Behaviour of finite-size floating particles in free-surface turbulence

Motivated by the need for a better understanding of marine plastic transport, we experimentally investigate finite-size particles floating in free-surface turbulence. Using particle tracking velocimetry, we study the motion of spheres and discs along the quasi-flat free-surface above homogeneous isotropic grid turbulence in open channel flows. The focus is on the effect of the particle diameter, which varies from the Kolmogorov scale to the integral scale of the turbulence. We find that particles of size up to approximately one-tenth of the integral scale display motion statistics indistinguishable from surface flow tracers. For larger sizes, the particle fluctuating energy and acceleration variance decrease, the correlation times of their velocity and acceleration increase, and the particle diffusivity is weakly dependent on their diameter. Unlike in three-dimensional turbulence, the acceleration of finite-size floating particles becomes less intermittent with increasing size, recovering a Gaussian distribution for diameters in the inertial subrange. These results are used to assess the applicability of two distinct frameworks: temporal filtering and spatial filtering. Neglecting preferential sampling and assuming an empirical linear relation between the particle size and its response time, the temporal filtering approach is found to correctly predict the main trends, though with quantitative discrepancies. On the other hand, the spatial filtering approach, based on the spatial autocorrelation of the free-surface turbulence, accurately reproduces the decay of the fluctuating energy with increasing diameter. Although the scale separation is limited, power-law scaling relations for the particle acceleration variance based on spatial filtering are compatible with the observations.