Numerical simulations of street canyon ventilation and pollutant dispersion

The relationship among air exchange rate (ACH), pollutant exchange rate (PCH) and friction factor (f) is proposed in this study to evaluate the performance of ventilation and pollution removal of hypothetical urban areas. Urban areas were simplified to arrays of idealised, repeated two-dimensional (2D) street canyons of different building-height-to-street-width (aspect) ratios (ARs) and building shapes were examined with the use of computational fluid dynamics (CFD). The Reynolds-averaged Navier-Stokes (RANS) equations with the Renormalization Group (RNG) k-epsilon turbulence model were adopted for the CFD. It was found that the turbulent component of ACH contributes more than 60% to the ventilation performance and is linearly proportional to the square root of friction factor (ACH'' proportional to f(1/2)) for the building shapes tested in this study. Unlike the well-defined relationship between ACH and f, the relationship of pollutant transport (or heat transfer) against f has not yet been formulated because of the different behaviour between building shapes, aspect ratios and locations of pollutant sources.