Mystery of the whistling kettle solved!

Researchers, including one of Indian-origin, have found how the kettle got its whistle - a problem which has puzzled scientists for more than 100 years.
In a basic sense, the reasons why kettles whistle are clear, but the physical source of the noise and the specific reason for the whistling sound have both remained elusive.
Now, two Cambridge University researchers 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 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.

Ross Henrywood, from the University of Cambridge Department of Engineering, who is the study's lead author carried out the research under the guidance of his supervisor, Dr Anurag Agarwal, a lecturer in aeroacoustics.
Drawing on previous research by Agarwal, which identified the source of noise in jet engines, the pair were able to show how sound is created inside a kettle as the 'flow' of steam comes up the spout.

The 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.

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.
The study found that above a particular flow speed, the sound itself is produced by small vortices - regions of swirling flow - which at certain frequencies 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.
Researchers also explain why this effect makes a whistle, rather than another noise, by showing that the mechanism is similar to that seen in an organ pipe or flute.
A specific frequency dominates among the sound waves because the note is determined by the size and shape of the opening, and the length of the spout. The longer the spout, the lower the note will be, researchers said.

The study was published in the journal The Physics Of Fluids.