The effect of canyon aspect ratio on flushing of dense pollutants from an isolated street canyon

This study presents an experimental investigation of the effect of canyon aspect ratio on the dense pollutant removal from a street canyon by a turbulent overflow. Four series of experiments for different aspect ratios (η=0.45, 0.75, 1, 2) were conducted for a range of Richardson numbers. The qualitative and quantitative results are discussed and compared. Increasing the Richardson number and decreasing the canyon aspect ratio resulted in an increasingly strong stratification within the canyon and longer trapping times for the pollutant. The aspect ratio strongly affects the initial flushing mechanics and subsequent flow regime within the canyon. Narrower street canyons limit the width of large-scale vortices in the canyon and hence reduce vertical mixing. Based on the initial Richardson number, for all geometric configurations, three different flow regimes were observed in the canyon. The Richardson number at which there is a transition between these regimes is a function of the canyon geometry.