Vibration Analysis of wind turbine blade

[SH Yang]

Hello to all,

I am doing the vibration analysis of wind turbine blades with 3 axis accelerometers after impact testing in order to obtain natural frequencies of the blade.
However, I also need to verify if each natural is the flat-wise or twist-wise with the phase frequency domain.
If anyone knows how to verify???
I have googled or read many modal analysis books; however, not usuful information of reading phase graph from them.

Thanks,

[BB.]

I assume that the twist wise vibration would be much higher frequency. If you can induce vibration with a frequency generator and a transducer--You may be able to use a strobe light to scan through the resonance points and see the movement (I don't know how large/stiff the blades are--So don't know the expected range of motion).

-Bill

[SH Yang]

Thank for ur reply.

Actually, the size of the blade is the only 36cm and weight is 0.9kg.
And I am gong to strike the blade with an impact hammer.
I was able to calculate the natural frequencies.
1st nf : 4.78Hz
2nd nf : 29.87Hz
3rd nf : 83.77Hz
4th nf : 164.29Hz
5th nf : 271.55Hz
Could I verify the flap or twistedge with Phase diagrm of FFT??

[BB.]

Those sound like the length of the blade, not "twist" frequencies (I am no expert).

Adding acceleration sensors to the blades edge(s) can change the frequency response--But it may be of interest to see if you can "get close" to your calculations.

Depending on the blade design and how you will limit RPM--Some small turbines use "blade flutter" to limit RPMs (as well as just natural flutter from RPM, blade flexing, side furling, etc.). These will be significant sources of torsional energy.

Here is a paper on calculating blade flutter for larger wind turbines--Perhaps it will give you some ideas/leads. And another paper on blade bending vs flutter.

http://windpower.sandia.gov/other/Delft-2004-Lobitz.pdf
http://orbit.dtu.dk/fedora/objects/o...318835/content

-Bill

[SH Yang]

Thanks for ur reply.

I have one more question.
I made tiny expansion-board for attaching an accelerometer on the blade.
Due to the low pass filter for anti-aleasing and noise, I added a capacitor on the board which is recommended by the spec sheet of the acceleromters like the attached photo.
(I can not make a bigger boards becuase of the blade size.)
However, the capacitor seems not to work as a low pass filter, at all.

I convertered the time domain to frequency domain with matlab, however, there are alot of high frequencies like 600 to 900 Hz.
(The filter (a capacitor) is supposed to remove ove 500 Hz.)
Did I make some mistake soldering the capacitor on the board??
( I attached 8 boards on the blade, but I can find that every accelerometers have over 600 Hz to 900 Hz.)

Thank,
Best Regards,
SH

[BB.]

SH,

I am not sure--I would have to see the schematic/type of accelerometer chip/digitizer you have there.

Some of those devices are really current sources and not voltage sources and a capacitor just loads the output in a parasitic manner... And even if they are voltage sources, you need a resistor or inductor to really make a low pass filter (a capacitor directly on the output of a low resistance voltage source will not do much. You need some sort of impedance in front of the capacitor to create a "knee" in the curve.

You can also look up "Pi" and "T" filters too...

If the knee is too close to your frequencies of interest--you can end up with phase shifting issues too (back those out with a calculation).

It has been many decades since I have done anything like this--So I may be of limited help (or even pointing you in the wrong direction).

-Bill