Simulation of two wind turbines with varying wind direction

Wind & Water Wind Farm Modeling
1

Hi all,

I’m running simulations of two aligned wind turbines to study direct wake impingement and its effect on power and fatigue.

Eg.** setup:**

  • NREL 5-MW reference turbine

  • Spacing: 8D

  • Wind speed: 12 m/s

  • Turbulence intensity: 12%

  • Two turbines in line (T1 upstream, T2 downstream)

  • For each wind direction, I used 6 seed pairs and averaged the power results

Attached is the results for the example showing the power of each turbine (T1 and T2) as well as the total power across different wind directions.

Method for changing wind direction:
I followed the approach discussed in the forum previously, where:

If you change the wind direction (PropagationDir) without changing the nacelle yaw angle (NacYaw), then you introduce a yaw error. To maintain zero yaw error, you should set NacYaw = -PropagationDir.

Regarding FAST errors: when changing NacYaw, YawNeut in the ServoDyn input file should also be updated. NacYaw defines the initial yaw angle, while YawNeut sets the neutral yaw position. A large mismatch between them can create unrealistic yaw moments. The recommendation is to either disable the yaw DOF or set YawNeut = NacYaw.

Questions:

  1. Why does the power production of T1 (the upstream turbine) vary slightly with wind direction? Shouldn’t it remain constant since it is not affected by the wake?

  2. For T2 (downstream turbine), why is its power not equal (actually larger) to T1’s for the largest wind direction offsets (±20°)? Could this be due to numerical effects, or is there a physical explanation?

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Any insights would be greatly appreciated!

Thanks,
Tiago Lucas

2

Dear @Tiago.Lucas,

Presumably you are using FAST.Farm with TurbSim-generated inflow; is that correct? Are you using the Python scripts from the OpenFAST toolbox to set up the inflow generation and FAST.Farm cases?

FYI: When running FAST.Farm with TurbSim inflow, we recommend not setting PropagationDir to angles other than 0, +/-90, or 180 so as to avoid misalignment between the TurbSim grid and the FAST.Farm grid points for the low- and high-resolution domains, and resulting interpolation and smoothing out of ambient turbulence. Instead, to change the wind direction, it is preferred to change the (X,Y) locations of the turbines, i.e., rotate the wind farm with the same inflow direction (in your case, only the X,Y coordinates of T2 would need change). I would guess misalignment of the grid points is the reason the results are not fully intuitive, i.e., the inflow is not being fully resolved. The OpenFAST toolbox should handle these cases directly.

Best regards,

3

Dear Jonkman,

Thanks you very much, your answer was very helpful!

Yes, I am using TurbSim-generated inflow.
I wasn’t using the OpenFAST toolbox scripts; I adapted some scripts I wrote myself for this case.

Initially, I had considered both approaches in my mind, but the possibility of undesired smoothing of turbulence intensity (or preference for 0, +/-90, or 180) escaped me. I chose the PropagationDir approach (as discussed in Wind farm modeling by using FAST and TurbSim - #15 by Shubham.Baisthakur ) because it felt more direct for calculating fatigue at specific points at the tower base, since the tower reference frame does not change with wind direction.

I’ll now explore the second option, rotating the (X,Y) turbine coordinates. When computing tower base fatigue, I’ll make sure to carefully convert the angles, since a specific fiber (angle) in the tower would correspond to a different one when Y is different.

Best regards,
Tiago Lucas

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