Tower top tilt moment and Tower bottom sideside moment looking strange
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 Posts: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Tower top tilt moment and Tower bottom sideside moment looking strange
Good morning,
I have been running the 5 MW onshore model with ElastoDyn, and I have obtained 10 minute results for different load channels for 3 yaw conditions 8 deg, 0 deg and 8 deg. I have used 6 turbulence seeds on TurbSim, NTM as the turbulence type and IECKAI as the turbulence model. The simulations I ran were from 4 m/s to 26 m/s in steps of 2.
While post processing the results, the way I have done it is for each yaw direction and wind speed, I have averaged the results for 600 seconds over the number of turbulence seeds, in my case 6. So essentially I will have 3*12 = 36 result files. My first question here is if my assumption of averaging over the number of turbulence seeds is correct (since the turbulence seeds are random, I did not see the point in assigning a particular weight).
After this, I have plotted some of the turbine loads for all 3 yaw conditions over the operating range. The load channels I have considered are Tower Top Tilt, Tower Top Roll, Tower Bottom ForeAft, Tower Bottom SideSide, Blade root flap and Blade root edge. The behavior of all the load channels except Tower Top Tilt and Tower Bottom SideSide are looking as expected, but these two look very strange. I have attached the plots for these two load channels below.
My understanding is that the mean Tower Top tilt moment should follow the thrust curve (inverted V with the peak at rated speed) and mean Tower Bottom SideSide moment should follow the power curve. However the Tower Top Tilt moment does not seem to follow the thrust curve (in fact resembling the power curve) and the Tower Bottom SideSide moment does not resemble the power curve.
I have looked at the steady state behavior of these two load channels and they are as expected. My suspicion is that adding turbulence appears to have brought some instability, but it is not consistent with other tower loads (in that they behave as expected). Another suspicion is that I may have used the wrong channel name to get these loads. I have used "YawBrMyp_[kNm]" for Tower Top Tilt and "TwrBsMxt_[kNm]" for Tower Bottom SideSide.
I hope to be able to get your guidance on this issue and what might be causing it.
Best regards,
Vishal S
I have been running the 5 MW onshore model with ElastoDyn, and I have obtained 10 minute results for different load channels for 3 yaw conditions 8 deg, 0 deg and 8 deg. I have used 6 turbulence seeds on TurbSim, NTM as the turbulence type and IECKAI as the turbulence model. The simulations I ran were from 4 m/s to 26 m/s in steps of 2.
While post processing the results, the way I have done it is for each yaw direction and wind speed, I have averaged the results for 600 seconds over the number of turbulence seeds, in my case 6. So essentially I will have 3*12 = 36 result files. My first question here is if my assumption of averaging over the number of turbulence seeds is correct (since the turbulence seeds are random, I did not see the point in assigning a particular weight).
After this, I have plotted some of the turbine loads for all 3 yaw conditions over the operating range. The load channels I have considered are Tower Top Tilt, Tower Top Roll, Tower Bottom ForeAft, Tower Bottom SideSide, Blade root flap and Blade root edge. The behavior of all the load channels except Tower Top Tilt and Tower Bottom SideSide are looking as expected, but these two look very strange. I have attached the plots for these two load channels below.
My understanding is that the mean Tower Top tilt moment should follow the thrust curve (inverted V with the peak at rated speed) and mean Tower Bottom SideSide moment should follow the power curve. However the Tower Top Tilt moment does not seem to follow the thrust curve (in fact resembling the power curve) and the Tower Bottom SideSide moment does not resemble the power curve.
I have looked at the steady state behavior of these two load channels and they are as expected. My suspicion is that adding turbulence appears to have brought some instability, but it is not consistent with other tower loads (in that they behave as expected). Another suspicion is that I may have used the wrong channel name to get these loads. I have used "YawBrMyp_[kNm]" for Tower Top Tilt and "TwrBsMxt_[kNm]" for Tower Bottom SideSide.
I hope to be able to get your guidance on this issue and what might be causing it.
Best regards,
Vishal S

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Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Vishal,
Are you plotting the mean of the timeseries mean across all turbulence seeds or something else, like the mean of the absolute maximums across all turbulence seeds?
I agree with your general observations, but it is difficult for me to guess why your results are not as you expect. Can you plot other output quantities, such as the mean of means and mean of standard deviations of blade pitch, rotor speed, blade deflection, and tower deflection to ensure the turbine response is as expected?
Best regards,
Are you plotting the mean of the timeseries mean across all turbulence seeds or something else, like the mean of the absolute maximums across all turbulence seeds?
I agree with your general observations, but it is difficult for me to guess why your results are not as you expect. Can you plot other output quantities, such as the mean of means and mean of standard deviations of blade pitch, rotor speed, blade deflection, and tower deflection to ensure the turbine response is as expected?
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: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Jason,
I am loading the result files for all windspeeds, yaw angles and turbulence seeds, then removing the first 100 seconds due to transient events so I am left with 600 seconds of data (I simulated for 700 s). Then I average all the load channels over the turbulence seeds (add all the values and divide by number of turbulence seeds, in this case 6). So I am left with 600 seconds of data averaged over turbulence seeds, so 1 set of data per wind speed per yaw angle.
The plots that I had attached previously are done by calculating the mean of the 600 s data averaged over turbulence seeds per wind speed and per yaw angle. So in essence, these are the mean of means (unless my understanding of it is incorrect).
I have attached the plots of RPM, Pitch and Tip Defl in x direction means along with Std Deviation.
I have not added the tower deflection as one of the output channels so I don't have them at the moment.
It seems that the turbine is behaving as expected. Just the tower top tilt and tower bottom sideside moments look strange.
Best regards,
Vishal
I am loading the result files for all windspeeds, yaw angles and turbulence seeds, then removing the first 100 seconds due to transient events so I am left with 600 seconds of data (I simulated for 700 s). Then I average all the load channels over the turbulence seeds (add all the values and divide by number of turbulence seeds, in this case 6). So I am left with 600 seconds of data averaged over turbulence seeds, so 1 set of data per wind speed per yaw angle.
The plots that I had attached previously are done by calculating the mean of the 600 s data averaged over turbulence seeds per wind speed and per yaw angle. So in essence, these are the mean of means (unless my understanding of it is incorrect).
I have attached the plots of RPM, Pitch and Tip Defl in x direction means along with Std Deviation.
I have not added the tower deflection as one of the output channels so I don't have them at the moment.
It seems that the turbine is behaving as expected. Just the tower top tilt and tower bottom sideside moments look strange.
Best regards,
Vishal

 Posts: 5738
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Vishal,
Thanks for clarifying. These new plots are all as I would expect them to look.
Regarding your original plots, it is only the towerbase sideside moment that is a bit odd to me. The towertop tilt moment has such a small moment arm from thrust, that much of the moment could by caused by asymmetry across the rotor, like shear. For the towerbase sideside moment, I question why the mean of the mean value does not plateau at wind speeds above rated. What does the generator torque look like?
Best regards,
Thanks for clarifying. These new plots are all as I would expect them to look.
Regarding your original plots, it is only the towerbase sideside moment that is a bit odd to me. The towertop tilt moment has such a small moment arm from thrust, that much of the moment could by caused by asymmetry across the rotor, like shear. For the towerbase sideside moment, I question why the mean of the mean value does not plateau at wind speeds above rated. What does the generator torque look like?
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: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Jason,
The generator torque looks sensible, it plateaus out after rated wind speed. I have attached the plot below.
Best regards,
Vishal
The generator torque looks sensible, it plateaus out after rated wind speed. I have attached the plot below.
Best regards,
Vishal

 Posts: 5738
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Vishal,
This generator torque input looks like what I would expect as well. Perhaps the additional towerbase sideside moment is coming from some sideside shear forces at the tower top, based on how the turbulent wind and associated aerodynamic loads have been applied. Do you have plots of that?
Best regards,
This generator torque input looks like what I would expect as well. Perhaps the additional towerbase sideside moment is coming from some sideside shear forces at the tower top, based on how the turbulent wind and associated aerodynamic loads have been applied. Do you have plots of that?
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: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Jason,
Unfortunately I did not have tower top sideside shear forces as one of the output channels during simulation. Do you have any other suggestion on which other output channel to look at? All the turbine operating parameters' response are exactly as expected, this would give indication that the turbine is operating normally. The other load channels like BR bending and Tower bottom foreaft are as expected. Wouldn't sideside shear forces at the tower top also affect the BR bending as well?
Best regards,
Vishal
Unfortunately I did not have tower top sideside shear forces as one of the output channels during simulation. Do you have any other suggestion on which other output channel to look at? All the turbine operating parameters' response are exactly as expected, this would give indication that the turbine is operating normally. The other load channels like BR bending and Tower bottom foreaft are as expected. Wouldn't sideside shear forces at the tower top also affect the BR bending as well?
Best regards,
Vishal

 Posts: 5738
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Vishal,
It would be hard to see the sideside shear force from the bladeroot bending moments, which are in a rotating frame of reference.
The towertop sideside shear force would be easiest to look at. The towertop sideside deflection may also be useful to review.
Best regards,
It would be hard to see the sideside shear force from the bladeroot bending moments, which are in a rotating frame of reference.
The towertop sideside shear force would be easiest to look at. The towertop sideside deflection may also be useful to review.
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: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Jason,
Apologies for the delay in replying. I have extracted the tower top sideside shear forces and sideside deflections.
Best regards,
Vishal
Apologies for the delay in replying. I have extracted the tower top sideside shear forces and sideside deflections.
Best regards,
Vishal

 Posts: 5738
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Vishal,
It definitely looks like the slow increase in mean towerbase sidetoside moment above rated is correlated with a slowincrease in towertop sidetoside shear force (and subsequent towertop sidetoside deflection). This was not seen when Figure 91 from the NREL 5MW baseline wind specifications report (https://www.nrel.gov/docs/fy09osti/38060.pdf) was first developed. I'm guessing this is related to the turbulent wind input you have used (the original work used steady uniform wind). If you want to examine this more, I would suggest comparing the mean response (motions/loads) between cases with steady uniform wind inflow and cases with turbulent wind inflow.
Best regards,
It definitely looks like the slow increase in mean towerbase sidetoside moment above rated is correlated with a slowincrease in towertop sidetoside shear force (and subsequent towertop sidetoside deflection). This was not seen when Figure 91 from the NREL 5MW baseline wind specifications report (https://www.nrel.gov/docs/fy09osti/38060.pdf) was first developed. I'm guessing this is related to the turbulent wind input you have used (the original work used steady uniform wind). If you want to examine this more, I would suggest comparing the mean response (motions/loads) between cases with steady uniform wind inflow and cases with turbulent wind inflow.
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: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Jason,
I have not focused much on this topic for the last few weeks, but I am trying to figure out what could be causing this abnormal load behavior. Like you suggested, I compared the mean loads for steady state condition like in the NREL 5 MW report with the mean loads for turbulent condition (both at 0 yaw). All the load channels are very close to each other except the Tower Top tilt and Tower bottom sideside. I tried changing the damping ratios for the Tower foreaft and sideside modes in the tower ElastoDyn input but to no avail, the results still remained the same.
Best regards,
Vishal
I have not focused much on this topic for the last few weeks, but I am trying to figure out what could be causing this abnormal load behavior. Like you suggested, I compared the mean loads for steady state condition like in the NREL 5 MW report with the mean loads for turbulent condition (both at 0 yaw). All the load channels are very close to each other except the Tower Top tilt and Tower bottom sideside. I tried changing the damping ratios for the Tower foreaft and sideside modes in the tower ElastoDyn input but to no avail, the results still remained the same.
Best regards,
Vishal

 Posts: 5738
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Vishal,
I would guess the key difference is between the mean wind speed profile between your simulations with steady wind and turbulent wind. How does the mean wind speed profile (shear, perhaps veer) compare across the rotor disk?
Best regards,
I would guess the key difference is between the mean wind speed profile between your simulations with steady wind and turbulent wind. How does the mean wind speed profile (shear, perhaps veer) compare across the rotor disk?
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: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Jason,
I have used the IEC recommended shear factor value for the TurbSim input when I generated .wnd files (I believe it is 0.2). I would have to look at the mean wind speed in X,Y and Z directions against azimuth to obtain a wind speed profile across the rotor disk, should I not?
Best regards,
Vishal
I have used the IEC recommended shear factor value for the TurbSim input when I generated .wnd files (I believe it is 0.2). I would have to look at the mean wind speed in X,Y and Z directions against azimuth to obtain a wind speed profile across the rotor disk, should I not?
Best regards,
Vishal

 Posts: 5738
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Vishal,
I agree. You should also be able to see/compare how the shear is set through the input file(s).
Best regards,
I agree. You should also be able to see/compare how the shear is set through the input file(s).
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: 35
 Joined: Mon Aug 10, 2020 7:50 am
 Organization: Power Curve ApS
 Location: Denmark
Re: Tower top tilt moment and Tower bottom sideside moment looking strange
Dear Jason,
I have plotted the time series of wind speed in X,Y and Z directions against azimuth for the 216 cases (426 m/s in steps of 2; 8,0,8 yaw and 6 turbulence seeds needed for DLC13), but I am not able to very clearly observe the shear.
What did you mean when you said "You should also be able to see/compare how the shear is set through the input file(s)." in your previous post?
Best regards,
Vishal
I have plotted the time series of wind speed in X,Y and Z directions against azimuth for the 216 cases (426 m/s in steps of 2; 8,0,8 yaw and 6 turbulence seeds needed for DLC13), but I am not able to very clearly observe the shear.
What did you mean when you said "You should also be able to see/compare how the shear is set through the input file(s)." in your previous post?
Best regards,
Vishal
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