On the study of ventilation and pollutant removal over idealized two-dimensional urban street canyons

Ventilation and pollutant transport in urban areas are major research problems nowadays due to the large number of urban inhabitants. However, our understanding of the street-level transport processes is rather limited because of the highly complicated urban morphology. This paper is thus conceived, using both laboratory wind tunnel measurements and computational fluid dynamics (CFD), to examine the pollutant transport behaviors from the ground level of idealized two-dimensional (2D) urban street canyons to the urban atmospheric boundary layer (ABL). Movable rectangular aluminum blocks are placed in the wind tunnel in cross-flow to construct street canyons of different building-height-to-street-width (aspect) ratios. The wind tunnel and CFD results complement each other to elucidate the pollutant removal mechanism that is in line with other results available in literature. From the experimental results obtained in this study, noticeable scaling effects are observed in the mass transfer behaviors even the flows have fulfilled kinematic similarity. Under this circumstance, a new indicator for pollutant removal, the scaled overall pollutant removal coefficient, is formulated for the assessment of pollutant removal performance. The accuracy of the new indicator is compared with our CFD results that shows an improved agreement. The newly proposed indicator could be used to account for the scaling effects occurred in laboratory experiments at finite Reynolds number (103 to 105 in this study) for comparison of pollutant removal performance in a more reasonable manner.

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