Pollutant Dispersion In Street Canyons Research Articles

The effect of noise barriers on viaducts on pollutant dispersion in complex street canyons

• The viaduct-noise barrier can remarkably influence the flow structure. • Using field synergy theory to analyze noise barrier-viaduct system. • Noise barrier-viaduct reduced the CO concentration in pedestrian breathing area. The noise reduction effect of noise barriers has been extensively studied, but the effect on pollutant dispersion remains unclear. A computational fluid dynamics (CFD) simulation is conducted to investigate the effects of different heights, lengths, and types of noise barriers and different wind speeds on pollutant dispersion in street canyons with viaducts. The field synergy theory of the convective mass transfer process is used for quantitative analysis of pollutant dispersion in street canyons. The results show that as the height and length of the noise barrier increase, the pollutant dispersion capacity decreases. As the wind speed increases, the rate of decrease in the average CO concentration declines. The effect of the wind speed on the synergistic improvement of the speed and concentration gradient vectors differs for different types of noise barriers. The performance follows the order: fully-closed noise barrier > left noise barrier > right noise barrier > semi-closed noise barrier. The different noise barrier types significantly impact the flow field and pollutant dispersion and reduce the CO concentration to varying degrees, except for the fully-closed type. The average CO concentration in the pedestrian breathing zone is reduced by a maximum of 55.85% on the leeward side and by 53% on the windward side, indicating that an appropriate noise barrier on the viaduct reduces noise pollution and improves the air quality in street canyons, especially in the pedestrian breathing zone.

Numerical investigation of the impact of urban trees on O3–NOx–VOCs chemistry and pollutant dispersion in a typical street canyon

Urban greening is one of major factors that influences flow, turbulence and air quality in street canyons. This paper aims to investigate the impact of urban trees on O3–NOx–VOCs (ozone -nitrogen oxides - volatile organic compounds) chemistry and pollutant dispersion in street canyons by Computational Fluid Dynamics (CFD). The Atmospheric Photolysis calculation framework (i.e., APFoam), which includes complex O3–NOx–VOCs chemistry into CFD, is employed to carry out the numerical simulations. The validation of the APFoam modelling results has been completed prior to further modelling works, including turbulent airflow, pollutant dispersion, and photochemical reactions. The influence of aerodynamic effects, biogenic VOC (BVOCs) emission and dry deposition of urban trees on air quality in a typical two-dimensional (2D) street canyon with aspect ratio H/W = 1 (where H is the building height and W the street width) are thoroughly examined. Moreover, the source contribution on ozone (O3) creation and the human health risk are also analyzed. Results show that, inside the street canyon, aerodynamic effects of trees have a greater impact on photochemical pollutant concentrations than BVOCs emission and dry deposition, the latter showing the smallest impact. In particular, the aerodynamic effects cause a wind reduction by 35%–45% at pedestrian level and subsequently an increase of nitrogen monoxide (NO) and nitrogen dioxide (NO2) concentrations by 95% and 66% near the ground, respectively, and an O3 concentration decrease by 35%. Further, the BVOCs emitted from trees, the vehicle VOCs and the background VOCs contribute 15%, 67%, and 9% to O3 production, respectively. These findings further suggest that the APFoam is an effective and promising tool which allows us to investigate the influencing mechanisms of trees on photochemical pollutant dispersion and urban air quality for the purpose of developing sustainable urban policy.

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.

Air pollutant dispersion in street canyons based on an outdoor scale model and machine learning

Air quality often deteriorates in street canyons owing to poor ventilation and increased emissions. In this study, the factors controlling air pollutant dispersion in street canyons were examined using outdoor scale models and machine learning. CO2 concentrations were measured at different heights for different ratios of building height (H) to street width (W) (H/W = 1, 2, 3). The results showed that when H/W increased from 1 to 2 and from 2 to 3, the mean CO2 concentration at a height of 0.25H (0.75H) in the street canyon increased by approximately 1 and 2 (0.5 and 1.5, respectively) times, respectively. An eXtreme Gradient Boosting (XGBoost) regression model for CO2 concentration was developed using machine learning based on different street canyon morphologies, monitoring locations, and meteorological conditions. Cross-validation demonstrated that the XGBoost model performed well on the test set, with an R2 value of 0.95. The SHapley Additive explanation (SHAP) values calculated for all samples showed that the five features that contributed most to the CO2 concentration were H/W, d (along-canyon position of the sensor), T (air temperature), W/2 (cross-canyon position of the sensor), and b_ws (atmospheric environmental background wind speed), with contributions of 34.1%, 19.1%, 12.1%, 10.5%, and 10.0%, respectively.

Investigation of O3–NOx–VOCs chemistry and pollutant dispersion in street canyons with various aspect ratios by CFD simulations

Based on the OpenFOAM environment, a new CFD code named as APFoam coupling turbulence with the complex photochemical mechanism of NOx-O3-VOCs reactions has been developed. The standard k-ε model is selected for turbulent airflow simulation and the SAPRC07 is for complex NOx-O3-VOCs photochemical reactions. Two-dimensional(2-D) full-scale street canyons with two aspect ratios(AR = H/W = 1 and 5, W = 24 m) are studied. By considering both physical effects and chemical reactions, the influences of three key factors on the photochemical pollutant dispersion and human health risk analysis are investigated, including the background wind speeds(Uref = 1.5 m/s, 3 m/s), pollutant concentrations from ambient air(O3-NOx-VOCs) and the emission(NOx-VOCs) source settings at the pedestrian-level. Besides, the effective measures to control O3 concentration is discussed.This study mainly concludes below. 1) The variations of NOx concentration increases about 20 times and O3 concentration decreases about 30% with the increase of AR. The chemical effect on photochemical pollutants is more important for the deep canyon(AR = 5) than that for the typical canyon(AR = 1). 2) The indoor and pedestrian-level exposure risk of NO2 is much larger than that of O3. The deep street canyon(AR = H/W = 5) has a higher exposure risk of NO2 than the typical canyon(AR = H/W = 1). 3) The NOx concentration increases about twice and O3 concentration decreases 6.5–13.5% with the decrease of Uref. 4) O3 concentration decreases after removing the background NOx and VOCs; it increases if there are no traffic emissions. 5) Simple traffic control measures can effectively reduce NOx but the reduction of O3 still requires additional VOCs control measures in a street canyons.

Impact of building façade geometrical details on pollutant dispersion in street canyons

The present study investigates the impact of building façade geometrical details on the pollutant transport mechanism in long street canyons. Large-eddy simulations (LES), extensively validated with experiments, are performed for four cases: (i) street canyon without façade balconies, (ii) street canyon with balconies at both windward and leeward façades, (ii) street canyon with balconies only at the windward façade and (iv) street canyon with balconies only at the leeward façade. The results show that the building balconies can strongly affect the wind flow field and pollutant dispersion in long street canyons. The most significant impact is observed for the two street canyon cases with balconies at the windward façade, which strongly obstruct the airflow from penetrating deep into the bottom of the canyon. The presence of balconies only at the windward façade and at both façades can increase the area-weighted mean pollutant concentration in the vertical center plane inside the canyon by 80% and 106%, respectively, and reduce the mean pollutant exchange velocity (Ue) by 46% and 54%, respectively. The analysis of the vertical mean convective and turbulent mass fluxes indicates that the presence of balconies mainly decreases the convective contribution to Ue, while the impact on the turbulent contribution is smaller.

Numerical evaluation of turbulence induced by wind and traffic, and its impact on pollutant dispersion in street canyons

In cities, street canyons are common features that affect the exposure level and health of pedestrians, drivers, and passengers. The purpose of this study is to analyze the characteristics of velocity, turbulence, pollutant dispersion, and pedestrian exposure levels inside a nearly 400 m asymmetrical street canyon in Dalian, China. We present a realizable k–ε turbulence model, with additional vehicle-induced turbulence (VIT) source terms in the momentum equation, turbulent kinetic equation, and turbulence dissipation rate equation, to simulate velocity and pollution diffusion under the predominant wind directions, varying inlet wind flow conditions, and different vehicle speeds. The results showed that the VIT effect dominated the velocity and pollution distribution under continuous traffic flow conditions. A significant underestimation of up to 33% for velocity, and obvious overestimation of up to 78% for CO concentration, appeared if the effect of VIT was neglected. The results also indicated that more attention should be paid to pedestrian health, as extremely high exposure levels occurred during traffic rush hours due to high vehicle exhaust emissions.

Numerical simulation of the effects of canopy properties on airflow and pollutant dispersion in street canyons

The air flow and pollutant concentration fields in a street canyon affected by trees could affect the comfort and health of residents. At present, the description of the non-uniform/discontinuous distribution of leaves is difficult. In this study, the leaf distribution in the canopy was characterized by establishing non-continuous (uniform/random) algorithm based on a numerical simulation method, and the effects of canopy properties including, height, porosity and uniform/random leaf distribution, on the airflow and pollutant concentration fields in urban street canyons were investigated. The position of the tree canopy was found to directly affect the airflow field form and the air velocity distribution in the street canyon at low inflows. The average air velocity in the street canyon could be reduced significantly when the top of the tree canopy is near the top of the street canyon. The air velocity and pollutant concentration in the street canyon would vary only slightly due to the canopy porosity. Due to the increasing canopy porosity, the air velocity would increase, and the pollutant concentration would be reduced. The leaves are non-continuous and uniformly distributed at constant porosity, which does not significantly change the velocity distribution and pollutant concentration in the street canyon.

A review of strategies for mitigating roadside air pollution in urban street canyons.

Urban street canyons formed by high-rise buildings restrict the dispersion of vehicle emissions, which pose severe health risks to the public by aggravating roadside air quality. However, this issue is often overlooked in city planning. This paper reviews the mechanisms controlling vehicle emission dispersion in urban street canyons and the strategies for managing roadside air pollution. Studies have shown that air pollution hotspots are not all attributed to heavy traffic and proper urban design can mitigate air pollution. The key factors include traffic conditions, canyon geometry, weather conditions and chemical reactions. Two categories of mitigation strategies are identified, namely traffic interventions and city planning. Popular traffic interventions for street canyons include low emission zones and congestion charges which can moderately improve roadside air quality. In comparison, city planning in terms of building geometry can significantly promote pollutant dispersion in street canyons. General design guidelines, such as lower canyon aspect ratio, alignment between streets and prevailing winds, non-uniform building heights and ground-level building porosity, may be encompassed in new development. Concurrently, in-street barriers are widely applicable to rectify the poor roadside air quality in existing street canyons. They are broadly classified into porous (e.g. trees and hedges) and solid (e.g. kerbside parked cars, noise fences and viaducts) barriers that utilize their aerodynamic advantages to ease roadside air pollution. Post-evaluations are needed to review these strategies by real-world field experiments and more detailed modelling in the practical perspective.

Large-eddy simulation of pollutant dispersion in generic urban street canyons: Guidelines for domain size

Pollutant dispersion in urban street canyons has been widely investigated by large-eddy simulation (LES). Many LES studies focused on generic street canyons under a wind direction perpendicular to the street axis. Accurate LES simulations require a sufficiently large domain size to minimize the effects of the artificial boundary conditions at the domain faces on the results. As opposed to RANS simulations, there is a lack of guidelines for an appropriate domain size for LES simulations of wind flow and pollutant dispersion in street canyons. The present study systematically investigates the effect of the domain width, domain height and upstream and downstream domain lengths on the wind flow and pollutant dispersion within a generic 2.5D street canyon with spanwise periodic boundary conditions. Following a validation study, 16 LES simulations are performed for different domain sizes. The results show that the minimum requirement for the domain width is 2.5H, where H is the roof height of the street canyon. For the domain height, upstream domain length and downstream domain length, 7.5H, 5H and 10H are recommended, respectively. These guidelines should help to reduce the computational costs of this type of simulation without significantly compromising the accuracy.

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.

Numerical study on flow field and pollutant dispersion in an ideal street canyon within a real tree model at different wind velocities

Studying the effects of trees on airflow and pollutant dispersion in urban street canyons is of considerable significance to clarify the laws of urban micro-scale airflow and pollutant dispersion. To characterize the trees in street canyons, different from the traditional vegetation resistance source method, in this paper, a fine tree model is used for numerical simulations of the airflow and pollutant dispersion in street canyons at varying inflow wind velocities. It is found during the study that the presence of trees physically blocks the airflow in street canyons. The airflow is sheared by the trunk, canopy, or branches, and then circumvents them, especially at a high inflow wind velocity. The low-velocity area in the street canyon distributes to the leeward side of the tree trunk as well as in the canopy area, and a discrete low-velocity distribution exists mainly in the canopy area. The average wind velocity in a street canyon with trees is approximately 39.5% lower at an inflow wind velocity of 1.7 m/s than that in a canyon without trees. In the presence of trees, the pollutant concentration in street canyons increases significantly, and the pollutants significantly accumulate between the tree trunk and the leeward side. With increasing inflow velocity, the pollutant concentration in a street canyon constantly changes but is much higher than that in the absence of trees. At a wind velocity of 5.7 m/s, the average pollutant concentration is 18.6% higher in street canyons with trees than in those without trees.

The effect of turbulence induced by different kinds of moving vehicles in street canyons

The turbulence induced by moving vehicles influences the flow field and pollutant dispersion in street canyons. In this paper, an ambient crosswind-vehicle movement mathematical model was established to analyze the turbulent kinetic energy(TKE) that is induced by a single vehicle for different wind directions; a new method--dynamic mesh updating method was employed to model the vehicle movement. Based on this, we analyzed the characteristics relating to turbulence induced by three types of moving vehicles (truck, SUV and car) which were commonly found on urban roads.The results showed that the movement of a vehicle has small influence over the the flow field at the front and both sides of the vehicle. However, its influence on the flow field in the wake of the moving vehicle was significant, and the influence increased as the distance the vehicle moved increased. It was shown that the shape of the vehicle had a significant influence on the quantity of TKE generated by the moving vehicle. At the rear of the vehicle, the TKE induced by the movement of the truck was about 1.6 times that of the SUV and 2.5 times that of the car. It was also observed that the wind direction had a significant influence on the TKE that was induced by the moving vehicles.

Numerical Study of the Composite Effects of Uneven Street Canyons and Time-varying Inflows on the Air Flows and Pollutant Dispersion

ABSTRACT The dispersion behavior of pollutants in realistic street canyons is dominated by the pattern of airflow, which is closely linked to the building layout and air inflow at the street canyon boundary. Although uneven building layouts and time-varying inflows have been individually investigated for their influence on air flow pattern and pollutant dispersion in street canyons, the combined effects of these two factors have not been examined. In this study, four street canyon models, one even and three uneven models with different occupying ratios of high-rise buildings, were numerically studied using large-eddy simulation under steady inflow and time-varying inflow conditions. The results showed that both the uneven building layouts and the time-varying inflow condition enhanced the air flow strength and pollutant removal efficiency inside the street canyon. However, the composite effects of these factors were not simply additive. The worst pollutant removal efficiency was observed for air flow in the even street canyon with the steady inflow, which is the typical benchmark case in studies. With the time-varying inflow, the induced updrafts prevented the relatively tall high-rise buildings from restricting the height of the pollutant dispersion during the inflow in the valley. The pattern of the pollutant isoconcentration plane and the time-series curves of normalized concentrations in the even and uneven street canyon models exhibited fluctuant features with the time-varying inflow.

LES simulation of flow field and pollutant dispersion in a street canyon under time-varying inflows with TimeVarying-SIMPLE approach

Field observation data have shown that the actual wind speed and direction in an urban built environment always vary with time. Realizing the flow field simulation under time-varying inflows can have a great significance in fully understanding the airflow and the transportation of air pollutants in the atmospheric boundary layer. A new scheme based on the standard SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm has been implemented in OpenFOAM, named as TimeVarying-SIMPLE approach, to simulate the time-series airflow field under time-varying inflows in this study. The accuracy of TimeVarying-SIMPLE approach compared with PISO (Pressure-Implicit with Splitting of Operators) algorithm and PIMPLE (merged PISO-SIMPLE) algorithm was validated by solving the fully developed laminar flow in a straight pipe under sinusoidally varying inflow. The wind field and pollutant dispersion in an ideal street canyon with an aspect ratio of 1 under time-varying inflows were calculated using the TimeVarying-SIMPLE approach. The flows show an obvious flapping of the shear layer near the roof layer and the expansion or shrinkage of the main vortex under time-varying inflows, resulting in a significant increase of the removal efficiency of pollutants inside the street canyon compared to that under steady inflow. The average time of actual high-frequency wind speed data is critical to produce the time-varying inflows for the boundary conditions of numerical simulations, which could produce different simulated removal efficiency of pollutants inside the street canyon. The appropriate average time should be estimated by the pre-multiplied power spectrum of the measured high-frequency wind-speed data.

Effect of moving vehicles on pollutant dispersion in street canyon by using dynamic mesh updating method

Moving vehicles exert a considerable influence on air flow and turbulence within urban street canyons, which is an important consideration for the pollutant dispersion and thus, local air quality management. In order to evaluate vehicle-induced turbulence and its contribution to pollutant dispersion, this study proposes a computational fluid dynamics based simulation methodology that uses a dynamic mesh updating method. The unsteady RNG k-ε turbulence model was employed to investigate the turbulent kinetic energy (TKE) induced by vehicle movement and pollutant profile in the target street canyon. The simulation accuracy is successfully validated against a published wind tunnel experiment. The results demonstrate that: (1) The TKE is mainly located behind the vehicle, and the magnitude of the TKE around the moving vehicle body far exceeds that of stationary vehicles; (2) Travelling speed has a significant influence on the TKE. When the moving speed is 15 m/s, its TKE near the tail is about 1.8 times and 6.3 times than that at 10 m/s and at 5 m/s, respectively; (3) Vehicle movement greatly contributes to pollutant dispersion. Compared to no vehicle movement (V = 0 m/s), vehicle-induced turbulence with V above 5 m/s can reduce the carbon monoxide concentration by about 4% at breathing height.

Qualitative and Quantitative Investigation of Multiple Large Eddy Simulation Aspects for Pollutant Dispersion in Street Canyons Using OpenFOAM

Air pollution is probably the single largest environment risk to health and urban streets are the localized, relevant hotspots. Numerous studies reviewed the state-of-the-art models, proposed best-practice guidelines and explored, using various software, how different approaches (e.g., Reynolds-averaged Navier–Stokes (RANS), large eddy simulations (LES)) inter-compare. Open source tools are continuously attracting interest but lack of similar, extensive and comprehensive investigations. At the same time, their configuration varies significantly among the related studies leading to non-reproducible results. Therefore, the typical quasi-2D street canyon geometry was selected to employ the well-known open-source software OpenFOAM and to investigate and validate the main parameters affecting LES transient simulation of a pollutant dispersion. In brief, domain height slightly affected street level concentration but source height had a major impact. All sub-grid scale models predicted the velocity profiles adequately, but the k-equation SGS model best-resolved pollutant dispersion. Finally, an easily reproducible LES configuration is proposed that provided a satisfactory compromise between computational demands and accuracy.

Investigation of air flow and pollutant dispersion in street canyon

With the rapid development of the city, more and more attention has been paid to the air quality of street canyon.We numerically investigate the air flow and pollutant dispersion in the street canyon with different window opening percentage, different wind speed and roof shapes. The results show that the window opening area leads to a significant improvement of removing pollutant out of street canyon, especially for pollutants distributed on upsides of leeward and windward walls.With increasing window opening percentage, the pollutant concentration K distributed on downsides of leeward and windward walls gradually decreases. With the growth in wind speed, the pollutant concentration K also drops. The slanted roof shape may be a better choice to improve the efficiency of pollutant removal.

Effects of Heat Intensity and Inflow Wind on the Reactive Pollution Dispersion in Urban Street Canyon

This paper investigates the impacts of heating intensity and inflow wind speed on the characteristics of reactive pollutant dispersion in street canyons using the computational fluid dynamic (CFD) model that includes the transportation of NO, NO2, and O3 coupled with NO-NO2-O3 photochemistry. The results indicated that the heat intensity and inflow wind speed have a significant influence on the flow field, temperature field and the characteristics of reactive pollutant dispersion in and above the street canyon. With the street canyon bottom heating intensity increasing, NO, NO2 and O3 concentrations in street canyon are decreased. The O3 concentration reductions are even more than the NO and NO2 concentrations. Improving the inflow wind speed can significantly reduce the NO and NO2 concentrations within street canyons. But the O3 concentrations have a slight rise with wind speed increasing. The results would be useful for understanding the interrelation among reactive vehicle emissions, and provide references for urban planners.

Lattice-Boltzmann Large-Eddy Simulation of pollutant dispersion in street canyons including tree planting effects

This study assesses the performance of a large eddy simulation (LES) based on the lattice Boltzmann method (LBM) in predicting near field dispersion in street canyons with tree planting. Based on a benchmark test case benefiting from wind tunnel measurements (CODASC), this study qualitatively and quantitatively discusses the prediction of traffic-induced pollutant concentration with respect to several reference studies. It also analyses the physics of the flow and concentration fields. Although the problem might seem rather simple, the flow is highlighted to be strongly three dimensional and transient. These properties enhance pollutant dispersion in the empty street canyon but air flow velocity and turbulence intensity tend to decrease in tree crowns. This effect of trees increases both mean and peak concentration levels at pedestrian level, which may be problematic in cities with dense traffic. These results show that LBM-LES is particularly well suited to study dispersion problems towards the development of more breathable cities.