Scientists finally unravel mystery of why kettles whistle

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LONDON (TIP): An Indian aero acoustics scientist has finally worked out why and how a kettle actually whistles – a problem which has puzzled scientists for more than 100 years. A team of British researchers from Cambridge University claim to have solved the conundrum and in the process developed the first accurate model for the whistling mechanism inside a classic stove kettle. Using the knowledge gained from the study, researchers now say they could potentially isolate and stop similar, but far more irritating whistles – such as the noise made when air gets into household plumbing or damaged car exhausts. It may come as a surprise but in all the years that people have been brewing tea, no-one has ever quite been able to work out why kettles whistle. The physical source of the noise and the specific reason for the whistling sound have both remained elusive until now. Two researchers – Dr Anurag Agarwal, a lecturer in aero acoustics at Cambridge and his student Ross Henrywood identified the source of the sound itself and have also been able to pinpoint two separate mechanisms, which not only create the sound but specifically cause a kettle to whistle, rather than making the rushing noise a flow might create in other household items, such as a hairdryer.

A basic kettle whistle consists of two plates, positioned close together, forming a cavity. Both plates have a hole in the middle, which allows steam to pass through. Although the sound of a kettle is understood to be caused by vibrations made by the build-up of steam trying to escape, scientists have been trying for decades to understand what it is about this process that makes sound. As far back as the 19th century, John William Strutt and author of the foundational text The Theory of Sound, was trying to explain it. In the end, he posited an explanation that Henrywood and Agarwal have proven to be flawed. Henrywood and Agarwal started by making a series of slightly simplified kettle whistles then tested these in a rig in which air was forced through them at various speeds and the sound they produced was recorded. This enabled them to plot the frequency and amplitude of the sound, and the data was then subjected to a nondimensional analysis, effectively a set of calculations using numbers without any units, which allowed them to identify trends in the data. Finally, they used a two-microphone technique to determine frequency inside the spout. 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.

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