Questions about yaw error output and periodic inflow velocity fluctuation when using AeroDisk in FAST.Farm

[xiaomi4356]

Hello,

Thank you for developing and maintaining FAST.Farm, and for your help with our questions.

We encountered two issues when using the AeroDisk model for FAST.Farm simulations.

1. When using AeroDisk model, we disabled the yaw degree of freedom in the ElastoDyn file. However, in the .fstf output file, YawErrT1 still shows yaw angle variation with an amplitude of about ±5 degrees and an oscillation frequency of about 0.1 Hz.

the YawErrT1 curve is :

It is worth noting that we disabled the yaw degree of freedom in all single-turbine .fst files through ElastoDyn. We also disabled the yaw control mode and yaw control enable time in ServoDyn. In the single-turbine .out files, the yaw angle is observed to be 0.

in ElastoDyn file,we set:

in ServoDyn file, we set:

Could you please explain this phenomenon? Also, does the yaw angle observed in the .fstf output affect the wake calculation results?

2. We implemented a two-turbine case using the AeroDisk model. We monitored the inflow wind speed of the second turbine. Under a steady inflow wind condition, the wind speed of T2 shows a periodic fluctuation, which seems strange.

the wind speed of T2 in FAST.Farm.out using AeroDisk model:

When we keep all other conditions unchanged and use the AeroDyn model instead, this periodic fluctuation does not appear.

Could you please help us understand why this happens?

THANKS!

[andrew-platt]

This looks like a bug in the code somewhere. The periodicity of the fluctuation is similar to what I expect for the rotor rotation rate. This hints at a coordinate issue someplace. Is the shaft tilt for this turbine set to 5 degrees?

[xiaomi4356]

This looks like a bug in the code somewhere. The periodicity of the fluctuation is similar to what I expect for the rotor rotation rate. This hints at a coordinate issue someplace. Is the shaft tilt for this turbine set to 5 degrees?

Thank you very much for your helpful reply. You were right — the shaft tilt of this turbine was initially set to 5 degrees. After we changed the shaft tilt from 5 degrees to 0 degrees, the fluctuation issue disappeared and the results became correct. Your suggestion about the possible coordinate issue was very helpful. Thanks again for your support!

[jjonkman]

I'm reopening this issue because the underlying bug has not yet been fixed.

[andrew-platt]

This confirms there is a bug with the coordinate transform used in calculating the yaw error (or in rotation of the disk).