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Bet You Never Knew a Teapot Had a Physics Mystery: Well, It's Just Been Solved

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"...the effect is similar to whistling by mouth."

Put a kettle on a burner and within a few minutes, it whistles, or "shouts" as the popular children's song "I'm a Little Teapot" goes.

Bet you didn't know just that how this teapot whistles was a mystery to scientists -- until now.

teapot whistle (Source: Shutterstock.com)

Cambridge University researchers recently published a paper in the journal The Physics of Fluids, describing what's considered the first accurate model for kettle whistling dynamics.

Think this is trivial research? It actually has more far reaching implications. According to the press release, these dynamics could be used to stop pipes in household plumbing from squealing or car exhausts from sounding, well, exhausted.

“The effect we have identified can actually happen in all sorts of situations - anything where the structure containing a flow of air is similar to that of a kettle whistle,” Ross Henrywood, the study’s lead author, said in a statement.

“Pipes inside a building are one classic example and similar effects are seen inside damaged vehicle exhaust systems," Henrywood continued. "Once we know where the whistle is coming from, and what’s making it happen, we can potentially get rid of it.”

What the fourth-year engineering student found with Dr. Anurag Agarwal, an aeroacoustics expert, were two different mechanisms at work to create the sound and direct its tone.

Here's the press release's explanation:

Their results showed that, above a particular flow speed, the sound itself is produced by small vortices – regions of swirling flow – which at certain frequencies can produce noise.

As steam comes up the kettle’s spout, it meets a hole at the start of the whistle, which is much narrower than the spout itself. This contracts the flow of steam as it enters the whistle and creates a jet of steam passing through it. The steam jet is naturally unstable, like the jet of water from a garden hose that starts to break into droplets after it has travelled a certain distance. As a result, by the time it reaches the end of the whistle, the jet of steam is no longer a pure column, but slightly disturbed.

These instabilities cannot escape perfectly from the whistle and as they hit the second whistle wall, they form a small pressure pulse. This pulse causes the steam to form vortices as it exits the whistle. These vortices produce sound waves, creating the comforting noise that heralds a forthcoming cup of tea.

how kettle whistles This diagram shows the mechanism by which a teapot whistles. (Image source: Cambridge University)

So why a whistle compared to another noise? This is determined by the length of the spout and size and shape of the opening.

The other mechanism discovered is that sound is also produced differently when the kettle comes to a boil. The researchers describe this mechanism as similar to the sound made when blowing over a bottle.

“In a kettle, of course, the air is blown through, rather than over, the neck – the effect is similar to whistling by mouth,” Henrywood said. “In some kettles, both these mechanisms are happening. Because our study enables us to assess the mechanisms in action, we can potentially make modifications to stop the noise – if we want to.”

And there you have it. That's how the teapot got its "shout."

Featured image via Shutterstock.com.

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