Street canyon flow and pollutant removal characteristics in different thermal stratification by large–eddy simulation

The transport of air pollutant in urban area is a complicated phenomenon that is related to the buildings geometry and atmospheric stability.  Recent studies have shown that unstable stratification could enhance the pollutant removal from street levels. In this study, we compare how atmospheric stratification affects the performance of ventilation and pollutant removal of urban street canyons.

Five sets of large-eddy simulation (LES), consisting of stable, neutral, and unstable stratification, are conducted to examine the mechanisms of ventilation and pollutant transport in two-dimensional (2D) street canyons. The building-height-to-street-width (aspect) ratio is equal to one and the bulk Richardson number Rb (= g(Q_ref-Q₀)/(U₀²Q_ref)) equal to -0.3, -0.11, 0, 0.33, and 0.8 are considered.

It is found that the mean wind inside a street canyon is weakened and strengthened, respectively, in stable and unstable stratification. In stable stratification, a significant suppression of mean flow is observed at the lower leeward corner where a relatively thick layer of air is trapped. This stagnant air mass is separated from the primary recirculation of the street canyon. On the contrary, an increase in mean wind speed, in which the flow pattern is similar to that in the neutral counterpart, is observed in unstable stratification.

A stronger turbulence intensity is observed inside the more stable street canyons except at Rb = 0.33 in which a slight turbulence enhancement is found in the roof-level windward region of the street canyon. This elevation in turbulence is likely caused by the specific turbulence production mechanism in idealized 2D street canyons. Whereas, turbulence is largely produced near the ground level in unstable stratification, suggesting the dominated convective turbulence.

An area source with uniform pollutant concentration on the street is included in the LES to investigate the pollutant transport in street canyons. In neutral and unstable stratification, the pollutant is quite well mixed inside a street canyon exhibiting a rather uniform distribution. On the other hand, in stable stratification, a substantial pollutant accumulation is observed at the street level. Taking into account the pollutant emission rate, the mean pollutant concentration inside a street canyon increases significantly in stable stratification.