Why do tower top side-to-side deflections have non-zero average?


I run a one hour long simulation, with a turbulent wind input file generated with turbsim. The misalignment of the wind component along x is set to zero. No other unbalance is (intentionally) designed in the turbine.
In the tower top, I can plot the side-to-side deflections (YawBrTDyp), in metres, along time, as shown in figure. I would expect a zero average, but that’s not the case. The tower kinda “stays much more” on one side than on the other.
I also would expect the displacements to be higher, not only a few centimetres. The same qualitative effects show up with steady uniform wind, with only the x component different from zero.

Is this due to some kind of error, or is there a logical explanation about this?


Y displ.jpg

Dear Emanuele,

I would expect the aerodynamic torque to induce some side-to-side deflection, including a nonzero mean offset.

Best regards,

As usual thank you very much for the reply.

I see what you mean, wasn’t thinking about that! Even when dealing with turbomachinery, I probably never put my mind to it. To see if I understand: it’s the shape of the blades that induces such an asymmetric behaviour.
To keep it short, if the blades had a “neutral” shape and were rotating driven by a motor in the nacelle, we would have a zero average of the side-to-side displacements. But the rotor is shaped to spin in one direction only, hence the asymmetrical side push. Am I right?


Dear Emanuele,

No, that is not really correct.

The mechanical power convertible to electricity is torque*speed. Unless the speed is very high, the torque must be sizeable to generate useable power. This torque not only counteracts the aerodynamic torque, but it has an equal and opposite effect on the tower–inducing a side-to-side bending moment and side-to-side deflection.

I hope that helps.

Best regards,