The news the other day was filled with the news of James Dyson's new bladeless fan which costs an arm and a leg (seriously, £199.99 for a desk fan!?!). There are a number of interesting things about this fan.
First, it appears to have no blades. It's just an annulus (or a ring) with a big space in the middle. Using Dyson Magic (tm) air magically flows through the ring.
Well, actually this thing does have blades which are hidden in the base and air is sucked in there and blown upwards into the ring. Dyson calls this part an impeller because instead of being a fan it's, well, an impeller: basically a pump enclosed in the base.
Impellers themselves are rather neat because if they are well made they are efficient and they can exploit the Venturi effect to get lots of flow.
But the really interesting thing about the Dyson Air Multiplier is how it gets so much air in your face. The air coming up from the base is directed into the ring and exits through a slot that runs around the back of the ring and faces forward.
This causes air to be blown towards you touching the ring itself. The ring is shaped like an airfoil and the blown air will 'stick' to the airfoil shape. Then the Dyson Magic (tm) happens: the Coandă Effect causes additional air to be 'sucked' into the flow (this is called entrainment).
This is where Dyson gets his 'multiplier' word from. The Coandă flow of air generated by his impeller coming out of the slot entrains the surrounding air causing it to flow.
The Coandă Effect is very well known, and Dyson hasn't invented something revolutionary here (although I assume the engineering is very good). The effect has been used on aircraft and experimental flying saucers. Here's a lovely YouTube video of a Coandă Effect flying saucer. The thing to realize when watching this is that the air flow causing this to float is going over the surface of the outside of the craft.
I'd probably pay £199.99 for one of those, but not for a fan.
PS If you want you can read Dyson's patent that makes all this clear.