Asymmetric Street Canyons Research Articles

Assessment of pollution removal mechanisms in steep-asymmetric city-type environments using wind deflectors

Traffic emissions directly impact indoor air quality in near-road buildings. Adjustable wind deflectors on building roofs were previously shown to be effective in mitigating air pollution for ideal city-type environments. This was based on the hypothesis that wind deflectors promoted higher pollutant removal by reducing the dependence on turbulent fluctuations. However, the question of whether such a deflector system would work in a more complex city-type environment such as an asymmetric street canyon remains unanswered. In addition, the fundamental impact a deflector could impart on flow dynamics within street canyons in the context of pollution removal through differing mechanisms also requires further research. The current study seeks to answer both questions by introducing adjustable wind deflectors for step-up and step-down asymmetric canyons with two traffic flow directions. For the step-down canyon, the deflectors promoted CO reduction in building facades by 73.55% and 34.79% from leeward and windward walls under a Cross Road Pollution (CRP) source. A 16.57% reduction was achieved on side walls under a Side Road Pollution (SRP) source. However, apart from the 13.87% CO reduction across windward walls under the CRP source, the wind deflectors predominantly resulted in detrimental results for step-up canyons. The ratio of pollution exchange rate achieved by mean flow-induced fluxes and total pollution exchange rate (θ), Sherwood number is the ratio of convective and diffusive mass transfer (Sh) and average canyon concentration (C‾canyon) were used as indices to investigate pollution removal mechanisms. Although both Shv/sC‾canyon and θv/sC‾canyon relationships exhibited good inverse correlations when the deflector was positioned at different locations, the Shv/sC‾canyon showed superior performance in distinguishing the scenarios where a deflector was involved and when not. This implies that the introduction of wind deflectors impacted more in effecting convective fluxes than fluctuations for pollution removal.

Effects of increasing the degree of building height asymmetry on ventilation and pollutant dispersion within street canyons

Rational urban design helps to build sustainable cities with high ventilation capacity and pollutant removal capacity, but the effect of building height on ventilation and pollutant dispersion inside asymmetric canyons has not been fully studied. In this paper, we studied the effect of increasing the degree of building height asymmetry (DBHA) on canyon ventilation and pollutant diffusion in shallow and deep asymmetric street canyons by considering six different building height ratios (BHR = 3/4, 1/2, 1/3, 4/3, 2/1 and 3/1). The results show that increasing the DBHA in asymmetric canyons can improve the ventilation and pollutant removal capacity. For step-up canyons, increasing the downwind building height is very useful to improve ventilation and pollutant removal. For shallow/deep step-up canyons with BHR = 1/3, the air exchange rate (ACH) increased to 211.2% and 380.1% of the flat canyons, respectively. The spatially-average pollutant concentration in the pedestrian zones (leeward Kavg* ang windward Kavg*) decreases significantly with the increase of DBHA, especially for the deep step-up canyon with BHR = 1/3, the leeward Kavg* and windward Kavg* decrease to 15.3% and 3%, respectively. Also, increasing the upwind building height can also improve the ventilation capacity in the step-down canyons. For the deep step-down canyon with BHR = 3/1, the leeward Kavg* and windward Kavg* decreased to 40.6% and 24.1% of the deep flat canyon, respectively. Notably, the ventilation capacity is very low for step-down canyons with BHR = 4/3, and for step-down canyons with BHR ≥ 2/1, the ventilation capacity and pollutant removal capacity increase significantly with the increase of DBHA. Therefore, in urban planning, step-down canyons with BHR = 4/3 should be avoided and designed to satisfy the condition of BHR ≥ 2/1. These findings will be a valuable reference for urban designers to build sustainable cities with high ventilation capacity.

Comparative assessment of ground-level air quality in the metropolitan area of Prague using local street canyon modelling

Metropolitan areas have recently been identified as hotspots in terms of air quality and pollution-related diseases caused by gaseous pollutants and particulate matter primarily from road emissions. This study was carried out to investigate the suitability of the air dispersion modelling for predicting the short-term concentration of selected pollutants within the asymmetric street canyon geometries. The geographic projection covered a modelling domain of 0.5 × 1.4 km, with a grid resolution of 2 m mapped to the receptor site of an operational ambient air quality monitoring station in Prague, Czech Republic. Basic street canyon simulation showed the best performance metric for the prediction of PM10 concentration. High spatiotemporal performance was found by the advanced street canyon simulation in predicting hourly averages of NOx and PM10. The underprediction of the modelled NOx concentration corresponded to those of previous studies with FB value of 0.11 and NMSE <4. The slightly overprediction of PM10 concentration with the obtained FB value of −0.19, the NMSE value of 0.04, and the FAC2 of 0.53 were in agreement with previous modelling work. The simulation results provided additional evidence for the underpredicted concentration of PM2.5 in different model configurations. Advanced street canyon simulation improves the accuracy of ADMS-Urban by a maximum of 34%. The underprediction of short-term emission was graphically analysed in time-series profiles. The validation results and significant remarks greatly contribute to the essential optimization of urban street canyon model to better represent observations in the street canyon geometries of historic European building architectures.

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 (3m 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 (4.5m 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.

CFD modeling on the canyon ventilation and pollutant exposure in asymmetric street canyons with continuity/discontinuity balconies

Balconies have significant impacts on the ventilation and pollutant dispersion inside the street canyon. However, as the balconies used in previous studies are continuity balconies, it cannot accurately describe the effects of balconies in a real street canyon with discontinuity balconies. This paper approaches the actual situation by exploring the effects of discontinuity balconies and their locations on the ventilation and pollutant exposure inside the asymmetric canyons. We found that discontinuity balconies significantly improve the ventilation and pollutant dispersion by allowing more fresh air to enter the canyon compared to continuity balconies. ACH (air exchange rate) values of the asymmetric canyons are the lowest when balconies are located at both buildings and are the highest when balconies are at only low building. Also, the effects of discontinuity balconies are more dominant for the step-up canyon. When balconies are located on both buildings, ACH values of the step-up and step-down canyons with continuity balconies are reduced by 89.0% and 94.4% of regular canyons without balconies, but they stay at 93.4% and 96% for the case of discontinuity balconies. In addition, the discontinuity balconies significantly reduce the pollutant exposure risk level for pedestrians and first-floor residents, compared to continuity balconies. Notably, the influence of discontinuity balconies is more pronounced at the canyon center than at the canyon laterals, so mean pollutant concentrations at the center of the wall are considerably reduced. That is, previous studies have overestimated the effects of balconies. These findings can contribute to deep understanding of the effect of balconies on the urban canyon design.

Effects of building layouts and envelope features on wind flow and pollutant exposure in height-asymmetric street canyons

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.

Toward PM2.5 Distribution Patterns Inside Symmetric and Asymmetric Street Canyons: Experimental Study

AbstractDistribution patterns of particulate matter 2.5 (PM2.5) inside urban street canyons varies according to the street canyon enclosure ratio (SCER) and traffic flow states, and therefore they ...

Field synergy analysis of pollutant dispersion in street canyons and its optimization by adding wind catchers

The microenvironment, which involves pollutant dispersion of the urban street canyon, is critical to the health of pedestrians and residents. The objectives of this work are twofold: (i) to effectively assess the pollutant dispersion process based on a theory and (ii) to adopt an appropriate stratigy, i.e., wind catcher, to alleviate the pollution in the street canyons. Pollutant dispersion in street canyons is essentially a convective mass transfer process. Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature, we apply the field synergy theory to the study of pollutant dispersion in street canyons. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind catcher, wind speed and the geometry of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons. The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%. Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.

Effects of height-asymmetric street canyon configurations on outdoor air temperature and air quality

This paper investigates the effects of height-asymmetric street canyon configurations on air temperature and air quality at the pedestrian level using the ANSYS Fluent® software. The study concerns the situation with a subtropical city where there is a predominant wind direction (as is the case in, e.g., Hong Kong) and where the direction of that wind is perpendicular to the street canyon, since this is the worst-case from air pollution and overheating point of view. In particular, this North-South oriented street has been studied with the realistic solar irradiance at two different sun directions, corresponding to morning (08:00) and afternoon (16:00) hours, respectively. Two step-up and two step-down North-South oriented street canyons are considered under two different incoming wind speeds (high and low). The corresponding ratios of upwind and downwind building heights are = 1/3, 2/3 and 3/1, 3/2, respectively.The results demonstrated that for the step-up canyon, a higher upwind building was found to produce a hotter air temperature only at a low wind speed and polluted more severely at both high and low wind speeds, compared with its lower upwind building counterpart. In contrast, for the step-down canyon, a higher downwind building was found to produce cooler air temperatures at both high and low wind speeds and accumulated more pollutants only at a low wind speed, compared with its lower downwind building counterpart. On the other hand, at the high wind speed, both air quality and thermal environment were better in the step-up canyon than in the step-down canyon. However, at the low wind speed, the air quality was higher in the step-down canyon than the step-up canyon, while the step-up canyon still provided better thermal environment than the step-down canyon. Moreover, a Richardson number (Ri) for the asymmetric street canyons is defined for the evaluation of the buoyancy force versus the inertial force. When |Ri| > 20, the flow field was mainly dominated by natural convection, and an increase of |Ri| resulted in an increase in the air temperature and a decrease in the pollutant concentration. In contrast, when |Ri| < 20, the flow field was dominated by forced convection, and the variation of |Ri| had an insignificant influence on air quality and air temperature. The simulated pollutant concentration and thermal environment results were further processed to obtain optimization guidelines for a north-south asymmetric canyon in the city centers of Hong Kong via the application of multivariate regression analysis with a group of dimensionless parameters. These guidelines will facilitate the renewal of north-south asymmetric street canyons while enhancing air quality and lowering air temperature by serving as a reference for architects.

Dynamic three-dimensional distribution of traffic pollutant at urban viaduct with the governance strategy

Urban viaducts influence air pollution due to their elevated emission sources and complex wind fields. This study investigates the three-dimensional distribution of a typical traffic pollutant (i.e., PM2.5) alongside a viaduct in Guangzhou, China, by using a movable measurement platform. Based on the measured data, the study explores the statistical relationship between the pollutant and the influencing factors. A hybrid method with validation using the measured data is developed to model the dynamic variations in the PM2.5 distribution. The distribution of PM2.5 is studied in an asymmetric street canyon with a viaduct under different wind speeds (i.e., 0.7 m/s, 4 m/s, 10 m/s), and the influence of canyon structures with different height–width ratios on the PM2.5 dispersion is evaluated. The study finally proposes a series of strategies for reducing roadside pollution along the viaduct. The well-validated hybrid model is also used to determine the optimal pollution reduction strategy for a specific scenario. This work not only reveals the dynamic distribution law and key influencing factors of traffic pollutants in urban viaducts but also provides an important practical basis for traffic infrastructure construction and roadside building design to effectively alleviate air pollution from traffic emission sources.

Influence of avenue trees on traffic pollutant dispersion in asymmetric street canyons: Numerical modeling with empirical analysis

The dispersion of traffic-related pollutants in urban street canyons is of importance for the health and quality of lives. To reveal the inherent principle, researchers have performed a lot of investigations; many dispersion phenomena have also been assessed during recent years. However, the presence of avenue trees in street canyons and their capacity for pollutant dispersion remains partly addressed. In this study, we investigated the effects of avenue trees in urban street canyons on traffic pollutant dispersion. The dispersion of CO concentration in asymmetric street canyons was simulated under varied situations. The computational results showed a good agreement with the experimental data, and the numerical model was validated to be adequate for investigating the pollutant dispersion in street canyons. Then, the numerical simulations were extended to explore the impacts of the effects of avenue trees on CO dispersion; the results indicated that avenue trees generally increase CO concentrations in asymmetric street canyons. When the wind direction is perpendicular to the street axis, a terraced building raises pollutant concentrations at the windward wall and reduces concentration at the leeward wall on the pedestrian levels. Findings of this study are expected to provide significant insight into urban road design and strategy making for avenue tree planting, particularly under the existing worldwide sustainable low-carbon urban development.

The impact of traffic-flow patterns on air quality in urban street canyons

We investigated the effect of different urban traffic-flow patterns on pollutant dispersion in different winds in a real asymmetric street canyon. Free-flow traffic causes more turbulence in the canyon facilitating more dispersion and a reduction in pedestrian level concentration. The comparison of with and without a vehicle-induced-turbulence revealed that when winds were perpendicular, the free-flow traffic reduced the concentration by 73% on the windward side with a minor increase of 17% on the leeward side, whereas for parallel winds, it reduced the concentration by 51% and 29%. The congested-flow traffic increased the concentrations on the leeward side by 47% when winds were perpendicular posing a higher risk to health, whereas reduced it by 17–42% for parallel winds. The urban air quality and public health can, therefore, be improved by improving the traffic-flow patterns in street canyons as vehicle-induced turbulence has been shown to contribute significantly to dispersion.

Study on Vehicle Emissions Dispersion in Asymmetrical Street Canyons and Proper Location of Air Intake for Ventilation System

Traffic–related pollutant has been recognized as an air pollution hot spot due to its large emission rate and great health impacts for the exposed population. In the present investigation, a computational fluid dynamics technique is used to investigate the effect of street layout on pollutant dispersion and find proper location for air inlet in ventilation systems. Four typical configurations of asymmetric street canyons are considered. It is found that pollutant retention time at people stayed zones is shorter, when buildings near the road are laid as down-wind building lower than the up-wind building. For buildings at down-wind side of a main road, air inlet should be located at the leeward façade of the building.

Traffic pollution modelling in a complex urban street

This study explores for the first time, the applicability of the Danish Operational Street Pollution Model (OSPM) in the city of Buenos Aires where street canyons are very irregular. The model is applied in an irregular and asymmetric street canyon of a five-lane avenue near a street intersection. Urban background concentrations estimated by the DAUMOD model are considered. Meteorological information registered at the domestic airport located in the city is used in calculations. Three months of hourly NOx, NO2 and CO estimated concentrations are compared with measurements inside the street canyon. Statistical evaluation of model results shows that OSPM performance is quite good.

Hourly NO&lt;SUB align="right"&gt;x concentrations and wind direction in the vicinity of a street intersection

This study relates NOx concentrations and wind (speed and direction) measured inside an asymmetric street canyon, with ambient wind. The monitoring site is located near a street intersection. The wind measured at the domestic airport located in the city, near the coast, has been considered as ambient wind. Wind direction at the monitoring site inside the canyon is mainly from the E, NE, W and NW for any ambient direction. In general, the behaviour of horizontal airflow at the monitoring site for different ambient wind directions seems to be quite similar to those observed at other street intersections reported in the literature.

Air pollution in a street canyon estimated considering different parameterisations of vehicle-induced turbulence

Leeward air pollutant concentrations (C) are estimated by incorporating four parameterisations of traffic-produced turbulence (TPT) into the scaling of C inside a street canyon. Three parameterisations consider expressions previously introduced by other authors, based on a theoretical formulation of TPT and a semi-empirical approach already incorporated in an operational street model. The fourth scheme introduces an empirical expression of TPT derived from four full-scale street canyon datasets. Hourly NO x concentrations are calculated for an asymmetric street canyon in Buenos Aires, using meteorological observations at local airport and modelled background concentrations. Statistical indicators show that the performance of the four schemes is satisfactory.

CFD 모형을 이용한 3차원 비대칭 도로 협곡에서의 흐름 및 오염물질 분산 연구

A three-dimensional computational fluid dynamics (CFD) model with the renormalization group (RNG) <TEX>$k-{\varepsilon}$</TEX> turbulence model is used to examine the effects of difference in building height on flow and pollutant dispersion in asymmetric street canyons. Three numerical experiments with different street canyons formed by two isolated buildings are performed. In the experiment with equal building height, a portal vortex is formed in the street canyon and a typical recirculation zone is formed behind the downwind building. In the experiment with the downwind building being higher than the upwind building, the ambient flow comes into the street canyon at the front of the downwind building and incoming flow diverges strongly in the street canyon. Hence, pollutants released therein are strongly dispersed through the lateral sides of the street canyon. In the experiment with the upwind building being higher than the downwind building, a large recirculation zone is formed behind the upwind building, which is disturbed by the downwind building. Pollutants are weakly dispersed from the street canyon and the residue concentration ratio is largest among the three experiments. This study shows that the difference in upwind and downwind building height significantly influences flow and pollutant dispersion in and around the street canyon.

Vertical variations of particle number concentration and size distribution in a street canyon in Shanghai, China

Measurements of particle number size distribution in the range of 10-487 nm were made at four heights on one side of an asymmetric street canyon on Beijing East Road in Shanghai, China. The result showed that the number size distributions were bimodal or trimodal and lognormal in form. Within a certain height from 1.5 to 20 m, the particle size distributions significantly changed with increasing height. The particle number concentrations in the nucleation mode and in the Aitken mode significantly dropped, and the peaking diameter in the Aitken mode shifted to larger sizes. The variations of the particle number size distributions in the accumulation mode were less significant than those in the nucleation and Aitken modes. The particle number size distributions slightly changed with increasing height ranging from 20 to 38 m. The particle number concentrations in the street canyon showed a stronger association with the pre-existing particle concentrations and the intensity of the solar radiation when the traffic flow was stable. The particle number concentrations were observed higher in Test I than in Test II, probably because the small pre-existing particle concentrations and the intense solar radiation promoted the formation of new particles. The pollutant concentrations in the street canyon showed a stronger association with wind speed and direction. For example, the concentrations of total particle surface area, total particle volume, PM2.5 and CO were lower in Test I (high wind speed and step-up canyon) than in Test II (low wind speed and wind blowing parallel to the canyon). The equations for the normalized concentration curves of the total particle number, CO and PM2.5 in Test I and Test II were derived. A power functions was found to be a good estimator for predicting the concentrations of total particle number, CO and PM2.5 at different heights. The decay rates of PM2.5 and CO concentrations were lower in Test I than in Test II. However, the decay rate of the total particle number concentration in Test I was similar to that in Test II. No matter how the wind direction changed, for example, in the step-up case or wind blowing parallel to the canyon, the decay rates of the total particle number concentration were larger than those of PM2.5 and CO concentrations. For example, CO concentrations decreased by 0.33 and 0.69 at the heights ranging from 1.5 to 38 m in Test I and Test II, while the total particle number concentrations decreased by 0.72 and 0.85 within the same height ranges in Test I and Test II. It is concluded that the coagulation process, besides the dilution process, affected the total particle number concentration.

Modelling the air flow in symmetric and asymmetric street canyons

The main objective of this paper is to describe flow features in sequences of two-dimensional street canyons. Several configurations of symmetric and asymmetric canyons are studied by CFD modelling. Streets with different aspect ratios (W/H), where W is the width of street and H is the building height, are investigated. In asymmetric situations, one building (in the central canyon) is taller (30 m) than the others (20 m). The presence of the taller building seems to have a large influence on the wind flow inside the central street canyon and in ones located downstream. These results show that simulations are Reynolds-independent.

Street canyon aerosol pollutant transport measurements

Current understanding of dispersion in street canyons is largely derived from relatively simple dispersion models. Such models are increasingly used in planning and regulation capacities but are based upon a limited understanding of the transport of substances within a real canyon. In recent years, some efforts have been made to numerically model localised flow in idealised canyons (e.g., J. Appl. Meteorol. 38 (1999) 1576–89) and stepped canyons (Assimakopoulos V. Numerical modelling of dispersion of atmospheric pollution in and above urban canopies. PhD thesis, Imperial College, London, 2001) but field studies in real canyons are rare. To further such an understanding, a measurement campaign has been conducted in an asymmetric street canyon with busy one-way traffic in central Manchester in northern England. The eddy correlation method was used to determine fluxes of size-segregated accumulation mode aerosol. Measurements of aerosol at a static location were made concurrently with measurements on a platform lift giving vertical profiles. Size-segregated measurements of ultrafine and coarse particle concentrations were also made simultaneously at various heights. In addition, a small mobile system was used to make measurements of turbulence at various pavement locations within the canyon. From this data, various features of turbulent transport and dispersion in the canyon will be presented. The concentration and the ventilation fluxes of vehicle-related aerosol pollutants from the canyon will be related to controlling factors. The results will also be compared with citywide ventilation data from a separate measurement campaign conducted above the urban canopy.