Turbulent diffusive transport of floating discs

Understanding floating particle transport is crucial for the global response to plastic pollution. One key issue is the rate at which floating particles of different shapes and sizes spread over the surface of turbulent waters. Here we study experimentally the diffusive transport of floating discs in free-surface grid turbulence using particle tracking velocimetry. We consider discs larger than the dissipative flow scales and investigate their inertial filtering, i.e., the ability to filter out small-scale fluctuations. This behaviour, long identified in studies concerning particles in three-dimensional turbulence, may result in longer time-correlated motion as compared to fluid elements, and thus increased diffusivity. However, the applicability of classic turbulence theory to free-surface turbulence is debated. To substantiate this notion, we consider the Lagrangian motion of the discs and compare them to that of floating tracers of the free-surface flow. Specifically, we determine the discs' diffusion coefficient by integrating the velocity auto-correlation function following the classical framework of Taylor diffusion. We verify their spreading rate by calculating the mean-square displacement due to turbulent fluctuations along the particle trajectories.