Kettles Stop Whistling in the Dark / 开水壶停止在黑暗中吹口哨

British physicist Lord Rayleigh is best known for his discovery of argon and for explaining, in 1871, why the sky is blue.

英国物理学家瑞利勋爵最为人所知的是他发现了氩，并在1871年解释了为什么天空是蓝色的。

But he also puzzled over this.

但他对下面这件事也感到困惑：

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.

瑞利知道，当蒸汽从水壶厚盖子中间的孔中喷射出来时，水壶就会发出这种声音。

He speculated that the jet becomes unstable inside that gap, setting up an acoustic feedback loop within the gap.

他推测蒸汽在缝隙内变得不稳定，产生了一个声学反馈回路。

But he couldn't prove it.

但他无法证明这一点。

Now two engineers at Cambridge University claim to have solved the puzzle and proved Rayleigh wrong.

现在，剑桥大学的两名工程师声称已经解决了这个难题，并且证明了瑞利的猜测是错误的。

The work is in the journal Physics of Fluids.

这项研究发表在《流体物理学》杂志上。

The engineers found that a kettle actually whistles in two distinct ways.

工程师们发现水壶实际上能发出两种不同的口哨声。

It starts off with air vibrating in the gap between the layers of the lid, like when you do THIS and THIS.

第一种口哨声由于空气在水壶盖子间隙中振动而产生，就像你做这个和这个一样（听音频）。

But as the pressure builds, vortices of steam peel off from the jet exiting the lid.

但是随着压力的增加，蒸汽漩涡会从壶盖喷出。

Each vortex creates sound waves at a frequency that depends on the length of the spout and the pressure inside it.

每个涡流产生声波的频率取决于喷口的长度和内部的压力。

Rising temperature means rising pressure, which produces a rising whistle which means it's time for tea.

温度的升高意味着压力的升高，这就产生了另一种哨声——这就意味着水开了，可以喝茶了。

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本文节选自《60-Second Science American》