Turbulence and momentum flux modification in the presence of sea spray

It has been suggested that the presence of sea spray in the marine atmospheric boundary layer may be in part responsible for the perceived reduction in drag at high winds. To study this effect, and more generally to determine the degree to which sea spray can modify the turbulence within the near-surface boundary layer, direct numerical simulations (DNS) were conducted in which a dispersed phase of individual Lagrangian particles were dynamically coupled to the surrounding fluid phase.

It is found, in general, that turbulence is attenuated in the presence of particles, and this effect is a strong function of particle mass concentration as well as the particle Stokes number (the ratio of particle time scale to characteristic flow time scale). The computed force exerted by the particles onto the surrounding flow field, when interpreted as a momentum flux, shows that sea spray may carry a significant portion of the total momentum flux reaching the ocean surface. For turbulent Couette flow, specifically, this is manifested as a shift of stress carried by the turbulent flux to that carried by the particles, thus appearing as a reduction in total momentum transport to the surface when considering only the turbulent contribution. If a drag coefficient is then determined from this turbulent momentum transport, it would therefore appear to decrease with increasing spray loading. In our presentation, the practical implications of this effect, as well as the mechanisms which lead to it, will be illustrated and discussed.