Dispersion and transfer of passive scalars in and above street canyons - Large-eddy simulations

This study applies a large-eddy simulation (LES) model to a street canyon in order to derive the fields of wind, turbulence, scalar concentration, concentration fluctuations, and scalar flux across the roof level. The wind blows at a right angle to the canyon axis and the emission is specified either as a line source with a constant emission rate along the street or as an area source with a constant concentration on the street and/or building surface, for which a wall function is introduced. Results have been compared with wind tunnel experiments. For the cases with a line source, the results of 2D spatial distribution of mean concentration and standard deviation demonstrate the promising capability of the LES model. Quantitative comparisons of mean concentration on the two walls provide convincing evidence that the LES model captures the main features of transport and dispersion processes in a street canyon. For the cases with an area source, simulations have been conducted for different canyon aspect ratios of H/W = 1/3,1/2, 2/3,1/1, 3/2, and 2/1, where H is building height and W is street width. LES results of spatial and temporal-mean scalar-flux-at-the-roof-level (SFRL) as a function of H/W agree fairly well with those of wind tunnel data. Profiles of temporal-mean SFRL across the canyon are indicative of flow regimes recommended by Oke [1988. Street design and urban canopy layer climate. Energy and Buildings 11, 103-113.1. Results of the cases with double facet source provide evidence that the LES model with an appropriate scalar boundary condition is promising for further LES studies of heat transfer inside a street canyon. (c) 2008 Elsevier Ltd. All rights reserved.