Dear Sir,
Thank you for the clarification. TwrAero was disabled previously.
Does FAST Calculate the normal and tangential thrust forces due to wind?
Akheel kk
Dear Akheel,
For the rotor, yes, both normal and tangential forces are computed by AeroDyn. For the tower, only a drag load is calculated.
Best regards,
Dear Sir,
- Is it possible to obtain the aerodynamic load acting in the fore-aft and side-side direction using FAST if I disable TwrAero?
- Does RtAeroFxh , RtAeroMxh represents normal and tangential forces?
Dear Akheel,
TwrAero enables/disables the aerodynamic drag loads on the tower. There can still be aerodynamic-applied fore-aft and side-to-side loads from the rotor when TwrAero is disabled.
RtAeroFxh is the total aerodynamic-applied thrust on the rotor and RthAeroMxh is the total aerodynamic-applied torque on the rotor. I would normally use the terms “normal and tangential” forces to refer to sectional values (force per unit length at a cross section); these are available outputs from AeroDyn via BαNβFx and BαNβFy (normal and tangential to the rotor plane) or BαNβFn and BαNβFt (normal and tangential to the chord). The full list of AeroDyn outputs is documented here: openfast.readthedocs.io/en/mast … endix.html.
Best regards,
Dear Jason,
i started using openFast v2.05 and i received the following warning at 3 of 600seconds
When i changed angle pitch to 17,47 and rotor speed to 12,1 (uref= 20 m/s) i received the same warning but at 275 of 600 seconds
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:BEMT_CalcOutput(node 18,
blade 1):UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be
repeated though the condition may persist.
can i try something else?
when i used Aerodyn v14.04 the simulation run without problems
Best regards
Georgia
Dear Georgia,
Can you provide more information about your simulation set up and other results you obtaining. What wind and wave conditions are you simulating? What response are you getting for rotor speed, blade pitch, and blade-tip deflection?
Best regards,
dear Jason,
im using windtype=3 and i have made some changes in the geometry (platform ,tower)
when i use Aerodyn v14.04 the simulation runs without problems and i got the following
Dear Georgia,
I’m not sure what you changed in the geometry, but these results are likely expected.
Due to the low gains in the blade-pitch controller of the baseline controller for the NREL 5-MW turbine atop the OC3-Hywind spar, the rotor speed is not tightly regulated around 12.1 rpm–in your plots, you see peaks reaching about 14.5 rpm. At this speed, the blade tips are approaching 0.3 Mach. This is likely at the upper limit of where BEM and the unsteady airfoil aerodynamics theories are valid.
Best regards,
Dear Jason,
I am also having issues with the ‘mach number exceeds 0.3’. The input parameter conditions I am using are as follows:
wave height = 11m
wind speed = 23,/s
peak spectral period = 15s
wind wave misalignment = 60 degrees
The image attached is from FAST aborting. I believe the problem is linked with high angle of attack and therefore large deflections of the blades, however, I am unsure how to rectify the problem.
Appreciate your time as always,
Thanks
Matthew
Dear Matthew,
I see a large blade deflection warning quite soon into the simulation. This suggest to me that your initial conditions are likely not set up properly, resulting in numerical problems with the start-up transients, specifically related to the controller. In general, it is recommend to–at a minimum–initialize the rotor speed and blade-pitch angle to their expected mean value for the given mean hub-height wind speed.
At 23 m/s, I would expect that you’d initialize your rotor speed at 12.1 rpm and blade pitch angle at about 21deg for the NREL 5-MW baseline wind turbine. Is that what you’ve used?
Best regards,
Dear Jason,
Thanks for getting back to me.
The turbine I am using is the IEA-15MW offshore wind turbine. I am trying to simulate the turbine for a range of input parameters, with associated bins:
Currently, the rotor speed is defined in ElastoDyn with initial conditions RotSpeed = 7.56 (corresponds to the max rotor speed).
Also I believe the blade pitch are all currently 0 degrees (assuming the image below is the correct variable where this is changed):
My two questions are:
- Do all 3 blades need their own value for pitch angle?
- Is it possible to set a value for both RotSpeed and blade pitch angle that will suffice ALL simulations for my intended range of parameter inputs?
Thank you as always for your help.
Regards,
Matthew
Dear Jason,
In addition to my last post, I have attached the ‘Bladed Interface’ section as it appears in my ServoDyn input file:
Am I correct in saying this section is the controller methodology (ROSCO) for the torque and blade pitch angles, as described on page 17 of IEA-15MW turbine:
corewind.eu/wp-content/uploads/ … urbine.pdf
Are there perhaps incorrect input values here that are causing my original OpenFAST error?
Thanks for your help.
Regards,
Matthew
Dear Matthew,
At BlPitch = 0 and 23 m/s winds, the thrust is very high and causing large blade deflection. It is generally recommend to at a minimum initial the rotor speed and blade-pitch angle to their expected (mean) values for the given (mean) hub-height wind speed simulated. For the IEA 15-MW reference wind turbine, a plot of these is given Figure 3-1 of its specification report: nrel.gov/docs/fy20osti/75698.pdf. You can set the initial blade pitch to be the same for each blade.
Best regards,
Dear Jason,
I have adjusted the initial blade pitch BlPitch for the 3 blades in my ElastoDyn file. However, I am still receiving the same error as above, regarding ‘high angle of attack’.
In general, I am trying to understand how the bladed interface and DISCON adjust the blade pitch angle and rotor torque during operation, and then if at all changing the BlPitch in ElastoDyn has any effect?
Perhaps is my error linked to my ServoDyn file where the bladed interface points to the ‘DISCON-Monopile.IN’ file? I have not changed the parameters in this file as default values are given for the IEA-15MW turbine. The snippet below is taken from my Bladed interface section in ElastoDyn where it points to the DISCON files.
I also understand that within the ‘DISCON-Monopile.IN’ file, it has a wind speed estimator that in particular reads values from the rotor performance tables “Cp_Ct_Cq.IEA15MW.txt”. This seems to be relating okay to each other but I still have the initial error of ‘Mach number exceeds 0.3’ as before when running OpenFAST. Can you suggest any error I am missing?
Thanks again for your time and help.
Best regards,
Matthew
Dear Matthew,
The values of BlPitch specified in the ElastoDyn input file are the initial blade-pitch angles used at the start of the simulation. With active blade-pitch control enabled, these really are just initial conditions and the actual blade-pitch angles will change throughout the simulation. But it is often important to specify the initial conditions to their expected mean value for a given mean wind speed to ensure that the overall time-domain solution is reasonable. I can’t tell from your forum post whether the response you are obtaining is expected or not.
You may get high angle of attack warnings at the inner-most nodes of a blade, which are typically not aerodynamically optimally designed. But I would not expect that you’d receive these warnings at most nodes along the blade. Can you confirm what nodes you receiving this error for?
My understanding is that the IEA Wind 15-MW reference turbine can experience high Mach numbers (exceeding 0.3) due to the high tip-speed ratio it is designed for. This was discussed on our forum here: Mach Number exceeds 0.3 for large wind turbines (15MW) - #4 by Garrett.Barter.
Best regards,
Dear Jason,
I am analyzing the impact various wind and wave loads have on the tower base (at mudline) of the 15MW turbine. In particular I am looking at the fore-aft moment (ReactMXss) and the side-to-side moment (ReactMYss), assuming these are the correct parameters which represent the moments?
The following images are results I have from MLife.
I was under the impression that the fore-aft moment would produce larger DELs than the side-to-side DEL as shown in the following document (iopscience.iop.org/article/10.1 … 052052/pdf).
However my results show the opposite, that the side-to-side moment have much larger DELs than the fore-aft moment.
Please could you offer a reason for this? And if I have made an error how best I can correct it?
Thanks as always for your help.
Regards,
Matthew
Dear Matthew,
You have the labels reversed. ReactMXss is the side-to-side moment (i.e., the moment about X caused by side-to-side forces along Y) and ReactMYss is the fore-aft moment (i.e., the moment about Y caused by fore-aft forces along X).
That said, I’m not sure I understand the results you are showing, which don’t seem to show clear trends. I would expect smoother curves if the results are converged.
Best regards,
Dear Jason,
Thank you for your quick reply.
That makes sense regarding the moments, thank you for clearing that up.
The reason for the non smooth curve which produces spikes is the limited dataset (only 240 input cases).
Also the input cases have varying wind wave misalignment and peak spectral period which may also cause the non smooth curve.
Best regards,
Matthew