Numerical simulation of dispersion in urban street canyons with avenue-like tree plantings: comparison between RANS and LES

Previous CFD studies on pollution dispersion problems have largely centred on employing Reynolds-averaged Navier–Stokes (RANS) turbulence closure schemes, which have often been reported to overpredict pollutant concentration levels in comparison to wind tunnel measurement data. In addition, the majority of experimental and numerical investigations have failed to account for the aerodynamic effects of trees, which can occupy a significant proportion of typical urban street canyons. In the present work, the prediction accuracy of pollutant dispersion within urban street canyons of width to height ratio, W/H = 1 lined with avenue-like tree plantings are examined using two steady-state RANS models (the standard k-ε and RSM), and Large Eddy Simulation (LES) to compare their performance against wind tunnel experiments available on the online database CODASC [1]. Two cases of tree crown porosities are investigated, one for a loosely (Pvol = 97.5%) and another for a densely (Pvol = 96%) packed tree crown, corresponding to pressure loss coefficients of λ = 80 m−1 and λ = 200 m−1, respectively. Results of the tree-lined cases are then compared to a tree-free street canyon in order to demonstrate the impact of trees on the flow field and pollutant dispersion, and it is observed that the presence of trees reduces the in-canyon circulation and air exchange, and increases the overall concentration levels. Between the two numerical methods employed, LES performs better than RANS, because it captures the unsteady and intermittent fluctuations of the flow field, and hence, successfully resolves the transient mixing process within the canyons.