78th Annual Meeting of the APS Division of Fluid Dynamics (Nov 23 — 25, 2025)

P006: Fluidic gears and pulleys: Hydrodynamic spin-coupling of rotors

• View Larger
• Download Original

Mechanical gears and pulleys transmit rotation through contact forces and friction. Can fluid forces be used to make contactless transmissions that never wear out?Consider an active rotor driven to spin and a nearby passive rotor that may be induced to spin by hydrodynamic coupling. Our system involves cylindrical rotors immersed in a water-glycerol mixture and confined within a cylindrical corral. By varying the separation distance between the rotors, we find that the rotation speed and even the direction of the passive rotor can be controlled. Flow visualizations explain why and how.The images are time-exposed photographs showing pathlines of air bubbles, and long shadows are cast due to the differences in refractive index between the spinners and the fluid. The top image shows that for moderate gaps between the spinners, they counterrotate like gears. The cause is stronger shearing forces on the inner side of the passive rotor. The photo on the lower left shows that smaller gaps lead to corotation as if coupled by a pulley or belt, and the cause is stronger shear on the outer side of the passive rotor. The photo on the lower right shows that larger gaps also induce corotation but here because of induced circulation that reverses the inner shear and acts as a middle gear.The result is a continuously variable gearbox with the special ability to change direction by switching between gear-like, pulley-like and gear-train coupling modes. Cartoons accompanying the flow visualizations show the mechanical analogies apt for each mode.