Tea time

Leave it to the English to solve the mystery of a tea kettle’s whistle.

English physicist Lord Rayleigh proposed in 1877 that water molecules bouncing back and forth in the spout produce the whistle, but new experiments show that little swirls of steam are responsible.

University of Cambridge engineers mimicked a tea kettle in the laboratory using tubing and a series of pressure sensors and microphones. The researchers found a two-step process; steam lazily rising from water that is just starting to boil vibrates within the spout to produce a faint tone, much the way a bottle neck hums when you blow across its mouth.

Then, as pressure builds and a strong jet of steam escapes through the lid’s opening, small vortices of steam break off and create pressure waves in the air that we hear as a high-pitched whistle, Ross Henrywood and Anurag Agarwal report in the October Physics of Fluids. 

The whistle rises in pitch as the speed of the jet grows. In addition to satisfying the curiosity of tea drinkers, Henrywood says the research could help engineers reduce pesky noises caused by fluid rushing through household plumbing and industrial pipelines.