The influences of building layouts on pollutant dispersion within urban street canyons have been widely studied, although they have been rarely considered with envelope features. Different envelope ...features, such as wing walls, balconies and overhangs, have not yet been quantitatively assessed for their effects on the wind flow patterns around street canyons to a certain degree. Adopting the evaluation indicators of personal intake fraction and daily pollutant exposure, this study aims to investigate the potential influences of building layouts and envelope features that affect pollutant exposure risks for pedestrians and near-road residents via computational fluid dynamics (CFD). The turbulence modelling approach and numerical methods are validated by reported experiments in the literature. Asymmetric street canyons with an aspect ratio of 2 and two typical building layouts, namely, step-up and step-down notch configurations, are further investigated to test their effects. The results indicate that the step-down configuration provides worse situations for wind environments and pollutant dispersion than symmetric and step-up configurations. More specifically, the presence of overhangs has the greatest impact on the personal intake fraction (P_IF) change ratio, followed by balconies. The largest P_IF change ratio occurs on the fifth floor of an upstream building when overhangs are applied to the step-down street canyon. The aforementioned findings are helpful to understand the influences of building layouts together with envelope features on the wind environment as well as pollutant exposure risks experienced by pedestrians and near-road residents.
•Influence of different building layouts on pollutant exposure is assessed.•Personal Intake fraction is adopted to evaluate exposure risks quantitatively.•Effect of wing walls on pollutant exposure is lowest.•Balconies have the most evident impact on exposure risks in the step-up canyon.•Overhangs bring the most dramatic change to the values of personal intake fraction in the step-down canyon.
Vertical greening is one of the effective measures to improve the microclimate and air quality of high-density urban street canyons, and its impact on the thermal environment is closely related to ...the street canyon morphology. In this paper, based on the characteristics of high-density urban street canyons in the context of Shenzhen City, 12 sets of cases (two orientations, two aspect ratios, and three height-to-width ratios) were simulated and designed using ENVI-met software to study the effects of vertical greening on the thermal environment of buildings under different street canyon morphologies, and to summarize the impact of vertical greening on the microclimate of different street canyons. The results of the study show that the orientation of the street canyon is the most influential factor in the thermal efficiency of vertical greening under the street canyon geometry with the same amount of greening and that the vertical greening effect of east-west oriented street canyons is generally better than that of north-south oriented street canyons in terms of cooling, humidification, and improvement of thermal comfort at the walking level.
This study examines the influence of building envelope features on interunit dispersion around multi-story buildings, when the presence of an upstream interfering building is also considered. ...Validated CFD methods in the steady-state RANS framework are employed. In general, the reentry ratios of pollutant from a source unit to adjacent units are mostly in the order of 0.1%, but there are still many cases being in the order of 1%. The influence of envelope features is dependent strongly on the interaction between local wind direction and envelope feature. In a downward dominated near-facade flow field, the presence of vertical envelope features forms dispersion channels to intensify the unidirectional spread. Horizontal envelope features help induce the dilution of pollutant to the main stream and weakens largely the vertical interunit dispersion. The large influences caused by the presence of envelope features extend the existing understanding of interunit dispersion based on flat-facade buildings.
Previous studies on the effects of envelope features on ventilation performance have neglected the buoyancy effect or simplified it as a uniform surface heating effect. However, envelope features can ...create a shading effect that changes the temperature distribution and enhances the buoyancy effect, thus resulting in inaccurate findings. We conducted computational fluid dynamics (CFD) simulations based on scaled outdoor experiments to investigate the effects of different envelope features (balconies, overhangs, and wing walls) on pollutant dispersion and ventilation performance. The buoyancy effect created a new local vortex and changed the wind flow pattern at the base of the flat-facade and wing wall street canyons. The age of the air at the base of the flat-facade, balcony, overhang, and wing wall street canyons increased by 30.23 %, 20.45 %, 21.74 %, and 30.95 %, respectively. Moreover, when investigating the effects of envelope features on ventilation performance while ignoring the buoyancy effect, there was a high error rate in the ventilation performance, and the rates were as follows: wing wall (24.03 %) > flat-facade (23.20 %) > overhang (18.56 %) > balcony (14.05 %). This study provides a suitable methodology for investigating the effects of different envelope features on pollutant dispersion and ventilation performance in two-dimensional street canyons.
•Effects of features on ventilation by CFD simulation based on outdoor experiments.•The buoyancy effect significantly affects the wind flow pattern and ventilation.•The flow pattern and ventilation for uniform/nonuniform heating are similar.•Wing wall case's ventilation difference between no surface/nonuniform heating is 24.03 %.
Traffic pollution has posed a serious threat to the health of near-road city residents and pedestrians, especially in high-density cities. However, the influences of envelope features, including ...balconies, overhangs and wing walls, on pollutant exposure to near road residents and pedestrians have not been fully understood. This paper investigates the effects of three commonly-used envelope features on wind flow and pollutant exposure to residents in street canyon with three different aspect ratios. The evaluation metrics of personal intake fraction and daily pollutant exposure are used to quantitatively assess the influences caused by different envelope features on healthy risk of near-road residents and pedestrians, alongside with wind flow pattern and pollutant distribution. The results show that these envelope features have increased the risk of pollutant exposure for the leeward side residents, while the risk of pollutant exposure for the windward side residents is reduced for most cases, in particular for the first floor. This observation is especially prominent when the canyon has the highest aspect ratio among the tested ratios, with the increased ratio of personal intake fraction reaching up to 540%. Moreover, the pollutant concentration is overall higher on leeward side of upstream building than that of windward side of downstream building. These findings can help urban planners and architects to build healthy and sustainable urban environment.
•Effects of different envelope features on pollutant exposure are investigated.•Intake fraction and daily pollutant exposure are used to quantify the exposure risk.•Envelope features increase the pollutant exposure risk for the leeward residents.•Envelope features reduce pollutant exposure risk of windward residents.
The configuration of an urban street canyon plays an important role in determining local microclimate including pedestrian level wind environment (PLWE). The semi-closed U-type street canyon has been ...widely used in high-density trans-oriented-development urban design for providing private space. However, a quantitative evaluation on PLWE of the U-type street canyon is still absent to date. This study quantitatively assesses PLWE of the U-type canyon via particle image velocimetry tests in a wind tunnel. A range of canyon aspect ratios (the ratio of the canyon height to width) and length ratios (the ratio of the canyon length to height) were investigated. The wind flow inside and in the vicinity of the street canyons was measured and analysed under the perpendicular, oblique and parallel approaching wind directions. It was found that the U-type canyon shows a higher PLWE inside canyon on the open side while a lower PLWE inside canyon on the closed side. Compared to parallel canyon, there is a significant enhancement of PLWE in the centre of the U-type canyon and in some spots on the leeward side of the canyon. This is further strengthened with increasing the canyon aspect ratio and length ratio. Nevertheless, the U-type canyon exhibits a lower wind speed at the pedestrian level than that of the parallel canyon both inside and in the vicinity of the street canyons in general especially under parallel wind direction. It is believed that these findings are greatly beneficial to build sustainable cities with a high quality PLWE.
•PLWE of two popular canyons model (U and parallel) in TOD cities were evaluated.•Three wind directions (perpendicular, oblique and parallel) were examined.•A significant enhancement of PLWE in some areas in U-type canyon was revealed.•Inside canyon MR values are the largest under oblique wind direction.
Interunit dispersion problems have been studied previously mainly through on-site measurements, wind tunnel tests, and CFD simulations. In this study, a scaled outdoor experiment was conducted to ...examine the interunit dispersion characteristics in consecutive two-dimensional street canyons. Tracer gas (CO2) was continuously released to simulate the pollutant dispersion routes between the rooms in street canyons. The wind velocity, wind direction, air temperature, and tracer gas concentrations were monitored simultaneously. Two important parameters, the air exchange rate and reentry ratio, were analyzed to reveal the ventilation performance and interunit dispersion of the rooms in the street canyons. Based on the real-time weather conditions, it was found that the ventilation performance of the source room varied according to the room location. The air exchange rate distribution of the leeward-side room was more stable than that of the windward side. The tracer gas was mainly transported in the vortex direction inside the street canyon, and the highest reentry ratio was observed at the room nearest to the source room along the transportation route. In addition, under real weather conditions, the rooms in the street canyon have a high probability of experiencing a high reentry ratio based on the maximum reentry ratio of each room. This study provides authentic airflow and pollutant dispersion information in the street canyons in an urban environment. The dataset of this experiment can be used to validate further numerical simulations.
•A scaled outdoor experiment was conducted to study pollutant dispersion in 2D streets.•The tracer gas method using CO2 was adopted to simulate interunit transmission routes.•Air exchange rate/reentry ratio were used to assess ventilation/pollutant dispersion.•Ventilation performance in street canyon varied with the room location.•Highest reentry ratio appeared at room nearest to the source room along transportation route.
Most of the related research on pollutant dispersion has mainly focused on flat-facade buildings. However, envelope features can significantly affect the near-wall flow, which in turn affects the ...natural ventilation and consequent pollutant dispersion in urban canyons. We conducted an outdoor experiment to investigate the influence of different envelope features (balconies, overhangs, and wing walls) on pollutant exposure in wide and narrow street canyons (H/W = 1, 2). Wing walls effectively increase pollutant dispersion in wide and narrow street canyons, which results in an increase in the personal intake fraction (P_IF). The highest growth rate of the P_IF, reached 291 % and 400 %. However, balconies can decrease the P_IF on the second, third and fourth floors of both the east and west buildings. Additionally, envelope features notably increase the daily pollutant exposure index (Ed) and the order of the impact is wing walls > balconies > overhangs. In wide and narrow street canyons, the presence of wing walls yielded a significant increase in the Ed values compare with flat-facade canyons on all floors of both the east and west buildings, with the highest growth rate of Ed reaching 280 %. Meanwhile, the presence of balconies caused a significant increase in the Ed values on the first floor of both the east and west buildings in the narrow street canyons, but a decrease in the Ed values on the other floors of both buildings. This study provides valuable insights into the impact of envelope features on pollutant exposure in street canyons.
•Envelope features effect on pollutant exposure are studied in an outdoor experiment.•Daily pollutant exposure index (Ed) is used to quantify the pollutant exposure risk.•Wing walls effectively enhance pollutant dispersion in street canyons (H/W = 1, 2).•Personal intake fraction rate of change in wing wall canyons (H/W = 1, 2), reached 291 % and 400 %.•Envelope notably increase Ed, the impact order is wing walls > balconies > overhangs.
Vertical greenery systems (VGSs) have been adopted in city planning operations to mitigate excess heat in hot and humid subtropical cities. This study focused on the influence of different ...arrangements of vertical greening on pedestrian thermal comfort and particulate matter with a diameter of 10 µm (PM10) in street canyons. In this paper, the ENVI-met computational fluid dynamics (CFD) method was used to investigate the effects of different façade greenery arrangements with the same amount of greenery in the Nan Hai Yi Ku (NHYK) industrial district. On-site measurements were used to validate the simulation results in a transition season. The results showed that greening façades could improve pedestrians' thermal comfort with physiological equivalent temperature (PET) value reductions varying from 0.17 °C to 1.4 °C. Under a certain amount of greenery, the critical factor determining pedestrians' thermal comfort was the coverage rate of the greening façade near the pedestrian level. Specifically, increasing the greening façade coverage near the lower parts of street canyons could enhance the pedestrian-level cooling effect. In addition, the VGSs positively affected the pedestrian-level air quality in the street canyons. Nevertheless, the changes in pedestrian-level PM10 concentration induced by the presence of VGSs were not very obvious under the building-parallel wind direction.
•5 façade greening are compared on the impact of pedestrian thermal comfort and air quality.•Greening coverage rate of façade’s lower part plays the key role on improving pedestrian thermal comfort.•Centralized façade greening within 3 m from the ground has the greatest thermal benefit at pedestrian level.•Façade greening has negligible effect of improving pedestrian air quality.
The focus of previous research on urban ventilation has primarily been on flat-facade buildings. However, envelope features can affect the surface temperature and ventilation performance. We ...conducted an outdoor scaled experiment to investigate the impact of different envelope features on the thermal and wind environment in street canyons. The envelope features effectively reduce the surface temperature, with the impact order being as follows: overhangs > wing walls > balconies. The dimensionless parameter B is used to assess the impacts of momentum and buoyancy on urban ventilation. In a wide canyon, when B < Bc (the critical value of different envelope feature cases), the momentum dominates urban airflow. The Bc values of the flat-façade, balcony, overhang, and wing wall were 0.335, 1.084, 1.320, and 1.529, respectively, while the normalized horizontal velocities (U0.25H/U2H) of the flat-façade, balcony, overhang, and wing wall street canyons remained relatively constant (i.e., 0.66, 0.54, 0.62, and 0.57, respectively). When B > Bc, under the combined influence of momentum and buoyancy, U0.25H/U2H increases nonlinearly with B. Moreover, street canyons with envelope features exhibit a smaller Bc value than flat-facade canyons. These findings provide valuable insights into the effects of envelope features on thermal environments and urban ventilation.
•Evaluating features' effect on thermal and wind environment by outdoor experiment.•Dimensionless parameter B is used to evaluate the main driven force in airflow.•Upedestrian/Ufree remains approximately constant when wind momentum dominates.•Upedestrian/Ufree is non-linearly with B as the buoyancy effect is significant.•Balcony is more susceptible to momentum and buoyancy effects than other features.
PDF
