Journal
JOURNAL OF FLUID MECHANICS
Volume 915, Issue -, Pages -Publisher
CAMBRIDGE UNIV PRESS
DOI: 10.1017/jfm.2021.72
Keywords
surface gravity waves; ocean processes; coastal engineering
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Funding
- Royal Academy of Engineering Research Fellowship
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Periodic water waves generate Stokes drift affecting the transport of floating marine litter. Different sizes and densities of objects are transported at different rates by waves, with larger buoyant objects potentially experiencing increased drift compared to Lagrangian tracers. This increased drift is due to variable submergence and dynamic buoyancy force components perpendicular to the water surface, leading to an amplification of drift compared to Stokes drift when averaged over the wave cycle.
Periodic water waves generate Stokes drift as manifest from the orbits of Lagrangian particles not fully closing. Stokes drift can contribute to the transport of floating marine litter, including plastic. Previously, marine litter objects have been considered to be perfect. Lagrangian tracers, travelling with the Stokes drift of the waves. However, floating marine litter objects have large ranges of sizes and densities, which potentially result in different rates of transport by waves due to the non-Lagrangian behaviour of the objects. Through a combination of theory and experiments for idealised spherical objects in deep-water waves, we show that different objects are transported at different rates depending on their size and density, and that larger buoyant objects can have increased drift compared with Lagrangian tracers. We show that the mechanism for the increased drift observed in our experiments comprises the variable submergence and the corresponding dynamic buoyancy force components in a direction perpendicular to the local water surface. This leads to an amplification of the drift of these objects compared to the Stokes drift when averaged over the wave cycle. Using an expansion in wave steepness, we derive a closed-form approximation for this increased drift, which can be included in ocean-scale models of marine litter transport.
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