Mathematicians have just reversed the machine gun

For 141 years, physicists like Richard Feynman have been puzzling the question of fluid flow: How could a sprinkler spin if it was underwater, sucking in matter rather than spewing it out? Now, a team of researchers has found an answer.

Although the idea was first proposed by experimentalists in the 1880s, Feynman republished it in the mid-20th century, to the point where it became known as Feynman machine gun. The problem was this: A regular sprinkler with S-arms would spout water, causing the arms to rotate, watering whatever needed watering. But whether the reverse sprinkler would spin at all remained an open question, and not for lack of trying.

Feynman used this idea for some time and even An experimental group was built to address the question while he was a graduate student at Princeton University. (The experiment ended when a large bottle filled with water exploded.)

Now, a team of researchers at New York University has rebooted it. The backspray system (as I call it – it rolls right off the tongue!) consists of a sprinkler submerged on an “ultra low friction” bearing, According to a statement from New York Universityto improve the ability of the device to rotate freely, and is designed in a way that allows them to easily monitor the flow of water through the device.

To that end, the team also dyed the water, added microparticles to it, illuminated it with a bright green laser, and videotaped the entire experiment using high-resolution, high-speed cameras. The resulting shots are pretty trippy:

Team research-published Last week in Physical Review Letters – I found that the backwash does just that. It rotates in the opposite direction to the sprinkler that spews water.

“A regular or 'front-end' machine gun is like a missile, because it propels itself by firing jets,” said Lev Ristrov, a researcher at New York University and lead author of the study at the university. launch. “But the reverse sprinkler is mysterious because the water being sucked in does not look like jets at all. We have discovered that the secret is hidden inside the sprinkler, where there are actually jets that explain the observed movements.”

Although the reverse sprinkler rotates (see notes) in the opposite direction, it does so at about 1/50 the speed of a regular sprinkler. Inside the reverse sprinkler, the absorbed water jets collide with each other, but not directly, and the mixing of that internal water causes the sprinkler to rotate slowly.

Brennan Sprinkle, an appropriately named researcher at the Colorado School of Mines and co-author of the study, added that the methods used in the experiment “will be useful for many practical applications involving devices that respond to air or water flow.”

Whatever the potential applications, the quandary that first arose in the 1880s has finally been answered, with precision testing and modeling that would not have been possible 140 years ago. I hope this gives you something to add to your next fluid dynamics talk.

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