Fantastic Fluid Flow

Take a beaker of water and a beaker of golden syrup and turn them upside down. Aside from making an enormous mess, you will notice that very different things are happening to each one. The water empties quickly in an almighty splash, whereas the heavier, stickier golden syrup slowly pours out in a stringy puddle.

Why Does This Happen?

To explain this behaviour, we need to look at fluid flow. A ‘fluid’ is generally a word used to describe a liquid, but it in fact describes everything that flows, be it liquid, gas, or even some varieties of solid. Unlike firm solids, they cannot support stress and thus deform under pressure.

The way a fluid flows depends on what it is made of. It is important to understand different types of fluid flow. For instance, when building a plane, the engineer needs to know exactly how the surrounding air will interact with the vehicle.

Vehicle designers also put a lot of attention into ‘streamlining’ their products. This means shaping them in such a way as to minimise air resistance, allowing air to travel smoothly over them as they travel through it, with as little turbulence (unstable, chaotic flow) as possible.

But fluid dynamics isn’t all about cars! Even the the humble cup of coffee provides a fantastic example of fluid flow. Have you ever tried walking with a freshly made cup of coffee, only to find it spills out of the edges after you’ve walked a few steps? What actually happens is that the coffee, as a fluid, moves in sync with your body movements. As you continue to walk, the coffee sways more and more, increasing with each stride, because it is being moved at it’s resonant frequency. Although resonance is something we normally investigate in relation to vibrations, it also applies to fantastic fluids!

Let’s Get Non-Newtonian

We couldn’t talk about fluids without mentioning non-Newtonian fluids. These specially named fluids aren’t as strange as you might think – it’s just a fancy name for fluids which can act like both liquids and solids. The classic example, which we use in a lot of our science busking, is just simple custard, made from cornflour and water.

Becky's hand covered in slimeCustard, as you may know, is a liquid. It pours out of a jug and falls of a spoon. Custard has a high level of ‘gloopiness’ – or as we scientifically, viscosity – which means it doesn’t flow very well. This is just the same as the syrup and water that was discussed at the beginning of the article. Now, while custard is a liquid, it can be transformed into a solid under pressure. If you hold custard in the palm of your hand and squeeze it, it will turn solid like a ball of dough. As soon as you let go, it becomes a liquid again. Some people have even filled paddling pools with custard, and using the pressure of their own weight, they can walk on custard!

Scientists and engineers are looking at finding useful applications for such non-newtonian fluids, such as in armoured vests. Imagine a police vest which acted like a liquid, and wasn’t restricted, yet would become really solid when an object tried to pass through it. The future of fluid dynamics could potentially be life saving!

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