RANS SPH CFD for Air Quality in and Around Urban Canopies

A Lagrangian mesh-less RANS CFD-SPH code is validated on the Urban Canopy of a wind-tunnel experiment in terms of mean concentration. Despite focusing on the most challenging 25% of the Canopy, the code satisfies the state-of-the-art thresholds (Chang and Hanna 2004) for the validation metrics and shows an above-average performance vs. 28 state-of-the-art deterministic codes. A smoothed Wall Function WF considers the actual depth of the Surface Boundary Layer around the walls within the current Urban Canopy. The combined replacement of a standard WF with a smoothed WF and of a constant turbulent Schmidt number ScT with the ScT-TGS turbulence closure reduces the Fractional Bias magnitude of 65% and the Normalised Mean Square Error of 78%. The simulated mean stream-wise velocity is validated and compared with RANS and LES datasets. A full-scale accidental release of hydrocarbons from a gas station involves 36 buildings.