Oil dispersion by turbulence and coherent circulations

Abstract

Oil spilled over the sea can be broken up into many small oil droplets, which can then be dispersed into the water column by turbulence and coherent circulations, such as Langmuir circulation and thermal convection. The oil dispersion depends critically on the droplet size. Small oil droplets having diameters in the range of a few to hundreds of micrometers are essentially neutrally buoyant particles and they can be dispersed as deep as the coherent circulations can penetrate. Larger oil droplets with diameters of millimetres have buoyant rise speeds comparable with the downwelling velocity of the coherent cells, and they can be suspended in a subsurface retention zone [Stommel (1949), Trajectories of small bodies sinking slowly through convective cells. J. mar. Res. 8, 24–29] at the downwelling sites of the cells. By extending the homogenization theory of Rhines and Young [(1983), How rapidly is a passive scalar mixed within closed streamlines? J. Fluid Mech. 133, 133–145], we can show that the joint effect of langmuir circulation and turbulence is to homogenize the oil concentration over the Stommel retention zone. When the buoyant rise speeds of oil droplets are much smaller than the downwelling velocity of the coherent cells, oil concentration will be uniformly distributed across whole cells.