Search

As most of the world’s population lives in cities, it is critical to understand dispersion processes of pollutants in urban areas. This study focuses on so called air exchange rate (ACH) index, which is frequently used by numerical studies to determine ventilation of street canyons without a simulation of a pollution source. These studies applied the ACH on idealised 2D street canyons, where the ventilation acts only through the one opening roof top. There are two pertinent questions: i) is the ACH really capable to predict the ventilation of a street canyon without knowing of a pollutant source; and ii) how much the ACH differs between 2D and 3D street canyons? To answer these questions, we performed large-eddy simulations of pollution of complex 3D street canyons from ground-level line sources. We computed ACHs and spatially-average concentrations for three different street canyons and compared these quantities with those from previous studies. Results clearly demonstrate that these quantities strongly depend not only on street-canyon geometry but also on geometry of surrounding buildings. It is also shown that 2D canyon gives unrealistic result for retention of pollutant within an urban street canyon. The ACH might lead to significant underestimation of the street-canyon ventilation if a source would be outside the canyon.

Results of computer simulation of flow over a series of street canyons are presented in this paper. The setup is adapted from an experimental study by [4] with two different shapes of buildings. The problem is simulated by an LES model CLMM (Charles University Large Eddy Microscale Model) and results are analysed using proper orthogonal decomposition and spectral analysis. The results in the channel (layout from the experiment) are compared with results with a free top boundary.