What’s happening?

You are watching fluid dynamics in motion. Fluid dynamics is the science of moving fluids. A fluid is something that flows and takes the shape of its container. Liquids like water, shampoo and volcanic lava are fluids. Even gases are fluids, although we can’t always see them flowing about.
The falling salt water in this activity has a higher density than the tap water in the jar. Having a higher density means that there is more stuff squashed into something; in this case there is salt dissolved in the water. When a fluid of a higher density falls through one with a lower density, interesting things start to happen.
Moving fluids can either be moving smoothly, called laminar flow, or be rough and mixed up, called turbulent flow. When a fluid is turbulent, friction between moving fluids creates amazing turbulent patterns. In this activity you can see some turbulence that is caused by a situation called the Rayleigh-Taylor instability, which occurs when two fluids of different densities coming into contact.
This type of turbulence can make vortex rings, and you can see these tiny donut shapes around the falling saltwater threads. They form as the downward moving water leaves a small area of lower pressure behind it as it falls. The pressure difference between the top and the bottom of the falling water keeps the ring swirling around in a donut shape.
This type of turbulence is seen in many situations, from nebulae in space to mushroom clouds that you might see above volcanoes.

Applications

Fluid dynamics has applications in industries like aviation, manufacturing, mining and food production.
Even our blood is a fluid. CSIRO has used research in fluid dynamics to develop a quick and easy way to mix blood in a blood sample for a pathology test. When researchers do a pathology test on our blood, they need to mix quite a large blood sample with certain compounds to see how the blood reacts. Usually, these samples are physically mixed and shaken, and this takes time. CSIRO researchers have placed tiny bubbles around the blood sample that move with sound vibrations. These moving bubbles mix the blood sample quickly and easily, saving time and allowing smaller blood samples to be used for the pathology tests.

Watch fluid moving like jellyfish in this simple activity that brings fluid dynamics and jellyfish together in the most beautiful thing you might see all week.

What you need

• A large glass jar
• A small plastic container that fits in the top of the jar – a plastic cup works well
• A pin or thumbtack
• Tap water
• Salt
• Food colouring
• A spoon or skewer to stir the salt and food colouring together

What to do

1. Fill the jar with tap water and let it sit on the bench for ten minutes so until the water is completely still.
2. With the pin or thumbtack, poke three small evenly spaced holes in the bottom of the plastic container.
3. Pour salt into the plastic container, until it is around one centimetre deep.
4. Add around ten drops of food colouring to the salt and stir until it is evenly coloured.
5. Place the plastic container into the jar so the bottom is touching the water. Make sure there are no air bubbles between the water and the bottom of the container.
6. Watch as the salt and food colouring trickle through the holes and into the water. It may take a minute or so for this to start to happen. Can you see the threads of salty water? Can you see small coloured rings around the thread?

	What you need.
	Poke holes in the bottom of the plastic cup.
	Place the cup in the top of the jar.
	Watch the stream of salty fluid fall.
	Can you spot the rings?