'Feynman and followers, you guys are off': Physicists disprove decades-old Richard Feynman theory on 'silly' sprinklers - Live Science

What the report says

Live Science reported that researchers have offered a new answer to a long-running fluid dynamics puzzle associated with Richard Feynman: how a sprinkler moves when it is submerged and made to draw water inward rather than spray it outward. The question, raised by Feynman as a Princeton graduate student in the 1940s, has produced conflicting experimental results over decades, with some tests showing different spin directions or little motion at all.

According to Live Science, a New York University team led by applied mathematician and experimental physicist Leif Ristroph tested the issue in a study published July 13 in Proceedings of the National Academy of Sciences. Building on earlier work that found a reverse sprinkler turns opposite to a normal sprinkler, the researchers made seven intentionally unusual sprinkler shapes, including spiraling arms and nozzles bent in unexpected directions, to compare rival explanations.

The tests challenged two prominent ideas: one linked to Ernst Mach, which emphasized the angular momentum of water inside the arms, and another associated with Feynman that focused on suction and pressure effects near the nozzles. Live Science reported that the altered designs did not behave as those theories predicted. Instead, measurements pointed to the central hub, where incoming water meets and swirls, producing a momentum flow that drives the sprinkler’s response.

The work involved Jesse Smith, who completed a physics doctorate at NYU, Ristroph’s students and computational fluid dynamics expert Brennan Sprinkle of the Colorado School of Mines. The team is now using simulations to test the model further. Beyond resolving a “silly” physics problem, the findings may help engineers understand devices that convert moving fluids into rotation, including turbines.