Large-eddy simulation and Lagrangian stochastic modeling of passive scalar dispersion in a turbulent boundary layer

A large-eddy simulation (LES) with the dynamic Smagorinsky-Germano subgrid scale (SGS) model is used to study the passive scalar dispersion in a turbulent boundary layer. Instead of resolving the passive scalar transport equation, fluid particles containing scalar are tracked in a Lagrangian way. The Lagrangian velocity of each fluid particle is considered to have a large-scale part (directly computed by the LES) and a small-scale part. The movement of fluid elements containing scalar at a subgrid level is given by a three-dimensional Langevin model. The stochastic model is written in terms of SGS statistics at a mesh level. The results of the LES are compared with the wind-tunnel experiments of Fackrell and Robins (1982, Journal of Fluid Mechanics, 117, 1-26) and with the LES results of Sykes and Henn (1992, Atmospheric Environment A, 26, 3127-3144), who used a completely Eulerian approach with a non-dynamic SGS model. Our simulations predict the quantitative features of the experiments of Fackrell and Robins (1982, Journal Fluid Mechanics, 117, 1-26). Moreover, by using the Lagrangian approach, scalar fluxes are computed with no additional modeling assumptions and show good agreement with the experimental data. A classic mean-gradient model of the scalar flux is calculated from the computed results. The agreement between the directly computed fluxes and the classic mean-gradient model calculation is remarkable.