Jennifer Ouellet reports via Ars Technica: A typical lawn sprinkler has various nozzles placed at an angle on a rotating wheel. When water is pumped, a jet is released, causing the wheel to rotate. But what happens if the water is sucked into the sprinkler instead? In which direction does the wheel then spin, or does it even spin at all? This is especially true for physicists like Richard Feynman. This is the essence of the “reverse sprinkler” problem he has been working on since the 1940s. Applied mathematicians at New York University now believe they have solved this conundrum, according to a recent paper published in the journal Physical Review Letters, and the answer may require challenging conventional wisdom on the subject. I'm throwing it. “Our study solves the problem by combining precise laboratory experiments with mathematical modeling that describes the behavior of reverse sprinklers,” said co-author Leif Ristlov of New York University's Coolant Research Institute. Ta. “We found that reverse sprinklers “reverse” or rotate in opposite directions when taking in water and when discharging water, but the reasons for this are subtle and surprising. ” […]
Leif Ristroph and colleagues built their own custom sprinkler system that incorporates ultra-low friction rotating bearings to allow the device to rotate freely. They immersed sprinklers in water and used special equipment to pump and pump water at carefully controlled flow rates. Of particular importance to this experiment was the fact that the custom sprinklers allowed the team to observe and measure how water flows inside, outside, and through the device. . By adding dye and particles to the water and shining it on with a laser, they were able to capture the flow in high-speed video. They ran experiments for hours at a time to precisely map the fluid flow patterns.
Ristolov et al. It turns out that reverse sprinklers rotate 50 times more slowly than regular sprinklers, but they work on a similar mechanism. This is amazing. “Normal sprinklers, or 'forward' sprinklers, are similar to rockets because they propel themselves with a jet of water,” Ristoroff said. “But reverse sprinklers are mysterious because the water they suck in doesn't look like a jet at all. We discovered that the secret is hidden inside the sprinkler. There it is: We have actual jets that explain the observed behavior.'' Reverse sprinklers function like an “inverted rocket,'' with the jets inside colliding with each other, but not head-on, according to Ristoloff. “The jet is not precisely centered because the flow is distorted as it passes through the curved arm,” Ball wrote. “As water flows around the bend of the arm, it is forced outward by centrifugal force, creating an asymmetric flow profile.” It's certainly a subtle effect, but the experimentally observed flow patterns are similar to the group is in good agreement with the mathematical model.