I just reviewed your derived Campbell diagram for the land-based 5 MW baseline wind turbine. It was in your post from Thu Sep 22, 2016 8:24 am.
Furthermore I reviewed the tower section 6 in Jonkman, J., Butterfield, S., Musial, W., and Scott, G: Definition of a 5-MW Reference Wind Turbine for Offshore System Development. February 2009.
In this section 6 on page 15 in the second paragraph, one writes:
This sounds like the wall thickness of the baseline tower is driven by its first two eigenfrequencies. But when I go back to your Campbell diagram I see, that these tower eigenfrequencies are very close to the three-per-rev frequencie at cut-in rotational speed. In fact, neglecting the proposed wall thickness increasement of 30 % would decrease the tower eigenfrequencies to about 0.282 Hz and a larger distance to the exciting three-per-rev frequency can be obtained. Note, that it would still have enough distance to the one-per-rev excitation frequency of 12.1 1/min / 60 s/min * 1.05 = 0.21 Hz.
If I recall correctly (although this was over 10 years ago now), I think the cut-in speed dropped a bit over the course of the NREL 5-MW development and perhaps the tower thickness was set before this and not later updated.
I agree that the final first tower fore-aft and side-to-side mode frequencies are bit close to the 3P frequency at the cut-in rotor speed. That said, I don’t recall finding a problem with resonance of the tower from 3P excitation.
I need some clarification with the Campbell diagram (Excel file) of the land-based NREL 5-MW turbine that you posted here (Campbell diagram for 5MW turbine rotor) in the forum on Sep 22, 2016. I think that the values for the 2nd blade regressive and progressive modes in rows 14 and 16 in the “CampbellDiagram” worksheet are reversed. Based on the data (natural frequencies) for the 1st blade flapwise regressive and progressive modes (6th and 8th row) on the CambellDigram worksheet, I understand that data corresponding to the 1st blade regressive and progressive modes was chosen from the rows 20 and 19 of the worksheets with names 0 RPM to 14 RPM. Likewise, the 1st blade edgewise regressive and progressive data was chosen from the rows 23 and 22 respectively. However, the reverse was done for 2nd flapwise regressive and progressive modes i.e. data from rows 25 and 26 are chosen for regressive and progressive modes shown in the rows 14 and 16 on the CambellDigram worksheet. Was this a mistake or do you have any particular explanation for doing this for only the 2nd blade regressive and progressive modes?
This is simply a naming convention. By “regressive”, I mean the asymmetric mode that reduces in frequency with increasing rotor speed. By “progressive”, I mean the asymmetric mode that increases in frequency with increasing rotor speed.
I am trying to recreate your campbell diagram .xls for the 5MW land turbine to understand the process of linearization. Unfortunately I am not making any progress. Would you be willing to sent me your input files to recreate the diagram so I can study them and understand the process?
Thank you very much,
best regards, Veith