There is a Skystream wind turbine outside my lab that has very fancy profiles to reduce noise. We hear a lot of high frequency noise from some fans where the edges are blunt, there are fancy things that can be done to reduce the sound they make. An unducted fan spills most of this of the ends of the blades in a radial direction, the air that has angular velocity as a bulk property in a ducted fan quickly loses the rotation when clear of the duct.
The momentum imparted by the blades has no rotational effect its direction only according the resulting vector of the air before it is effected and the angle and instantaneous direction when the blade interacts with the air - pushes the air.
There isn't even a rotational motion any distance from the fan as we easily intuit. If the air simply moved constantly with no loss, there would be no sound at all but instead you have edge effects, the center velocity is slower than the outer - until they blend, there are vortices shed, etc., each effect creating its own characteristic noise and they all bend together. If the blades move at a constant speed and the sound that is made is very complicated. The rotating blades don't really work as separate entities they create a bulk pressure a difference and the air moves to equalize the difference. When peaks coincide in the same direction they are additive in amplitude, when they are opposed they cancel out, and they all dissipate according to the medium and environmental conditions. Sound waves are merely superimposed - in all cases.
Click to expand.I take exception with this.
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