Dear Jason,
Thanks for your reply. I provided the spreadsheet for the sake of supporting my query related to picking the full system frequencies when the maximum values of the modes shapes highlighted with green backgrounds refer to the same column (natural frequency). A similar question was asked in this forum (Eigenanalysis FAST) in the past. Although I do agree with your reply to this question, I do not understand how the phase values are being used in the spreadsheet to pick the right modes in case of a DOF belonging to a certain natural frequency when there are many mode shape magnitudes with green background. I thought the right mode for a DOF (say tower) is first picked by finding out the maximum value (green cell) along its row and then the value in the corresponding column gives the natural frequency. In case, if the several modes contain green cells pointing to the same column then I should look for the column with the maximum value (red cell) in the same row as the DOF. But this seems not always true. Please let me know if this is true.
I do agree it certainly takes time and expertise to interpret the Eigenanalysis results and as a non-expert, I am trying to learn the techniques followed in this community from the discussions here in the forum. I understand how linearization and MBC transformation are used to compute the full-system natural frequencies, mode shape vectors, and damping ratios for offshore wind turbines as nicely described here (FAST Modularization Framework for Wind Turbine Simulation: Full-System Linearization and User’s Guide to MBC3) but nowhere I found information on the procedure used in the spreadsheet. It would be really helpful if you can refer to any report/article where I can find technical information related to this procedure.
In fact, I have already tried linearization i) for the tower only DOFs with all other DOFs disabled, ii) for the tower, blades only DOFs enabled without any aerodynamics and rotor rotation. In the first case, I can see the spreadsheet is able to pick the right modes as there are only 4 dofs. However, for the second case, I see the spreadsheet is not picking up the right modes. Moreover, I cannot rely on the first case results to identify the modes for the second case as the mode magnitudes for the tower changes. I do have some idea about the values of the natural frequencies for the 1st tower modes and platform DOFs based on my simulations run using BModes and white-noise excitation PSDs. The problem is in picking the modes for the blades as their natural frequencies are closely spaced.
Best regards,