Mixing of a passive scalar near a free surface

We study numerically and analytically the mixing of a passive scalar in tur bulent shear flow with a free surface. The Navier-Stokes and scalar mixing equations are solved by direct numerical simulation. We find that the mean concentration of the scalar exhibits a two-layer structure near the free su rface: an outer layer, characterized by a fast reduction in the value of th e turbulent diffusivity of the scalar, and a much thinner inner layer chara cterized by a fast reduction of the mean concentration gradient. We develop a similarity theory for the variation of the mean concentration, which agr ees very well with the numerical results and provides us with scaling relat ions for the thickness of each layer. (C) 2001 American Institute of Physic s.