Why the kettle whistles

For more than 100 years, scientists have been puzzled as to why a kettle on the stove whistles – what is the physical source of the noise and the specific reason for the whistling sound?

Two University of Cambridge researchers claim to have solved the problem, and in the process developed an accurate model for the whistling mechanism inside a classic stove kettle.

But the research – published in the Physics of Fluids – has much wider ramifications.

“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,” said Ross Henrywood, lead author.

“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.”

[caption id="attachment_36284" align="alignright" width="300"] Why a kettle whistles Credit Ross Henrywood[/caption]

Together with Dr Anurag Agarwal, Henrywood showed how sound is created inside a kettle as the ‘flow’ of steam comes up the spout. Having identified the source, they pinpointed two separate mechanisms which not only create the sound, but specifically cause a kettle to whistle.

A basic kettle consists of two plates positioned close together forming a cavity – both with a hole in the middle to allow steam through. The pair created a series of simplified kettle whistles and tested these in a rig in which air was forced through them at various speeds and the sound produced recorded.

Results showed that, above a particular flow speed, the sound itself is produced by small vortices which at certain frequencies can produce noise. The mechanism is similar to that seen in an organ pipe or flute.

Kettles below the flow rate at which the vortices emerge will still whistle. Just as the water begins to boil, an entirely different mechanism also makes a sound – the difference was that the tone at the stage was fixed at one frequency.

“The fixed frequency was intriguing and not something that we had expected to see,” Henrywood said. “We eventually established that below a particular flow rate the whistle behaved like a Helmholtz resonator – the same mechanism which gives you a tone when you blow over an empty bottle.”

“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.”

 The Aeroacoustics of a steam kettle http://scitation.aip.org/content/aip/journal/pof2