Passive Propulsion of Anisotropic Objects in Turbulence

Aquatic animals have evolved different strategies to swim through water effectively. These strategies combine both active (flapping, undulating or oscillating appendages) and passive (a non-smooth compliant skin, hair, feathers and other passive protrusions) mechanisms.  Animals use passive mechanism to harvest the energy of an external flow directly so that they can propel themselves without spending energy.  Passive mechanisms also offer clear advantages when swimming in unfavourable environmental condition, especially in the presence of strongly turbulent flows. However, identifying passive mechanism is challenging. Here we demonstrate that an anisotropic object can tap the energy from wave-driven two-dimensional turbulent flows to fuel its directed motion. The mechanism of the propulsion relies on (1) the underlying fabric of the turbulent flow and (2) the coupling between the translation and rotation of the object. This knowledge gives us the ability to switch the "swimming gait'' of the objects from a propulsion dominated regime to a random walk regime. Our experimental results yield insights into passive swimming in turbulence and suggest a new method of designing floating objects capable of converting energy of turbulence into deterministic, directional motion.