[bobkoure]

Originally Posted by JWFokker

So you're saying slapping two fans together that happen to counter rotate won't help?

Yup, that's what I'm saying. I might even be right - at least as far as the noise part goes.
It'd be interesting to get one of these SD units and dissect it - maybe there'd be an obvious mod we could make to the blades of the "secondary" fan.
I don't claim to understand axial fans other than that they behave like a wing - so angle of attack and top of blade curvature (coanda effect) are relevant - and they behave a bit like propellers and helicopter rotors in that the outer circumference is going faster than the inner, so the wing needs to be different between inner and outer sections.
If you've seen the air coming off a wing (like from an airplane flying just over clouds - so it's visible) the air doesn't go straight down, but down and forward (same direction as the wing is going) and loops in a vortex parallel and below the wing trailing edge. I would guess that air flow coming off a rotary wing is a lot more complicated than that. Anyway, I wasn't at all surprised that stacking counter-rotating fans got unpleasantly loud. In retrospect I guess I should have installed them anyway, just to see what sort of airflow I was getting (my airflow gauge is my temp reading - really inaccurate but at least something) but I figured it was a lost cause noise wise right away and didn't go any farther.
Please remember that I tried this once. If you've already got some counter-rotating fan pairs, by all means slap 'em together and see what happens. I'd suggest using a couple of fan controllers and varying speed of the two fans individually - and if you've got a way to separate them with some sort of plenum, that'd probably be worth trying as well.
I would think we've already got data on stacked fans with a "flow straightener" (like in multi-stage compressors). The straightener we've been using is called a "radiator" - and we've been calling those experiments "push-pull" - which got no noise/performance gains for me. I'm now curious about push-pull with counter rotating fans. I expect the same non-improvement, but that's a guess.