## NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Discuss the theory and modeling of rotor aerodynamics.

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Jon.Martinez
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jason,

Thank you very much for your anserwer.

Best regards,

Jon Martínez Rico

Chenxu.Zhao
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jason,

I have some doubts about thrust of NREL 5MW given in this article [Definition-of-a-5-MW-reference-wind-turbine-for-offshore-system-development](Figure 9-1. Steady-state responses as a function of wind speed).

I used ORIGIN to obtain the data of RotThrust vs wind speed, and then it was easy to get the CT(coefficient of thrust, the black line) with the formula(air density=1.225, Ad=3.1415*63^2)

At the same time, I used FAST to calculate Ct in different average wind speed ranged from 3-25 m/s(the red line).

The strange things is that those 2 line has huge difference. Why has this happened? Is it normal Ct is as high as 2.5?

Best regards,
Chenxu.Zhao
Attachments
CT vs wind speed.png (6.34 KiB) Viewed 2207 times
RotThrust vs wind speed.png (8.36 KiB) Viewed 2207 times
formula-Ct.png (4.33 KiB) Viewed 2207 times

Jason.Jonkman
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Chenxu.Zhao,

The FAST (ElastoDyn) output RotThrust is the reaction force transmitted from the rotor to the shaft, so, not only includes the applied aerodynamic forces, but also the forces resulting from rotor weight (and in transient analysis, rotor inertia); see related forum posts on this topic. So, normally, RotThrust is shifted upward relative to the aerodynamic applied Thrust. If you are using AeroDyn v15, you can output the aerodynamic applied rotor thrust via AeroDyn output RtAeroFxh.

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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Location: China

### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jason,

But there is still one little problem. The red line was exactly caculated by using your method of getting 'RtAeroFxh' from FAST. And still when wind velocity equals 3m/s, the CT is somewhat higher than 1 as 1.1 . If this is the right way to do it, then what's the reason for this abnormal phenomenum? From the textbooks I have read, they told me that CT was not allowed to be more than 1.

Best regards,
Chenxu.Zhao

Jason.Jonkman
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Chenxu,

Actually, the aerodynamic thrust is able to exceed 1, but in this situation, flow will recirculate around the rotor and momentum theory no longer applies. At high thrust, AeroDyn uses Glauert’s empirical correction with Buhl’s modification in place of momentum theory.

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
Posts: 11
Joined: Mon May 20, 2019 4:36 am
Organization: Chongqing University
Location: China

### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jason,

Thanks again! It turns out I need to do more researches about the theories used by FAST. Your guidence is really useful to me.

Best regards,
Chenxu.Zhao

Jian.Zhang
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Location: China

### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Hi, everyone

I have some problems with the output parameters, "B1N1DynP", "B1N1Fx" and "B1N1Fy". Based on the "OutListParameters.excel" in the archive of FAST V8, I know the "B1N1Fx" and "B1N1Fy" refer to the normal and tangential forces to plane per unit length for Node 1 of Blade 1, and "B1N1DynP" means the dynamic pressure for Node 1 of Blade 1. If I want to employ these output parameters in my finite element model as the wind loading, How could I utilize these parameters? I have some ideas based on the attached figure, but I do not know whether it is right.

If I employ the parameters "B1N1Fx" and "B1N1Fy", I prefer to adopt two methods:
1) assuming only the forces per unit length at the nodes are accurate, the centering normal force at Node 3 could be calculated by B1N3Fx*(L2+L3)/2
2) assuming the forces along the blade could be linearly interpolated, the concentrated force at Node 3 should be computed by ((B1N2Fx+B1N3Fx)/2+B1N3Fx)/2*L2/2+((B1N3Fx+B1N4Fx)/2+B1N3Fx)/2*L3/2

If I adopt the parameter "B1N1DynP", How should I do?
Any hints would be much appreciated.

Best regards,
Jian
Attachments
schematic diagram.JPG (19.38 KiB) Viewed 1534 times
Last edited by Jian.Zhang on Fri Jul 24, 2020 7:34 am, edited 1 time in total.

Jason.Jonkman
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jian,

Are you referring to AeroDyn outputs? These are the aerodynamic applied loads per unit length along the blade.

I'm not sure I really understand your question, but if your FE model accepts applied forces per unit length, but the nodes are not co-located with the AeroDyn nodes, then, yes, you could interpolate the AeroDyn outputs before using them in your FE model.

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

Jian.Zhang
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Joined: Sun Apr 26, 2020 9:26 pm
Organization: Hong Kong Polytechnic University
Location: China

### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jason,

Thank you for your timely response.
Yes, I refer to the AeroDyn outputs. I just edited my questions for clarity. I wonder if I want to impose the concentrated force on the blade, whether the method I mentioned is right? I am still confused about how to employ the parameter "B1N1DynP", could you give me more clues. Thank you.

Best regards,
Jian

Jason.Jonkman
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jian,

I think methods (1) and (2) will give very comparable results, especially as the element lengths decrease. We use a slightly different method in the internal Line2-to-Point mesh-to-mesh mapping in the FAST modularization framework (that also ensures that the overall moments balance between the two meshes), but again, I would guess the results would be very comparable as the element lengths decrease. You can read about the mesh-to-mesh mapping inherent within FAST in the attached paper.

The dynamic pressure is defined such that:
B1N1Fx = B1N1Cx*B1N1DynP*BlChord

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

Younes.Oudich
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Jason,

I tried to implement a BEM code in MATLAB, using the Distributed Blade Aerodynamic Properties of the 5MW NREL wind turbine (Table 3-1 in the 5MW NREL reference pdf: https://www.nrel.gov/docs/fy09osti/38060.pdf) and the airfoils data. I find the maximum power coefficient of Cp_max=0.486 at TSR=7.82 and Pitch=0.0°, which are slightly off what is in the pdf reference (Cp_max=0.482 and TSR=7.55). Here is a plot of Cp with TSR and Pitch angle.

CpTSRPitch.png (57.5 KiB) Viewed 157 times

I have two questions:
_ Is this tiny difference comes from the number of blade nodes taken?
_ Are those results that I found make sense to you?

Jason.Jonkman
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### Re: NREL 5-MW reference turbine - CP, CQ, CT Coefficients

Dear Younes,

Yes, these results are quite similar to the ones generated by NREL. Differences could be the result of discretization (which could easily checked by studying the convergence with increasing number of nodes) or by differences in the underlying BEM assumptions (more difficult to assess).

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