Hydrodynamic coupling of rotating bodies

Flow-mediated interactions between spinning bodies are important in active matter, mixing, and renewable energy contexts. We use the rigid penalty immersed boundary (pIB) method to simulate the two-rotor interaction problem, where one is prescribed an active rotation and the other is free to passively rotate in response. These rotors can be thought of as 'gears' where the coupling occurs through the fluid rather than through mechanical contact. We confirm a surprising result from previous tabletop experiments, which is that the passive gear can either co- or counter-rotate with the active gear depending on their separation distance. This simulation provides an idealized fluid-dynamical setting which gives insight into the role of spin-coupling in more complex fluid-structure interactions.