Dear NREL, I am new to FAST.Farm but I am currently trying to run some simulations without wind. However, I am obtaining some errors. The idea is to get the static condition, run some static offsets or perform decay tests, everything without wind, as I can do in OpenFAST. I tried both with 0m/s or even very low wind speed (0.05m/s), but getting the following error:
“Wake model is not valid in the propeller-brake region, i.e., Ct > 2.0”
I also tried to disable AeroDyn and InflowWind modules in the fst file.
So my question is if it is possible to set up these conditions (no wind).
I would greatly appreciate guidance on this.
Kind regards and thank you so much.
Antonio.
Dear @Antonio.Medina,
Actually, FAST.Farm is not really set up well to support simulations without wind. While it is easy in standalone OpenFAST to remove wind altogether, the same is not true for FAST.Farm given that much of the software revolves around coupling the wind turbines of a wind farm together through wind.
Can you clarify your use case? The one use case I could see is if you are trying to model free-decay simulations without wind of floating offshore wind turbines of a wind farm with shared mooring systems. Without coupling of the wind turbines in a wind farm through shared moorings, there is likely no other couplings between wind turbines that FAST.Farm would support without wind, so without wind, you may just as well run independent OpenFAST simulations of each wind turbine. (FAST.Farm also couples wind turbines of a wind farm together through farm-level control, but this is mainly valid with wind as well).
Best regards,
Dear @Jason.Jonkman,
Thank you so much for your kind reply. To be honest the idea is to model an experimental campaign of two turbines tested without wind. In the following steps, we will add the wind and simulate it with FAST.Farm. As you mentioned, in the first order shared mooring is not important.
However, we need to perform a decay to validate our shared mooring setup and analyse the different eigenmodes. So we were wondering if it is possible to use FAST.Farm.
Thanks for your time,
A.
Dear @Antonio.Medina,
Thanks for clarifying. That makes sense. To avoid receiving errors from FAST.Farm, I would recommend setting up a simulation with a low wind speed and a low rotor thrust. I would also recommend setting a low value of NumPlanes (e.g., NumPlanes = 2) to ensure that there are no wake effects between the turbines.
Best regards,
Dear @Jason.Jonkman,
Thank you so much for your help and suggestions. With these recommended settings, it works in static conditions, with negligible motions. However, when I tried to give an initial position and release to analyse a surge decay, I got an error.
I tried with both ElastoDyn module, using PtfmSurge != 0, and also in the main .fstf file, changing the WT_X. I excited only a single turbine. The returned error was:
T1:FARM_UpdateStates:WD_UpdateStates:The time-filtered nacelle-yaw error has reached +/- pi/2.
Aborting FAST.Farm.
I was trying several things and looking for this error, but I couldn’t find anything in the Forum. Some hints that you could provide will be really helpful to keep going with these simulations.
Dear @Antonio.Medina,
My guess is your wind speed is very low and floater motions are triggering this error. To bypass this error, I would suggest increasing the wind speed (perhaps up to a few m/s or so), which shouldn’t cause much forcing if the thrust is very low.
Best regards,
Dear @Jason.Jonkman,
Thank you for your answer. I started working on this matter with my teammate too.
We still want to come to an acceptable solution on the decay test without wind. The bypass that we think is quite okay is the following: In AeroDyn we set: WakeMod to 0, AFAeroMod to 1, TwrPotent to 0, TwrShadow to 0, and TwrAero to False. In InflowWind we set: WindType to 1. In the .fst files of the turbines, we set CompServo to 0. Would you say this approach is reasonable to minimize wind effects?
Since we cannot find any similar error in the forum, can you elaborate on what is the meaning of this error message (The time-filtered nacelle-yaw error has reached +/- pi/2)? We do not understand it and the purpose of it.
We tried to increase the wind speed up to 3 m/s, but we still encountered this error. Can it be that we are setting wrong the ambient wind parameters? We followed the recommendations provided in the user guide, but still, we could be missing something.
The turbines are in line and the separation between them is around 6 D_{rotor} – 5MW Baseline Turbine:
WT_X WT_Y WT_Z WT_FASTInFile
(m) (m) (m) (string)
-1411.500 0.00 0.00 “FOWTs/5MW_Spar_WT1.fst”
-2161.500 0.00 0.00 “FOWTs/5MW_Spar_WT2.fst”
With that said, we have changed the radial increment and number of radii for our 5MW turbine and set NumPlanes to 2, but the other parameters have been left to the default values. Would you say that we are having a problem here too?
We have tried also different initial amplitudes (always performing decays in the horizontal DoFs) but still, we find the error.
Best regards,
Tomás.
Dear @Tomas.Lopez.Olocco,
Regarding your setting of CompServo and your settings in AeroDyn and InflowWind, these make sense to me. How you are defining the airfoil data in AeroDyn to minimize the rotor thrust?
The nacelle-yaw error is used by the wake dynamics module of FAST.Farm in the wake-deflection correction. The nacelle-yaw error is low-passed filtered and must be in the range -pi/2 to pi/2 for the model to be valid. I was guessing that that nacelle-yaw error in your case was higher because of your use of a very low wind speed. But if increasing the wind speed doesn’t solve the issue, could it be that your rotor is not aligned with the wind?
Best regards,
Dear @Jason.Jonkman,
I can attach the files, but they are the same for both turbines and the 5MW baseline turbine, both AeroDyn_Blades and ElastoDyn_Blades. So, nothing to reduce the rotor thrust, but should be negligible, right? Also, are you saying that I should modify somehow for example the BlSpn from 61.5m to a lower value to reduce the wake produced by the upwind turbine?
Regarding if the rotor is aligned with the wind or not, intuitively that was one of the first things we thought that could be wrong. However, I don’t think that is the case. I have set the following:
PropagationDir to 0, YawNeut to 0, YawDOF to False, and NacYaw to 0.
Am I missing something here?
Best regards,
Dear @Tomas.Lopez.Olocco,
To reduce the thrust, I would suggest either reducing the chordlength in AeroDyn (BlChord) or reducing the lift and drag coefficients at the airfoil level.
Regarding the nacelle-yaw error error, can you confirm that the nacelle-yaw angle remains near zero degrees? I would also suggest looking at AeroDyn output RtVAvgxh (as well as y and z) to ensure that the rotor-disk-averaged wind speed used in the nacelle-yaw error calculation is making sense.
Best regards,
Dear @Jason.Jonkman,
Regarding the nacelle-yaw error I think that was not the cause. Nacelle-yaw angle remains zero degrees during the simulations since the DoF flag had been set to False. Attached are the rotor-disk-averaged wind speed in x, y and z. Do they seem reasonably for you? The inflow wind speed has been set to 2 m/s.
However, we followed your suggestions to reduce the thrust. What I did was reduce the chordlength one order of magnitude and the drag/lift coefficients at the airfoil level by a 50%. The solution has now substantially improved and I think that the wind effects are minimized enough to do the decay tests. I will show the final results of this work once we finish. Thank you!
Best regards,
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Dear @Tomas.Lopez.Olocco,
My guess is the rotor thrust before reducing BlChord and the aerodynamic coefficients was significant enough that it was leading to induction that resulted in high yaw errors. Now that you’ve dropped the thrust lower by reducing BlChord and the aerodynamic coefficients, the solution now works as expected.
Best regards,
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