As the summer holidays come to an end, so does our 2021 STEM Summer Challenge. With another fun experiment to try, Neil Downie shows how the Bernoulli effect creates a vacuum through a blast of air from a balloon or hairdryer.
STEM Challenge #54: How Blow Can Suck, Bernoulli Roundabouts and Hoversuckocraft on Your Ceiling
“P + ½ ρ.v2 is a constant” – Daniel Bernoulli
You find this little equation in things everywhere. And one of the far from obvious results is that blowing can suck.
You may have seen a small hovercraft made from a balloon and a CD. They’re easy enough to make: you take a CD, stick a piece of plastic tubing on it, on the side with the rim for the balloon. Blow up a balloon and hold the neck while attaching it to the CD. Release the balloon neck and the air will rush out from the center and it will float and glide beautifully over a smooth table.
But try this now. Check that your ceiling is smooth, then try your small hovercraft upside down on the ceiling. That’s crazy, isn’t it? It will blow itself off the ceiling and fall to the floor faster than if you just let it go under gravity, right? New. It sucks itself up but then floats just below the ceiling. It will slide under the ceiling for a few seconds until it runs out of air and then it falls. Try it and you will see. (And if your ceiling isn’t smooth, look for something like a table that’s smooth underneath.)
Why does this ‘hoversuckocraft’ work? It all comes down to Daniel Bernoulli and his equation. But first, you need to add another simpler equation: A.v is a constant, which just means air is created out of nowhere as it flows down a pipe. If you get a smaller cross-sectional area A, then you get a greater velocity v.
Now Bernoulli says that if v goes up, P must go down. P becomes negative with respect to atmospheric pressure as it decreases. And if it goes negative, you’ve got a hoversuckocraft. So positive pressures can cause negative pressures: the principle behind many useful things, from flow meters to spray guns.
You could make a motorized version of the hoversuckocraft, but it would be tricky – you would need a powerful lightweight fan. However, there is a simpler motorized demonstration of the Bernoulli equation that we can all do. First find a flat sheet of wood with a cross-section of 25 cm, square or round, and a piece of polystyrene or other foam plastic of a similar size, for example, 20 or 40 mm thick, and a hairdryer. Now make a hole in the wood the size of the hairdryer outlet and somehow join the two together.
Using a cool blowing position, hold the blow dryer vertically downwards and place the foam squarely underneath. It is blown away by the blast of air when held at a distance. But once it is close enough, the foam is not blown away but sucked up until it is a millimeter away from the wood.
It will also swing sideways if it is the smallest angle to horizontal. You can stop this by sticking a small pin in the center of the foam. Then hold the hairdryer vertically down again and you can rotate the foam 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 current cross-section at the halfway point of the edge may be ~10cm2, while in the hairdryer it may be 20cm2. If the hairdryer has a flow rate of 2ms-1, it will double to 4ms-1.
Pressure reduction = ½ ρ.v2 = ½. 1.5 km-2. 42 between the wood and the plastic → 12Pa
12 Pa pressure over the 0.06 m2 between wood and foam gives 0.72 N, 72 g. Now this estimate is only rough because the pressure varies with radius: the graph below shows the type of variation.
How much negative pressure ‘lift’ can you get? Can you somehow use the airflow to keep the Magic Roundabout running continuously? And finally, if you have enough lift, why not add some carousel horses? Or characters from the famous ‘Magic Roundabout’ animation, such as Dougal, the shaggy dog, Zebedee on his feather, or Dylan, the relaxed rabbit.
If you liked this, you’ll find plenty more science fun stuff in Neil Downie’s books, like Princeton University’s “The Ultimate Book of Saturday Science,” and a host of other things (and a free copy of the book “Exploding Disk Cannons’ ), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new breathing system to support people with breathing difficulties – learn more about this great project here: Exovent.org.