Consider a sprinkler system with S-shaped arms. The water flows out and the fountain moves; So far everything seems simple enough. Now imagine the opposite scenario: Your sprinkler system is submerged and sucking in water. The question physicist Richard Feynman asked was: In which direction does it spin? We now have an answer that demonstrates the complexity of fluid motion.

If you have a simple and clear idea of ​​how to behave, you are in good company. Feynman believed that people would either be in the reverse rotation camp or the same rotation camp, with reasonable logic or how this would work. Experiments since 1985 (then the book “You must be kidding, Mr. Feynman!») is rather indeterminate, exhibiting counter-rotation, non-stationary rotation reversing direction, and motion that depends entirely on the geometry of the system. This is a big mess.

Recent studies aim to provide a global understanding of the mechanics of the system. Thanks to a precise experimental setup and consistent simulations, the team solved the puzzle. The sprinkler changes direction, but this movement is irregular and much slower. Therefore, reversing the flow of water in a sprinkler system is not the same as seeing the system backfire.

The first step to understanding the problem is to submerge the sprinkler in water and let it spin. This should happen with as little friction as possible in both directions. In standard forward motion the sprinkler is driven by a jet engine. In the reverse version, the sprinkler is still driven by a jet engine, but the average rotation speed is about 50 times slower.

The opposite approach is still confusing if you can’t keep track of what’s going on inside the sprinkler. As a result, the inward flow should be damped and not create a net torque. The team used dyes and light to monitor the flow’s behavior. In the front housing, the sprinkler moves nicely as water flows through the S-shaped arms.

To help visualize the internal behavior, the inverted fountain-shaped arms, which remain stationary in the video above, push the water slightly off center, creating a small but measurable movement. The asymmetric flow gives rise to the strange profiles observed in various experiments.

“A traditional or ‘front’ sprinkler is like a rocket in that it moves to launch jets,” said senior study author Leif Ristroff of New York University. “But the reverse jet is confusing because the water being sucked in is not jet-like at all. “We discovered that the secret lies within the fountain, where jets account for the observed movements.”

There is no need for sprinklers that absorb water, but applications for devices whose flow may be similar now have reliable simulations that can be relied upon. While this is specific to water, its mechanics are common to liquids.