A team of physicists at New York University has reportedly resolved a longstanding question in fluid dynamics known as the Feynman sprinkler problem, which has puzzled scientists since the 19th century. The problem involves a submerged lawn sprinkler that, instead of spraying water outward, draws water inward through its arms. Researchers sought to determine whether such a sprinkler would rotate in the same direction as a conventional sprinkler, spin in the opposite direction, or remain stationary.

The investigation, led by applied mathematician and experimental physicist Leif Ristroph, culminated in a paper published in the Proceedings of the National Academy of Sciences. According to the researchers, when water is sucked into the sprinkler arms, the device rotates in the opposite direction to that of a standard sprinkler that expels water. However, the rotation occurs at a significantly slower rate—roughly one-fortieth of the speed.

This phenomenon is attributed to the shape of the sprinkler’s arms, particularly the bends near the central pivot. These curves generate forces that cause the incoming water jets to be offset slightly rather than colliding directly, resulting in a twisting torque that influences the sprinkler’s rotation. Brennan Sprinkle, a collaborator on the project, explained the mechanics using an analogy: just as a car turning right experiences an inertial force in the opposite direction, the fluid dynamics here produce a similar effect on the sprinkler.

The original problem traces back to Austrian physicist Ernst Mach in 1883 and was famously discussed by Richard Feynman during his time at Princeton University in the 1940s. Feynman himself conducted an experiment intended to resolve the question but faced a catastrophic failure when his apparatus exploded, preventing a conclusive answer. Since then, the problem has remained a subject of debate, with various hypotheses and conflicting arguments.

Skepticism surrounding the NYU study has emerged, largely due to the complexity of fluid flows and concerns about whether results from a single experimental setup could be generalized. To address these issues, the team conducted additional experiments testing various sprinkler designs, including models with multiple bends in the arms to alter water intake direction. Findings showed that the rotation’s direction remained consistent regardless of these modifications, suggesting that the key factor is the curvature close to the pivot that affects the jets’ offset.

Ristroph emphasized that alternative explanations for the sprinkler’s motion do not align with their experimental evidence. While the direct applications of reverse-flow sprinklers may be limited, the researchers suggest their findings enhance broader understanding of interactions between solid objects and fluid flows. Such insight could inform the design of technologies like devices that harvest energy from ocean waves or wind.

The study represents a significant step in addressing a classic physics question and provides a clearer framework for analyzing related fluid-structure interactions.