In the past few years, the Dyson Air Multiplier has revolutionized the field of recreational air transportation. It's for this reason that I feel it warrants a blog post, all to itself. Lauded for a lack of (visible) fan blades, it is safer than the more common axial-style fan.
The Man Behind the Magic
But how is this trickery pulled off? Allow me to explain.
The Dyson Air Multiplier does, in fact, have typical fan blades. But instead of being open to the air, they are hidden in the base of the fan, and air intake is through the little gratings along the circumference of the tube. That air is then "pumped" through the upper ring and exits the fan.
But then why is it called an "air multiplier"? Dyson claims that the fan outputs 15 times the input air volume, and it does (or at least comes close). It does this as a result of Bernoulli's Principle, which states that faster moving air has a lower pressure. Because the air in the center of the ring is slower, and therefore higher pressure, it tends to get "dragged" along behind the small amount of air that is output, bringing more air into the mix.
Conservation of energy still applies, so yes, the air does get decelerated during this process to account for that. It still does have a fan in the base, which can be noisy in getting the air up to an acceptable speed to "multiply" it. But it is a unique and interesting concept nonetheless, and a mark in the record books for "fan"-atics like me.