Pretty neat! I guess I'm one of those naive folks who would have said that it's obvious it goes in reverse. I also don't see why this would be so hard to just test. Submerge a sprinkler, attach a pump to the hose, observe it moving in reverse as each arm sucks water up, right?
this post was submitted on 02 Feb 2024
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The big question then becomes: "is that behaviour inherent to all systems like this, or just this one?" Like, if you go to the store, buy a basic sprinkler, and then test it and it behaves exactly opposite to how you might expect it to. Or it does something completely unexpected, like phases into another dimension and starts pumping strawberry jam. Your next step shouldn't be to say "Oh, weird, I guess that's that." You'd start knocking down variables. Is it the same with every sprinkler or just this one? Does the amount of suction applied affect it? If I replace the water with something else does the outcome change?"
If you're doing research like this, you're kind of expected to do the same sort of elaboration even if the result of a basic experiment conforms precisely to your hypothesis, because the question isn't if any given sprinkler setup behaves in this way, it's about whether this is a universal phenomenon across all similar setups. Because there's an xkcd for everything, it's this.
There is a difference between observing that the apple hit your head because of gravity and figuring out its number (and why), and how to potentially use it to further science in general.
same
I'm with you. As an engineer, I know a sprinkler works through the transition of potential energy (pressure) to kinetic energy (water jet) and the the law of momentum requires an opposite reaction to the ejected water. For the opposite you would still have the energy of the pump and the momentum of water which must change direction through the flow. OTOH, also as an engineer, I know that there are some effects we ignore or intentionally discount as being insignificant to "real world" applications. Depending on the application, 10% error may be more than close enough, or 0.1% might be. It's rare that anything beyond the third significant digit affects something an engineer would care about, but physicists deal almost exclusively in those fine differences (having worked with them) and weird things really do happen.
Answer:
NYU's Applied Mathematics Laboratory Ristroph et al. found that the reverse sprinkler rotates a good 50 times slower than a regular sprinkler, but it operates along similar mechanisms, which is surprising. “The regular or ‘forward’ sprinkler is similar to a rocket, since it propels itself by shooting out jets,” said Ristroph. “But the reverse sprinkler is mysterious since the water being sucked in doesn’t look at all like jets. We discovered that the secret is hidden inside the sprinkler, where there are indeed jets that explain the observed motions.”
A reverse sprinkler acts like an "inside-out rocket," per Ristroph, and although the internal jets collide, they don't do so head-on. "The jets aren’t directed exactly at the center because of distortion of the flow as it passes through the curved arm," Ball wrote. "As the water flows around the bends in the arms, it is slung outward by centrifugal force, which gives rise to asymmetric flow profiles." It's admittedly a subtle effect, but their experimentally observed flow patterns are in excellent agreement with the group's mathematical models.