Hi Everyone,
I have been running a complete set of ultimate design load cases on the IEA 10MW (GitHub - IEAWindTask37/IEA-10.0-198-RWT)) on OpenFAST v3.5.3.
This is being done for the purposes of extracting rotor and shaft loads for some drivetrain related works.
I have encountered some odd behaviour for certain load cases which mostly come from scenarios where a stop is initiated.
In short, where I initiate a stop the core behaviour appears fine but after a certain amount of time (usually 25-50s after the stop), there are what appear to be extreme vibrations observed in the blade root and shaft with exceptionally high loads (see below example - as an added note I subtracted the initial 100s that I placed at the start of the simulation from those plots).
These issues seem somewhat related to a previous post here: IEA 10MW DLC2.3 using openFASTv220
In line with that post, I have experimented with different Aerodyn setups such as changing AFAeroMod (options 1 and 2), WakeMod (None, BEMT and DBEMT) and UAMods (1, 2, 3 and 6).
However, this did not seem to reduce these vibrations and the order of magnitude remains the same (100,000+ kN moments) throughout.
Given that I am primarily interested in shaft loads here, I also experimented with the shaft brakes but again with little success.
I have run the complete load cases, including power production and parked events, whilst the loads there are little higher than what I was anticipating, they are still far below what is occurring in the above case.
In terms of initiating the stop in OpenFAST, I simply adjusted the Servodyn file to fully pitch after the select time period (TPitManS and BlPitchF) and for the generator to also switch off at the same time (TimGenOf).
So I am looking for some advice on what might be the cause of these extreme fluctuations in the loads? For example are there any known issues regarding say the 10MW’s blade design or could it be something in my setup?
Happy to provide more info if needed.
Thanks,
Dylan
Dear @Dylan.Duncan,
You mention experimenting with different shaft brake settings; which settings have you used in the plots you shared?
It sounds like you have tried several aerodynamic options; which options have you enabled in the plots you shared? I would first start with the simplest aerodynamics option: AFAeroMod = 1, WakeMod = 0; are you still seeing odd oscillations in that case?
Best regards,
Hi Jason,
Many thanks for the support.
In terms of brake settings for the above, it was enabled with HSSBrTqF = 50000Nm and a HSSBrDT = 30s as mentioned I’ve experimented with variations for both of these parameters.
As an example, the below comparison compares 50000Nm against 100Nm with little difference between the two.
Worth mentioning that this is a direct drive turbine so I wasn’t sure if the brake settings in OpenFAST work the same way as they would for a geared as there isn’t a High-Speed Shaft here so I’m conscious that I might be applying these incorrectly.
In terms of AFAeroMod = 1 and WakeMod = 0, here are the plots, oscillations still observed which has puzzled me.
As an extra point, wind direction and yaw direction are aligned (both 0 degrees) so there are no issues there.
Thanks,
Dylan
Dear @Dylan.Duncan,
Thanks for clarifying. I would focus first on the case with AFAeroMod = 1, WakeMod = 0; can you also share GenSpeed for this case? Do you have the drivetrain torsion DOF enabled in ElastoDyn (DrTrDOF = TRUE)? Do you have BeamDyn enabled, and if so, doesn’t switching from BeamDyn to ElastoDyn for modeling the blades eliminate the load oscillations?
For a direct drive wind turbine, there is no distinction between the low-speed shaft and high-speed shaft, so, OpenFAST inputs labled as “high-speed shaft” should be considered relative the direct drive shaft.
Have you run the OpenFAST r-test with high-speed shaft brake deployment and do you get similar results to the ones obtained by NREL: r-test/glue-codes/openfast/AWT_WSt_StartUp_HighSpShutDown at main · OpenFAST/r-test · GitHub (if not, can you)?
Best regards,
Hi Jason,
Thank you for the suggestions.
In terms of GenSpeed, I’ve included it below. Generally speaking it consistently matches the RotSpeed.
In terms of DT DOFs I had it disabled, I did try enabling but it had no bearing on the results. I have BeamDyn disabled here (Elastodyn = 1 in the FST).
As per your suggestion, I tried mimicing the setup for that AWT model.
For avoidance of doubt relating to wind input, I changed that to a steady profile. I set Aerodyn, AeroMod and WakeMod = 1 and the relevant twr influences to false.
In terms of Elastodyn, my setup was largely the same barring my initial conditions (Pitch and RotSpeed being adjusted inline with the input wind), I also set DT DOFs to true here to match that setup.
Unfortunately, I’m still seeing oscillations with that particular setup.
I then tried again by changing Servodyn inline with the AWT file by setting VSContrl and PCMode to 0. I also disabled any pitch activity (TPC > sim time). These changes have yielded far different results with reductions in overall shaft and blade root loads. So maybe that indicates a problem caused by the pitching?
I did compare this to the AWT out file but there seems to be quite a big discrepancy between the two (although I assume the turbines and initial conditions are different there). I’ll maybe run some comparisons between the IEA 10MW and one or two other RWTs under the exact same setup to compare performance.
Thanks,
Dylan
Dear @Dylan.Duncan,
Thanks for clarifying.
Regarding the AWT comparison, I was asking if you could run the same AWT simulation as the r-test and compare that you are getting the same result as provided in the OpenFAST r-test (i.e., the NREL baseline); I just want to make sure that your installation of OpenFAST is functioning the same as the NREL baseline.
When you don’t pitch, it doesn’t appear that the rotor speed ever stops; the oscillations only appear after the rotor stops.
Best regards,
Hi Jason,
Ah, apologies for the late reply and misunderstanding your message
Yes so I ran the r-test simulations and they align with the NREL output so I believe my OpenFAST install is functional.
I did try a similar simulation on the 15MW and it stops with no oscillations. This is using the same setup as my prior runs of the IEA 10MW. So I believe it is to do with the model itself, I did produce updated controller files via ROSCO for these simulations. Could an issue with the controller have an impact on these oscillations? Otherwise, I’m unsure of what the cause could be, maybe something involving the blade design?
Thanks,
Dylan
Dear @Dylan.Duncan,
OK, thanks for confirming.
I wouldn’t expect ROSCO to be doing anything once you’ve shut off the generator and feathered the blades.
I would next try to isolate the issue in the IEA Wind 10-MW RWT to a specific structural degree of freedom. I assume this model would not oscillate if you disabled all DOFs except the GenDOF (which needs to be enabled to trigger a shutdown). Other than GenDOF, is there a specific DOF, when enabled, that results in the oscillations?
Best regards,
Hi Jason,
That was a good suggestion as disabling separate DOFs yielded some interesting results.
Namely that disabling edgewise DOFs still resulted in a huge fluctuation after the stop is completed.
However, disabling either Flapwise dofs, tower dofs or platform dofs removed those huge fluctuations reducing the overall load magnitudes significantly.
I’m going to do some extra investigation with regards to damping for the blades and maybe the tower as well.
Thanks,
Dylan
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