3P frequency experimental vs simulation

Hi all,

I have to make a comparison between experimental measurements from an accelerometer and Fast simulation.The wind tunnel- tested turbine is a small turbine( blade length is 1meter).The accelerometer measures the acceleration in FA direction of the wind turbine tower(placed near tower-top) subjected to a costant 6 m/s wind velocity(in Fast I used a HH file to simulate it). Doing an FFT of the acceleration signal I see that the signal has really an high 3P content while FFT from Fast simulation does not have it…can you say me which are the main causes to 3P loading? I have tried to consider a small yaw error and i can see the 3P but also other frequencies( it seems 6P and 9P) that are non shown in the experimental signal FFT!

Dear Alessandro,

As discussed in the following forum topic: http://forums.nrel.gov/t/oscillation-in-local-axial-wind-component-and-aoa/938/1, loads in the rotating frame due to yaw error, shear, turbulence etc. show up as harmonics of the rotor speed (1P, 2P, 3P, etc.), but loads in the fixed frame show up as harmonics of 3-times the rotor speed (0P, 3P, 6P, etc.).

Does your accelerometer have the bandwidth to measure 6P and higher frequencies?

Best regards,

Dear Jason,
thank you for your answer! I need to understand if i correctly understood what you explaind in the post you suggested me to read!
Suppose I have a 3-bladed balanced wind turbine .Many reasons (turbulence,shear wind,shaft tilt,tower deflection,blade passing near tower…) can crate load that are harmonics of the rotating speed(1P,2P,3P etc…) in the rotating frame( i.e. blade reference (xb,yb,zb in Fast). This loads “translate” into harmonics of 3-times the rotation speed (3P,6P,9P eccc…) in the fixed frame(for example loads in tower base reference) right?Now I have two questions:

  1. When you said “translate” you mean that the reason is simply that a 3_bladed rotor has got 3 blades,so if a load in one blade appears each rev. (1P) it will be 3 times per rev. in the fixed frame (3P)because the rotor has three blades?

  2. Assuming my previous supposition is correct is it right to say that a 1P load in the rotating frame means a 3P in fixed , a 2P in the rotating frame means a 6P in fixed…a 3P in the rotating frame means a 9P and so on ? I asked you this question because in the figure below I reported the measurement made by the accelerometer in wind turbine-tesing in a wind tunnel. As you can see (red circle on the left) I noticed 3P frequencies(they translates as the velocity in wind tunnel grows) and on the right (second red circle) i noticed frequencies contents that are 48 times the rotor rotational speed (Is it possibile)? So is it right to think that the 3P contents are the result of 1P loads in the rotating frame while 48P are due to 16P loads in the rotating frame?

Dear Alessandro,

I agree that 3P, 6P etc. excitation in the fixed frame results because of 3 rotor blades, but it is not true that 3P excitation in the fixed frame results solely from 1P excitation in the rotating frame. In fact, 3P excitation in the fixed frame results from 1P, 2P, 3P, 4P, and 5P excitation in the rotating frame. See Table 2 in the following paper for more information:

Stol A, Moll HG, Bir G, and Namik H. “A Comparison of Multi-Blade Coordinate Transformation and Direct Periodic Techniques for Wind Turbine Control Design.” 47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exhibition, 5–8 January 2009, Orlando, FL. AIAA-2009-479. January 2009.

Best regards,