77th Annual Meeting of the APS Division of Fluid Dynamics (November 24, 2024 — November 26, 2024)

P2676730: Secret swirls give sprinklers a whirl

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Authors

• Leif Ristroph, New York University (NYU)
• Jesse Smith, New York University
• Mingxuan Zuo, New York University
• Brennan Sprinkle, Colorado School of Mines
• Kaizhe Wang, New York University (NYU)
• Will Kuhlke, New York University

DOI: https://doi.org/10.1103/APS.DFD.2024.GFM.P2676730

Some types of sprinklers can freely spin while spraying out water. What happens if instead water is sucked in? This question posed by Mach and popularized by Feynman is notoriously difficult. We turned to flow visualizations for answers. The regular sprinkler works like a rocket. Jets swirl in one direction, and the sprinkler twirls in the other. The images to the right are photos of fluorescein dye and microparticles in a laser sheet that are recolored to highlight the jets issuing from the moving arms. The suction case below shows that the flows are not reversible: there are no jets streaming into the arms.The secret to the reverse sprinkler is its internal jets that collide and spin up vortices. For low flow rates, the vortices are symmetric, and the sprinkler does not turn. For higher rates, the vortices are asymmetric, and the sprinkler slowly rotates in reverse. The asymmetry first appears around Reynolds number Re = 100. The internal jets meet in a glancing collision as if aimed slightly away from the center. This is like an inside-out rocket with thrusters directed inward and whose misalignment produces rotation. The ultimate cause is the curved arms, where the flows curl one way as the sprinkler whirls in the other.