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Guess what? For more than a century, scientists have been wondering why kettles whistle. That's right. And we all thought it was part of the design. Thankfully, that has finally been answered. Thanks to two scientists from the University of Cambridge that is.

The whistle is created when steam passes through two plates, positioned close together. Scientists have been trying and failing for decades to understand just why this process makes such a high pitched sound.

Ross Henrywood and Anurag Agarwal used insights gained from analyzing noise creation in jet engines to try and answer the question. They analyzed the flow of steam which travels up the spout of the kettle and were able to pinpoint what exactly creates the whistle.

The researchers published their results in the journal The Physics of Fluid, which is that sound is produced by small vortices - regions where the steam swirls. They are able to produce noise at certain frequencies. They explain:
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.
Henrywood adds:
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. Pipes inside a building are one classic example and similar effects are seen inside damaged vehicle exhaust systems. Once we know where the whistle is coming from, and what’s making it happen, we can potentially get rid of it.
[The Physics Of Fluids via University of Cambridge Image by niseag03 under Creative Commons license]