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Centrifugal Flows Drive Reverse Rotation of Feynman’s Sprinkler

Kaizhe Wang^1,2, Brennan Sprinkle ³, Mingxuan Zuo¹, and Leif Ristroph ^1,*

¹Applied Math Lab, Courant Institute, New York University, New York, New York 10012, USA
²Department of Physics, New York University, New York, New York 10003, USA
³Department of Applied Mathematics and Statistics, Colorado School of Mines, Golden, Colorado 80401, USA

^*Corresponding author: ristroph@cims.nyu.edu

Phys. Rev. Lett. 132, 044003 – Published 26 January, 2024

DOI: https://doi.org/10.1103/PhysRevLett.132.044003

Abstract

The issue of reversibility in hydromechanical sprinklers that auto-rotate while ejecting fluid from S-shaped tubes raises fundamental questions that remain unresolved. Here, we report on precision experiments that reveal robust and persistent reverse rotation under suction and a model that accounts for the observed motions. We implement an ultralow friction bearing in an apparatus that allows for free rotation under ejection and suction for a range of flow rates and arbitrarily long times. Flow measurements reveal a rocketlike mechanism shared by the reverse and forward modes that involves angular momentum flux, whose subtle manifestation in the reverse case stems from centrifugal effects for flows in curved conduits. These findings answer Feynman’s long-standing question by providing quantitatively accurate explanations of both modes, and they suggest further inquiries into flux-based force generation and the roles of geometry and Reynolds number.

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Focus

Feynman’s Reversed Sprinkler Puzzle Solved

Published 26 January, 2024

Which direction would an S-shaped lawn sprinkler rotate if it were submerged and the flow were reversed? Experiments now provide a definitive answer.

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