Overview
- The paper published in PNAS on Monday confirms that the angular momentum of internal water flows, called momentum flux, drives both forward and reverse sprinkler rotation.
- Researchers used ultra-low-friction bearings, controlled pumping and suction, dyes, microparticles, laser illumination and high-speed video to map internal jets and measure torque across many custom 'silly sprinkler' shapes.
- The team found reverse sprinklers rotate in the opposite direction by an 'inside-out rocket' mechanism where internal jets collide off-center and that reverse rotation is about 50 times slower than forward rotation.
- The experiments contradict long-running explanations from Ernst Mach and the interpretation of Richard Feynman’s experiments by showing outer-arm flows do not set the torque while internal momentum flux does.
- Authors led by Leif Ristroph and co-authors from NYU and Colorado School of Mines report design rules linking arm shape to torque, a result that could help engineers tune devices such as small turbines to harvest or control fluid-driven forces.