Effects of building envelope features on airflow and pollutant dispersion within a symmetric street canyon.

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 numerical models validated by wind-tunnel experiments, to explore the effects of the BEFs on characteristics of the airflow and pollutant distribution inside a symmetric street canyon under perpendicular incoming flow. Three different BEFs (balconies, overhangs, and wing walls) and their locations and continuity/discontinuity structures were considered. For each canyon with various BEFs, the air exchange rate (ACH), airflow patterns, and pollutant distributions were evaluated and compared in detail. The results show that compared to the regular canyon, the BEFs will reduce the ACH of the canyon, but increase the disturbances (the proportion of ACH') inside the canyon. The BEFs on the leeward wall have the least influence on the in-canyon airflow and pollutant distributions, followed by that on the windward wall. Then when the BEFs are on both walls, the ventilation capacity of the canyon is weakened greatly, and the pollutant concentration in the ground center is increased significantly, especially near the windward side. Moreover, the discontinuity BEFs will weaken the effect of the continuity BEFs on the in-canyon flow and dispersion, specifically, the discontinuity BEFs reduced the region of high pollutant concentration distributions. These findings can help optimize the BEFs design to enhance ventilation and mitigate traffic pollution.

Evaluation of the ventilation and pollutant exposure risk level inside 3D street canyon with void deck under different wind directions.

With continuous global warming, growing urban population density, and increasing compactness of urban buildings, VD (void deck) street design has become increasingly popular in city planning, especially in tropical countries. However, understanding on traffic pollutant dispersion inside the street canyons with VDs is still at early stage. This paper evaluates quantitatively the effects of VD location and wind direction on the ventilation and traffic pollutant exposure inside the street canyon with VDs. The results show that under seven wind directions (0°, 15°, 30°, 45°, 60°, 75°, and 90°), the VD provides higher ACH than that of the regular canyon, especially at high α (angle between the approaching wind and the canyon axis). Also, mean K (dimensionless pollutant concentration) values of the canyon wall and pedestrian respiration plane on one side where VD is located are significantly reduced compared to the regular canyon. Therefore, when VDs are at both buildings, both pedestrian respiration planes and walls have the lowest K values, thus providing the best living environment for pedestrians and near-road residents. In addition, as α increases, the K values on both respiration planes significantly decrease except for the leeward respiration plane of the canyon with the windward VD. These findings can help to design urban street canyons for mitigating traffic pollution risk and improving ventilation in tropical cities with frequently changing wind directions.

New insights on precise regulation of pollutant distribution inside a street canyon by different vegetation planting patterns.

Mixed-vegetation planting patterns are commonly seen in urban areas for specific reasons like aesthetic, cooling, and particle deposition effects of the vegetation. However, they may have a negative impact on human health by worsening the air quality inside the street canyon due to the decreased air exchange rate. From the view of precise control of pollutant concentration in the sensitive areas of people's concern in the existed street canyons, thirty-four cases with different vegetation planting patterns and pressure loss coefficients (λ) are studied numerically to investigate the effects of vegetation on airflow and pollutant dispersion inside the canyon. The cases of treeless and 2 rows of tree planting patterns in wind-tunnel measurements were selected for the model validation. The results demonstrate that compared to the treeless case, the greenbelts can greatly change the airflow features and reduce the pollutant concentration at the leeward side, while the only-tree planting patterns have little impact on the flow and deteriorate dispersion within the street canyon. Moreover, rows of greenbelts planted under the corresponding trees can reduce the average pollutant concentrations on the leeward wall and the footpath of the street canyon by up to 22.6% and 33.2%, respectively. Besides, the pattern of 1 row of trees with 1 row of greenbelts planted in the street canyon center should be suggested as the optimal mixed vegetation configuration in this study. That is because compared to the treeless case the pollutant concentration on leeward wall, windward wall, leeward footpath, and windward footpath can be reduced by 14.2%, 10.0%, 24.6%, and 37%, respectively. It is helpful to the city planners to consider whether the disadvantages of planting vegetation inside the street canyon would overwhelm the advantages.

Evaluation on ventilation and traffic pollutant dispersion in asymmetric street canyons with void decks.

With continuous global warming, growing urban population density, and increasing compactness of urban buildings, the "void deck" street canyon design has become increasingly popular in city planning, especially for urban streets located in tropical areas. Nevertheless, research on traffic pollutant dispersion in street canyons with void decks (VDs) is still at its early stage. This study quantitatively evaluates the effects of void deck height and location on the canyon ventilation and pollutant dispersion in asymmetric street canyons with void decks, and the pollutant exposure risk level for pedestrians and street dwellers. Void decks introduce more fresh air, thereby greatly improving the ventilation properties of the asymmetric canyon. The air exchange rate (ACH: 147.9%, 270.9%) and net escape velocity (NEV*: 416.7%, 915.8%) of the step-up and step-down canyons with VDs (3 m high at full scale) at both buildings are higher than those of regular asymmetric canyons. Moreover, the mean dimensionless pollutant concentration (K) on the building wall and pedestrian respiration plane in which VDs are located stands at a low level, because pollutants are removed by the airflow entering or exiting through the void decks. Increased VD height (4.5 m at full scale) enhances the strength of airflow flowing into and out of the canyon, significantly increasing ACH (177.3%, 380.9%) and NEV* (595.2%, 1268.4%) and decreasing the mean K on both pedestrian respiration planes and canyon walls. In particular, the K values on both pedestrian respiration planes and both walls are almost zero for the canyons with VDs at both buildings. Therefore, among the three VD locations, both VDs provide the best living environment for pedestrians and near-road residents. These findings can help to design urban street canyons for mitigating traffic pollution risk and improving ventilation in tropical cities.

Influence of wind direction on the ventilation and pollutant dispersion in different 3D street canyon configurations: numerical simulation and wind-tunnel experiment.

Configuration of street canyon and the wind environment have a great influence on the self-ventilation capacity of the canyon, but the couple-effect of these two factors could not be considered in the previous study. The purpose of this study is to clarify the couple effect of street canyon configuration and wind environment on the ventilation and pollutant dispersion inside the street canyon. For this purpose, five wind directions of α = 90°, 60°, 45°, 30°, and 0° (α is the angle between the approaching wind and street canyon) and three canyon configurations (flat, step-up, and step-down canyons) were considered with numerical simulation and wind-tunnel experiment. Meanwhile, ACH (air exchange rate) and NEV (net escape velocity) were used to evaluate the ventilation capacity of the canyon. The results reveal that the wind direction has a vital influence on the ventilation in the different canyon configurations. Under the parallel wind direction (α = 0°), the airflow and ventilation capacity inside the three canyons are similar. Relative difference of ACH, named as RDA ((ACHasymmetric-ACHsymmetric)/ACHsymmetric [Formula: see text] 100%), is 1.82%. However, under the oblique (α = 30°, 45°, and 60°) and perpendicular wind direction (α = 90°), the airflow of the step-down canyon is very different from the step-up and flat canyons. In step-down canyons, reverse flow occurs under the oblique and perpendicular wind direction, and the strength of the reverse flow increases as α increases. Due to this reverse flow, the ventilation capacity of the step-down canyon is lower than that of the step-up and flat canyons. As for the ventilation capacity in the pedestrian respiration domain, the ventilation capacity of the leeward pedestrian domain (leeward NEV) is higher in the step-down canyon than in the step-up canyon and the flat canyon (when α = 90°, leeward NEV of step-down canyon is 2.47 times the flat canyon). Conversely, the ventilation capacity of the windward pedestrian domain is lower in step-down canyons than in step-up or flat canyon (when α = 90°, windward NEV of step-down canyon is 0.1 times that of step-up canyon). The aforementioned findings are helpful to understand the effects of canyon configurations together with wind directions on the airflow as well as pollutant concentration inside the canyon. Although further researches are still required to provide practical guidelines, this study present effective methodologies to quantify the influences of street configurations and wind directions on street canyon ventilation for urban design purpose.

Effects of void deck on the airflow and pollutant dispersion in 3D street 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. However, previous studies have aimed to improve the pedestrian-level wind speed with void deck in single buildings or short canyons. 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. The results show that the void decks have a significant effect on the airflow and pollutant distribution inside the canyon. 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. 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. In particular, when the void decks are at both buildings, they are increased by 38.3 times. Also, for the canyons with the void deck, traffic pollutants are removed out of the canyon by the strong airflow through the void deck. 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. 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. These findings can help ease traffic pollution inside the street canyons composed of high-rise buildings, especially in tropical cities.