Took the weans a walk away up the back of The Three Towns in Ayrshire where they are installing these big bad boys.
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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
Helps them chop up birds better
As others have said, mostly about noise reduction. There’s actually work looking whether the same concept could be developed for electric aircraft.
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.
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.
So why don’t my CPU/case fans have serrated edges as well? Because they are too small?
Some do, e.g. Noctua: https://noctua.at/en/vortex-control-notches
