"Changing gears:" flow-mediated coupling of spinning cylinders

Solid body interactions mediated by a flow are ubiquitous in the natural world. Flow-mediated interactions between spinning degrees of freedom play a basic role in more complicated fluid-structure interactions and have been variously studied in active matter, mixing, and renewable energy contexts. To bring these disparate experiments into a canonical fluid-dynamical setting, we study the interactions of two immersed cylinders at intermediate Taylor number. Inspired by mechanical gears, one of the cylinders is driven to rotate while the other can passively respond with rotation of its own. Unlike mechanical gears which rotate in opposite directions, we find that the passive cylinder can rotate in either direction relative to the active cylinder. We demonstrate that this bifurcation is caused by a three-dimensional Taylor instability between the cylinders. We also find that confining boundaries have a strong effect on the phase space of these spin-switching bifurcations, suggesting new directions in the geometric control of flow-coupled interaction.