Blade passing frequency and Tower Shade

Wind & Water Structural Analysis
1

Dear Jason,

I’m doing a simulation of a small wind turbine (tower height is only 1.2 meters) with Fast because I have the experimentally-measured acceleration of the tower top in xt direction and I want to make a comparison.
I noticed that in the experimental data the acceleration signal has the lowest frequency component(it seems like a sine wave +random signal) that is numerically the same as “blade passing frequency” 3P. Than making a comparison between sperimental acceleration and fast-simulated acceleration i noticed that:

-if i use a “TwrShad” index of 0.3 the xt acceleration from Fast is different from the experimental data(I can not even see the low frequency component)

-if i use “TwrShad” index o 1 the Fast acceleration resambles the sperimental because it has the same lowest frequency component(BPF) and amplitude variation!!

Do you think this is reasonable?I think the difference is that ,in some way that I don’t understand , the higher is TwrShad the higher is the effect of aerodynamic forces that grows from the blade passing near the tower…am I wrong?..can you please explain me this better or send me some reference that explains the “link” between the blade passing aerodynamic forces and TwrShad(if they are linked)?

Thank you very much.

Best Regards,

Alessandro

2

Dear Alessandro,

I don’t recall having papers that show this effect. Regardless, for downwind rotors, increasing TwrShad will increase the 3P excitation of the tower. Of course, while guesses can be made, specifying an appropriate TwrShad requires the use of experimental measurements or computations (e.g. CFD).

Another way to increase the 3P excitation of the tower is to increase the turbulence of the wind inflow.

Best regards,

3

Dear Jason,

Thank you for your answer.
Do you know about any relation between 3P frequency and TwrShad for Upwind rotors that could justify my results?

Best Regards,
Alessandro

4

Dear Alessandro,

AeroDyn v14 and earlier input TwrShad only applies a tower shadow for downwind rotors. I would not expect any impact of TwrShad for upwind rotors.

Best regards,