Yaw loads at skewed inflow

Wind & Water Rotor Aerodynamics
1

Hi all,

In theory, with a yawed rigid turbine (or skewed inflow), I would expect no yaw moment at the rotor (LSSTipMzs = 0), because the angle of attack should change at positions 0° and 180° only and not from side to side.

Instead I get quite high yaw moments in the partial load range. I found out that the maximal blade bending moment occurs at 60°, not at 0° as anticipated.

My simulation parameters:
Wind: HH-Windfile, const. partial load wind speed and horizontal skew only.
FAST fst: All DOF false, except for GenDOF and CompAero, 0 tilt, 0 precone, symmetric rotor, const pitch
AeroDyn ipt: StallMod steady, UseCm no, InfModel EQUIL, IndModel SWIRL (or WAKE), Prandtl tip and hub losses, no tower effects

I read about sources for yaw loads in nrel.gov/docs/legosti/old/4822.pdf and tried to exclude all sources for yaw moments with the choice of parameters but I still get these loads.
I hope the code is correct, but what am I disregarding that causes yaw loads?

Thanks for any ideas!

Best regards,
Stefan

2

Dear Stefan,

The combination of skewed wake and the relative motions between the structural motions and the wind will generate a periodic varation in the induced velocities and aerodynamic loads. These will produce a yaw moment on a rigid rotor operating with a yaw error, even with steady uniform unsheared winds, no structural DOFs, no tilt, no precone, no tower shadow, and no dynamic stall. In the present version of FAST with AeroDyn, it is not possible to disable the skewed wake model and relative motion calculation, so, you will always get these loads.

I hope that helps.

Best regards,