Interpretation of State Vector during linearization with AeroDyn

Wind & Water Structural Analysis
1

I am trying to interpret the results of a Modal analysis around an Operational Point. To do that i am using the linearization capabilities of OpenFast. I am using the standard NREL 5-MW reference turbine and have enabled the following modules:

  1. ElastoDyn
  2. InflowWind
  3. AeroDyn
  4. ServoDyn

The results after running the python script ex2a_MultiLinFiles_OneOP.py are as follows:

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1. Why are aerodynamic wind velocities part of my state vector?

I see that the state vector includes quantities like AeroDyn wind velocities, which I didn’t expect. I had assumed the aerodynamic influence would be captured in the system matrices (e.g., the A matrix), while the state vector would contain only the structural degrees of freedom. Is there a way to exclude these AeroDyn states from the state vector or configure the linearization to focus only on the structural states?

2**.** How should I interpret mode shapes that include wind velocities and structural displacements together?

This is the Main_lin.1.lin output file and as we can see there are 258 continuous states. This many eigenmodes and frequencies make the results less clear and hard to interpret.

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These are my input files in case they are needed:

NREL

Thank you very much in advance,

Best regards,

Vasileios Xenodochidis

2

Dear @Vasilis.Xenodochidis,

Here are my responses:

  1. Aerodynamic states are included in your linearized model because you’ve dynamic wake (DBEMT_Mod = 3) in AeroDyn. While inclusion of aerodynamic states (from dynamic wake or unsteady airfoil aerodynamics) makes interpretation of the eigensolution more difficult, it is a very valuable tool to include the effect of aerodynamics on the stiffness and damping of the eigensolution. See our TORQUE 2022 paper for more information: https://docs.nrel.gov/docs/fy22osti/82343.pdf. To eliminate these aerodynamic states from the linearization, set DBEMT_Mod = -1 to enable frozen wake, which mimics the dynamic inflow effect without directly considering aerodynamic states).
  2. To interpret the eigensolution, I would focus on the the structural state contributions to the modes. Our Automated Campbell Diagram Code (ACDC) is the most convenient way to set-up, analyze, and interpret eigensolutions: GitHub - OpenFAST/acdc: ACDC: Automated Campbell Diagram Code .

Best regards

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