Dear All

I am trying to reduce the cyclic loads of blades such as blade root bending moment, blade root torsion and blade tip out of plane deflection by an algorithm, I am wondering what types of wind could cause the loads mentioned above periodic. I have found that constant wind speed will cause the wind turbine to rotate in a fixed rpm so that the loads will be periodic with the same frequency of rotating rotor. Besides, wind shear will have the same effect. Is there any other wind types which will have the same effect? By the way, what about the turbulence? I simulated with turbulence finding that it is not clear to see the period of the loads. I am wondering if the turbulence will have some influence on cyclic loads or it only affects stochastic loads.

Best

XING WEI

Dear Xing,

Yes, turbulence will have a strong impact on cyclic loads. Because of the how the wind is “sampled” by the rotating blades, turbulent wind induces loads in the rotating frame that are harmonics of the rotor speed (0P, 1P, 2P, 3P, etc.), but loads in the fixed frame show up as harmonics of 3-times the rotor speed (0P, 3P, 6P, etc.) (for a 3-bladed rotor).

Best regards,

Dear Jason

I am wondering where I can find the specific explanation of how turbulence influence cyclic loads? If I want to reject the cyclic loads, I need to generate the cyclic loads first such as blade root bending moment. I am wondering what is the best wind type (constant wind speed or wind shear or something else) I should use to generate cyclic loads on blade root bending moment?

Best

XING WEI

Dear XING WEI,

I’m sure you can find a discussion of “rotational sampling” of blades passing through turbulence in most any wind energy text book.

Cyclic loads in the blade are caused by many things (gravity, shear, tilt, yaw error, tower shadow/influence, turbulence), but I’m not sure I can comment on what is the “best type”, as this depends on how you plan to go about your analysis. Certainly for controls design for reducing cyclic loads, you’ll want to demonstrate that your controller reduces loads in all conditions.

Best regards,

Dear Jason

Thank you very much for your reply! Let me rephrase my question. I found on some book saying that turbulence could be divided to two parts. One is the constant speed part and the other are Gaussian variables with zero mean. So I think that the way in which turbulence causes cyclic loads on blades is due to the constant speed part. The other part will superimpose on the blade loads causing the loads curves look not like exact sinusoidal curves. Am I understanding right?

Best

XING WEI

Dear Xing,

By “constant speed part”, I assume you are referring to the mean wind speed profile that includes the mean wind speed and shear. I agree that this shear (as well as tilt and or yaw error) will induce cyclic loads. But the “Gaussian” part–i.e. the fluctuations about the mean–will also induce cyclic loads on the blades due the “rotational sampling” of the blades passing through these fluctuations.

Best regards,