Prediction of four concentration moments of an airborne material released from a point source in an urban environment

Abstract

Ιn case of the dispersion of an airborne material from a point source in an urban environment the reliable prediction of the concentration statistical distribution by a numerical dispersion model presupposes the capability of the model to predict at least four statistical moments (mean, variance, skewness and kurtosis). In the present study, the beta distribution, the selection of which is justified based on a previous study, is incorporated in the Reynolds Averaged Navier Stokes (RANS) methodology. The shape parameters of the beta distribution are calculated using the numerical results of the mean, variance and maximum concentration. The latest is calculated through a deterministic model which uses also the numerical results of the mean and variance concentration as well as a hydrodynamic time scale. The validation of the new hybrid model “RANS-beta” is performed using the experimental dataset of the MUST wind tunnel experiment. The performance of the “RANS-beta” model for the skewness is very good (FAC2 = 0.811) while for the kurtosis it is acceptable (FAC2 = 0.557). The discrepancies are observed mainly at the edges of the plume. Future research will be focused on the optimization of the mean flow field, turbulent quantities (Reynolds stresses, turbulent kinetic energy) and on new parameterizations for the turbulent diffusion term.