>I don't know how well the rotating aluminum impeller breaks-up the boundary layer surrounding the fins. It won't eliminate it completely but it certainly could be much improved from the case of stationary fins with low-speed air going across them. If you had a heatsink with stationary fins and air moving at the same speed as in the case of the rotating impeller (by using fans, of course) you'd probably get similar boundary layer effects.
I think the point is that it's a lot easier to get a piece of solid metal moving very quickly with little noise than to get air to do the same.
It has to get that air moving at comparable speeds to be as efficient as the traditional way. The trick here is to get the air move faster with less sound. It's possible because of larger airfoils, so they can get more laminar flow and with more laminar flow comes less sound.
Right, if you were building a wing. The problem is that here we are after heat exchange and, as I understand it, the last thing you want is laminar flow due to the accompanying boundary layer.
I think the point is that it's a lot easier to get a piece of solid metal moving very quickly with little noise than to get air to do the same.