Particle Clustering in Free-Surface Turbulence

The transport of floaters on the surface of turbulent waters has been the object of growing interest, due to its relevance to the global challenge of micro-plastics pollution. Here we investigate the behaviour of densely concentrated particles in free-surface turbulence. Experiments are carried out over the fully-developed turbulent region behind a square mesh grid in an open channel facility. This has a 1-m wide, 6-m long test section, and the flow regime produces negligibly small surface waves. Floating particles are illuminated by pulsed LEDs and tracked by two high-speed cameras. We utilised 2-mm, slightly buoyant spheres that behave as surface flow tracers in the dilute limit. Initially, these particles are dispersed homogeneously, but cluster over the compressible free surface. As their concentration is increased up to covering ~20% of the area, the inter-particle interactions profoundly alter the transport: the particles aggregate and stick to each other, forming large clusters. Varying particle concentration and Reynolds number, we investigate how the underlying turbulence determines the cluster topology, characterized by Voronoi tessellation. Furthermore, we analyse the Lagrangian dispersion and relative velocities in comparison to the dilute case.