A question concerning Cp surface generation with OpenFAST

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Jason.Jonkman
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Re: A question concerning Cp surface generation with OpenFAST

Postby Jason.Jonkman » Wed Apr 28, 2021 7:57 am

Dear Bartosz,

It is always good to set proper initial conditions to reduce the simulation start-up transients.

My understanding is that you are asking why your simulation is resulting in a Cp (of 0.467) that is higher than the maximum Cp (of 0.452) that you calculated in prior simulations...is that correct? Can you clarify what is different between these two sets of simulations (e.g., in terms of DOFs or features enabled/disabled)?

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

Bartosz.Stachowicz
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Re: A question concerning Cp surface generation with OpenFAST

Postby Bartosz.Stachowicz » Wed Apr 28, 2021 10:16 am

Dear dr Jonkman,

Exactly, this is the reason for my initial question.
While checking the differences I noticed I had the tip, hub and tower losses disabled in the simulation, and after enabling them the Cp is much closer at 0.4533 so I would say this solves my problem.
Thank you for your suggestion!

as for my RotTSR, now it stabilizes at 9.88 instead of 9.35. I calculated VS_Rgn2K using the formula
kform.PNG
kform.PNG (2.91 KiB) Viewed 715 times

for Cpmax i put 0.452 and for CpmaxLambda 9.35 corresponding to the maximum point

I obtained the Cp distribution using the rigid system with all elastodyn DOF disabled.
After that I enabled the following DOFs:
True FlapDOF1 - First flapwise blade mode DOF (flag)
True FlapDOF2 - Second flapwise blade mode DOF (flag)
True EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
True GenDOF - Generator DOF (flag)
False 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)
True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
True PtfmHvDOF - Platform vertical heave translation DOF (flag)
True PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)
True PtfmYDOF - Platform yaw rotation DOF (flag)
and ran some simulations to check if the difference would be important. I had good agreement for most cases I managed to run so I'm using my Cp data from the first set of simulations (rigid rotor, tower, static aerodynamics).
So now it's a floating turbine with flexible blades and tower. I'm using Simulink interface now.
Could rotor plane tilt due to platform pitch be the reason for the turbine to stabilize at TSR 9.88?

Regards,

Bartosz Stachowicz
Last edited by Bartosz.Stachowicz on Wed Apr 28, 2021 11:23 am, edited 1 time in total.

Jason.Jonkman
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Re: A question concerning Cp surface generation with OpenFAST

Postby Jason.Jonkman » Wed Apr 28, 2021 10:24 am

Dear Bartosz,

The floating platform pitch could certainly play a role in the rotor performance, as could blade deflection. You've also enabled the GenDOF, so, the controller will now also play a role if the rotor speed or blade-pitch angle differ between the two sets of simulations.

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

Bartosz.Stachowicz
Posts: 50
Joined: Wed Jul 29, 2020 4:37 am
Organization: Politechnika Gdanska
Location: Gdansk

Re: A question concerning Cp surface generation with OpenFAST

Postby Bartosz.Stachowicz » Sun May 02, 2021 6:23 pm

Dear Jason,

I have run some more simulations and I am happy with the Cp I get most of the time if the initial conditions are OK. At least it seems to me to be close to the max value I obtained from the rigid simulations. I mean the Cp in Region 2 of variable speed control. I am attaching some plots of a simulation like this. Can you please comment on the torque ripple right after the controller goes into region 2.5? Is this normal? I think I set the speeds and slopes OK but I was thinking that maybe this is because the turbine was accelerating in this case and the ripple was due to the inertia. What is your opinion?

Best Regards

BS

Ps: I am attaching more plots in my next post, cheers
Attachments
Generator.png
Generator.png (22.19 KiB) Viewed 661 times
Rotor Parameters.png
Rotor Parameters.png (68.79 KiB) Viewed 661 times
Torque Regions.png
Torque Regions.png (67.73 KiB) Viewed 661 times

Bartosz.Stachowicz
Posts: 50
Joined: Wed Jul 29, 2020 4:37 am
Organization: Politechnika Gdanska
Location: Gdansk

Re: A question concerning Cp surface generation with OpenFAST

Postby Bartosz.Stachowicz » Sun May 02, 2021 6:25 pm

Here are some blow-up plots of torques 2 and 2.5 and the pitch angle. Wind speed is in the image with platform motions. Can I linearize about any of the operating points from this simulation?

Thank you!
Attachments
Platform Displacements.png
Platform Displacements.png (59.55 KiB) Viewed 656 times
Pitch_degrees.png
Pitch_degrees.png (7.02 KiB) Viewed 660 times
Trq_2_25_Transition.png
Trq_2_25_Transition.png (14.01 KiB) Viewed 660 times

Bartosz.Stachowicz
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Organization: Politechnika Gdanska
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Re: A question concerning Cp surface generation with OpenFAST

Postby Bartosz.Stachowicz » Mon May 03, 2021 5:43 am

Dear Jason,

I'm afraid I have some more questions. The simulation result from the previous post were obtained for still water conditions. In this post, I am attaching similar graphs for two cases with regular waves. One has Tp=10s and Hs=1. The other one has Tp=33 and Hs=1. The second one is really a crash test case because the 33 period is close to my spar platform natural frequency in roll and pitch, but the one with Tp=10s is close to real conditions prevailing in my localization (Hs can be much greater, but the Tps range from 6 to 11s for the measurement data I am planning to compare against later).
My questions:
-In the picture named Torque Regions, I have an indicator of the current torque region of the controller. When the turbine is in region 3 for a still water case, but close to rated power production, for the case with wave excitation, there is rapid switching of torque regions. this is for times t: 600<t<900.
What could be done to prevent this changing of torque region?
-When the turbine is in the linear region with respect to the rotational speed of the shaft( region 2.5. Times t 400<t<600s.) The torque sensibility to the wave excitation seems to be greater than for region 2 just before. What could be the reason for that?

If you find some time to have a look at these plots, your opinion would help me a lot.

Thank you,

BS
Attachments
Tp_33s_Hs_1m.zip
(254.92 KiB) Downloaded 22 times
Tp_10s_Hs_1m.zip
(175.84 KiB) Downloaded 19 times

Jason.Jonkman
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Re: A question concerning Cp surface generation with OpenFAST

Postby Jason.Jonkman » Mon May 03, 2021 7:11 am

Dear Bartosz,

I'm not sure I fully understand what you are doing. As far as I could tell, you are assessing the steady-state response at 3, 11.4, 13, and 15.5 m/s by running one long simulation with slow wind-speed ramps in between. I'm not sure what you are plotting in the Torque Regions.png images; these appear to be hand calculations of the torque in various control regions rather than an actual OpenFAST output; is that correct? Which control settings have you used within the OpenFAST simulations?

I'm not sure what you "ripple" you are referring to.

In your zipped figures, again, I'm not sure what you are plotting in the Torque Regions.png files; the torques here don't seem to match what I assume is the OpenFAST output plotted in Generator.png.

The torque can jump quickly if the torque-speed curve is very steep, which I would not recommend. In the transition region between Regions 2.5 and 3, the torque controller is often limited to maintaining Region 3 torque if the blade-pitch angle is above some threshold (even if the generator speed falls into Region 2.5) so as to avoid toggling often between Regions 2.5 and 3.

You can linearize the OpenFAST model at any point in time, but the linearized solution is most useful when the model is in (periodic) steady state.

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

Bartosz.Stachowicz
Posts: 50
Joined: Wed Jul 29, 2020 4:37 am
Organization: Politechnika Gdanska
Location: Gdansk

Re: A question concerning Cp surface generation with OpenFAST

Postby Bartosz.Stachowicz » Mon May 03, 2021 7:41 am

Dear Jason,

The Torque Regions is the output from the scope with the same name, from the Simulink torque control. I'm attaching a graph print. The second scope is used to produce the 'blow-up' image.
Thank you for taking the time to answer.
I like the idea to use the blade pitch angle to stay in Region 3. I guess this was the initial purpose of having it in the model of torque control. I just didn't quite grasp that before. (input 2 in the graph is blade pitch. I guess I'll have to calibrate it better in the condition for region 3 or work more on pitch controller...
By 'ripple' I mean the jump in torque when control switches to region 2.5. In the graphs from previous posts that happens when time is 400s. I'll Try to change slopes as you have suggested.

Best Regards,

BS
Attachments
Torque Controller.pdf
(47.39 KiB) Downloaded 21 times


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