Vortex

My lovely kids have been asking about vortexes recently.  Which way does the water spin when it goes down a plug hole? Does this change in different parts of the world?  That sort of thing.

I want to demonstrate the physics involved in an interesting way by experimentation.  Something safe and easy that the kids can do themselves that will keep their interest.  Here is what I’ve got so far from a suggestion from Prof. David Houghton that I found on t’Internet.  I would love to see some more if anyone has any suggestions.  I’d also be interested in hearing of any attempts at this experiment.

Vortex experiment

Equipment

• Two empty clear plastic bottles.  Clean and remove all labels.
• A rubber washer.  It should be bigger than the bottle mouth on the outside but the hole should be small.
• Duct tape.

Method

• Fill one bottle 2/3rds with water.
• Place the washer on top of the bottle so the it sits over the top.
• Put the second bottle onto the washer and tape them together firmly.  The bottles should be mouth to mouth with the washer in the middle.
• Turn the two bottles over so the full one is on top.
• Swirl the bottle round to start the water spinning.
• Set the bottles down in a flat place and watch the vortex form.  One bottle should be standing upright.

Additional Notes

• You can make the vortex more visible by colouring the water or by adding some foam beads or something that floats in the water but doesn’t block the funnel.
• How fast can you get the water to spin?  Does this make the vortex bigger?  How do you think you could record your observations.
• Experiment with washers of different sizes.  What happens for a wide hole or a very small one?  Record your results in a table and think of a way to present them.
• Does the angle that the bottle is at change the vortex?  See what happens when you tilt the bottle after a vortex has formed.  Try different angles and record your results.
• Is there any difference in the experiment if you spin it clockwise or anti-clockwise?  Record your results.

What have you observed in your experiments?  Talk about what you can see as you perform each test.  Try to predict what will happen if you change one thing (the angle of the bottles, the speed of rotation, the washer size) and then test your prediction against the evidence with a new experiment.

What does this show you about meteorology?  On a large scale what have you learnt?  If a vortex forms in a bottle of water and a vortex can form in other fluids (salt water, river water, air, cloud) where do you think you will see vortexes in the natural world?  Armed with this knowledge how could human beings use this information is something like city planning, road building or travel?

Additional Experiments

Take a large mug, a spoon and some ice. Pour some very hot water into the cup and stir it slowly in one direction so the water is rotating slowly.  You could add something to the water to see it better. 

Next place a small ice cube at the centre of the water as it rotates. With a little luck you should see the ice cube start to spin faster and faster until it is turning much faster then the water around it. 

Why do you think this is?*

The answer is below but try to work it out first and then see if you get it right.

*This occurs because the melting ice causes water to sink underneath it.   The cold water draws in some of the warm water from the top edges.  The motion of water coming in from the edges is what spins the ice cube faster.  This is the same principal which causes some tornados and whirlpools in rivers to spin up.  You may have to experiment with the position of the ice cube to make this work well. everyday items?