So, you've read about the Dyson Air Multiplier, and you've looked at the photos, but you're still baffled about how the damn thing works. You've seen vague promises of "Engineering" being responsible (which we've come under criticism for, for some reason), but here's - in gory physics-y detail - roughly what's going on inside Dyson's mysterious new bladeless fan.

The Air Multiplier uses friction in the air to push out its cool breeze. Around the rim of the circular fan is a little opening from which jets a very thin (1mm or so) stream of air at 55 mph. That thin stream of air pulls more air into the stream thanks to the aforementioned friction - Physicists call this process entrainment.

At the same time, the air that gets pushed away from the ring towards your beautiful face creates an area of low pressure - not quite a vacuum, but the same effect - in the ring. That low pressure pulls in more air from behind the machine (filling the gap) which is then in turn drawn into the air stream. That's called inducement.

The engineering challenge, however, is in keeping that stream of air flowing smoothly - balancing how fast the air is travelling (its "inertia") with how thick it is (its "viscosity"). Physicists have an equation which represents this balance, and it results in something called the Reynolds number.

Some quick calculations (using 55mph air flow, a 1mm gap, 1.2 kg/m^3 air density and 18.27 * 10^-6 Pa s viscosity) show that the Air Multiplier has a Reynolds number of around 1615 - relatively low. That means that the air flow out is pretty smooth (or "laminar", as physicists call it) - something seen in this image of air speeds:

 

If all those equations confused you massively, think of a garden hose. If you turn on the tap half way, then the flow out of the hose is smooth - it stays together in a stream when it exits the end of the pipe until it hits your flowerbed. But if you squeeze the hose, making the exit smaller, or if you turn the tap on full, then the water sprays all over the place - much less easy to control. That would have a high Reynolds number.

In a fan, you want a smooth airflow which stays aimed at whatever you point it at for upto 5 or more metres away without going all over the place, wasting energy in turbulence. The Air Multiplier appears to have achieved this with its low Reynolds number.

The "multiplier" part of the name refers to how much efficiency it saves. It sends approximately 15 times more air at you thanks to taking advantage of these physical processes than it actually has to drive through its own internal spinning turbine. Of course, regular fans gain some of these effects, too, though they're more likely to suffer inefficiencies due to turbulence caused by the speed of the blades and the "open" air around it. Dyson's turbine is enclosed in the body of the unit - the "handle" of the magnifying glass, if you will.

So there you go. Of course, we're tech fanatics, not professional physicists, so if we've got something slightly off in the explanation above, and you think you know better, then don't hesitate to drop us a comment below.

• Reviewed - Dyson DC31 handheld vacuum

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