Mathematicians finally solve Feynman’s “reverse sprinkler” problem

In normal operation, an S-shaped lawn sprinkler rotates because of the ‘jets’ of water shooting from its nozzles. But if the sprinkler is underwater and sucking in water, which way does it spin? The riddle of ‘Feynman’s sprinkler’ was popularized by physicist Richard Feynman. “The answer is perfectly clear at first sight,” he wrote in his memoir Surely You’re Joking, Mr. Feynman. “The trouble was, some guy would think it was perfectly clear one way, and another guy would think it was perfectly clear the other way.”

The complexities of flow and turbulence mean that past experiments have given inconclusive or contradictory results (or, in Feynman’s case, broken glass and a thorough soaking). Now, researchers have carefully designed a sprinkler to remove confounding effects and found that the underwater sprinkler rotates in the opposite direction to the normal one, but unsteadily, and about 50 times slower. Detailed observations backed up by mathematical modelling suggest that a weak jet effect inside the device dominates the sprinkler’s motion.