Enter Leif Ristroph and colleagues, who built their own custom sprinkler that incorporated ultra-low-friction rotary bearings so their device could spin freely. They immersed their sprinkler in water and used a special apparatus to either pump water in or pull it out at carefully controlled flow rates. Particularly key to the experiment was the fact that their custom sprinkler let the team observe and measure how water flowed inside, outside, and through the device. Adding dyes and microparticles to the water and illuminating them with lasers helped capture the flows on high-speed video. They ran their experiments for several hours at a time, the better to precisely map the fluid-flow patterns.
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.
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Source : https://science.slashdot.org/story/24/02/03/0220248/mathematicians-finally-solved-feynmans-reverse-sprinkler-problem?utm_source=rss1.0mainlinkanon&utm_medium=feed