科学美国人60秒：茶壶为什么会嗖嗖叫?

This is Scientific American — 60-Second Science. I'm Wayt Gibbs. Got a minute?

这里是《科学美国人》的60秒科学，我是韦特·吉布斯。准备好了吗?

British physicist Lord Rayleigh is best known for his discovery of argon and for explaining, in 1871, why the sky is blue. But he also puzzled over this: [sound of a kettle whistling].

英国物理学家瑞利勋爵(Lord Rayleigh)因发现了氩，以及在1871年解释了天空为什么是蓝色而闻名于世。但他也对一个问题感到困惑：茶壶为什么会嗖嗖叫?

Rayleigh knew that a kettle makes that sound when steam jets through the hole in a thick lid that has a gap in the middle. He speculated that the jet becomes unstable inside that gap, setting up an acoustic feedback loop within the gap. But he couldn’t prove it.

瑞利知道，当急速的蒸汽从有夹层的厚茶壶盖盖眼中喷出时，就会发出声响。他推测，气流会在夹层中变得很不稳定，从而形成一个声反馈回路。然而，他没法证明这个推测。

Now two engineers at Cambridge University claim to have solved the puzzle—and proved Rayleigh wrong. The work is in the journal Physics of Fluids. [R. H. Henrywood and A. Agarwal, The aeroacoustics of a steam kettle]

两位剑桥大学的工程师宣布他们解决了这个茶壶盖谜题——还证明了瑞利是错的。这一论文发表在《流体物理学》(Physics of Fluids)期刊上。

The engineers found that a kettle actually whistles in two distinct ways. It starts off with air vibrating in the gap between the layers of the lid, like when you do this [blow over an empty bottle] and this [whistle].

两位工程师发现，壶盖其实有两种截然不同的发声方式——刚开始时，是由于空气在夹层中的上下振动，就像你对着空瓶子吹气和吹口哨时一样。

But as the pressure builds, vortices of steam peel off from the jet exiting the lid. Each vortex creates sound waves at a frequency that depends on the length of the spout and the pressure inside it. Rising temperature means rising pressure, which produces a rising whistle. Which means it’s time for tea.

但当压力不断增大，喷出壶盖的气体会剥离出一股股涡流，并发出声音。声音的频率取决于喷口的长度与其中压力的大小。升温意味着加压，从而会提高音调，也意味着——你可以来一杯热茶了~

Thanks for the minute for Scientific American — 60-Second Science. I'm Wayt Gibbs.

感谢收听《科学美国人》的60秒科学。韦特·吉布斯报道。