Dear @Emanuel.Rergis,
I’m glad you got your simulation to converge. Please note that reducing DT by a factor of 2 would be a similar computational expense to increasing NumCrctn from 0 to 1 and reducing DT by a factor of 3 would be a similar computational expense to increasing NumCrctn from 0 to 2. That said, it is sometimes beneficial for the coupling algorithm to increase NumCrctn rather than reduce DT.
Regarding your additional questions:
- There is no hard limit on
WaveTMax, but eventually you could run into memory allocation issues if WaveTMax is very large.
- I’m not sure I understand why want to run such a long simulation of 24 hours. We typically recommend running multiple seeds of shorter simulations (such as 24 simulations of 1 hour each) rather than one really long simulation.
- I agree that if
WaveTMax is less than TMax, than the waves will repeat after WaveTMax. But the simulation start-up transients are a result of initialization of the model and do not start again if waves start to repeat.
Best regards,
Thank you very much, Dr. @Jason.Jonkman. I followed your advice and am now running a massive number of simulations. During this time, two questions arose in my mind:
- Would there be any shortcut I can take to run a Montecarlo simulation?
- Is there any way to see a sample of a Prescribed force/moment time series file? I would need this to create my “own TMD”, and I would like to explore if that’s feasible with this option. What can you advise?
Thanks a lot for your help.
Best wishes!!!
Emanuel
I’m not sure I understand your first question.
You can find an example OpenFAST model with a prescribed force/moment time series in the following r-test: r-test/glue-codes/openfast/StC_test_OC4Semi at main · OpenFAST/r-test · GitHub.
Best regards,
Thank you one more time, Dr. @Jason.Jonkman, for your hints. The link you provided was very helpful. What I meant by my first question was if there’s any way to run multiple simulations in parallel since I’m planning to do some Monte Carlo simulations.
And one more question: I need to create various wind profiles with TurbSim. My limitation is that these profiles do not inject extremely low frequencies into the OWT model. This is because I discovered that the created wind profile file generates this problem in some cases. How can I prevent this from happening?
Thanks a lot!
Kind regards!
Emanuel M. Rergis
Dear @Emanuel.Rergis,
You can certainly run multiple OpenFAST simulations in parallel (in batch mode), including multiple simulations on different cores, or if you are running on an HPC, on different nodes. Typically you are running OpenFAST through some script that automates the process. Are you using a specific script?
I’m not sure I understand your question about low frequencies; can you clarify?
Best regards,
Dear Dr. @Jason.Jonkman:
Thank you for your response. I didn’t know anything about these scripts you mentioned in your previous message. Do you know where I can find an example of this batch mode? I feel that the method you suggested can help me save some time. On the other hand, you can see what I meant about the low frequencies if you look at these snapshots (find them attached). As you can see, I get very low frequencies acting on the system. These low frequencies are the ones responsible for my TMD not working properly (this is an assumption I might be wrong). It is important to point out that I turned off HydroDyn. I’m just computing InflowWind and AeroDyn. Is it now clear what I meant?
Thank you very much for your help.
Kind Regards!
Dear @Emanuel.Rergis,
Regarding scripts that automate the generation of input files and executing TurbSim and OpenFAST simulations for IEC DLCs, see my post dated Aug 31, 2022 in the following forum topic: Wind & Wave data - #67 by Jason.Jonkman .
Can you clarify what the low-frequency excitation is caused by? Are these system natural frequencies or excitation frequencies, e.g., harmonics of the rotor speed? Are you running an OpenFAST model provided by NREL (e.g., from the r-test); if so, which one? Or is this a model that you created yourself? What is the rotor speed the wind turbine is spinning at for this case?
Best regards,
Dear Dr @Jason.Jonkman:
Thanks a lot for your reply. After some attempts, I managed to automate the generation of some input files. Regarding your second question, I haven’t succeeded in identifying what causes this low-frequency excitation. You recommended checking the rotor speed; what is the default value? I’m running a 5MW OC3 Monopile Offshore Wind Turbine model (I assume NREL provided this one). Is there anything else I should check to eliminate this low-frequency excitation source?
Thanks a lot, and kind regards!
Emanuel M. Rergis
Dear @Emanuel.Rergis,
The NREL 5-MW baseline wind turbine atop the monopile does not have any full-system natural frequencies below 0.2 Hz.
The rotor speed of the NREL 5-MW baseline wind turbine is between 6.9 and 12.1 rpm (0.115-.202 Hz, depending on the wind speed).
I would guess the lower frequencies you are seeing in the base moment are related to wave-excitation. Presumably you are using an irregular sea state with a peak-spectral period near 14 s (0.07 Hz).
Best regards,
Thanks for your reply, Dr. @Jason.Jonkman. I firmly believe this was my fault. I wasn’t specific enough about what I did. This time, I’ll try to explain my problem more clearly.
In order to isolate the problem, I ran four experiments. For the first two I deactivated the wind loads with the following parameters:
CompElast - 1
CompInflow - 0
CompAero - 0
CompServo - 1
CompHydro - 1
CompSub - 1
CompMooring - 0
CompIce -0
MHK - 0
I’m attaching some snapshots of the HydroDyn file that I used in these two experiments and in the following two. These last two experiments have the following parameters:
CompElast - 1
CompInflow - 1
CompAero - 2
CompServo - 1
CompHydro - 1
CompSub - 1
CompMooring - 0
CompIce -0
MHK - 0
I’m also attaching some snapshots of the InflowWind and TurbSim files used in these late simulations.
In other words, two experiments were run under two different conditions (just wave loads and wave loads + wind loads), one without TMD and the other with a TMD whose nacelle mass ratio was equal to 10%. As you can clearly see in the corresponding plot (please find it attached as well), the wind profile injects very low frequencies that make the TMD useless.
My question is: How can I eliminate these very low frequencies? I understand that this scenario doesn’t truly represent natural wind profile behaviour. I’m giving you extra information:
11x11 grid (170m wide, 5m to 175m above the ground). Characteristic wind speed: 12m/s.
Thank you very much one more time.
Kind regards!
Emanuel M. Rergis
Dear @Emanuel.Rergis,
The low-frequency excitation of the tower below 1P (0.2 Hz) is the energy from the turbulent wind spectrum, which is maximum at low frequency. It looks like you are using the Kaimal wind spectrum in TurbSim, so, I would encourage you to plot this Kaimal spectrum to see it yourself, as well as the corresponding spectrum of the rotor-thrust force.
Best regards,
Dear Dr. @Jason.Jonkman:
Thanks for your time and patience. I’m a newbie to using TurbSim. Therefore, I need to ask where I can get the information to plot the PSD of the Kaimal wind spectrum. Does it come from the BTS file? The same question applies to the rotor-thrust force spectrum. One last thing: what kind of adjustments can I make to my TurbSim model to reduce this low-frequency excitation?
Thanks again.
Kind regards!
Emanuel
Dear @Emanuel.Rergis,
You can process the PSD from any time-series output of OpenFAST, e.g., InflowWind output Wind1VelX (or the like) should show the Kaimal wind speed for your case, or AeroDyn output RtAeroFxh for aerodynamic rotor thrust. An analytical expression for the Kaimal wind spectrum as implemented in TurbSim is also documented in the TurbSim User’s Guide: https://openfast.readthedocs.io/en/main/_downloads/cb14d3e2d3533d76e405d730fea19846/TurbSim_v2.00.pdf.
TurbSim supports various built-in wind spectra models, but all of these have low-frequency energy in the wind. You can provide your own user-specified wind spectrum in TurbSim if you don’t want to use one of the built-in options, but wind naturally has turbulent energy in the low-frequency range, so, I wouldn’t think you should reduce it artificially.
Best regards,
Dear Dr. @Jason.Jonkman:
Thank you very much for your comments. They are always very useful. Now, I am trying to run some simulations to analyze the free vibration response of the tower top of a 5MW NREL wind turbine. In these experiments, I set these flags as follows;
CompElast 1;
CompInflow 0;
CompAero 0;
CompServo 0;
CompHydro 0;
CompSub 0;
CompMooring 0;
CompIce 0;
MHK 0;
It should be noted that there were four experiments with initial conditions of x(0)=1, x(0)=0.75, x(0)=0.50,x(0)=0.50. Additionally, I have attached the ElastoDyn file that I used in my simulations. My question is, why do you think the displacement does not oscillate around 0 as expected? What do you think could be the parameter or force that seems to inject a negative offset level? (Please see the snapshot I also sent you. If I want to add TMDs, I must activate the CompServo =1 flag, right? I would appreciate it if you could help me with your comments.
Thanks once again for your help.
Kind regards!
Emanuel M. Rergis
Dear @Emanuel.Rergis,
The nonzero static displacement of the tower-top in the absence of aerodynamic loads comes from the overhanging weight of the rotor-nacelle assembly (RNA), whose center of mass is upwind of the tower.
Yes, you must enabled ServoDyn to model TMDs through its structural control (StC) submodel.
Best regards
Thanks a lot for your comments, Dr. @Jason.Jonkman. I added some TMDs, and I got some irregularities. (I am attaching the snapshot where I highlighted with black dotted circles what I considered an inconsistency). Are these circled areas also
related to the overhanging weight of the rotor-nacelle assembly (RNA)? What is your opinion about it?
Thanks and kind regards!
Emanuel
Dear @Emanuel.Rergis,
I’m not certain, but I would guess the “irregularities” are caused by nonlinearities in the model, particularly relevant at larger displacements, e.g., geometry stiffness or the hitting of positive/negative stops in the TMD.
Best regards,
Thank you Dr @Jason.Jonkman . I think the problem is that when I disconnect all the degrees of freedom, maintaining only the Fore-Aft first bending moment, the system’s natural frequency changes, and therefore, my TMD seems to be detuned. I’ll double-check this.
On the other hand, I would like to ask you one more question. What is the monopile foundation model that OpenFAST assumes? I’m using the OC3 Monopile 5MW NREL Offshore wind turbine. In what openFAST module can I check this information?
I’m asking this because, after reading the Offshore Code Comparison Collaboration (OC3) report, it is still unclear to me which of those models are implemented in my simulations and how I can verify or change values if possible. Please find attached the snapshot.
Thanks again, and kind regards!
Emanuel M. Rergis
Dear @Emanuel.Rergis,
If you are using the OpenFAST model of the OC3-monopile from the following r-test, you are modeling the foundation rigidly: r-test/glue-codes/openfast/5MW_OC3Mnpl_DLL_WTurb_WavesIrr at main · OpenFAST/r-test · GitHub. By changing the SubDyn input file, you could change this model to treat the foundation via the apparent fixity or coupled springs model. Using the distributed springs model would require a source code change.
Best regards,
