It is the most conducive to pollutant removal for the void decks at both buildings, while the construction with void deck at the upstream building causes pollutant accumulation on the windward side. Comparing with the regular street canyon (a main clockwise vortex is obtained therein), the void deck can cause several vortices when a strong stream of air passes through the canyon. The results reveal that the street geometry affects significantly the in-canyon flow structures and thus the pollutant distributions. In this study, a validated CFD model is used to analyze the flow field and pollutant distribution in an isolated canyon (street aspect ratio, W/H = 1) by considering different street categories and arrangements of void deck under a perpendicular inflow wind. These findings can help ease traffic pollution inside the street canyons composed of high-rise buildings, especially in tropical cities. The mean pollutant concentration on the wall of the building with the void deck and on the pedestrian respiration plane close to it is near zero. Therefore, unlike the regular canyons, as the void deck and the building height increases, the strength of the airflow through the void deck becomes stronger, and as a result, the mean pollutant concentration is significantly reduced at both walls and the pedestrian respiration level. Also, for the canyons with the void deck, traffic pollutants are removed out of the canyon by the strong airflow through the void deck.
In particular, when the void decks are at both buildings, they are increased by 38.3 times. For the windward void deck, purging flow rate (PFR) and normalized net escape velocity (NEV*) increase by 6.4 times compared to the regular canyon, and for the leeward void deck, increase by 13 times.
Air exchange rates (ACH) of the canyons with the void deck are much larger than that of regular canyons, and the perturbation changes of turbulence (ACH′) decrease. The results show that the void decks have a significant effect on the airflow and pollutant distribution inside the canyon. This study investigated the effects of void deck height and location, and the building height on the airflow field and the traffic pollutant diffusion in a long canyon with L/H = 10, validated by wind-tunnel experiment data. However, previous studies have aimed to improve the pedestrian-level wind speed with void deck in single buildings or short canyons. In general, urban canyons are the areas most clearly affected by traffic pollutants since the ability of the canyon to self-ventilate is inhibited due to blockage of buildings or other urban structures.
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