Impact of a surfactant on a turbulent shear layer under the air-sea interface

The effects of surface active material (surfactant) on a turbulent shear layer near the air-sea interface are studied by direct numerical simulation of the flow. The study is motivated by the observation of the reduction of the scalar exchange rate across the air-sea interface by a surfactant film. Though the model, based on the concept of surface renewal by turbulent eddies, explains the observation well, there has been little experimental or numerical validation of the detailed hydrodynamic mechanism. The present work aims to identify the differences in the underlying turbulence structures between clean and contaminated interfaces and to provide a basis for parameterizing the scalar transfer rate across a contaminated interface. The simulation results show that the inviscid blocking effect of a clean interface, the attenuation of vertical fluctuating velocities and the increase in horizontal turbulence intensities, is significantly reduced by surfactant contamination. The viscous sublayer adjacent to the interface is thickened with the presence of a surfactant. Within this viscous sublayer, both horizontal and vertical fluctuating turbulent intensities are attenuated. The upwelling (splatting) events, which are responsible for renewing the fluids in contact with the interface, are reduced drastically. In addition, the vortex connection onto the interface, which frequently occurs on a clean interface, is also diminished. The impact of a surfactant on the sheared turbulence is explained by the interaction between the transport of the surfactant and the underlying coherent horseshoe vortical flow. For turbulence beneath a clean interface, the impingement of the horseshoe vortex causes upwelling and, subsequently, a vortex connection onto the interface. if the surface is covered with a surfactant, surfactant concentration gradients are formed by the impinging turbulent eddies. Surface shear stresses are induced, which retard the approaching of the horseshoe vortices toward the interface, diminish the renewal of near-surface fluids, and, consequently, reduce the scalar transfer rate.