Prediction of pollutant dispersion in buildings: Analysis of the gradient-diffusion hypothesis

Numerical simulations of room airflow using Computational Fluid Dynamics (CFD) are often based on the Reynolds-averaged Navier-Stokes (RANS) approach. Using this approach, only the averaged quantities are computed, whereas the effect of turbulence on the mean flow is modeled. Since the RANS approach does not provide information on velocity and concentration fluctuations, also the turbulent mass fluxes should be modeled. In the majority of the cases this is done by employing the standard gradient-diffusion hypothesis, which relates the turbulent mass flux to the mean concentration derivatives. In this paper a CFD analysis of pollutant dispersion in an enclosure ventilated by a transitional wall jet (Re ≈ 2,500) is presented, using validated high-resolution RANS and Large Eddy Simulations (LES). The LES simulation shows that a counter-gradient turbulent mass flux is present, indicating that the standard gradient-diffusion hypothesis used in RANS is not valid in the entire flow domain. However, it is shown that for this particular case, the convective mass fluxes dominate over the turbulent mass fluxes, and that the predicted pollutant concentrations by RANS will therefore not differ significantly from those by LES.