In this work a new approach for CFD RANS modelling of dispersion of airborne point source releases is presented. The key feature of this approach is the model capability to predict concentration time scales that are functions not only of the ﬂow turbulence scales but also of the pollutant travel time. This approach has been implemented for the calculation of the concentration ﬂuctuation dissipation time scale and the maximum individual exposure at short time intervals. For the estimation of travel time in the Eulerian grid the new ‘radioactive tracer method’ is introduced. The new approaches were incorporated in the CFD code ADREA. The capabilities of the new approaches are validated against the Mock Urban Setting Trial ﬁeld experiment data under neutral conditions. The comparisons of model and observations gave quite satisfactory results.
The concentration fluctuations of a dispersing hazardous gaseous pollutant in the atmospheric boundary layer, and the hazard associated with short-term concentration levels, demonstrate the necessity of estimating the magnitude of these fluctuations using predicting models. To predict and estimate the maximum expected dosage and the exposure time within which the dosage exceeds certain health limits, the knowledge of the behaviour of concentration fluctuations at the point under consideration is needed. The whole effort is based on the field experiment MUST (Biltoft, 2001) and the computational simulations have been performed with the CFD code ADREA.