The height dependence of the turbulent Schmidt number within the boundary layer

The gradient diffusion theory with the exchange approximation by Boussinesq has been used successfully to predict dispersion of passive scalars in boundary layers. The knowledge of the turbulent diffusivity is required. Based on the analogy between the exchange of mass and of momentum, usually the turbulent viscosity (momentum) is used instead of the turbulent diffusivity of mass. This fact requires good knowledge of the turbulent Schmidt number. The latter is determined by means of wind tunnel experiments in a turbulent boundary layer above a flat plate. In contrast to literature where a constant is assumed mostly, it has been demonstrated for the turbulent Schmidt number that a strong dependence on height existed within the boundary layer.