Undesirable aerodynamic fluctuations at rated wind speeds

Dear all,

In order to obtain frequenty-dependent aerodynamic damping of floating wind turbine,I want to obtain the aerodynamic thrust(RtAeroFxh in outlist) through simulation satisfying the following relation:




OpenFAST only seems to define nacelle forces, not nacelle speed, so I decouped hydrodyn and moordyn by ExtPtfm module, given the forced motion.
Here is my ExtPtfm input file:

In order to get the ideal motion and ideal RtAeroFxh, my Elastodyn and Aerodyn have the following Settings:

The Nacelle motion met my expectations.

At non-rated wind speed, the trend of RtAerofxh is ideal, but the peak value fluctuates slightly when the period is larger.

Here are the questions:
At the rated wind speed, the output RtAeroFxh has unexpected fluctuations, and the fluctuations become more significant as the period of forced oscillation increases:

There seems to be a frequency disturbance, and I want to know what exactly is causing the aerodynamic force to fluctuate? Why does it get more and more significant as the motion period become longer?
As a final supplement, the model I used is 5MW DeepCwind semi-submersible wind turbine.The controller I have used is 5MW-OC3Hywind.dll.

Please forgive me for saying so much and for making some mistakes in English grammar. If you can read it and give me some suggestions, I would be grateful!

Best regards,

Dear @He.Li,

I haven’t reviewed the values you set in ExtPtfm, but your approach sounds reasonable to me.

I would guess the fluctuation from ideal sinusoidal behavior is related to the variable-speed generator and controller. Do you get the response you expect if you disable the generator degree of freedom in ElastoDyn, GenDOF = FALSE?

Best regards,