Numerical Investigation of Pollutant Transfer Coefficient in Street Canyons of Different Aspect Ratios

Building-height-to-street-width (aspect) ratio (AR) is one of the most important parameters of building geometry affecting the flows in urban street canyons. A numerical study was performed to investigate the influence of AR on the ventilation and pollutant removal performance. The large-eddy simulation (LES) with the one-equation subgrid-scale (SGS) model was applied to the idealized two-dimensional (2D) street canyons of different ARs. The local convective pollutant transfer coefficient (ωT) was illustrated as a spatial function on the roofs, façades, and streets of the street canyons to examine the pollutant removal performance. The LES was validated by comparing the Nusselt number, the convective heat transfer counterpart, along the street of the street canyon. The characteristic flow regimes, isolated roughness, wake interference, and skimming flow, were taken into account.

In the isolated roughness regime (ARs = 0.0667 and 0.0909), two primary reciculations were developed in which one is on the leeward side and the other on the windward side. Persistent flow reattachment and separation were observed inside the street canyon. Moreover, ωT is tightly coupled with the recirculating flow in which the maximum and minimum coincide with the points of reattachment and separation, respectively. The fresh air entrains from the urban canopy layer down to the street level dominating the pollutant removal. In the wake interference regime (ARs = 0.25 and 0.5), the flow reattachment and separation were replaced by a single primary recirculation. A monotonic increasing ωT, from the leeward side to the windward side, was perceived instead of peaks and troughs in the wider street canyons. Similarly, in the skimming flow regime (ARs = 1 and 2), neither flow reattachment nor separation was observed. Instead, the width of the street determined the characteristics of ωT of the street canyon. The narrower the street, the more the number of vertically aligned primary recirculations. In the street canyon of AR = 1, an isolated clockwise-rotating recirculation was developed in which ωT is monotonically increasing from the leeward side to the windward side. On the contrary, two counter-rotating recirculations were layered in the street canyons of AR = 2, leading to the decreasing ωT from the leeward side to the windward side. In view of the monotonic ωT, the pollutants are mainly removed by turbulence in the wake interference and skimming flow regimes.