Passive propulsion in turbulent flows

The ability of a device to exploit the energy of a flow to generate thrust is the main feature of passive propulsion. In a turbulent flow, such energy conversion is challenging due to the unpredictable and disordered fluid motion. In wave-driven turbulence, it has recently been demonstrated that asymmetric floating rotors can tap the energy of ambient fluctuations to fuel directed rotation. Here we report on the dynamics of asymmetric floating vehicles capable of passively propelling themselves in two-dimensional turbulence. We show experimentally how the shape of a floater and its rotational dynamics conspire to allow harvesting energy of the turbulent fluid motion. The translation and rotation of the floater are shown to be strongly coupled. The propulsion velocity and the rotational diffusion timescale depend on the relative size of the floating vehicle with respect to the turbulence forcing scale. The geometry of the floater is investigated in the range of circular-sector-shaped objects and a shape optimizing its propulsion is identified. At times larger than the rotational diffusion timescale, our results shed light on a substantial increase of the turbulent diffusion coefficient of anisotropic objects due to the coupling between propulsion and rotational diffusion.