The relationship among air exchange rate (ACH), pollutant exchange rate (PCH) and friction factor (
) 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)
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 (
) for the building shapes tested in this study. Unlike the well-defined relationship between ACH and
, the relationship of pollutant transport (or heat transfer) against
has not yet been formulated because of the different behaviour between building shapes, aspect ratios and locations of pollutant sources.
Keywords: air quality, computational fluid dynamics, CFD, city ventilation, pollutant transport, turbulence model