As I understand it AM is partly due to wind shear - the difference in wind speed between the top and bottom of the disc. Unless they can vary the blade pitch as the blade rotates this means the pitch has to be a compromise to suit the average wind speed. Means the tip vorticies change as the blade rotates.

Yes that's exactly what I mean. I've heard it explains why short dumpy towers are worse than tall ones, why trees near turbines increase AM and even why it's worse at night when wind shear apparently reduces.

References in here..
http://www.dickbowdler.co.uk/wp-content/uploads/2010/03/AM-of-Wind-Turbines.pdf

http://www.maine.gov/dep/ftp/bep/ch375citizen_petition/pre-hearing/AR-40%20chapter%20375%20-%20r%20brown%20hearing%20submission%20-%20wtn2011_siponen.pdf

"Recent research work suggests that the varying angle of attack of incoming wind caused by the wind shear is the main source of amplitude modulation of wind turbine noise [3]. Depending on the blade profile, the angle of attack has an optimal value related to the generated lift of the blade, as illustrated in Figure 1. If the angle is differed from its optimal value, the turbulent boundary layer on the low pressure side of the blade grows, decreasing power performance and increasing sound level. Figure 1.

Because the blade length of modern wind turbines can be more than 60 meters, the difference in wind speed at different blade positions can be several meters per second. As wind speed varies in different blade positions, also the angle of attack varies resulting periodic fluctuation in the generated sound level. Therefore growing the size of wind turbines and the diameter of their blades may yield increasing problems with amplitude modulation."

I wish they'd use the correct terminology though!
Wind shear is a very specific atmospheric effect, quite different from wind gradient. Shear can be quite aggressive and contain vortices created entirely by viscous drag between the shearing layers, quite unlike wind gradient with is a purely ground viscous drag induced effect.

The key to reducing the impact of tip vortices is to get them into clear air so that they can't interact with anything. They will move outwards and downwind from the turbine, a bit like a cone extending back from the turbine location, with local velocities that are highly dependent on blade tip speed and wind speed. Slowing the blade tips down will reduce the velocity and total energy in the vortices.

Low frequency sound can travel for quite astonishing distances, as it doesn't get attenuated by structures etc to anything like the same degree as higher frequency sound. When you get to the near-subsonic region (below 20 Hz or so) then it's extremely difficult to effectively attenuate, plus there will be components in many houses that may well have resonant frequencies of a few Hz (windows, doors, stud walls etc), and these can be effective "re-transmitters" if they are excited to resonance (the common effect of "the windows rattling" in response to a low frequency external sound wave is an example).

One consequence of the move towards bigger turbines is that they will tend to produce lower frequency noise than their smaller cousins, as they will have lower blade passing rates due to their larger diameter (turbines are tip speed limited, so increasing the diameter decreases the maximum allowable rpm). I suspect they will cause far worse low frequency noise than the first generation turbines we've been used to until now, yet because of the way the EU standards are worded this will not be counted in terms of their approval, as I understand it.

When my dad worked for Yorkshire TV for a science program they looked at setting off a ton (in those days, without the "ne") of TNT, ahem, to see what it would be like, and to do a mass experiment getting viewers to listen out for the rumble as it passed them all over the UK. (I could digress onto the very good rainfall survey they did that involved rushing outside for 30s after the show with a sheet of toilet paper on the palm of your hand...)

They indeed did conclude that the very low frequency stuff would travel a huge distance IIRC.

Sadly, the insurers pulled the plug on the idea though fears of fraudulent damages claims, even though YTV was going to set off the charge on a Lancashire moor where clearly there would be nothing of value to be lost... B^>

Rgds

Damon

Posted By: DamonHDYTV was going to set off the charge on a Lancashire moor

YORKSHIRE TV destroying part of a LANCASHIRE moor!!!!

Wars have been started for less. Hrrmph, hrmpph.

Actually djh, the crater would only have been about 6' deep at most I think. And YTV was prepared to buy the only building in line of sight in that desolate windswept spot I think ... which was a pub, natch... B^>

Rgds

Damon

Posted By: DamonHDthe crater would only have been about 6' deep at most I think.

I think you miss the subtlety here. The idea that a Yorkshireman would destroy one blade of Lancashire grass (or sprig of heather, or beetle, or whatever there is there) would be enough to start a war!

I'm a Yorkshireman as it happens, though I hide it well: not one jot of that undercurrent was lost on any of us. Indeed, something like generic smirk was in the air... B^>

Rgds

Damon

I mentioned it earlier in this thread, Colin, as it does indeed (IMHO) make some low frequency noises more irritating. It's not just surfaces, either, as the volume of air in a room can itself resonate if driven via a controlled leak (ventilation source) to the room.

At a slight tangent to all this, you might all find this work from Caltech interesting: suggesting that much higher power density per m2 land area can be had with lots of small (~10m) VAWT than a few big (~120m) HAWT despite each turbine being less efficient. No idea if this is noisier or not, but it's certainly less visually intrusive.

This is the full hour lecture which explains the numbers and research: http://www.youtube.com/watch?v=Bc4GRaAyE9c

This is the 4-min potted version without any real sums: http://www.youtube.com/watch?v=cZu-4Plk_5A

If this fellow is right then we could see a big change in the way large windfarms are constructed sometime over the next decade.

Perceived wisdom is that bigger and higher is better for windfarms though because of the surface area being a square law and the power output being a cube law. Though this does depend on how you calculate 'efficiency'. If it is a fraction of the annual windspeed distribution or the land area used, the sums are different.
Efficiency is often misused and compared to 'other efficiencies' on a like for like basis.

Thankfully though, we all know what we mean

What should really be happening is that once a suitable site is found, rather than put several small turbines on it, one really large one should be put there. Trouble is that the public do not like big, they prefer little and often. Probably because they fell asleep during maths and science lessons and started dreaming of being an X-Factor star (think I read something about this yesterday on the front cover of the Sun)

Yes, all those other factors skew it dreadfully.
I think it is a 'scaling' problem but not sure if there is an adjusting factor for density. I suspect that it is not hard to work out and compare.

Take a domain of 100m by 100 meters and place turbines with optimal spacing for the windspeed and direction distribution and compare with a single turbine with 50m blades sited in the middle. Easy stuff that.

Then add in the limitations of safety and noise.

How on earth can you make the sweeping statement "investment in wind farms is something of a fraud"?

That is an extraordinary claim and requires extraordinary support.

Rgds

Damon

Is it a re-write of the high cost, low production, need for back-up, not investing elsewhere, blah, blah, blah argument.
Well I am going to make an incredible claim. Every Dane I have met likes saunas (only time I have met one was in one and we were all naked). But I do know someone that worked for Vesta and he has never claimed that a community turbine will solve all the energy problems that the world is faced with.

Pick any technology and I will find you an expert that disagrees with the consensus view point.

Off to feed the Unicorns Sprouts now as we all know that is really the only solution

Steamy - did you watch the vid? It explains why the conventional wisdom is wrong. It concentrates on individual turbine efficiency measured in terms of energy extracted per swept area. The point is that given an area of land and a wind resource that's the wrong thing to optimise. What you want to optimise is total energy output per m2 of land. That's what they modelled and then tested.

Wilko, wind turbines are not 'a fraud' - that's a foolish thing to say (and yes I have read your link, which is little more than a long selection of cherry-picked and often dubious stats). I can post plenty of others that show why it's nonsense. This is a good one for a start:
http://www.eurotrib.com/story/2009/5/1/174635/6513
And the whole series here is very informative: http://www.eurotrib.com/story/2008/6/5/172819/2079

Read that lot and compare with Mr Rosenbloom.

Wind turbines are the most cost-effective renewable energy we have available. Yes, insulation is more cost-efective than generation given the UK building stock, but that applies to all forms of generation, not just wind. Nothing like enough effort is spent on getting our millions of badly-insulated houses done, and the green deal is a fairly feeble step in the right direction. Have you done yours?