A subgrid Lagrangian stochastic model for turbulent passive and reactive scalar dispersion

A large-eddy simulation (LES) with the dynamic Smagorinsky–Germano subgrid-scale (SGS) model is used to study passive and reactive scalar dispersion in a turbulent boundary layer. Instead of resolving the 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. Diffusion is taken into account by a particle pairing exchange model. A second order, irreversible chemical reaction is considered to take place within each fluid particle. The mixing fraction, that behaves as a passive scalar is compared to the experimental results of [Fackrell, J.E., Robins, A.G., 1982. Concentration fluctuations and fluxes in plumes from point sources in a turbulent boundary layer. J. Fluid Mech. 117, 1–26] and to the LES of [Sykes, R.I., Henn, D.S., 1992. Large-eddy simulation of the concentration fluctuations in a dispersing plume. Atmos. Environ. 26A, 3127–3144]. A model for the intensity of segregation is presented and the results of the computations are in good agreement with the model. Finally, the spatial evolution of the intensity of segregation is compared to the dynamic and reactive scalar LES of [Meeder, J.P., Nieuwstadt, F.T.M., 2000. Large-eddy simulation of the turbulent dispersion of a reactive plume from a point source into a neutral atmospheric boundary layer. Atmos. Environ. 34, 3563–3573].