapwise/1730/4 Actually, I'm more interested in figuring out if my calculations via FFT make sense or not.

Best regards,

from openfast.

Dear @126qoq,

The preferred way to calculate the full-system natural frequencies is through an OpenFAST linearization analysis. I suggest using that to verify your estimation of natural frequencies through FFT.

Best regards,

from openfast.