Dear Aina,
The old Eigenanalysis.m script simply takes the azimuth average of the periodic linear model to obtain a constant-coefficient linear model. It is known that this process causes many problems if the linear model contained periodicity (i.e., if the model was linearized with a spinning rotor). It is far better to perform a multi-blade coordinate (MBC) transformation before azimuth averaging. Several years ago, we developed the MBC3 code that can be used to post-process the output of a FAST linearization analysis. You can find that code here: wind.nrel.gov/designcodes/postprocessors/mbc/. Since then, we’ve suggested that everyone use the MBC3 code in place of the Eigenanalysis.m script in all circumstances (in fact, we eliminated Eigenanalysis.m from the FAST archive in the last two releases of FAST). My remaining comments assume you’ve switched to using the MBC3 code.
You must compare the eigenvalues with the eigenvectors to see which frequency corresponds to which mode. The eigenvalues contain complex conjugate pairs that contain information about the natural freqeuncies and damping of each mode. The largest components of the eigenvectors represent the dominant components of each mode. I’ve created an MS Excel workbook titled “CampbellDiagram.xls” to aid in analyzing the eigenvalues and eigenvectors. (This workbook is attached.)
CampbellDiagram.xls (242 KB)
The process is as follows:
*Once a .lin file has been generated by FAST, run the GetMats.m and mbc3.m MATLAB script.
*Open up CampbellDiagram.xls and enter the information into the gray cells. Most of the information to be put into the gray cells is computed by the mbc3.m script and, so, can be copied from MATLAB and pasted in.
The CampbellDiagram.xls workbook sorts the eigenvalues and eigenvectors by frequency and highlights the dominant components of each mode. The spreadsheet is fairly self-explanatory; I hope you find it useful.
Please note the spreadsheet is meant to be used to process separate results from a range of rotor speeds in order to make the complete Campbell diagram. Typically, the first solution is always made at 0 RPM. Also, please note that the MBC3 script changes the mode descriptions as follows (when we get the chance, we plan to modify the mbc3.m script to output the MBC state descriptions accordingly):
FAST Output → MBC Output
Blade 1 DOF → Collective DOF
Blade 2 DOF → Cosine DOF
Blade 3 DOF → Sine DOF
I hope that helps,
Best regards,