Large-eddy simulation of turbulent diffusion with chemical reactions in the convective boundary layer

Abstract

Bottom-up and top-down diffusion of two reacting air constituents in a dry homogeneous convective boundary layer with zero mean wind are simulated using large-eddy simulations. The model includes a simple binary, irreversible reaction without heat release. Subgrid-scale contributions of reactions are neglected as justified by spectral analysis of the results. The results depend upon the ratio R of time scales for convection relative to that of the chemical reaction, on the concentration ratio of the two reacting components, and on the initial conditions. Cases with zero, finite and infinite reaction rate are considered. Without reactions, the bottom-up diffusivity is about twice as large as top-down diffusivity due to buoyancy effects. For infinite reaction rate, the thin reaction layer becomes highly convoluted. For R > 0.1, i.e. for parameter values which are typical for the reaction between ozone and nitrogen oxide in the atmosphere, the reaction rate is strongly influenced by turbulence. The effective eddy diffusivities may be enhanced or reduced by chemical reactions, depending on the importance of gradient forcings of mass fluxes relative to other source and sink terms. A simple ‘box-model’ describes the time evolution of the mean concentrations in the mixed layer fairly well for R ≤ 0.1 and for infinite reaction rate.