# Positive vs Negative yaw Error Effect

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 ‘took vertical shear (0.2) into account’.

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 “odd” 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 = “ES10.3E2” in FAST’s 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: http://forums.nrel.gov/t/fast-integrator-step-size-precision/247/1.

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?

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 you’ve enabled the upwind tower-influence model in AeroDyn, “yes” 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 ‘NEW TOWER’ feature in Aerodyn (with potential flow around the tower) and the ‘tower shadow’ option in set to False. So am not sure how does Aerodyn account for this tower influence?

Thanks,
Neelabh

Dear Neelabh,