Counter-gradient diffusion in a slot-ventilated enclosure assessed by LES and RANS

In Computational Fluid Dynamics (CFD) studies for the prediction of room airflow the Reynolds-averaged Navier-Stokes (RANS) approach is often used, in which only the averaged quantities are computed, whereas the effect of turbulence is modeled. Since the RANS approach does not provide information on the velocity and concentration fluctuations, turbulent mass transport is often modeled using the standard gradient-diffusion hypothesis, which relates the turbulent mass flux to the mean concentration derivatives. This paper presents a CFD analysis of pollutant dispersion in an enclosure ventilated by a transitional wall jet (Re. ≈. 2500), 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 the results obtained with LES.