Hello all,
I wanted to check the effect of yaw error on NREL 5MW reference turbine for a steady wind speed (no vertical shear) of 9 m/s when I came across this strange plot:
- Why there are variations in generator speed for negative yaw error and not for positive yaw error (as yaw error creates 1P excitation in rotating frame)
- Moreover, these excitations repeat after every 2 s or so it not 1P in any ways?
- Why is generator power fluctuating in all the three scenarios?
I have removed the initial computational transients from the run
Hope to get more understanding on this!
Thanks a lot,
Neelabh
I am sorry, I âtook vertical shear (0.2) into accountâ.
Dear Neelabh,
The frequency of osciallation seen in the generator power and generator speed plots seems to be around 0.52 Hz, which is the 3P frequency for a rotor speed of around 10.4 RPM that is consistent with the generator speed of around 1010 RPM (= ( 10.4 RPM rotor speed ) * ( 97:1 gearbox ratio ) ) and makes sense for the NREL 5-MW turbine at a wind speed of 9 m/s. The 3P oscillation results from the asymmetry of the flow due to wind shear and yaw error. (Even without shear in the wind, the NREL 5-MW has a shaft tilt that will induce shear.) (Asymmetry in the flow will cause a 1P oscillation in the rotating frame, which leads to a 3P oscillation in the fixed frame; a 1P oscillation in the fixed frame would only result from an imbalance in the rotor.)
The âoddâ square shape of the generator speed time series is likely due to the small round-off error that results from the fact that FAST is only outputting 4-digits of precision based on the setting of OutFmt = âES10.3E2â in FASTâs primary input file. My guess is that the shape would look more rounded if you added at least one more digit of precision in the output format. This roundoff is also likely the reason why you only see generator speed oscillation in one of the three time series. See the following forum topic for more information on OutFmt: FAST Integrator, step size & precision.
I hope that helps.
Best regards,
Thanks a lot Jason for this insight. It does help in understanding it. Meanwhile, you are correct about the round off error in FAST,
- why does negative yaw error gives less power (mean value) than positive error. Is it due to the effect of positive and negative torque acting due to cross wind component in respective scenarios?
- also when I plotted root moment (flapwise) at blade 1, I came across this plot which shows 1P fluctuatuion. Is this because of difference in reference frame used for Generator speed and root moments?
Thanks again for your help,
Neelabh
Dear Neelabh,
I would expect negative and positive yaw error to give the same power in the absence of wind shear. It is the combination of yaw error and wind shear that causes the differences in power output.
Yes, in the rotating frame (e.g., the blade root), you will see loads that are harmonics of the rotor speed, 1P, 2P, 3P, etc. But in the fixed frame (e.g., rotor speed, rotor torque, or tower), these loads are translated into harmonics of 3-times the rotor speed, 0P, 3P, 6P, etc., unless the rotor loads are imbalanced.
Best regards,
Dear Jason,
I understand your 2nd point but to your 1st reasoning, I switched off wind shear (no vertical shear and shaft tilt= 0 degree) and still observed some fluctuations in generator power for 0° yaw error. Though, for positive and negative yaw errors the mean generator power became significantly closer.
Thanks,
Neelabh
Does tower influence has to do something with this oscillation? But this being an Upwind turbine, this effect must be minimal?
Dear Neelabh,
If youâve enabled the upwind tower-influence model in AeroDyn, âyesâ this would influence the torque, power, etc. of the response. The upwind tower influence would have a 1P effect in the rotating frame a 3P effect in the fixed frame.
Best regards,
Dear Jason,
I use the âNEW TOWERâ feature in Aerodyn (with potential flow around the tower) and the âtower shadowâ option in set to False. So am not sure how does Aerodyn account for this tower influence?
Thanks,
Neelabh
Dear Neelabh,
Please see the following post form information about how to enable the upwind tower-influence model: Tower shadow effect in FAST (1.5MW, test13).
Best regards,
Thanks a lot Jason for all your inputs. I really appreciate your responses. This gave me a good insight on harmonics seen in fixed and rotating frames.
Hope to learn from from you in future 
Neelabh
Dear @Jason.Jonkman
I ran simulations using the IEA 15MW UMaine Semi type to check the change in power generation under yaw error and wind-wave misalignment conditions.
- Wind-wave misalignment angle: 0°, 20°, 40°, 60°, 80°, 90°
- Yaw error angle: -15°, -8°, 0°, +8°, +15°
But there is a difference between positive yaw error and negative yaw error.
For example, power generation when the yaw error is +15° and -15° is different.
According to my limited knowledge, it should have the same power generation.
I checked and found that the wind-wave misalignment doesnât affect much power generation.
Is it because of the rotorâs rotation orientation?
You described it well above, but I am still confused.
Here are my results.
- âβâ means wind-wave misalignment angle.
Dear @Sangwon.Lee,
Iâm guessing the slight asymmetry between positive and negative yaw error is a result of wind shear. Even if youâve set the wind shear to zero in the simulation, there is likely some shear across the rotor anyway as a result of shaft tilt, tower deflection, or floater heel.
FYI: The following more recent forum topic is likely of interest to you: Power loss with yaw misalignment.
Best regards,
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Dear @Jason.Jonkman
Now that it makes sense to me, I thank you for your detailed response.
Also, I appreciate your link for information.
Have a nice day!
Best regards,
Sangwon
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Dear @Jason.Jonkman
I have another question about power generation under yaw error and wind-wave misalignment conditions.
According to my power generation standard deviation (StDev) results, I found some doubts.
I thought wind-wave misalignment conditions might affect each yaw error condition the same.
But there are certain points where StDev is higher than I expected. (marked by a black circle in the bar graph)
They have in common that the wind direction is over 90 degrees.
1st mark: 98° (wind direction) = 90° (wind-wave misalignment angle) + 8° (yaw error angle)
2nd mark: 105° (wind direction) = 90° (wind-wave misalignment angle) + 15° (yaw error angle)
(The wave direction is fixed at 0°.)
I know Platformâs pitch and roll motion influence power generationâs StDev.
But according to my results, I couldnât find a correlation between platform 6-DOF and my question.
I understood that the asymmetry of power generation might be caused by wind shear.
Is this phenomenon also caused by the same reason?
I set the parameter as follows:
- Wind-wave misalignment angle: 0°, 20°, 40°, 60°, 80°, 90°
- Yaw error angle: -15°, -8°, 0°, +8°, +15°
- Wind type: NTM (IEC Class 1B)
- Wave type: Irregular wave
- Wind speed: 10.59 m/s
- Wave height(Hs): 1.641 m
- Wave period(Tp): 7.544 s
Here are my simulation results:
- âβâ means wind-wave misalignment angle.
Best regards,
Sangwon
Dear @Sangwon.Lee,
Iâm not sure what platform motions (means or standard deviations?) in your lower figure, but I do see several asymmetries here with respect to yaw error, so, it is not surprising that you are seeing asymmetries in the power with respect to yaw error as well.
Best regards,
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Dear @Jason.Jonkman
Platform motion data are mean values.
As you mentioned, these might result from a combination of asymmetries.
Thank you for your response, and I will keep studying.
Have a nice day!
Best regards,
Sangwon
