LSShftFys under yaw error

I have a question regarding the vertical force (Fz on fixed coordinate system) acting on the rotor under yaw condition. In principle, the wind turbine yaw will induce lateral force (horizontal component of force Fy). There will be negligible amount of vertical component of rotor loading (LSShftFzs).

So I setup a small simulation case for 5 MW wind turbine in FAST. I turn off the flexibility of blade, control of rotor speed (fixed rotor speed), tower motion, and use uniform flow, set gravity to 0, use WAKE as my induction model. I expect to have lateral force (LSShftFys) after certain degree of yaw and obtain negligible vertical force component (LSShftFzs). But there is even higher value of Fz than Fy.

Then I used CP-Lambda (aero-elastic code from our lab) to run the similar case. It turns out that Fz is very low for yaw case.

I also tried to turn off the induction model (From WAKE to NONE, which is not correct), Fz in this case is minimized. Also if I reduced the rotating speed intentionally, Fz is decreased.

Could you give me some help about this Fz(LSShftFzs)? It should stay close to zero when yaw is activated. Is there any setting needs to be turned?

Thank you very much. I am appreciated your help.

Dear Jesse,

I set up a case myself to look into what you are reporting and don’t really see any problem.

Introducing a yaw error introduces a periodic variation of induction, which leads to a small variation of shear forces transverse to the rotor plane (both horizontal/LSShftFys and vertical/LSShftFzs); however, the majority of the force is still the thrust force normal to the rotor disk (LSShftFxs), which is reduced relative to a case without yaw error. The relative contribution of the horizontal and vertical forces also depends on the skew induced by shaft tilt.

When examining the tower-top forces (YawBrFxp, YawBrFyp, YawBrFzp), which are aligned with the wind direction and don’t yaw with the nacelle, introducing a yaw error leads to a reduction of the thrust force (YawBrFxp), a large horizontal shear force (YawBrFyp), and a very small vertical force (YawBrFzp). I suspect it is the large change to the horizontal force that are that are you are expecting to see, but you need to examine YawBrFyp, not LSShftFys.

Best regards,