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Toroidal vortex gun

Make a toroidal vortex 'gun' that shoots doughnut-shaped vortices you have to see to believe.

Toroidal vortex

Toroidal vortex (Source: Sleek Geeks)

Mathematicians call the shape of a doughnut a 'toroid'. Physicists call a swirling fluid a 'vortex'. A toroidal vortex, then, is a swirling doughnut of fluid.

This video from the Sleek Geeks archive (above) shows that toroidal vortices are surprisingly easy to make and, once formed, are incredibly stable. They're also heaps of fun and you can even use them to shoot Styrofoam cups off your mates' heads.

Here's how to make big, beautiful vortices with an old rubbish bin, a sheet of plastic and a fog machine. If you don't want to go to this much trouble, can still make miniature toroidal vortices the same way using a cardboard tube, some cling wrap, and the smoke from a blown-out candle, too.

To make a jumbo toroidal vortex gun, sacrifice an old plastic garbage bin and cut a round hole out of the bottom. The size of the hole is not critical but about half to a third of the diameter of the base works well. You can still use the bin afterwards by using a bin liner. (Alternatively, cut a round hole in one side of a big cardboard box, and a much larger hole on the opposite side, and cover the large hole with a sheet of plastic).

Cover the open end of the bin with a loose sheet of strong plastic film such as the kind a new mattress comes in (clear plastic is great but any kind will do). Secure with packing or gaffer tape. Add a tab of tape to the middle of the plastic so you can pull it in and out.

You're ready to start blasting Styrofoam cups of volunteers' heads. Ask them to stay perfectly still.

Pull the plastic sheet out, take aim, and then gently hit the plastic sheet. An invisible toroidal vortex will shoot out of your 'gun'.

The volunteer will feel a strong puff of wind and the cup will go flying.

To make the toroidal vortices firing out of your 'gun' add some fog from a fog machine. Alternatively, you could use a beekeeper's smoker.

NOTE: make sure you have isolated any smoke detectors if you are shooting your toroidal vortices indoors!

Fill the 'gun' with fog or smoke, fire it again, and this time the room will fill with 'oohs' and 'aaaahs'. The spectacular toroidal vortices are now clearly visible. A gentler tap on the 'gun' produces a slower fog ring in which the swirling is toroidal clearly visible.

Each tororidal vortex drags a trail of fog or smoke through the air until the ring itself contains hardly any fog at all.

The addition of fog makes aiming your 'gun' much easier. If you miss, you'll know how to adjust your aim for the next shot.

The force of the puff is surprisingly powerful, even when the vortices are moving slowly. The sensation is hilarious so make sure you get someone to fire a couple of fog rings at you too.


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What's going on?

Force a large volume of just about any fluid through a small round hole and you will almost always form a toroidal vortex.

Amazingly, you can blow toroidal vortices made of air underwater, too and you'll see them in action in this Catalyst video. Or, you can make tiny ones with a drop of food dye in a glass of water with this nifty trick.

The scariest toroidal vortices are the mushroom clouds formed by nuclear explosions which form spontaneously from the rising ball of heat. You can't see the swirling doughnut in a mushroom cloud because it's obscured by all the dust and smoke, but that's exactly what's lurking inside.

Even volcanoes, like Mount Etna, are known to occasionally blow huge smoke rings, which must have scared the living daylights out of our ancestors!

So why do toroidal vortices form so easily, and why are they so incredibly stable? The secret lies in the surprising fact that moving fluids exert lower pressure on their surroundings than 'still' fluids. To understand why, you need to know a few basic, but amazing facts about the molecules in fluids. Physics students learn about this in a subject called 'the kinetic theory of gases' and that name will soon make sense.

Consider the still air inside a nice warm room. Those molecules are far from 'still'. At room temperature, they're jittering and jostling about at average speeds around 1500 kilometres per hour. Whack a heater in the room and they'll start moving even faster. Turn the heater off, and they'll slow down again.

If you're wondering where the air molecules on Earth get their zippiness from, the answer is the Sun. The rotating Earth is bathed in its electromagnetic radiation (light), and some of it is absorbed by air molecules, which makes them move. In other words, electromagnetic energy is absorbed by air molecules and is transformed into kinetic (moving) energy. Turn off the Sun, and the kinetic energy would slowly dissipate, the air would cool down, and we'd end up with oceans of liquid oxygen and nitrogen but they, too, would cool down and eventually become solid. But don't panic because the Sun still has billions of years left before it peters out and becomes a red giant, so the Earth won't be cooling down any time soon.

Back to those warm, jostling air molecules. They're zipping to and fro at phenomenal speeds, and therefore, each have a tiny bit of kinetic energy. In a room full of air, though, they can't go very far before colliding with another air molecule and bouncing back in some random direction. On average, they only travel a few millionths of a centimetre bumping into another molecule and this happens millions of billions of times per second.

Now consider the walls in a room full of nice, warm air. Air molecules continually collide into it at the same astonishing rate of around ten million billion collisions per square centimetre, per second. And it's the force of all those collisions we feel as air pressure.

Each molecule only has a tiny bit of mass, which, measured in kilograms, is a tiny number with 27 zeros before the decimal place. But with millions of billions of them bashing into every square centimetre around you, the force of all those tiny collisions adds up to the mind-boggling pressure of 10,000 kilograms per square metre. You don't notice air pressure until it changes, but you are submersed in it. The first place you feel a change in air pressure is in your ears because the pressure behind your ear drum stays the same. A sudden drop in external air pressure causes the ear drum to bulge out a little and a rise in pressure pushes it inwards.

It's a long story but if this is all making sense, then you're not far from understanding why those swirling doughnuts are so stable.

When air starts moving along a surface due to some disturbance (for example a rotating fan, or you blowing through a straw) the air molecules sweep along it. They all still have the roughly the same total amount of kinetic energy, but they now have slightly more of it in the direction of the 'wind' and slightly less in the downward direction onto the surface. As a consequence, the pressure on the surface is now slightly lower than it is when the air is still.

Similarly, the jet of air blasted out of a straw when you blow through it exerts a bit less pressure on the surrounding air perpendicular to the jet than still air does. As a result, the surrounding still air squeezes and confines the jet. You don't normally notice this because air is invisible.

A swirling doughnut of air also exerts lower pressure on the still air around it and, so, the higher pressure air surrounding the doughnut 'squeezes' and confines the doughnut, which helps to maintains its incredible shape.

Tags: preschool, physics, play-and-learning

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Published 17 July 2012

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