this post was submitted on 16 Jun 2023
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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.
I can't hear the word eddies without thinking Hitchhiker's Guide
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?
So why don't my CPU/case fans have serrated edges as well? 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.
Some do, e.g. Noctua: https://noctua.at/en/vortex-control-notches
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.