This might shock you, but for over a century scientists have been pondering why kettles whistle -- and completely failed to find an answer. That's all changed now though, thanks to two scientists from the University of Cambridge who have worked out how it happens.
The whistle in a kettle is created when steam passes through two plates, positioned close together, each with a hole in them. But scientists have been trying and failing for decades to understand exactly what it is about this process that makes the high-pitched sound.
Ross Henrywood and Anurag Agarwal used insights gained from analysing noise creation in jet engines to try and answer the question. By analysing the flow of steam which travels up the spout of the kettle, they were able to pinpoint what creates the whistle.
Their results, which are published in the academic journal The Physics Of Fluids, show that the sound is produced by small vortices -- regions where the steam swirls -- which at certain frequencies can produce noise. 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.
Which is fascinating. But it could also prove useful, because the knowledge gained from the study could help other scientists and engineers find and stop other similar -- but more annoying -- noises. Henrywood explains:
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.