Summer STEM Challenge: The Bernoulli Magic Roundabout
Image credit: Neil Downie
As the summer school holidays come to an end, so does our 2021 Summer STEM Challenge. With one more fun-filled experiment to try out, Neil Downie shows how the Bernoulli Effect creates a vacuum from a blast of air from a balloon or from a hairdryer.
STEM Challenge #54: How blowing can suck, Bernoulli Roundabouts and Hoversuckocraft on your ceiling
“P + ½ ρ.v2 is a constant” – Daniel Bernoulli
You’ll find this little equation inside things all over the place. And one of the far-from-obvious results is that blowing can suck.
You might have seen a little hovercraft created from a balloon and a CD. They’re simple enough to make: you take a CD, stick a stub of a plastic tube on it, on the side with the ridge for the balloon. Inflate a balloon and hold the neck while you attach it to the CD. Let go of the balloon neck, and air will whoosh out from the middle and it’ll hover and glide beautifully across a smooth table.
But now try this. Check your ceiling is smooth, then try your little hovercraft upside down on the ceiling. That’s nuts, right? It’ll blow itself away from the ceiling and fall down to the floor quicker than if you just let it go under gravity, right? Nope. It’ll suck itself up but then hover just below the ceiling. It’ll glide underneath the ceiling for a couple of seconds or so until it runs out of air and then it falls. Try it and you’ll see. (And if your ceiling isn’t smooth, look for something like a table that is smooth underneath.)
Why does this ‘hoversuckocraft’ work? It’s all down to Daniel Bernoulli and his equation. But first, you need to add another simpler equation: A.v is a constant, which is just saying that air is created from nowhere as it flows along a pipe. If you get a smaller cross-section area A, then you get a bigger speed v.
Now Bernoulli says that if v goes up, then P must go down. P goes negative relative to atmospheric pressure when it goes down. And if it goes negative, then you have a hoversuckocraft. So positive pressures can make negative pressures: the principle behind lots of handy things, from flow meters to spray-guns.
You could make a motorised version of the hoversuckocraft but it would be tricky – you’d need a powerful lightweight fan. However, there is a simpler motorised demonstration of the Bernoulli equation that we can all do. First find a flat plate of wood 25cm across, square or round, and a similar-sized piece of polystyrene or other foam plastic, say 20 or 40mm thick, and a hairdryer. Now put a hole in the wood the size of the hairdryer outlet and join the two together somehow.
Using a cool blow setting, hold the hairdryer facing vertically downwards and put the foam square underneath. It’s blown away by the air blast when held at a distance. But once it is close enough, the foam isn’t blown away, it’s sucked up until it’s a bare millimetre away from the wood.
It will also whoosh away sideways if it’s the slightest angle to horizontal. You can stop this by putting a small peg in the middle of the foam. Then hold the hairdryer vertically down again and you can set the foam turning without friction. You’ve made a Bernoulli Magic Roundabout!
Does Bernoulli’s theory work? If the space between wood and foam is 1mm thick, the flow cross-section halfway to the edge might be ~10cm2, while in the hairdryer it might be 20cm2. If the hairdryer has a flow speed of 2ms-1, then it will double to 4ms-1.
Pressure reduction = ½ ρ.v2 = ½. 1.5kgm-2. 42 between the wood and the plastic → 12Pa
12Pa pressure over the 0.06m2 between wood and foam gives 0.72N, 72g. Now this estimate is only rough because the pressure varies with radius: the graph below shows the kind of variation.
How much negative pressure ‘lift’ can you get? Can you use the airflow, somehow, to make the Magic Roundabout turn continuously? And finally, if you’ve got enough lift, why not add some carousel horses? Or characters from the famous ‘Magic Roundabout’ animation, like Dougal, the shaggy dog, Zebedee on his spring, or Dylan, the laid-back rabbit.
If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project here: Exovent.org.
There is a back catalogue of STEM-related challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.
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