The Dust Boundary

While sitting on the couch watching TV the other night I looked up at the ceiling fan my wife had turned off earlier and it was covered in dust. How could that be…? That fan had been going non-stop, on a medium speed, since March. How does it collect dust? It must be Dust Ferries.


For four nights straight I stayed up and sat on the couch in a full camo with my trusty M-16 waiting for these little intruders to show themselves and try and place dust on my fan. I could hear them around the corner and down the hallway waiting for me to nod off so they could deliver their dirty cargo but when I’d creep around the corner they’d be gone….ah the Dust Ferry is indeed worthy prey. After night four I decided to do some research and get to know my enemy and it turns out that my enemy is the laws of fluid dynamics.


So it would seem that the air right on top of your fan blades is moving at the same speed as the fan blade itself. This is called the Boundary Layer and the faster an object is moving the thicker the boundary layer is. Other than the dust not being blown off your fan another practical ramification of this is fighter jets. If you look at the air inlets on most fighter jets you’ll notice that they are not right next to the body of the plane. There is a gap. This is because the designers are trying to get the inlet out of the boundary layer next to the plane. In fact, in some planes, the large inlet surface next to the plane is actually moveable so as the plane increases speed the inlet moves further out to account for the growing boundary layer.

As your fan blade turns it slams into the dust which collects and unless you have a Binford 2000 fan motor which propels your fan from 0 to 1000 rmp in .4 seconds the dust is going to stay. I guess I’ll put up the shotgun and get out the ladder…time to dust the fan.