Setting up to determine wind loads (thrust forces) acting on RNA

Thank you for all your help.

How do I set up OpenFAST to determine the wind loads (thrust forces) acting on the RNA?

I would like to reproduce the thrust force shown in Fig3-1(b) of this document:
I also want to find the thrust force when the wind speed is 50 m/s.

I have made a trial calculation and the thrust force I have obtained is about 1/5 of the value in this literature.

To simplify the problem, I am making the following assumptions
…I use the published model of the IEA 15MW wind turbine.
The .fst file is set to CompElast=0, CompInflow=1, CompAero=2, and 0 for the others.
ElastoDyn has FlapDOF1 to PtfmYDOF all set to False. I am checking the response when wind acts on the rigid body. Note that even if FlapDOF1 etc. are set to True, the blades will only oscillate, and the average value obtained is almost the same as when the settings are set to False.
BlPitch(1)~BlPitch(3) are changed according to the wind speed. In the calculation for a wind speed of 50 m/s, it is set to 90 degrees.
In the InflowFile, WindType=1 is used. The angle of incidence of the wind is 0 degrees.
AeroDyn15 uses public data as it is.
I thought that YawBrFxn was consistent with the thrust force.

Looking at the calculation results, I think the wind is hitting the blades, but not the nacelle.

Dear @Yosuke.Matsumoto,

Presumably you mean that you using CompElast = 1 and you are running separation simulations at each mean wind speed? Are you setting the rotor speed correctly (along with the blade pitch angles) at each wind speed?

Note that if you don’t want to see the influence of structural flexibility, you can run the standalone AeroDyn driver, uncoupled from OpenFAST.

Typically a power curve is generated by running a series of OpenFAST simulations at a number of given, steady, and uniform wind speeds (WindType = 1). Separate simulations are run at each wind speed and each simulation is run long enough to ensure that all transient behavior dies out (typically all relavent DOFs are kept enabled). Then recorded the steady-state output values from each simulation and plotted them as a curve (thus, only the steady-state response is shown).

Best regards,

Thanks for the reply.

As you mentioned, the following settings.
CompElast = 1.
Rotor speed was not entered in my previous post.

I have corrected the calculation but still no success.

I attach the conditions and results.

I believe the thrust force corresponding to the rated wind speed (10.59 m/s) is about 2800 kN. Looking at the results, with GenDOF =True it is about 3800 kN; with GenDOF =False it is about 2400 kN; with GenDOF =False it is about 2400 kN; with GenDOF =False it is about 2400 kN.

Question 1: Why does GenDOF =True result in a larger thrust force at the rated wind speed (10.59 m/s)? And why does GenDOF have little effect at very high wind speeds (57 m/s)?

Question 2: How do you set the wind load acting on the nacelle, which is about 400 kN smaller than 2800 kN? I imagine the difference is due to the presence or absence of wind loads acting on the nacelle.

Question 3: Are there any other settings that need to be changed?

Dear @Yosuke.Matsumoto,

Regarding (1), presumably, when you toggle on/off GenDOF between cases 1/2, the rotor speed is changing, resulting in different rotor thrust. When GenDOF = True in case 1, you must also enable the controller in ServoDyn to ensure that the rotor speed is regulated properly; have you done that?

For cases 3/4, the blade pitch is 90 degrees, which should result in minimal rotor thrust at zero rotor speed.

Regarding (2), I’m not sure I understand your question.

Regarding (3), when you pitch the blades to 90 degrees in high winds, you must also change the aerodynamic calculation (e.g., WakeMod = 0, AFWeroMod = 1), because the wake/induction and unsteady airfoil aerodynamics models are not applicable during parked/idling conditions.

Best regards,

Thanks for your reply.

  1. I understand that if GenDOF=True, the ServoDyn controller must also be enabled.

  2. Attached are the details.

  3. I understand that WakeMod = 0 and AFAeroMod = 1 are required to reproduce the idling state. I will be performing various DLC calculations. I would like to know what parameters must be changed for each DLC calculation. Please let me know if you have a document that summarizes the parameters that need to be changed.

Dear @Yosuke.Matsumoto,

NREL has not developed a universal document that lists the simulation recommendations for each load case, but the “Modeling Guidance” sections (or equivalent) of the user documentation for each OpenFAST module provides some guidance (4. User Documentation — OpenFAST v3.5.0 documentation), and this topic has been discussed several times on this forum.

The AeroDyn module of OpenFAST does not currently support direct modeling of wind loads on the hub and nacelle.

Best regards,

Thank you for your response.

I understand that the settings for each state have been discussed in previous forums and that hubs and nacelles cannot be considered in AeroDyn.

I would like to know of alternatives to account for wind loads acting on the hubs and nacelles. Because if we do not consider them, we will underestimate the loads acting on the tower and floating structure.

As an alternative, I have considered the following two methods. Are these alternatives feasible in OpenFAST? Also, if you have any other good ideas, please let me know.

  1. Consider a height, diameter, and drag coefficient at the top of the tower modeled in AeroDyn that corresponds to the wind-receiving area of the hub and nacelle.
  2. Consider a concentrated load equivalent to the wind load acting on the hub and nacelle.

Dear @Yosuke.Matsumoto,

I have not considered (1) before, but that sounds like an interesting work around.

We have applied (2) before, using the ability in the Structural Control (StC) submodel of ServoDyn to apply arbitrary point loads in the nacelle. That is, compute the aerodynamic load on the nacelle separately and apply this load to the OpenFAST model via the StC_DOF_MODE = 4 (prescribed load time history) option of StC.

Please note that for situations with an operating rotor, that the direct wind loads on the hub and nacelle are likely much less than the loads on the rotor. And for parked/idling conditions, the direct wind loads on the hub and nacelle are likely much less than the loads on the tower. I agree that we need to add wind loading on the nacelle and hub to AeroDyn in the future, but in most cases, these loads are much less than wind loads on the rotor and tower.

Best regards,