Separate/Individual frequencies for NREL5MW tower and blade

Wind & Water Structural Analysis
1

I am working on a reduced order NREL5MW model for control design. I want to have tower and collective blade included. I need the frequencies for individual parts I have started by taking the frequencies from table 9-1 in Jonkmann2009. As I understand this is not really right as the frequencies in table 9-1 is for the whole combined system. Am I right.

In that case, where can I find the individual frequencies?

Another question I have is how to explain the 0.2Hz spectral peak for OoPDefl1 and IPDefl1 I get under 17m/s turbulent simulation.

2

Dear @Torben.Knudsen ,

I am Riad, a wind turbine enthusiast.

I dont bnow the answer for you first question.

Regarding your second question, i can say that 0.2 Hz corresponds to 1P auto-excitation frequency. The 1P auto-excitation frequency is the frequency of the rotor. Indeed, in case of 17 m/s mean wind speed, the rotor speed is 12.1 rpm = 0.2 Hz.

Best Regards,

Riad

3

Thanks for the response. I forgot about the 1P. Indeed it makes sense that gravity and perhaps wind sheer will give 1P in blade edge but also flap when the pitch is around 13deg at 17m/s.

4

Dear @Torben.Knudsen,

I agree with @Riad.Elhamoud regarding 1P.

Regarding the natural frequencies of the NREL 5-MW turbine, I agree that the values in Table 9-1 of its specifications report are full-system natural frequencies. I don’t recall if the isolated blade and tower natural frequencies are published somewhere, but you could compute these quite easily with OpenFAST by disabling all but the DOFs you want to isolate. For the tower, you’ll have to decide if you want the frequencies with the mass/inertia of rotor-nacelle assembly on top, or without.

Best regards,

5

Dear @Jason.Jonkman

Thanks for the response. I will try that. I like to have tower plus nacelle (without blades) as one model and then blades on a stiff tower as the other model. I will then combine these myself. I hope that is possible fixing DOFS. An alternative could be to use system identification in some way.

6

Dear @Torben.Knudsen,

It is quite easy in OpenFAST to enable/disable the tower and blade degrees of freedom independently to isolate each contribution. The ElastoDyn module will not eliminate the blade mass when disabling the blade DOFs for the case where you want to isolate the tower + nacelle modes, but you can eliminate the blade mass by changing the mass or geometry of the blades in ElastoDyn.

Best regards,