Wind turbine does not alineate with incoming wind

Dear Jason,
I have been doing some FAST V8 simulations for a SWT based on the SWRT CertTest data and I have encountered some behaviour that I am sure you could send me help about. The turbine is a 3 bladed, with twisted,tapperded 3.22 m long blades, and I have used the tail geometry of the SWRT furl file in order to keep the rotor orientated to the wind.I have run 3 different simulations:

  1. EOG with hub height horizontal wind velocity of 13 m/s.
  2. EDC with hub height horizontal wind velocity of 13 m/s (constant).
  3. Steady wind with horizontal wind velocity of 13 m/s (no direction change).
    My concern is about the NacYawPos channel of FAST outputs (see attached figures).
  4. I would expect the nacelle position angle to remain more or less around 0 degrees of NacYawPos for the EOG simulation, as the incoming wind direction is 0 in ths case. I understand that due to aerodynamic inbalances inherent to a twisted blade the average value of the nacelle position will be different from zero, but the oscillations seen in the figure EOG.png seem excesive for me (up to 50 degrees!).
  5. For the EDC, a oscillation between -20 and 20 degrees is seen before starting the transient, then it follows more or less correctly the wind direction imposed by the incoming wind. When the transitient ends, the nacelle starts to oscillate around the position where it ended.
  6. Finally, from the steady 13 m/s wind the first oscillation of the EDC case (before starting the transitient) is seen. Then, the amplitude of this oscillation decreases and it ends with a yaw position of about 16 degrees, which I understand that is the angle that the nacelle rotates due to aerodynamic inbalances at 13 m/s.
    To sum up, I would expect the turbine to keep aligneate to the wind more tightly as I understand the tail makes in a real wind turbine. Maybe I am wrong and under EOGs such oscillations are normal due to aerodynamic imbalances. I also notice that I have set by default YawSpr and YawDamp to zero, what makes a no friction, perfect hinge. If the key are these two parameters, do you know some orientative values, or a method to determinate them in order to obtain a quite accurate modelling of what a SWT with tail orientation and free-yaw (like the NREL SWRT) in reality does.
    Thank you very much in advance. I really appreciate your help as I am in the last part of my bachellor thesis.
    PS: If you need more information, I could show you the FAST input files used for the simulations.
    Best regards,
    Alvaro Olcoz

Dear Alvaro,

The wind turbine furling and tail-fin aerodynamic capability of FAST v7 and earlier has not yet been migrated into FAST v8 and OpenFAST. So, the SWRT models/test cases from the FAST v8 CertTest and OpenFAST r-test are not really valid (they are simply carryovers from FAST v7 and will be updated once the furling and tail-fin aerodynamic capability has been added to OpenFAST). The SWRT model does not align itself properly with the wind without tail-fin aerodynamics.

For now, if you wish to model a wind turbine (such as the SWRT) with furling degrees of freedom and/or tail fin aerodynamics, you’ll will have to switch to using the older FAST v7 tool.

Best regards,

Dear Jason,
Thank you one more time for your rapic reply.
You have been helpful as always.
Alvaro Olcoz