[aerohead]

Originally Posted by Otto

Yes. the sawtooth edge improves on the ducted fan, where the fastest inner core is buffered from the ambient air, reducing the shearing effect between layers, which causes the noise. The sawtooth makes for micro-vortices that to some extent cancel each other out, as the upside flow from one micro vortex impinges on the downside flow of the next.

Where could I get a look at the Soderman paper?

The owl, btw, uses his serrated edge feathers more for noise reduction than for most efficient lift. That's why raptors fly faster, though more noisily--they get breakfast by dint of speed and power, vs. being sneaky.

Otto,if you can't find the paper through GOOGLE,as CobraBall recommended,I'll dig it out and mail you a copy.Might take a little time to find it but I'm pretty sure I can put my hands on it.