BladeElement/Momentum Theory and Implementation
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Re: BladeElement/Momentum Theory and Implementation
Dear Guilherme,
I'm sorry, but I'm not sure I understand what your question is enough to answer it. Also, the images you uploaded are a bit too coarse to interpret e.g. I'm not sure which curve is FAST and which is your model. Please clarify your question.
Best regards,
I'm sorry, but I'm not sure I understand what your question is enough to answer it. Also, the images you uploaded are a bit too coarse to interpret e.g. I'm not sure which curve is FAST and which is your model. Please clarify your question.
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
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

 Posts: 0
 Joined: Thu Nov 10, 2016 6:23 pm
 Organization: Universidade Federal do Ceará
 Location: Brazil
Re: BladeElement/Momentum Theory and Implementation
Dear Jason,
I'm sorry for the confusing question. First of all by clarifying the image, the FAST response is in blue, while the response taken from the model is black. Second, in the image it can be seen that the outofplane displacement are very close in both simulations (Fast and Model), but the inplane displacement is very different.
For example, the tip blade inplane displacement on FAST oscillates around 0.5 whereas in the model the mean is zero. And in the tower, the model's displacement is very small compared to the FAST's displacement.
My question is wider and I'm sorry for that, but I checked the model and I did not find something that would make that difference. I would like to know, with your experience, what could cause this difference only in displacement in the plane?
Thank you for your patience and cooperation.
Best Regards
Guilherme Terceiro
I'm sorry for the confusing question. First of all by clarifying the image, the FAST response is in blue, while the response taken from the model is black. Second, in the image it can be seen that the outofplane displacement are very close in both simulations (Fast and Model), but the inplane displacement is very different.
For example, the tip blade inplane displacement on FAST oscillates around 0.5 whereas in the model the mean is zero. And in the tower, the model's displacement is very small compared to the FAST's displacement.
My question is wider and I'm sorry for that, but I checked the model and I did not find something that would make that difference. I would like to know, with your experience, what could cause this difference only in displacement in the plane?
Thank you for your patience and cooperation.
Best Regards
Guilherme Terceiro

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 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
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Re: BladeElement/Momentum Theory and Implementation
Dear Guilherme,
Does your model account for structural pretwist, as FAST does? As discussed in the following forum topic: viewtopic.php?f=4&t=424, because the blade flapwise stiffness is typically quite a bit less than the edgwise stiffness, the influence of the edgwise bending on the flapwise tip deflection (due to pretwist) will be much less than the influence of flapwise bending on the edgewise tip deflection. Perhaps this is why the results between FAST and your model match for outofplane deflection, but not for inplane deflection.
Best regards,
Does your model account for structural pretwist, as FAST does? As discussed in the following forum topic: viewtopic.php?f=4&t=424, because the blade flapwise stiffness is typically quite a bit less than the edgwise stiffness, the influence of the edgwise bending on the flapwise tip deflection (due to pretwist) will be much less than the influence of flapwise bending on the edgewise tip deflection. Perhaps this is why the results between FAST and your model match for outofplane deflection, but not for inplane deflection.
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
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

 Posts: 0
 Joined: Thu Nov 10, 2016 6:23 pm
 Organization: Universidade Federal do Ceará
 Location: Brazil
Re: BladeElement/Momentum Theory and Implementation
Dear Jason,
Thanks for your answer.
Yes, my model account for structural pretwist. But after reading the topic you recommended, I do not know if I'm doing it right.
In the model I used the second area product of inertia obtained through the principal second area moments to take into account the coupling. I used the mode shape of the first edgewise mode as inplane mode shape and the first flapwise mode as outofplane mode shape.
Can this big difference in results be due to these considerations? The equations that I used are attached.
Again, thank you very much for the help.
Best Regards
Guilherme Terceiro
Thanks for your answer.
Yes, my model account for structural pretwist. But after reading the topic you recommended, I do not know if I'm doing it right.
In the model I used the second area product of inertia obtained through the principal second area moments to take into account the coupling. I used the mode shape of the first edgewise mode as inplane mode shape and the first flapwise mode as outofplane mode shape.
Can this big difference in results be due to these considerations? The equations that I used are attached.
Again, thank you very much for the help.
Best Regards
Guilherme Terceiro
 Attachments

 Potencial Energy (only elastic stiffness).PNG (8.82 KiB) Viewed 6563 times

 Structural pretwist.PNG (7.33 KiB) Viewed 6563 times

 Posts: 5069
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 Location: Boulder, CO
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Re: BladeElement/Momentum Theory and Implementation
Dear Guilherme,
Your approach for accounting for the structural pretwist is definitely different than the approach used within FAST. Without going into the details, your approach seems to be missing the influence of the twist on the beam curvatures. Many years ago back in graduate school, I wrote a paper about finite elements of pretwisted beams; please find this paper attached and compare equation (21) to your formulation of the strain energy of the beam. Hopefully a review of this paper will help you resolve the problem.
Best regards,
Your approach for accounting for the structural pretwist is definitely different than the approach used within FAST. Without going into the details, your approach seems to be missing the influence of the twist on the beam curvatures. Many years ago back in graduate school, I wrote a paper about finite elements of pretwisted beams; please find this paper attached and compare equation (21) to your formulation of the strain energy of the beam. Hopefully a review of this paper will help you resolve the problem.
Best regards,
 Attachments

 PretwistedBeamReport.pdf
 (390.81 KiB) Downloaded 207 times
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
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

 Posts: 0
 Joined: Thu Nov 10, 2016 6:23 pm
 Organization: Universidade Federal do Ceará
 Location: Brazil
Re: BladeElement/Momentum Theory and Implementation
Dear Jason,
Thank you very much for the answer and for the article. Your article explained me a lot about pretwist beam and made me find the axis orientation error in my model.
I checked the equation (21) of your article and noticed that it is in the local coordinate. When I transformed the whole equation to the global coordinate, it equals the equation I used (in my last post). The term that adds the influence of the twist on the beam is the second area product of inertia, it was exactly in that term where the error was. So you hit it right when you said that my model was not represented the coupling correctly. Many thanks for your guidance.
Now the only difference between my response and the FAST response is in the tower displacement in the plane of rotation. I am attaching an image with the answers. In the following forum topic (viewtopic.php?f=4&t=1619&p=7533&hilit=generate+torque#p7533) you comment on the application of torque at the top of the tower. I think that this error in the response occurs by the lack of the addition of this torque on the top of the tower. I did not add the rotor torque on my model, I just considered the forces. Am I right in suspecting the lack of torque at the top of the tower? How does FAST add that torque to the tower, considering only the following DOFs (FlapDOF1; EdgeDOF1; TwFADOF1; TwSSDOF1)?
Again, thank you for the great collaboration.
Best Regards
Guilherme Terceiro
Thank you very much for the answer and for the article. Your article explained me a lot about pretwist beam and made me find the axis orientation error in my model.
I checked the equation (21) of your article and noticed that it is in the local coordinate. When I transformed the whole equation to the global coordinate, it equals the equation I used (in my last post). The term that adds the influence of the twist on the beam is the second area product of inertia, it was exactly in that term where the error was. So you hit it right when you said that my model was not represented the coupling correctly. Many thanks for your guidance.
Now the only difference between my response and the FAST response is in the tower displacement in the plane of rotation. I am attaching an image with the answers. In the following forum topic (viewtopic.php?f=4&t=1619&p=7533&hilit=generate+torque#p7533) you comment on the application of torque at the top of the tower. I think that this error in the response occurs by the lack of the addition of this torque on the top of the tower. I did not add the rotor torque on my model, I just considered the forces. Am I right in suspecting the lack of torque at the top of the tower? How does FAST add that torque to the tower, considering only the following DOFs (FlapDOF1; EdgeDOF1; TwFADOF1; TwSSDOF1)?
Again, thank you for the great collaboration.
Best Regards
Guilherme Terceiro
 Attachments

 tower in plane displacement.jpg (121.62 KiB) Viewed 6553 times

 Posts: 5069
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
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Re: BladeElement/Momentum Theory and Implementation
Dear Guilherme,
Yes, I would assume that the absence of torque in your model is the cause of this difference.
The tower DOFs are related to bending, so, a bending moment (induced by rotor torque) is natural to induce bending of the tower. Regardless, FAST considers all loads (both forces and moments) in its equations of motion.
Best regards,
Yes, I would assume that the absence of torque in your model is the cause of this difference.
The tower DOFs are related to bending, so, a bending moment (induced by rotor torque) is natural to induce bending of the tower. Regardless, FAST considers all loads (both forces and moments) in its equations of motion.
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
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

 Posts: 0
 Joined: Thu Nov 10, 2016 6:23 pm
 Organization: Universidade Federal do Ceará
 Location: Brazil
Re: BladeElement/Momentum Theory and Implementation
Dear Jason,
Forgive me for the stupid mistakes. I added the momentum induced by the rotor at the top of the tower. As we suspected, the model response approached the FAST response, but has a slightly higher frequency. What would cause this behavior (attached image)? I have already added aerodynamic and structural damping and the influence of gravity on the stiffness.
Thank you for your help.
Best Regards
Guilherme Terceiro
Forgive me for the stupid mistakes. I added the momentum induced by the rotor at the top of the tower. As we suspected, the model response approached the FAST response, but has a slightly higher frequency. What would cause this behavior (attached image)? I have already added aerodynamic and structural damping and the influence of gravity on the stiffness.
Thank you for your help.
Best Regards
Guilherme Terceiro
 Attachments

 tower.PNG (76.68 KiB) Viewed 6544 times

 Posts: 5069
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: BladeElement/Momentum Theory and Implementation
Dear Guilherme,
The higher frequency in your model would imply a higher stiffness or lower mass in your model.
Best regards,
The higher frequency in your model would imply a higher stiffness or lower mass in your 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
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
Re: BladeElement/Momentum Theory and Implementation
[quote="Guilherme.Terceiro"]
I checked the equation (21) of your article and noticed that it is in the local coordinate. When I transformed the whole equation to the global coordinate, it equals the equation I used (in my last post). The term that adds the influence of the twist on the beam is the second area product of inertia, it was exactly in that term where the error was. So you hit it right when you said that my model was not represented the coupling correctly. Many thanks for your guidance.
[/quote]
Dear Guilherme,
Could you explain a bit more what was the error in the product moment of inertia? Are the equations in the above post (Fri Nov 18, 2016 6:00 pm) correct to obtain the product inertia (in/out of plane)?
Best regards,
Koen
I checked the equation (21) of your article and noticed that it is in the local coordinate. When I transformed the whole equation to the global coordinate, it equals the equation I used (in my last post). The term that adds the influence of the twist on the beam is the second area product of inertia, it was exactly in that term where the error was. So you hit it right when you said that my model was not represented the coupling correctly. Many thanks for your guidance.
[/quote]
Dear Guilherme,
Could you explain a bit more what was the error in the product moment of inertia? Are the equations in the above post (Fri Nov 18, 2016 6:00 pm) correct to obtain the product inertia (in/out of plane)?
Best regards,
Koen

 Posts: 7
 Joined: Wed Sep 07, 2016 9:18 am
 Organization: 
 Location: Canada
Re: BladeElement/Momentum Theory and Implementation
Dear all,
Regarding to know Cp, Ct, power and thrust load in each element of the airfoil, thought I set "Print" (PrnElm) in AeroDyn input file, the fast does not make element.plt file, I was wondering to know what would be the probable solution or is there any other way to know lift, drag, extracted power for each blade element in FAST.
Thank you for your time.
regards;
Saeedeh
Regarding to know Cp, Ct, power and thrust load in each element of the airfoil, thought I set "Print" (PrnElm) in AeroDyn input file, the fast does not make element.plt file, I was wondering to know what would be the probable solution or is there any other way to know lift, drag, extracted power for each blade element in FAST.
Thank you for your time.
regards;
Saeedeh

 Posts: 5069
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: BladeElement/Momentum Theory and Implementation
Dear Saeedeh,
Which version of FAST and AeroDyn are you using?
Best regards,
Which version of FAST and AeroDyn are you using?
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
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

 Posts: 7
 Joined: Wed Sep 07, 2016 9:18 am
 Organization: 
 Location: Canada
Re: BladeElement/Momentum Theory and Implementation
Dear Jason
As I'm working on small wind turbine I need tailfin motion, I am using FAST version 7 (v7.02.00) and I think my AeroDyn version is 12.50.
Best Regards;
Saeedeh
As I'm working on small wind turbine I need tailfin motion, I am using FAST version 7 (v7.02.00) and I think my AeroDyn version is 12.50.
Best Regards;
Saeedeh

 Posts: 5069
 Joined: Thu Nov 03, 2005 4:38 pm
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Re: BladeElement/Momentum Theory and Implementation
Dear Saeedeh,
In FAST v7.02, if you used enabled "PRINT" for one or more aerodynamic elements in AeroDyn, then you should get an output file with a *.elm extension that contains the AeroDyn output. Do you get that file?
Best regards,
In FAST v7.02, if you used enabled "PRINT" for one or more aerodynamic elements in AeroDyn, then you should get an output file with a *.elm extension that contains the AeroDyn output. Do you get that file?
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
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

 Posts: 7
 Joined: Wed Sep 07, 2016 9:18 am
 Organization: 
 Location: Canada
Re: BladeElement/Momentum Theory and Implementation
Dear Jason
Thank you for your prompt help, according to 'FAST User’s Guide' I was looking for element.plt but now I find " *elm" as an output file.
Best regards;
Saeedeh
Thank you for your prompt help, according to 'FAST User’s Guide' I was looking for element.plt but now I find " *elm" as an output file.
Best regards;
Saeedeh
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