Hi Rafael,
Thank you very much for your opinion!
I have tried replicating the data of the 5MW reference turbine and everything seems fine (except differences due to different version of Aerodyn).
Regards,
Matteo Rovelli
Hello everyone,
I run the 5MW_Land_BD_DLL_WTurb.fst test with OpenFAST v2.5.0. I could not understand why the pitch angle signal oscillates even when the wind speed is constant at the rated wind speed of 12 m/s. can anyone explain this?
Best regards
Sebastian
Dear Sebastian,
It looks like this model has not yet reached equilibrium and you are seeing the start-up transient. The oscillations in blade pitch are commensurate with the oscillations in rotor speed. The system is not in equilibrium because the blades and tower are initialized in the undeflected state whereas in equilibrium the blades and tower will be deflected as a result of the aerodynamic forces applied to the rotor. As with any aeroelastic simulation, it is best to neglect the start-up transient when post-processing the output.
Best regards,
Dear Jason,
Thank you for your quick reply.
In the figure below the wind speed rises from 0 m/s to 12 m/s in the first 100 s. After that the wind speed keeps constant over the next 100s till the simulation time of 200 s. Finally, the wind speed jumps to 14 m/s and keeps constant till the end of the simulation. As you see the oscillations in the rotor speed and the pitch angle cannot be damped at wind speed of 12 m/s (unlike the jump to wind speed of 14 m/s). These oscillations have a period of circa 13 s, which do not match any of the natural frequency at 12 rpm in the Campbell diagram of the 5-MW WT. Therefore, I would expect that these oscillations are due to an excitation from the control system.
do you agree with that? Can you tell me which signal could I plot to figure out the reason of such oscillations?
Best regards
Sebastian
Dear Sebastian,
I agree that the oscillations appear to be coming from the controller. I would guess the 13-s period (0.077 Hz frequency) is coming from the PI-based blade-pitch controller. Did you change this OpenFAST model (of the NREL 5-MW baseline wind turbine with blades modeled in BeamDyn) in any way, other than the wind speed and initial conditions? It appears that the gains in the gain-scheduled pitch controller may need to be adjusted (increased) at low pitch angles. Perhaps this is because the controller was originally designed for this model with the blades modeled in ElastoDyn, which does not consider blade torsion. I’m curious how much blade twist you are seeing from BeamDyn during this simulation (TipRDzr)?
Best regards,
Dear Jason,
Thank you for your reply
I only changed the initial rotor speed from 12.1 to 7 rpm and I set the WindType to the uniform wind file as shown in the previous figure. I did not change the gains in the gain-scheduled pitch controller.
If I understand you correctly, you mean if I attempted to use the NREL 5-MW rotor blade design in BeamDyn I have to tune the gains in the gain-scheduled pitch controller. Can you give me some hints to adjust the values of KP and KI values?
Best regards.
Sebastian
Dear Sebastian,
Right now, I’m simply suggesting that the gains may need to changed, based on the results you’ve shown. We’ve verified the BeamDyn model of the NREL 5-MW baseline wind turbine against the ElastoDyn model, using the same controller in both, but we at NREL have not performed a rigorous loads analysis with the BeamDyn model across many conditions (as far as I know).
Regarding the setting of the PI gains in the pitch controller, you can refer to the specifications report for the NREL 5-MW baseline wind turbine, which explains how the original gains were derived: nrel.gov/docs/fy09osti/38060.pdf.
Best regards,
thank you for your suggestion and for the quick response 
.