# NREL OC3 Monopile Questions.

Dear Arash,

It is difficult to answer your question without knowing more about your model, e.g. did you somehow generate white-noise turbulence or is this based on a given wind spectrum, what rotor speed is the turbine is running at, etc? I’m not really seeing any clear peaks in your second plot, except maybe at 0.28 Hz. When a turbine operates under turbulent wind excitation, I would expect to be strong peaks at 0 Hz (the mean thrust), at three-times-per revolution (3P), at 6P, and if there is a rotor imbalance, at 1P. In your second plot, I don’t see a strong 0P. Perhaps 0.28 Hz is 3P (corresponding to a rotor speed of 5.6 rpm)?

Best regards,

Hi Jason,

I generated turbulent wind file in Turbsim with Kaimal Turbulence model with 14.6% turbulence intensity and 12m/s wind speed. As wind speed is larger than rated wind speed 11.4, rotor speed should be 12.1 rmp.

Regarding rotor speed, I don’t change RotSpeed in Elastodyn for different wind speeds. Should I change it? I assumed it (RotSpeed) is initial rotor speed and I do not need to change it as after transition period it will balance with control system. So, I just kept that 12.1 rmp. Is it correct?Do I need to change RotSpeed in Elastodyn for every wind speed?

Thank you very much again.

Best Regards,
Arash,

Dear Arash,

See the following forum topic for my recommendations on setting initial conditions for FAST simulations: http://forums.nrel.gov/t/initial-conditions-for-tower-and-rotor/609/1.

For a simulation at 12.1 rpm, I would expect to see a strong excitation of the tower at 3P = 0.6 Hz, which I don’t really see in your plot.

Best regards,

Dear Jason,

Thank you for your reply. According to the post, I need to find the initial condition from figure 9-1 FAST user manual.

Regarding calculating FFT from time history, I plotted time history of the result I used (TwHt1TPxi for-aft top tower displacement). Do you think my time history looks reasonable given the wind condition (Wind speed is 12m/s with 14.6% turbulence intensity and 12.1 rmp rotor speed)?Or something is wrong?

Thank you very much.

Best Regards,

Arash,

Dear Arash,

It is hard to look at one plot in isolation to judge the correctness of the response. I suggest that you also look at the rotor speed, blade-pitch angle, blade deflections, and loads.

Best regards,

Hi,

I have a question regarding Platform inertia for yaw rotation about the platform CM in Elastodyn input file.

In OC3-Monopile and OC3-Tripod Elastodyn input files, Platform inertia for yaw rotation about the platform CM is 1.534E+06 kg m^2 and 2.48448E+06 kg m^2, respectively.
How are these values calculated?
Both values have the same order of magnitude (10^6). Are they arbitrary values?

Regards,

Gabriel.

Dear Gabriel,

As discussed in section 5.4 of the draft SubDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/SubDyn_Manual.pdf, when SubDyn is enabled in a FAST model, we recommend setting the platform-yaw inertia (PtfmYIner) in ElastoDyn equal to the total rotational inertia of the undeflected tower about its centerline. The two values you mention are these inertias (the towers in both models differ).

Best regards,

Hi Jason,

Thank you for your input. It will help me.

Regards,

Gabriel.

Dear Jason,

I have a basic question. I have modelled my foundation and tower in Matlab and want to apply time history of aerodynamic loading that I get from FAST output (YawBrFxp) to the top of the tower. In my foundation and tower model in Matlab I have RNA mass around 400 tons, shall I remove this mass when I apply time history of YawBrFxp? Thank you very much.

Regards,
Arash

Dear Arash,

ElastoDyn output parameter YawBrFxp includes not only the applied aerodynamic loads, but also the loads from the oscillating inertia of the tower-top as a result of tower deflection. So if you have modeled the RNA inertia in your MATLAB model, you’d be double-booking the inertia loads if you used YawBrFxp. If you want the pure aerodynamic load, you must disable the structural degrees of freedom in ElastoDyn when outputting YawBrFxp or output the pure aerodynamic loads–from AeroDyn v15 this would be e.g. output parameter RtAeroFxh.

Keep in mind that–because of the coupled aero-elastic solution of FAST–that the aerodynamic loads depend on the motion of the rotor (including the displacement and velocity of the tower), so unless the motion of the tower in the FAST model is quite similar (including amplitude and phase) to your MATLAB model, the one-way coupling you are proposing may have some inaccuracies. (A similar statement applies to the inertia loads if you choose to remove the RNA inertia from your MATLAB model.)

Best regards,

Dear Jason,

Thank you for your reply. According to your comment, I disabled structural DOFs (TwFADOF1,TwFADOF2, TwSSDOF1,TwSSDOF2, PtfmSgDOF, PtfmSwDOF, PtfmHvDOF, PtfmRDOF, PtfmPDOF, and PtfmYDOF) in ElastoDyn and compared the time history and FFT of YawBrFxp (the attached figures).

Then I applied these forces (YawBrFxp) on the top of tower in my Matlab code ( the model has RNA mass as well), and I have TMD and TLCD dampers tuned to the first natural frequency of the system modelled in my Matlab code. But the problem is that TMD and TLCD have negative effects on the responses compared to the baseline model when there is no structural control devices. However, I expect the devices to reduce the responses as they do when I apply the time history of (YawBrFxp) where structural DOFs in ElastoDyn are not disabled.

Do you think it is something wrong about my loading applied? or should’nt I tune the dampers to the first natural frequency of the system?

Any hint is highly appreciated. Thank you very much.

Best Regards,
Arash

Dear Arash,

Normally a TMD or TLCD would be tuned to the first natural frequency of the tower.

What you are using is a one-way coupled model rather than two-way coupled model between structural dynamics and aerodynamics, so, inaccuracies would be expected depending on the influence of tower displacement and velocity on the aerodynamic loads. Is there a reason why you don’t just use the two-way coupled FAST solution, which can also be used to model TMDs in the nacelle and/or tower?

Best regards,

Dear Jason,

Thank you for your quick reply. The reason I am not using TMD model in FAST is that I need to use my code for structural analysis and also TLCD can not be modelled in FAST so I modelled TMD and TLCD in my code.
When I do not disable Structural DOFs in ElastoDyn and use the time histories in my model, both TMD and TLCD have reasonable suppression effects. But when I use the time histories of forces where structural DOFs in ElastoDyn is disabled, TMD and TLCD have negative effects on responses.

Best Regards,
Arash

Dear Jason,

Another problem I have is that when I use AeoroDyn15 in FAST to output parameter RtAeroFxh, it gives the error as attached. It is related to reading BldNodes, but I didnt change anything else, I just changed AeroDyn to AeroDyn15 in .fst file. Thank you very much for your help.

Regards,
Arash,

Dear Arash,

From the error message you’ve received, my guess is that you’ve changed AeroFile in the FAST primary input (*.fst) file from an AeroDyn v14 to an AeroDyn v15 file, but you didn’t change CompAero from 1 to 2.

Best regards,

Dear Jason,

Thank you very much for your reply. I changed CompAero but it gives another error. Is there anything else I should change? It refers to Mesgses.11.dbg file but that file is empty. Thank you very much.

Regards,
Arash,

Dear Arash,

It appears that FAST is aborting because of an error in the mesh-mapping search for ED_L_2_AD_L_T, which is the mesh-mapping search between the ElastoDyn Line2 tower motion output mesh and the AeroDyn Line2 tower motion input mesh. FAST will trigger this error e.g. if the tower in AeroDyn starts lower than the tower in ElastoDyn or if the tower in AeroDyn ends higher than the tower in ElastoDyn. This error has been discussed on this forum before e.g. here: http://forums.nrel.gov/t/meshing-error/1527/2.

I hope that helps.

Best regards,