Simulating the parked turbine

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Jason.Jonkman
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Re: Simulating the parked turbine

Postby Jason.Jonkman » Mon May 04, 2020 9:32 am

Dear Chenxu.Zhao,

Just a few comments:
  • Do you mean that you are looking at the tower-base fore-aft moment, i.e., moment driven by fore-aft forces?
  • Is your simulation with steady wind and in steady state? Otherwise, the solution is likely oscillatory and effect by inertia terms.
  • How is your gravity-induced moment calculated and does it include the effect of tower and blade deflection?
  • Are you considering direct aerodynamic loading on the tower, which will also contribute to the moment?
Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Chenxu.Zhao
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Organization: Chongqing University
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Re: Simulating the parked turbine

Postby Chenxu.Zhao » Mon May 04, 2020 7:34 pm

Dear Jason

Thanks for your reply.

Here are my answers.

1.Yes, the tower-base fore-aft moment, is driven by fore-aft forces
2.My simulation is with turbulence wind. I set analysis time to 630s, I think it is in steady state. And I feel in mean bending moment,the inertia terms has no contribution.
3.When I calculate gravity-induced moment,I set the steady wind velocity at hub to 0.01m/s. The tower and blade DOFs are set to ‘ture’. Under such conditions, I take the tower bending moment as gravity-induced moment. It doesn’t include the effect of tower and blade deflection in turbulence wind.
4.When I calculate the side-side moment, the wind load on the tower (F=0.5*ρ*U^2*CL*A) should be 0.

Best regards,
Chenxu Zhao

Jason.Jonkman
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Re: Simulating the parked turbine

Postby Jason.Jonkman » Tue May 05, 2020 7:03 am

Dear Chenxu.Zhao,

Here are my responses.
  • In your post dated May 03, you said you were using the side-to-side moment, but I gather based on your posted dated May 04 that are actually using the fore-aft moment.
  • I agree that the inertia terms will not contribute to the mean; I did not realize you were talking about means.
  • The rotor-thrust force will also induce tower and blade deflection that will change the mean gravity-induced moment; this could be the reason for the discrepancy you noted.
Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

George.Chan
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Re: Simulating the parked turbine

Postby George.Chan » Fri Jul 10, 2020 10:40 pm

Dear Jason
I am George, a research student who major in wind turbine technology.
I am simulating a wind turbine under extreme wind speed (35m/s) and parked with brake condition under different pitch angles.
According to your advice in previous post, the major settings are listed as below:

ElastoDyn:
True DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
False GenDOF - Generator DOF (flag)
True YawDOF - Yaw DOF (flag)
True TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
True TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
True TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
True TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
!! The nearest 4 items care changed to be False for a rigid tower consideration

X BlPitch(1) - Blade 1 initial pitch (degrees)
X BlPitch(2) - Blade 2 initial pitch (degrees)
X BlPitch(3) - Blade 3 initial pitch (degrees) [unused for 2 blades]
!! X is set to range from 0~90 for different pitch angle settings

AeroDyn:
0 WakeMod - Type of wake/induction model (switch) {0=none, 1=BEMT}
1 AFAeroMod - Type of blade airfoil aerodynamics model (switch)

ServoDyn:
0 PCMode
0 VSContrl
1 HSSBrMode - HSS brake model
0 THSSBrDp - Time to initiate deployment of the HSS brake (s)
0 HSSBrDT - Time for HSS-brake to reach full deployment once initiated (sec)
0 YCMode - Yaw control mode

I specified different pitch angles (0~90 degree) in ElastoDyn and want to investigate the RotThrust variation trend with the pitched angle of this parked wind turbine, and I also investigated the RotThrust of a rotor upon a rigid tower by changing "TwFADOF" and "TwSSDOF" to be "False" in ElastoDyn.

The major result are shown in the following picture:
RotThrust of a Parked 5MW Onshore Turbine under Different Pitch Angles.png
RotThrust of a Parked 5MW Onshore Turbine under Different Pitch Angles.png (159.67 KiB) Viewed 481 times


From the result, my current understanding and questions are listed as below:

* The top 3 picture seems to be reasonable because the aerodynamic mode has been switch to steady and the RotThrust should bear similar trend as the wind speed (proportional to V^2).

* Because the aerodynamic damping of a parked rotor is relatively small, therefore, the RotThrust won't vary too much when the tower was set to be rigid (the 1st natural frequency of blade is ranging from 0.66-1.08Hz [flapwise to edgewise], which is away from the 1st natural frequency of tower, is it means resonating effect is negligible?).

* What is the principle behind the totally different variation trend for the RotThrust when the pitch angle is larger than certain degree (around 50 hereby)?

* Why the disparity between the red and blue curves tends to be salient when the pitch angle is larger than certain degree?

* Did I do anything wrong in the settings that lead to the seemingly strange results?

I am looking forward to receiving your reply!


Yours Sincerely
George

Jason.Jonkman
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Re: Simulating the parked turbine

Postby Jason.Jonkman » Mon Jul 13, 2020 7:00 am

Dear George,

Your input file settings make sense to me.

The response is likely very different for the different pitch angles because of the change in angle of attack at the blade airfoils. At the high pitch angles, the angle of attack is low such that the lift is high and drag is small; the other cases are dominated by high angle of attack, low lift, and high drag. I would suggest plotting the power spectra (PSD) of the time series to see if various blade modes are being excited differently between lift- and drag-dominated excitation. It appears disabling the tower has some effect for all cases, but plays a larger role as the mean thrust drops with increasing blade-pitch angle.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

George.Chan
Posts: 5
Joined: Fri Jul 10, 2020 3:51 am
Organization: UNSW Australia
Location: Australia

Re: Simulating the parked turbine

Postby George.Chan » Tue Jul 14, 2020 8:22 am

Dear Jason

Thank you for your reply and conducive advice.

In my humble opinion, the inflow angle would remain 90 degree when the rotor is parked, therefore the lift force is perpendicular to the wind velocity so as to the RotThrust (if the shaft tilt angle is 0), and the lift force will not affect the magnitude of RotThrust.

Following your precious advice, I drew the PSD of RotThrust of a parked wind turbine under 35m/s wind speed and different blade pitch angles (blue curves in the following Figures).

For a comparison purpose, I investigated the RotThrust of a rotor with rigid blade (flap and edge-wise mode disabled), whose PSD (red curves) is plotted together with the previous set of PSD.

As shown in the following graphs:
blade_excitation3.png
blade_excitation3.png (1.03 MiB) Viewed 463 times

* Both the red and blue curves are peaked around the 1st and 2nd tower frequency, which is 0.32Hz and 2.9Hz (marked by 2 vertical red lines through all the graphs)

* In addition to the aforementioned two peaks, the PSD of a flexible rotor (blue curves) are significantly excited around 1.10Hz when the pitch angle is larger than 50 degree.

By checking the natural frequency of a 5MW turbine in the manual, the blade may be excited in a mode of '1st Blade Asymmetric Edgewise Pitch (1.07Hz)' or '1st Blade Asymmetric Edgewise Yaw (1.08Hz)', which is understandable because it corresponds to a high blade pitch angle and the blade may vibration edgewise.


However, I still have some doubts and want to hear your advice~

* Why the blade mode would be significantly excited only when blade pitch angle is high?
(Since the incoming turbulence for all cases are unchanged, and the 'Kaimal Spectrum' seems to have no peak around 1.10Hz, so I would expect all the PSD bears similar trends).

* Have I ignored anything that is significant to account for this phenomenon? And through what theory can I derived similar results as shown in the bottom 2 pictures?


Best Regards
George

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
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Re: Simulating the parked turbine

Postby Jason.Jonkman » Tue Jul 14, 2020 9:14 am

Dear George,

At small pitch angles, the angle of attack is high for a parked rotor, but for large pitch angles, the angle of attack is small. My guess is that the edgewise-bending modes of the blade(s) are being excited by lift forces when the angles of attack are past the stall peak at high pitch angles. We've seen the blade-edgewise mode get excited in post-stall conditions in other parked situations, e.g. see section 3.3 of the following Wind Energy journal paper: http://onlinelibrary.wiley.com/doi/10.1002/we.442/pdf.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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