Validation of linear model for IPC control

Wind & Water Controls
2

Dear @Yarong.Zhou,

Here are my answers to your questions:

  1. In the linearized model, all states, inputs (including disturbances), and outputs represent perturbations about the operating point. So, the wind disturbance input is also a perturbation about the operating point.

  2. Yes, blade root flapwise moments are typical blade loads used to assess control impact on blade loads. The operating point values of states, inputs, and output are written to the linearization (.lin) output files of OpenFAST.

  3. The MBC3-transformed blade states represent the collective (0) and asymmetric (C, S) rotor modes associated with the blade flapwise deflection.

  4. You can always ignore inputs and outputs that you don’t want. Other than the azimuth state, I wouldn’t eliminate other states unless they have no influence on the response you are concerned with.

  5. It sounds like you have done everything correctly. To compare the linear and nonlinear models, I would run equivalent simulations in each. For example, a stepwise wind input or a stepwise pitch input. Of course, you’ll have to ensure that the inputs are set up correctly in each model (with the inputs to the linear model being the perturbation about the operating point) and the outputs should be compared by adding the perturbation to the operating point from the linear model.

  6. I wouldn’t change the frame of reference of the nonlinear model. Instead, I would convert the output of the linear model from the nonrotating to the rotating frame and add the OP perturbation to compare to the nonlinear solution, e.g., as discussed in the following forum topic: OpenFAST: Linearization of NREL 5MW Onshore Turbine - #9 by Jason.Jonkman.

Best regards,