AeroDyn v15 convergence issues

Provide feedback, request enhancements, and get help with wind-turbine computer-aided engineering tools.

Moderators: Bonnie.Jonkman, Jason.Jonkman

Joseph.Saverin
Posts: 4
Joined: Sun Jul 12, 2015 10:04 am
Organization: TU Berlin
Location: Germany

AeroDyn v15 convergence issues

Postby Joseph.Saverin » Wed Mar 23, 2016 8:46 am

Hello NREL team,

Thanks again for the great work on FASTv812, we are already looking forward to the next release!

Myself and a student of mine are having issues with the convergence in AeroDyn v15.
We have run a number of simulations with both ElastoDyn and BeamDyn, but the simulations with turbulent windfields (TurbSim), which produce the following error:

"FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UnCoupledSolve:DeterminePhiBounds:There is
no valid value of phi for these operating conditions! psi = -1.8325, Vx = -4.0579, Vy = 37.139,
rlocal = 28.136, theta = 0.13636"
(The values psi, vx,vy etc are different every time the crash occurs)

There is of course a number of issues which could be occuring here, but we have done our best to eliminate sources of error.
The problem appears to occur whenever we use the Pitt-Peters wake model (BEMT=1, SkewMod /=1), particularly at small time steps or low wind velocities.
We have inspected the source code (particularly BEMT:TestRegion), but have thus far not found where exactly the problem could be coming from. Is there perhaps
a condition hard-coded into the BEMT module somewhere for low wind speeds which is causing the issues?

Any help would be appreciated.

Jason.Jonkman
Posts: 4750
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: AeroDyn v15 convergence issues

Postby Jason.Jonkman » Wed Mar 23, 2016 9:20 am

Dear Joseph,

There is nothing in AeroDyn hard-coded for low wind speeds.

A negative Vx is a bit odd. Does the structural solution look satisfactory up until the point of failure, or is there perhaps a numerical instability whereby the structural motions become very large?

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

Joseph.Saverin
Posts: 4
Joined: Sun Jul 12, 2015 10:04 am
Organization: TU Berlin
Location: Germany

Re: AeroDyn v15 convergence issues

Postby Joseph.Saverin » Thu Mar 24, 2016 8:18 am

Thanks for the quick answer,

Occasionally the structural appears to fail (small rotation assumption invalid error), but this is generally the exception.
The structural (along with aerodynamic) outputs appear to be quite stable (albeit even if the simulation only runs for a few seconds).

We have inspected the IP, OOP tip deflections, generator power, rot speed, pitch angles, and have trialled deactivating a number of DOF's.
Switching back to Aerodyn14 completely eliminates the error and the simulation runs without problem to the end. We also focussed first on the -ve Vx, but found no reason for it...

If we find any other error sources, we will update.

Thanks again,

Joe & Arved.

Yannick.Debruyne
Posts: 3
Joined: Mon Jun 29, 2015 5:27 am
Organization: WavEC - Offshore Renewables
Location: Portugal

Re: AeroDyn v15 convergence issues

Postby Yannick.Debruyne » Mon Jul 18, 2016 9:26 am

Dear all,

I was wondering whether there was any update related to the post of Joseph above ?

I am running into a similar problem when simulating a 660 kW wind turbine using FAST 8.15 with Aerodyn v15. In some very specific cases (Class A turbulent wind with 5 m/s average), I often get an error of the type:

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 11, blade 3):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating conditions! Vx = -7.74500E-03, Vy = 11.171, rlocal = 7.0683, theta = 0.14425


The values quoted in the error message can change, but Vx is always somewhere between say -0.15 and -0.0001 m/s. I tried deactivating structural vibrations of the blades and tower with no success. Could you please tell me which variables to track in order to find why Vx < 0 m/s ? I looked at variables such as B3N11Vundx, B3N11VDisx, B3N11VRel, but could not identify which one leads to the error message.

Many thanks,
Best regards,
Yannick

Jason.Jonkman
Posts: 4750
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: AeroDyn v15 convergence issues

Postby Jason.Jonkman » Mon Jul 18, 2016 10:42 am

Dear Yannick,

I hadn't heard from Joe and Arved since their last post on Mar 24, 2016, so, I had assumed there problem was related to numerical instability that they managed to solve.

Do you run into this problem when the blades are modeled with ElastoDyn, BeamDyn, or both?

The Vx in the error message is the relative wind speed (wind speed minus structural speed) at the AeroDyn blade analysis node, not including the influence of induction. While this speed is not directly an output of AeroDyn, it can be calculated with existing outputs i.e.

Vx = BαNβVDisx - BαNβSTVx

for node β of blade α. Note that the disturbed wind speed, BαNβVDisx, is used here in place of the undisturbed wind speed, BαNβVUndx (the former includes the influence of the tower disturbance on the wind, if enabled).

I hope that helps.

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

Yannick.Debruyne
Posts: 3
Joined: Mon Jun 29, 2015 5:27 am
Organization: WavEC - Offshore Renewables
Location: Portugal

Re: AeroDyn v15 convergence issues

Postby Yannick.Debruyne » Mon Jul 25, 2016 8:48 am

Hello Jason, and thank you for the suggestions.

I am modelling the blades with ElastoDyn only - I have no experience of BeamDyn.
I have been tracking the variables that you suggested for the blade node that issued an error. It seems that the x-disturbed wind speed at the blade node of interest is getting too close to zero, and so the difference between the disturbed speed and the structural speed is negative (see figure below).

Is there a way to prevent this from happening ? My wind input is a full-field file generated with TurbSim, class A turbulence with 5m/s average.

Best regards,
Yannick
[img]
B3N1speeds.png
[/img]
Attachments
B3N1speeds.png
B3N1speeds.png (15 KiB) Viewed 4251 times

Jason.Jonkman
Posts: 4750
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: AeroDyn v15 convergence issues

Postby Jason.Jonkman » Mon Jul 25, 2016 2:40 pm

Dear Yannick,

So, are you saying that you get the "no valid value of phi" error when Vx goes negative e.g. at time = 140 s for the case you've plotted? Does this happen often? My guess is this would happen less frequently at higher mean wind speeds or lower turbulence levels--is this what you've seen?

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

Yannick.Debruyne
Posts: 3
Joined: Mon Jun 29, 2015 5:27 am
Organization: WavEC - Offshore Renewables
Location: Portugal

Re: AeroDyn v15 convergence issues

Postby Yannick.Debruyne » Tue Jul 26, 2016 1:44 am

Jason,

Yes exactly, this is when I get the error. I run a large set of tests at different wind speeds in the context of a fatigue analysis. The error occurred in most of the cases with a 5m/s average and class A turbulence, and some cases with a 7m/s average and class A turbulence.
It rarely happened for higher wind speeds, or lower turbulence, as you guessed.

Best regards,
Yannick

Jason.Jonkman
Posts: 4750
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: AeroDyn v15 convergence issues

Postby Jason.Jonkman » Tue Jul 26, 2016 9:47 pm

Dear Yannick,

OK, it is good to know what is triggering the error. But I doubt this issue will be easy to solve. The AeroDyn source code already has logic to prevent the code from throwing an error for the case where Vx equals zero; perhaps the tolerance around zero needs to be expanded? I'll report this as a known issue, but I can't promise an immediate solution.

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

Yingyi.Liu
Posts: 99
Joined: Thu May 01, 2014 3:16 am
Organization: Kyushu University
Location: Japan

Re: AeroDyn v15 convergence issues

Postby Yingyi.Liu » Mon Nov 07, 2016 7:45 am

Dear Jason,

I have a problem when using AD15.ipt instead of AD.ipt as the AeroDyn inputs.
I ran the FAST Test04 example after changing the inputs "AeroFile" and "CompAero" in the Test04.fst file into "AWT27/Test04_AD15.ipt" and "2" {2=AeroDyn v15}, I got the following errors:
AD15.ipt_error.jpg
AD15.ipt_error.jpg (35.77 KiB) Viewed 4122 times

How can I fix the errors? Thanks a lot in advance.

Yingyi Liu

Jason.Jonkman
Posts: 4750
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: AeroDyn v15 convergence issues

Postby Jason.Jonkman » Mon Nov 07, 2016 9:02 am

Dear Yingyi Liu,

It appears that we forgot to update the AeroDyn v15 input file for Test04, when converting from AeroDyn v15.02 to v15.03 in FAST v8.16. To resolve this error, simply add the following line after line for input TwrAero:

False FrozenWake - Assume frozen wake during linearization? (flag) [used only when WakeMod=1 and when linearizing]

See the draft AeroDyn v15 User's Guide and Theory Manual for more information: https://wind.nrel.gov/nwtc/docs/AeroDyn_Manual.pdf.

We'll fix this in the next release of FAST.

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

Yingyi.Liu
Posts: 99
Joined: Thu May 01, 2014 3:16 am
Organization: Kyushu University
Location: Japan

Re: AeroDyn v15 convergence issues

Postby Yingyi.Liu » Tue Nov 08, 2016 2:12 am

Dear Jason,

Many thanks. It works. Yet here comes another error, see below screenshot:

AD15.ipt_error2.jpg
AD15.ipt_error2.jpg (71.69 KiB) Viewed 4112 times

I tried to enlarge the grid size to even GridHeight=80.0 and GridWidth=100.0 in the file of 42m_12mps.inp, and then run by using Turbsim v1.06.00. However, the resulting new 42m_12mps.wnd file still cannot resolve the above error. Do you know the possible reason?

Best regards,
Yingyi Liu

Jason.Jonkman
Posts: 4750
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: AeroDyn v15 convergence issues

Postby Jason.Jonkman » Tue Nov 08, 2016 6:21 am

Dear Yingyi Liu,

Looking at the AWT27/Test04_AD15.ipt file, I see that the downwind tower shadow model is enabled (TwrShadow = True), with tower analysis nodes starting at ground level (TwrElev = 0). To run this model, you must use wind with data starting at the ground level.

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

Yingyi.Liu
Posts: 99
Joined: Thu May 01, 2014 3:16 am
Organization: Kyushu University
Location: Japan

Re: AeroDyn v15 convergence issues

Postby Yingyi.Liu » Wed Nov 09, 2016 6:58 am

Dear Jason,

Thank you for you advice, they are very helpful.
Now the problem is solved. FAST can run freely with this Test04 example.

Best Regards,
Yingyi Liu

Rene.Mebus
Posts: 35
Joined: Tue Jan 10, 2017 5:37 am
Organization: Hochschule Aalen
Location: Germany - Baden Wuerttemberg

Re: AeroDyn v15 convergence issues

Postby Rene.Mebus » Wed Aug 02, 2017 3:08 am

Dear Jason,

I refer to your post:
Jason.Jonkman wrote:Dear Yannick,

OK, it is good to know what is triggering the error. But I doubt this issue will be easy to solve. The AeroDyn source code already has logic to prevent the code from throwing an error for the case where Vx equals zero; perhaps the tolerance around zero needs to be expanded? I'll report this as a known issue, but I can't promise an immediate solution.

Best regards,


Is it possible to fix this issue in the source code in AeroDyn? May you instruct me to do this?

Thank you very much.

Best regards,
René


Return to “Computer-Aided Engineering Software Tools”

Who is online

Users browsing this forum: No registered users and 1 guest