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jonasforssell
PostPosted: Sat Feb 23, 2008 8:14 pm    Post subject: Silent blade design Reply with quote



Joined: 23 Feb 2008
Posts: 3

Hello!

I have just mounted my first windmill:
http://24volt.eu/phpBB3/viewtopic.php?f=16&t=221

Unfortunately, it gives off a whining noise which gets quite annoying at higher windspeeds.

As you can see on the pictures, the blades are quite simple.
What is the reason for the noise and what can be done about it?

I read somewhere that Bonus experimented with tips on the wings to lower noise...?

Best Regards
/Jonas Forssell, Sweden
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erik
PostPosted: Sat Feb 23, 2008 11:22 pm    Post subject: Reply with quote

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Joined: 28 May 2007
Posts: 35

Hello Jonas

Did the manufacturer of the turbine supply a graph showing the power at different windspeeds, and also the rotor speed when working...?
It might be a good idea to measure the rpm of the turbine, to get an idea, wether the turbine is "working" or just speeding up like a runaway..

The pultruded blades - with same shape from root to tip - are of course not very effective - the concave (high camber) airfoil, is not good at the tip area... especially at high speed, it will make a lot of turbulence/noise

(You could look at the blades for other well renowned small turbines like the american Bergey, I guess your new turbine is made in China ?)
So, a better shape of the blade would give higher power and lower noise..

Yes, Bonus made tests at a 300 kW Turbine at Gierslev, Denmark 1993 (?) "Torpedoes" mounted at the tip, and different shapes proposed by Bonus and Risų. I think one of the best was the so called Ogee tip...see if I can find some pictures...or drawings... I may upload the old report to WindsOfChange, so you can download it there.

Others have tried winglets like it is used on modern planes (German Enercon did). To reduce noise on some garden lawn movers the rounded L.E. tip has also been used, if I remember right.

But, even if you grind off, and shape some of your bladetips, you will not stop the noise, with this bad airfoil (too much camber at the tip)..

best regards and greetings to "broderfolket"

Erik / W.O.C.
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erik
PostPosted: Sun Feb 24, 2008 2:53 pm    Post subject: Reply with quote

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Joined: 28 May 2007
Posts: 35

Hello Jonas

You can find the noise-reduction report from 1993 - (Bonus 300 kW Combi at Gierslev) on WindsOfChange - under Danish Turbines and Bonus (Danregn). In the "brochures" download.
You have to register and subscribe - then go to:
http://www.windsofchange.dk/WOC-danturb.php and click the green button for brochures at the Bonus Turbines area. The file is : aero-noise-1993.pdf

The tests were paid for by EFP of the Danish Department of Energy

Good luck

Erik / WOC
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jonasforssell
PostPosted: Fri Feb 29, 2008 7:58 pm    Post subject: Reply with quote



Joined: 23 Feb 2008
Posts: 3

Hej Erik,

It turns out that the turbine is an exact replica of the Bergley turbine.

They copied all but forgot a small rivet on the top of the blade. Probably thought this was a casting mark from the process.

Here is a guy with the same problem as me:
http://www.youtube.com/watch?v=5KH9qWODSPc

And here is how he solved it:
http://www.thebackshed.com/windmill/articles/ChineseBlades2.asp

So, I have glued a nylon thread onto the top of the profile.
I'll keep you updated on any improvement when I get the turbine running again.

Thanks for all the info Erik. I'll have a look!
/Jonas
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erik
PostPosted: Fri Feb 29, 2008 10:14 pm    Post subject: Reply with quote

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Joined: 28 May 2007
Posts: 35

Hej Jonas

Nice links to the other owners with the same problems !

Good idea to try a turbulator wire to reduce separation bubbles on the lee side of the blade. Hope it works !

However it may turn out that the big camber that I think you have on your blades (to judge from the photos of your blades shown earlier) still gives you a noisy rotor - not sure ...
The blades on your link: http://www.thebackshed.com/windmill/articles/ChineseBlades2.asp seems to have less camber (concavity)...

(I used turbulator wires on my Wakefield model airplanes back in 1965 to help create turbulent flow over the airfoil and increase the performance : see http://www.windsofchange.dk/WOC-49-75.php )

Also the US made StormMaster Turbine, with very slender blades used a wire turbulator : http://www.windsofchange.dk/WOC-usaturb.php - at the StormMaster Brochures download you can find a "carbon copy" of the airfoil - drawn at the backside of a brochure....

Also see references: http://en.wikipedia.org/wiki/Turbulator and http://www.mh-aerotools.de/airfoils/turbulat.htm

Best regards

Erik
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jonasforssell
PostPosted: Tue Mar 04, 2008 5:29 pm    Post subject: Reply with quote



Joined: 23 Feb 2008
Posts: 3

Hej,

Well, it is up and running again. I've put up some pictures on the site:
http://24volt.eu/phpBB3/viewtopic.php?f=16&t=221&st=0&sk=t&sd=a

There is a change in the noise. It is more quiet but not completely silent.

There has not been any significant wind yet so we will see if the improvement works also at higher wind speeds.

Thanks for your great feedback Erik!

/Jonas
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