They disrupt the trailing edge eddies - which is what causes any noise - and so make them quieter. They are often fitted in areas where sound might be a particular issue.
this post was submitted on 16 Jun 2023
78 points (98.8% liked)
UKCasual
6073 readers
2 users here now
A friendly place to chat.
No politics please. Don't be a dick.
founded 1 year ago
MODERATORS
I can't hear the word eddies without thinking Hitchhiker's Guide
"Eddies," said Ford, "in the space-time continuum."
"Ah," nodded Arthur, "is he. Is he."
So why don't my CPU/case fans have serrated edges as well? Because they are too small?
Because they are too small?
Could be. As is said so often: physics doesn't scale. Trailing edge eddies may not be the major source of the noise in this anyway - since these are pushing air not being pushed by it.
Yeah that makes sense I guess.
They are surrounding a farm with hunners of cows in multiple fields, so I wonder if the noise is quite scary for them, hence efforts to make them quieter. Or the farmer insisting. Are they usually quite loud?
Trailing edge you say? So not designed to mince up birds, that's a relief.
Oh they’ll still mince birds, just with the blunt side
Most of the bird deaths are not actually physical strikes - it is a result of the massive change of air pressure as the blades pass at speed. Evidently painting one of the blades black - or some contrasting colour - keeps a lot of the birds well away.
do they pass out? does it affect their breathing? this is very interesting to me
I realise that I had mis-remembered this. It is primarily bat deaths that are caused by the air pressure changes around turbines - not birds. This is because bats have a particularly large and thin lung internal membranes which gives them high efficiency. The rapid decompression in the immediate wake of the turbine blades - particularly towards the tips, which are the fastest moving part of course - damages this membrane.
Birds have more compact lungs and hollow bones which aid in their breathing - a different solution to the efficiency problem and one that is not as susceptible to these pressure changes.
Unless if they be milling grain those are turbines if I'm not mistaken
Aye but doesn't sound as quaint though, does it?
True, maybe they could be both? They should have little mill yer own grain stations at the bottom lol
I guess a lot of grain mills use electric motors now, so in a way they are.
Huh, TIL. Had no idea they were serrated either.
Helps them chop up birds better
It reduces the turbulence at the trailing edge, reducing noise. Bernouli's principle dictates that part of the force on the blade is due to the airflow on each side moving at different speeds, if the crashed into each other all at once, you'd get a lot of turbulence, and hence noise. The serrations ensure that the two flows meet over a longer distance, reducing said turbulence.
Same reason you see chevrons on the back of some modern turbofan engines
brb
Look at the size of those bad boys!
I'm sure that makes sense for some reason but all I can think of is giant comb.
Is it true they make noise? Or were they not running at the time?
Is it true they make noise?
We have some quite close (a few miles away) and I've cycled right past them. I've never heard anything from them.
Riding my moped past these and I can tell you they make swooping noises, especially if they're aimed 90 degrees to you. These windmills are quite small and old, however, and are quite low. Its a bit intimidating seeing something that massive moving that fast, that close.
They make a rainbow though
Apparently one collapsed recently
There's something oddly disturbing about this fact. I don't like it!
As others have said, mostly about noise reduction. There's actually work looking whether the same concept could be developed for electric aircraft.
The Bird-Slurry 3000 is almost complete!