Building envelope features (BEFs) have attracted more and more attention as they have a significant impact on flow structure and pollutant dispersion within street canyons. This paper conducted CFD simulations validated by wind-tunnel experiments, to explore the effects of the BEFs on the airflow and pollutant distribution inside a symmetric street canyon under perpendicular incoming flow. The three different BEFs(balconies, overhangs, and wing walls) and their locations and continuity/discontinuity structures have been considered. The air exchange rate, airflow and pollutant distributions, including mean and maximum dimensionless pollutant concentrations, were evaluated within urban streets with different BEFs.
The results of this paper are as follows.
First, the continuity/discontinuity and location of BEFs have a great influence on the ventilation of urban streets. As you can see from the figure, a relatively large clockwise vortex is formed inside the reference street canyon, but the BEFs weaken this vortex, reducing the air exchange rate of the urban street. In the case of the balconies, the air exchange rate is maximum at 99.4% of the reference canyon when the continuous balconies are on the windward wall and minimum at 96.3% when they are on the leeward wall.
For the wing walls, it has a maximum value of 99.5% when the discontinuous wing walls are on the leeward wall, and a minimum value of 95.8% when the continuous wing walls are on the leeward wall. In the case of the overhangs, it has a maximum value of 95.9% when the discontinuous overhangs are on the windward wall, and a minimum value of 83.1% when the continuous overhangs are on both walls. Therefore, among the three BEFs, the air exchange rate of urban streets in the order of balconies, wing walls and overhangs is reduced. Also, compared to the reference canyon, the BEFs of the leeward wall have the least influence on the airflow and pollutant diffusion inside the urban street, and then when they are on the windward wall. When the BEFs are on both walls, the ventilation capacity of the street canyon is weakened significantly, resulting in a large increase in the pollutant concentration at the bottom center, especially near the winward wall.
Second, the street canyons with discontinuous BEFs have a greater air exchange rate compared to the cases with continuous BEFs as more airflow can pass through the space between BEFs. Discontinuous BEFs show more dispersed pollutant distributions, weakening the effect of continuous BEFs on the airflow and pollutant distribution inside the street canyons.
Third, whether the BEFs are continuous or discontinuous, it has no significant effect on the mean pollutant concentrations near the leeward pedestrian respiration plane and the leeward wall. However, the discontinuous BEFs reduce the mean pollutant concentration near the windward pedestrian respiration plane and the windward wall.
These findings can be used to optimize the design of BEFs to enhance ventilation and mitigate traffic pollution inside urban streets.
The research result has been published in "Environmental Science and Pollution Research" under the title of "Effects of building envelope features on airflow and pollutant dispersion within a symmetric street canyon" (https://doi.org/10.1007/s11356-024-33343-5).
