Dispersion in a street canyon for a wind direction parallel to the street axis

We investigate pollutant dispersion in a street canyon for an external wind direction parallel to the street axis, a case which has been poorly documented in the literature. The study is performed numerically and analytically by means of a model based on a series of simplifying assumptions. The range of validity of these assumptions is discussed by comparing analytical and numerical results for two different street aspect ratios. Our results show that, for a critical length of the street, ground level concentration can be higher than those observed in a street canyon whose axis is perpendicular to the external wind direction. We show that this critical length depends on the street aspect ratio.     

Airflow and pollutant transport in street canyons

In this work the dispersion of gaseous and particulate exhaust emissions in different street canyons were studied. For two-dimensional sections of canyon models airflow, pollutant dispersion and deposition patterns in the streets and on the surrounding buildings were analyzed. Effects of building size, street width, and wind velocity on the pollutant transport were examined. While the stress transport turbulence models were used in most of the analysis, the predictions of other turbulence models were also examined. Depending on wind speed, building height, and street width, it was found that large recirculation regions in canyons might form. Under certain conditions, also pollutants emitted from vehicle exhaust may trap inside the street canyon. Variations of transport and deposition of emitted particulate pollutants with particle size and relaxation time were also studied. It was shown that the amount of deposited particles in street canyons reduces when the wind speed increases. The simulation results were compared with the available wind tunnel experiments and favorable agreement was found.     

Effect of wind direction and speed on the dispersion of nucleation and accumulation mode particles in an urban street canyon

There have been many studies concerning dispersion of gaseous pollutants from vehicles within street canyons; fewer address the dispersion of particulate matter, particularly particle number concentrations separated into the nucleation (10-30 nm or N10-30) or accumulation (30-300 nm or N30-300) modes either separately or together (N10-300). This study aimed to determine the effect of wind direction and speed on particle dispersion in the above size ranges. Particle number distributions (PNDs) and concentrations (PNCs) were measured in the 5-2738 nm range continuously (and in real-time) for 17 days between 7th and 23rd March 2007 in a regular (aspect ratio approximately unity) street canyon in Cambridge (UK), using a newly developed fast-response differential mobility spectrometer (sampling frequency 0.5 Hz), at 1.60 m above the road level. The PNCs in each size range, during all wind directions, were better described by a proposed two regime model (traffic-dependent and wind-dependent mixing) than by simply assuming that the PNC was inversely proportional to the wind speed or by fitting the data with a best-fit single power law. The critical cut-off wind speed (Ur,crit) for each size range of particles, distinguishing the boundary between these mixing regimes was also investigated. In the traffic-dependent PNC region (UrUrUr,critUr,crit), concentrations were inversely proportional to Ur irrespective of any particle size range and wind directions. The wind speed demarcating the two regimes (Ur,critUr,crit) was 1.23+/-0.55 m s(-1) for N10-300, (1.47+/-0.72 m s(-1)) for N10-30 but smaller (0.78+/-0.29 m s(-1)) for N30-300.     

Dispersion characteristics of vehicle emission in an urban street canyon

The dispersion of vehicle emission is limited by various factors existing in an urban environment, which may produce a poor air quality in an urban street canyon environment. This poor air quality has a high potential to be easily delivered into indoor air environment through building ventilation. In this study, the dispersion of vehicle emission was characterized by conducting wind tunnel tests and applying tracer gas techniques. The aspect ratio of a street canyon (i.e. the ratio of the width of a street and the average height of buildings) and the direction of external wind are the major test parameters. In addition to the simple data analysis of the results, a series of statistical analysis was also introduced to formulate the complex effects on the dispersion of vehicle emission. The updated result is presented in this article.     

Study on the influence of meteorological conditions and the street side buildings on the pollutant dispersion in the street canyon

As cities grow, automobile exhaust pollution is worsening, which has become a major problem of air pollution, even it is a serious threat to the physical and mental health of residents. Thus, to study its diffusion law and influential factors occupies a count for much position. The paper analyzes the factors that affect the dispersion of urban vehicle emissions in street canyon to evaluate the research method of pollutant dispersion. In addition, the influences of different wind speeds and wind directions, the roof shape of buildings on both street sides, and the relative height of the two sides of buildings on the street canyon, on airflow field and pollutant dispersion are simulated. It is shown that the wind speed, the wind direction and the buildings on both sides have a great impact on the airflow and contaminant dispersion in the street canyon. The results provide scientific basis for controlling, monitoring and evaluating the urban motor vehicle emissions, besides the reasonable layout and the programme of urban streets.     

Measuring and modelling the airborne particulate matter mass concentration field in the street environment: model overview and evaluation.

This paper discusses the outline structure and preliminary evaluation of an emission-dispersion model for predicting the temporal and spatial distribution of vehicle-derived airborne particulate matter mass concentration in street canyons. The model is called Street Level Air Quality (SLAQ). SLAQ is semi-empirical, in that it uses not only results from field and wind tunnel experiments but also theory and models derived from multiple runs of numerical routines in order to simulate the basic physical processes within the street canyon. A combination of a plume model, for the direct contribution of vehicle exhaust, and a box model for the recirculating part of the pollutants in the street, is used to predict concentration for receptors within the canyon. Emission rates of vehicle-derived particulate matter are calculated within SLAQ, which serve as input to the dispersion module. Exhaust emission rates are scaled element by element along the street for each of the lanes according to the direction of traffic flow to account for modal operation of vehicles near signalised intersections. This refinement allows SLAQ to account for non-uniformity in along-canyon emission rates and to model a street that has several intersections along its length. Thermal turbulence due to environmental surface sensible heat and vehicle-generated heat is accounted for in the model. Other features of SLAQ include correction for the urban heat island effect, dry deposition, wet deposition, particle settling and estimation of wind direction standard deviation, when this latter data is not available. SLAQ has been evaluated in a street in Loughborough, Leicestershire, United Kingdom and correlation coefficient of 0.8 between the modelled and measured concentrations has been obtained.     

Concentration and flow distributions in urban street canyons: wind tunnel and computational data

The goal of this paper is to present bluff body flow and transport from steady point sources of pollutants, or chemical and biological agents in an idealized urban environment This paper includes ventilation behavior in different street canyon configurations. To evaluate dispersion in a model urban street canyon, a series of tests with various street canyon aspect ratios (B/H) are presented. Both open-country roughness and urban roughness cases are considered. The flow and dispersion of gases emitted by a point source located between two buildings inside an urban street canyon were determined by the prognostic model FLUENT using four different RANS turbulent closure approximations and in the model fire dynamics simulator using a large eddy simulation methodology. Calculations are compared against fluid modeling in the Industrial Meteorological Wind Tunnel at Colorado State University. A basic building shape, the Wind Engineering Research Field Laboratory building (WERFL) at Texas Tech University, was used for this study. The urban street canyon was represented by a 1:50 scale WERFL model surrounded by models of similar dimensions. These buildings were arranged in various symmetric configurations with different separation distances and different numbers of up- or downwind buildings. Measurements and calculations reveal the dispersion of gases within the urban environment are essentially unsteady, and they are not always well predicted by the use of steady-state prediction methodologies.     

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.     

Wind tunnel simulation studies on dispersion at urban street canyons and intersections—a review

Increased traffic emissions and reduced natural ventilation cause build up of high pollution levels in urban street canyons/intersections. Natural ventilation in urban streets canyons/intersections is restricted because the bulk of flow does not enter inside and pollutants are trapped in the lower region. Wind vortices, low-pressure zones and channeling effects may cause build up of pollutants under adverse meteorological conditions within urban street canyons. The review provides a comprehensive literature on wind tunnel simulation studies in urban street canyons/intersections including the effects of building configurations, canyon geometries, traffic induced turbulence and variable approaching wind directions on flow fields and exhaust dispersion.     

A numerical investigation of the impact of low boundary walls on pedestrian exposure to air pollutants in urban street canyons

A previous investigation into methods of exposure reduction for the pedestrian in the urban commuter environment highlighted the impact of a low boundary wall on the dispersion of air pollutants from adjacent traffic sources. The impact of low boundary walls on the dispersion of air pollutants in street canyons has been brought forward in this investigation to examine them, in more generic terms, with a view to highlighting exposure reduction strategies for pedestrians. 3D Computational Fluid Dynamics (CFD) models were used to examine this effect for varying wind speeds and directions in different street canyon geometries. The results of this investigation show that a low boundary wall located at the central median of the street canyon creates a significant reduction in pedestrian exposure on the footpath. Reductions of up to 40% were found for perpendicular wind directions and up to 75% for parallel wind directions, relative to the same canyon with no wall. The magnitude of the exposure reduction was also found to vary according to street canyon geometry and wind speed.     

Pollution removal effectiveness of the pedestrian ventilation system

Dispersion of vehicular pollution through street canyons has been widely studied in order to find strategies for reducing concentration level. Recently, a pedestrian ventilation system (PVS), an active mitigation strategy, has been proposed to enhance pedestrian comfort indices and to induce appropriate air movement. This paper investigates the performance of PVS to control pollution dispersion within street canyons. Pollution control is achieved by exhausting/supplying air from/to the street canyon through the PVS. In the present paper, the effectiveness of these strategies was studied by varying the parameters that affect dispersion, such as aspect ratios (AR) and thermal stratifications.Computational Fluid Dynamics (CFD) has been selected as the investigation tool. Prior to simulations, the proposed model was successfully validated using two sets of experimental data. Four case-studies were also used to investigate the aspect ratio and the stratification effect. These test cases were developed based on small scale studies in a wind tunnel. Results show the ability of the PVS to change the airflow pattern through the street canyon, resulting in significant pollution removal, especially from the pedestrian level. Moreover, the air and pollution exchange rate concepts have been used for better evaluation of the PVS performance. Furthermore, a breakthrough index was proposed to evaluate the effect of the PVS airflow rate.     

Effect of uneven building layout on air flow and pollutant dispersion in non-uniform street canyons

Uneven building layouts and non-uniform street canyons are common in actual urban morphology. To study the effects of building layouts on air flow in non-uniform street canyons, various building arrangements are designed in this study. Simulations are carried out under four cases (i.e., a uniform street canyon as Case 1 and three non-uniform canyons as Cases 2–4) with parameter change of the occupying ratio of high buildings (ORHB) in the computational domain and their bilateral allocation as well as the combinations of stepup and/or stepdown notches. In the three non-uniform canyons, stepup and stepdown notches are separating (with ORHB of 25% for Case 2 and 75% for Case 4) or adjoining (with ORHB of 50% for Case 3). The air flow and pollutant dispersion in these street canyons are investigated using Large-eddy Simulation (LES). The air flow structures in the non-uniform street canyons are more complicated than in the uniform street canyon. Inside the non-uniform street canyons, the tilting, horizontal divergence and convergence of wind streamlines are found. Large-scale air exchanges of air mass inside and above the street canyons are found as well. At the pedestrian level, the concentrations of simulated pollutants (e.g., the mean and maximum concentrations) in the non-uniform street canyons are lower than those in the uniform one, suggesting that uneven building layouts are capable of improving the dispersion of pollutants in urban area. Further studies on Case 2–4 show that the separation of stepup and stepdown notches along the street increases the wind velocities in the vicinity of high buildings, while the adjoining of stepup and stepdown notches decreases the wind velocities. Low concentrations of pollutant at the pedestrian level are found in Case 2 compared to Cases 3 and 4. Thus, the separation of stepup and stepdown notches in non-uniform street canyons might be a good choice for uneven building layout arrangements from the point of view of pollutant dispersion and human health.     

Effects of inflow turbulence intensity on flow and pollutant dispersion in an urban street canyon

The effects of inflow turbulence intensity on flow and pollutant dispersion in an urban street canyon with a street aspect ratio of 1 are examined using a two-dimensional numerical model. As the inflow turbulence intensity increases, turbulent kinetic energy and turbulent diffusivity in the street canyon increases. Also, the mean horizontal velocity near the roof level increases and the street-canyon vortex strengthens. The analyses of the time series and residue ratio of pollutant concentration show that the inflow turbulence intensity significantly affects pollutant concentration in the street canyon. As the inflow turbulence intensity increases, the pollutant concentration in the street canyon becomes low and hence more pollutants escape from the street canyon.     

街谷行道树种植对环境热舒适度的影响及适应性设计研究 进展

行道树种植通常被作为改善城市街谷 近地微气候的重要策略,如何发挥行道树对街 谷热舒适度的提升潜力受到诸多学者的广泛关 注。近年来“行道树种植与街谷热舒适度”相关 研究获得了丰硕成果，通过总结梳理可将其归 纳为行道树树木个体形态对热舒适度的调控、 行道树绿带空间配置与街谷热舒适度整体提升 的关联性、适应不同街谷空间形态的行道树种 植设计策略等三个研究主题。在深入分析既有 研究成果基础上，提出了一套改善热舒适度的 街谷行道树种植设计方案技术框架。最后，分 别从建立响应地域气候特征的街谷行道树种植 设计模式、构建街谷环境热舒适度模拟评估导 则、制定街谷环境热舒适度评价标准等方面开展深入讨论，以期为后续城市街谷绿化提升热环境的研究提供思路和借鉴。     

Experimental study of temperature and airflow distribution inside an urban street canyon during hot summer weather conditions. Part II: Airflow analysis

This paper presents the results of an urban measurement campaign performed in a street canyon in Athens, Greece. A number of field experimental procedures were organized during hot weather conditions, on a 24-h basis for five consecutive days during July 2002. Wind velocity measurements were conducted inside and outside the street canyon together with air and surface temperature measurements. Based on the results of air and surface temperature measurements, a further analysis is performed for the investigation of airflow inside the canyon when the ambient flow is parallel, perpendicular and oblique relative to the long canyon axis. The observed airflow characteristics are associated with the impact of thermal effects mainly induced from ground heating due to the incident solar radiation. However, the role of the finite length canyon effects related to wind circulation near street intersections, on the observed airflow patterns, is also identified.     

Measurements of particles in the 5-1000 nm range close to road level in an urban street canyon

A newly developed instrument, the 'fast response differential mobility spectrometer (DMS500)', was deployed to measure the particles in the 5-1000 nm range in a Cambridge (UK) street canyon. Measurements were taken for 7 weekdays (from 09:00 to 19:00 h) between 8 and 21 June 2006 at three heights close to the road level (i.e. 0.20 m, 1.0 m and 2.60 m). The main aims of the measurements were to investigate the dependence of particle number distributions (PNDs) and concentrations (PNCs) and their vertical variations on wind speed, wind direction, traffic volume, and to estimate the particle number flux (PNF) and the particle number emission factors (PNEF) for typical urban streets and driving conditions. Traffic was the main source of particles at the measurement site. Measured PNCs were inversely proportional to the reference wind speed and directly proportional to the traffic volume. During the periods of cross-canyon flow the PNCs were larger on the leeward side than the windward side of the street canyon showing a possible effect of the vortex circulation. The largest PNCs were unsurprisingly near to road level and the pollution sources. The PNCs measured at 0.20 m and 1.0 m were the same to within 0.5-12.5% indicating a well-mixed region and this was presumably due to the enhanced mixing from traffic produced turbulence. The PNCs at 2.60 m were lower by 10-40% than those at 0.20 m and 1.0 m, suggesting a possible concentration gradient in the upper part of the canyon. The PNFs were estimated using an idealised and an operational approach; they were directly proportional to the traffic volume confirming the traffic to be the main source of particles. The PNEF were estimated using an inverse modelling technique; the reported values were within a factor of 3 of those published in similar studies.     

Impact of wedge-shaped roofs on airflow and pollutant dispersion inside urban street canyons

The objective of this study is to investigate numerically the effect of wedge-shaped roofs on wind flow and pollutant dispersion in a street canyon within an urban environment. A two-dimensional computational fluid dynamics (CFD) model for evaluating airflow and pollutant dispersion within an urban street canyon is firstly developed using the FLUENT code, and then validated against the wind tunnel experiment. It was found that the model performance is satisfactory. Having established this, the wind flow and pollutant dispersion in urban street canyons of sixteen different wedge-shaped roof combinations are simulated. The computed velocity fields and concentration contours indicate that the in-canyon vortex dynamics and pollutant distriburtion are strongly dependent on the wedge-shaped roof configurations: (1) the height of a wedge-shaped roof peak is a crucial parameter determining the in-canyon vortex structure when an upward wedge-shaped roof is placed on the upwind building of a canyon; (2) both the heights of upstream and downstream corners of the upwind building have a significant impact on the in-canyon vortical flow when a downward wedge-shaped roof is placed on the upwind building of a canyon, due to flow separation as wind passes through the roof peak; (3) the height of upstream corner of the downwind building is an important factor deciding the in-canyon flow pattern when a wedge-shaped roof is placed on the downwind building of a canyon; (4) the characteristics of pollutant dispersion vary for different wedge-shaped roof configurations, and pollution levels are much higher in the “step-down” canyons relative to the “even” and “step-up” ones.     

CFD modeling of pollution dispersion in a street canyon: Comparison between LES and RANS

CFD modeling using RANS and LES of pollutant dispersion in a three-dimensional street canyon is investigated by comparison with measurements. The purpose of this study is to confirm the accuracy of LES in modeling plume dispersion in a simple street canyon model and to clarify the mechanism of the discrepancy in relation to RANS computation. Simple LES modeling is shown by comparison with wind tunnel experiments to give better results than conventional RANS computation (RNG) modeling of the distribution of mean concentration. The horizontal diffusion of concentration is well reproduced by LES, mainly due to the reproduction of unsteady concentration fluctuations in the street canyon.     

Pollutant dispersion near roadways — Experiments and modeling

Since the 1970's, many field and wind tunnel experiments have been conducted to study pollutant dispersion from roadways. For an at-grade situation, field experiments have revealed that mechanical mixing dominates effects due to ambient stability, that plume rise is important under very low crossroad winds, that regions of large shear enhance the mixing volume, and that the wake region grows rather slowly in the vertical direction. The models that have been developed based on recent experimental results are briefly described. For the street canyon situation, both field and wind tunnel experiments have revealed that ambient stability does not play an important role, that corner vortices near an intersection cause an increase in pollutant concentrations near the bottom corners of the leeside buildings, that in the midsection of a street block the vortex circulation causes high pollutant concentrations to be advected toward and up the leeside wall. No general street canyon models are available except an empirical model for the midsection of the street block. The complicated flow field must first be ascertained before a reliable concentration model can be developed.     

Effects of building aspect ratio and wind speed on air temperatures in urban-like street canyons

The objective of this study is to simulate the characteristic role of building aspect ratio (AR) and wind speed on air temperatures during different street canyon heating situations. A two-dimensional Renormalization Group (RNG) k–? turbulence model is employed to solve the Reynolds-averaged Navier–Stokes (RANS) and energy transport equations. A comparison of the results from the adopted model with those reported by similar experimental and numerical works demonstrated that the model is quite reliable when simulating temperature and wind profiles. The model is employed to predict air temperatures in idealized street canyons of aspect ratios (building-height-to-street-width ratio) of 0.5–8 with ambient wind speeds of 0.5–4 m/s. Three situations were identified for simulating diurnal heating of street canyon. It is noted that air temperatures are positively correlated with the bulk Richardson number (R_b) in most of the cases. The results show that the air temperature difference between high and low AR street canyon (Δθ_AR) was the highest during the nighttime (i.e., around 7.5 K between AR8 and AR0.5), but low or even negative during the daytime. It is also found that air temperatures rose as high as 1.3 K when ambient wind speed decreased from 4 m/s to 0.5 m/s. It is also revealed that the Δθ_AR during different diurnal situations and the nighttime and daytime air temperature difference between urban and rural areas (Urban Heat Island, UHI) closely resemble one another. Conclusively, the results of this study have highlighted the importance of street canyon AR and wind speed on urban heating.