Reynolds-averaged Navier-Stokes simulation of nearshore Langmuir circulation and the formation of oil-particle aggregates

Abstract

Langmuir turbulence in the inner shelf coastal ocean is characterized by Langmuir circulation (LC) or cells that can span the full depth of unstratified water columns. A Reynolds-averaged Navier-Stokes (RANS) simulation strategy resolving full-depth LC coupled with an oil-particle aggregate (OPA) formation model is introduced. It is seen that full-depth LC generated by wind and waves under storm conditions can result in sediment resuspension and oil droplet entrainment (in the case of an oil spill) and subsequent mixing between these two, leading to significant OPA formation. This conclusion comes from a simulation in which various classes of oil droplets (with diameters ranging between 40 and 140 μm) were released at the surface, each class initialized with a 0.1 kg m−3 concentration. The full-depth LC led to most of the oil becoming trapped within OPAs in the first 5 min. The majority of the larger oil droplets were quickly aggregated with sediments near surface, whereas the smaller oil droplets were first submerged by the downwelling limbs of the Langmuir cells, and eventually aggregated with sediment while being carried upwards by the upwelling limbs. The OPAs conglomerated in the form of clouds transported by the action of the cells, with the heavier OPAs eventually settling within the upwelling limb of the cells while slowly depositing to the bed over time.