Power coefficient/Aerodynamic torque

Dear Dr. Jonkman,

I need the power coefficient or aerodynamic torque output related to the 5MW offshore WT. Could you tell me what are the outputs for them in the outlist in Matlab? In some of messages I found out that RotCp is the output for power coefficient but I couldn’t see it in the list of output!

Thanks
Best regards,
Sina

Dear Sina,

I’m assuming you are referring to the Simulink interface of FAST v8 or OpenFAST. The outputs sent out of the FAST S-Function (OutData) are the same outputs written to the time-series output file, as defined by the inputs specified in each FAST module.

RotCp was an old output from FAST v7 not available in FAST v8 or OpenFAST. I would now recommend using AeroDyn v15 output RtAeroCp for aerodynamic power coefficient of the rotor and RtAeroMxh for the aerodynamic torque.

Best regards,

Dear Dr. Jonkman,
Thanks for your prompt response. Actually I have two questions.

1- I cannot see any “RtAeroMxh” ,in the OutList in matlab, which lists 99 output parameters for test 24 in FAST 8. You mentioned this is the output of AeroDyn v.15. Where are its output located to enter them to the Matlab?
2-Is it possible to give a fixed value of the rotor speed and wind speed but variable pitch angle and then get the value of aerodynamic torque? What I mean is that the aerodynamic torque has only one time-varying parameter and based on it, FAST sends me its value during the simulation.

Thanks,
Best regards,
Sina

Dear Sina,

Regarding (1), The AeroDyn v15 outputs are set in the AeroDyn primary input file. In Test24, this is the file named, “CertTest/5MW_Baseline/NRELOffshrBsline5MW_OC3Hywind_AeroDyn15.dat”.

Regarding (2), yes. To fix the rotor speed, disable the generator and drivetrain degrees of freedom in ElastoDyn (GenDOF = DrTrDOF = False). To fix the wind speed, use WindType=1 in InflowWind. You can vary the blade-pitch linearly by setting the override pitch maneuver inputs in the ServoDyn input file.

Best regards,

Dear Dr. Jonkman,
Thanks for your reply. I made those signals output but when I run the “Run_OpenLoop.m” for test 24, it gives me this error:

Error using Run_OpenLoop (line 30)
Error reported by S-function ‘FAST_SFunc’ in ‘OpenLoop/FAST Nonlinear Wind
Turbine/S-Function’:
FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 5, blade
1):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these
operating conditions! Vx = -1.1284, Vy = 22.627, rlocal = 11.757, theta = 0.23218

I also have another question about initialization. I need to initialize the blade pitch angles (all of them) not to start at zero. How can I initialize them?

Thanks,
Best regards,
Sina

Dear Sina,

There were issues in AeroDyn v15.03 within FAST v8.16 that led to the error “no valid value of phi” in some cases. However, these issues have been fixed in AeroDyn v15.04 and newer in OpenFAST. I suggest you upgrade to OpenFAST: github.com/OpenFAST/openfast/pull/204.

The initial blade-pitch angles for each blade are set in the “Initial Conditions” section of ElastoDyn input file.

Best regards,

Dear Dr. Jonkman,
Thanks for your prompt response and sending me those links. Regarding the openfast-master file, when I run the “Run_OpenLoop.m” I get this error:
Error using Run_OpenLoop (line 30)
Error reported by S-function ‘FAST_SFunc’ in ‘OpenLoop/FAST Nonlinear Wind
Turbine/S-Function’:
FAST_InitializeAll:FAST_Init:FAST_ReadPrimaryFile:OpenFInpFile:The input file,
“…\CertTest\Test01.fst”, was not found.
I had a look at the openfast-master files, it seems there is no CertTest in them.
Regarding the second link, could you tell me what is it?
Thanks
Best regards,
Sina

Dear Sina,

In the transition from FAST v8 to OpenFAST, what used to be the CertTest of FAST v8 has been changed to the regression tests (r-test) of OpenFAST: github.com/OpenFAST/r-test.

I’m not sure I understand your second question; are you asking about the Simulink Pull request? OpenFAST development happens within github. See the github workflow documentation for more information: openfast.readthedocs.io/en/mast … kflow.html.

Best regards,

Dear Dr. Jonkman,
Thanks for your prompt response. So far, you have sent me several files including “openfast-master”, and “r-test”. Now, I am a little confused. The tests located in a different place with different names, while the “Run_OpenLoop.m” asks me for “CertTest”! What I need is to plot the power coefficient and aerodynamic torque in Matlab related to test 24. Could you help me ease this procedure?
Thanks
Best regards,
Sina

Dear Sina,

My guess is you obtained Run_OpenLoop.m from FAST v8, not OpenFAST. The Master branch of OpenFAST does not yet support the Simulink interface. Instead, the Simulink interface was added through Pull Request #204, which has not yet been merged into the Dev(elopment) (or Master) branches of OpenFAST. You’ll have to merge the changes from #204 into your branch of OpenFAST in order to use the Simulink interface.

Best regards,

Dear Dr. Jonkman,
Thanks for your reply and online supports. Let’s discuss a little analytically. I plotted the power coefficient (CP) in the closed-loop system when the wind speed is below the rated value (10m/s) with the TI around 25%, and the controller tries to regulate the rated generator speed. Weirdly, I got this CP:


while the CP in wind turbines cannot go beyond 0.45. On the other hand, its plot above the rated value are below 0.45 like this (wind speed is 18m/s):

Thanks,
Best regards,
Sina

Dear Sina,

I’m not sure which power coefficient output from FAST / OpenFAST you are plotting (in general, I would recommend outputting RtAeroCp from AeroDyn v15). Regardless, as has been discussed on this forum, by nature of its calculation within the aero-elastic solution, I would only use the power coefficient output in a time-averaged sense to avoid apparent violations in the Betz limit that may occur instantaneously as a result of the calculation approach. The instantaneous values are effected by rotor acceleration / deceleration, and so, can instantaneously exceed the Betz limit.

Best regards,

Dear Dr. Jonkman,
Thanks again for your prompt response. I used FAST 8 and Aerodyn v15.03!
Best regards,
Sina

Dear Jason,

I have performed simulation of WP1.5MW using FAST for the conditions specified below. I observe that Cp is greater than 1 as shown in figures. But it should be below Betz limit. Right? Could you please help me how to interpret this.

Uniform wind speed: 6 m/s.
Pitch controller: No pitch controller. (4 deg constant pitch angle).
Tower and blades are rigid.
Only GenDof is kept on for the gear box.
“WakeMod =0” is chosen.
Simple VScontrol .
Power.JPG Cp.JPG

From output:
Torque at steady state: 385.4 kN-m (plot not attached)
Rotor speed at steady state:14.8 RPM (plot not attached)

Also, Cp is calculated as the ratio of output power of rotor to the (0.5* air density * Disk Area * wind vel^3). Correct?

Regards,
Kumara

Dear Kumara,

Are you plotting AeroDyn output RtAeroCp?

With WakeMod = 0 in AeroDyn v15, the induced velocity calculation is disabled (meaning the induction is zero and the angle of attack will be determined geometrically), so, you are not using BEM and the Betz limit can be violated. The WakeMod = 0 option should only be used when simulating parked/idling conditions (when not producing power).

I suspect you’ll get a result that makes more sense for an operational wind turbine when enabling BEM by setting WakeMod = 1.

Best regards,

Yes Jason. I am plotting AeroDyn output RtAeroCp.
Yes, Induced velocity calculation is disabled in my simulation. I did not think about the fact that Betz limit is valid only when BEMT is used.
Thanks for pointing that. Now I understand that exceeding Betz limit is sensible, but how is it possible to generate more power than that is available in the wind? Kindly advice me incase i’m missing something fundamental here. Thanks.

Regards,
Kumara

Dear Kumara,

Without induction, energy is not preserved in this simulation, so, energy-related quantities are not accurate. I would set WakeMod = 1.

Best regards,

Dear Jason,

Many thanks for your quick response. I’ll do that.

Regards,
Kumara

Dear all,

Issue: Difference in the aerodynamic power of rotor between Standalone AeroDyn calculation and OpenFAST calculation.

I have calculated Rotor aerodynamic power for WindPact 1.5MW turbine at (pitch=2.2 deg, rotor speed=24RPM, wind speed=12 m/s Uniform) using

  1. Standalone version of AeroDyn15.04 and 2) OpenFAST-V2.40 but i see the difference in aerodynamic power of the rotor.

  2. RtAeroPwr = 1954.472 kW

  3. ‘RtAeroPwr’= 2003.206 kW

In the OpenFAST simulation case some of the settings are GenDOF = 0, CompAero=2, shafttilt=0,precone=0.

I have made sure that the settings are same for both. What could be the reason for this discrepancy?
Also, does the Elastodyn play role in the calculation of the rotor aerodynamic power (“RtAeroPwr’”) in OpenFast?

Regards,
Kumara

Dear Kumara,

If all structural DOFs are disabled in ElastoDyn and if the geometry and wind are set up identically between the OpenFAST and standalone AeroDyn driver simulations, then I would expect the standalone AeroDyn driver to give the same aerodynamic power as OpenFAST. If the power predictions are different, I would expect that structural DOFs are enabled in ElastoDyn or some geometry or wind differs. Enabling structural DOFs in OpenFAST–e.g., blade deflection–can impact the aerodynamic power calculated by AeroDyn.

Best regards,