Dear all,

I’m currently working with IEA-15-240-RWT-UMaineSemi and I’m trying to extrapolate the non-dimensional damping of the platform pitch using the linearization of OpenFAST for different wind speeds. The goal was first to create the state matrix A (considering as dofs platform pitch and its derivative + rotor speed and its derivative: 4x4 matrix) , then evaluate the eigenvalues and so the non-dimensional damping. I followed the guidelines for which in a rotating case the recommended approach was :‘it is recommended to use NLinTimes=36 (corresponding to on linearization every 10-degrees of azimuth rotation)’. The dofs activated were PtfmPDOF and GenDOF and HydroDyn still water. Once I applied that, I extracted the 4x4 A matrices from .Lin file and then I’ve averaged them for each wind speed. The results at the end are not as expected: firstly, from manual calculation I was expected a natural frequency (as conseguence of the eigenvalues) of about 0.04 Hz but I got 0.4 Hz, secondly I was expecting to have the poles related to the first derivative of the rotor speed to be in 0, but it was just close to it.

I was wondering if someone could help me to understand if I’m extracting in the wrong way the data from .lin files, in fact I encountered the following issue:

Here I write part of the .lin file

Row/Column Operating Point Rotating Frame? Derivative Order Description

```
1 -8.756E-04 F 2 ED Platform pitch tilt rotation DOF (internal DOF index = DOF_P), rad
2 4.721E+00 F 2 ED Variable speed generator DOF (internal DOF index = DOF_GeAz), rad
3 -2.294E-07 F 2 ED First time derivative of Platform pitch tilt rotation DOF (internal DOF index = DOF_P), rad/s
4 3.643E-01 F 2 ED First time derivative of Variable speed generator DOF (internal DOF index = DOF_GeAz), rad/s
```

A is a 984x984 where from the info above, I know lines 1,2,3,4 and columns 1,2,3,4 are associated to the 4 dofs i want. But looking at the matrix it seems that the first two lines are actually associated to the derivatives of the PtfmPDOF and GenDOF in opposition to what the order is telling me

0 0 1 0

0 0 0 1

-8.36600000000000 -1.22321111111111e-05 -0.125091666666667 0.00250797222222222

|-0.00313063888888889 -0.000102025277777778 -0.955677777777778 -0.0860683333333334

Here the plot of the eigenvalues:

Thank you very much.