Vertical Distribution of Weakly Neutrally Buoyant Floating Macro- and Megaplastic Marine Litter Using Direct Forcing / Immersed Boundary Method and Large Eddy Simulation

Abstract The current knowledge in modelling of plastic marine debris transport remains mostly limited to the presence of small fragments in all scales of ocean processes. As the cleanup systems developed by The Ocean Cleanup captures mostly large plastics marine debris, there is pressing need to understand their dynamics. In this work, we show that using well-known numerical methods in fluid dynamics such as Large Eddy Simulation combined with Level Set for interface tracking and Immersed Boundary method for rigid body motion can help us to shed some light on the dynamics of idealised weakly neutrally buoyant large plastics categorised as macro- and megaplastics. We show all the necessary steps that led us to correctly modelling floating large plastics. These encompass the space convergence needed to capture the dynamics of a rising buoyant object in still water, the wave numerical resolution and the relation between the former and the latter. Despite modelling floating plastics being computational demanding, we show that plastics undergo excursions as deep as two wave heights when the wave steepness S is decreasing.