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.     

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。     

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.     

Impact of urban geometry on outdoor thermal comfort and air quality from field measurements in Curitiba,Brazil

Urban climate can have severe impacts on people who use outdoor spaces within a city. In its essence, urban climate is directly linked to the configuration of street axes, building heights and their attributes. Thus, the role of urban planners can be crucial for guaranteeing outdoor thermal comfort and air quality in open spaces. This paper presents observed and estimated relations between urban morphology and changes in microclimate and air quality within a city center. Two approaches are presented, showing results of field measurements and urban climate simulations using the ENVI-met software suite. From measured microclimatic data and comfort surveys, carried out in downtown Curitiba, Brazil, the impact of street geometry on ambient temperatures and on daytime pedestrian comfort levels was evaluated, using the sky-view factor (SVF) as indicator of the complexity of the urban geometry. The impact of street orientation relative to prevailing winds and the resulting effects of ventilation (air speed and spatial distribution) on the dispersion of traffic-generated air pollutants were additionally analyzed by means of computer simulations. Results show the impact of urban geometry on human thermal comfort in pedestrian streets and on the outcomes of pollutant dispersion scenarios.     

The effect of street dimensions and traffic density on the noise level and natural ventilation potential in urban canyons

High external noise levels are often used to justify the use of air conditioning in commercial and residential buildings. Methods of estimating noise levels in urban canyons are necessary if the potential for naturally ventilating buildings is to be assessed. A series of noise measurements were made in ‘canyon’ streets in Athens with aspect ratio (height/width) varying from 1.0 to 5.0. The main purpose of the measurements was to examine the vertical variation in noise in the canyons in order to give advice on natural ventilation potential. A simple model of the noise level has been developed using a linear regression analysis of the measured data. The model can be used to predict the fall-off (attenuation) of the noise level with height above street level. The attenuation is found to be a function of street width and height above the street, but the maximum level of attenuation (at the top of the canyon) is almost entirely a function of the aspect ratio except in narrow streets. Background noise (L₉₀) suffers less attenuation with height than foreground noise (L₁₀). Measurements of acoustic comfort in a survey throughout Europe are used to estimate the potential for natural ventilation in canyon streets.     

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.     

On the development of an urban passive thermal comfort system in Cairo,Egypt

This paper investigates how urban form can be designed to act as a passive thermal comfort system in Cairo. The system utilizes two main elements, the urban fabric form, with its green structure, and thermal comfort adaptation by introducing urban green scene stimulation with the time of exposure to the urban environment. The courtyard form on a large scale is used to study comfort levels, the effect of compactness on solar access and the local radiant heat island potential in a theoretical neighborhood design using a medium population housing concept as required by Egyptian urban planning laws. Urban canyons in a grid network, with three mid latitude orientations 15°, 45° and 75° from the E–W axis, were examined, with one canyon having virtually no shade. Other canyons had a green structure containing two types of native Egyptian trees. Numerical simulations using ENVI-met were performed for hot climate conditions. Although some very hot conditions were recorded, there were evident examples of more acceptable comfort levels and cooling potential for some orientations and degrees of urban compactness due to the clustered form with green cool islands and wind flow through the main canyons. Some design guidance on how to form urban passive cooling systems is presented.     

Influence of urban planning regulations on the microclimate in a hot dry climate: The example of Damascus, Syria

Urban planning regulations influence not only the urban form; they also have a great impact on the microclimate in urban areas. This paper deals with the relationship between the urban planning regulations and microclimate in the hot dry city of Damascus. The main purpose is to highlight the shortcomings of the existing urban planning regulations. The microclimatic parameters necessary for the thermal comfort assessment of pedestrians were determined through simulations with the software ENVI-met. It is shown that the street design—as regards aspect ratio, orientation and the presence of trees—has a great influence on ground surface temperatures and thermal comfort. Moreover, the type of buildings—whether detached or attached (street canyons)—has an impact. For deep canyons there is an interactive relationship between aspect ratio, orientation and vegetation. However, for streets with detached buildings, there is only a weak influence of street orientation and aspect ratio but a strong influence of vegetation on surface temperatures and outdoor thermal comfort. The study shows the importance of modifying the planning regulations in Damascus for new areas by allowing smaller setbacks, narrower streets, higher buildings, etc.     

Numerical simulation of the thermal environment of urban street canyon and a design strategy

The thermal environment of urban street canyons is closely related to design factors. By establishing a dynamic model of street canyons numerical simulations of temperature fields for typical street canyons are conducted. Through numerical analysis the influence of the most common variables related to environment in China’s urban residential quarters is compared and contrasted. Environmental factors include the height-width ratio of the street canyon, ground paving materials, canyon directions, and facades of buildings on canyon sides. Simulation results reveal how these factors influence the street canyon thermal environment. Results also give the recommended optimal height-width ratios for urban street canyons and question and revise some design ideas prevalent in China today.     

Influence of urban geometry on outdoor thermal comfort in a hot dry climate: A study in Fez,Morocco

There are few studies on the microclimate and human comfort of urban areas in hot dry climates. This study investigates the influence of urban geometry on outdoor thermal comfort by comparing an extremely deep and a shallow street canyon in Fez, Morocco. Continuous measurements during the hot summer and cool winter seasons show that, by day, the deep canyon was considerably cooler than the shallow one. In summer, the maximum difference was on average 6 K and as great as 10 K during the hottest days. Assessment of thermal comfort using the PET index suggests that, in summer, the deep canyon is fairly comfortable whereas the shallow is extremely uncomfortable. However, during winter, the shallow canyon is the more comfortable as solar access is possible. The results indicate that, in hot dry climates a compact urban design with very deep canyons is preferable. However, if there is a cold season as in Fez, the urban design should include some wider streets or open spaces or both to provide solar access.     

An application of the thermo-radiative model SOLENE for the evaluation of street canyon energy balance

This paper presents a validation of the thermo-radiative model SOLENE and its application for analysing the street canyon energy balance. The validation data were selected from the temperature and radiation measurements obtained during the JAPEX campaign, previously described by Idczak et al. [16]: a set of four lines of steel containers buildings composing three parallel street canyons at an approximate 1:5 scale. Reference weather data and micrometeorological conditions within the canyon were measured. Numerical simulations were carried out using the meteorological measurements as model inputs. The simulated surface temperatures and radiation fluxes are compared with the measurements for a full week period, with a focus on a day with clear sky conditions. The street canyon energy balance analysis demonstrates that the most energetic surface was the street ground due to its thick surface layer of tar-coated gravels while the walls had a low heat capacity. The thermal radiation balance was negative for all canyon surfaces. The sensible heat was transferred mainly from the canyon surfaces to the ambient air, but also from the air to the ground in the morning. The effective albedo of the canyon had a diurnal value of 0.20–0.25, but dropped to 0.10 in the afternoon when the ground strongly transformed the direct and reflected solar radiation into sensible heat. This narrow street configuration enhanced solar radiation absorption and longwave radiation trapping.     

Some characteristics of the wind flow in the lower Urban Boundary Layer

The characteristics of the wind flow at low levels in the urban environment (Roughness Sublayer (RS)) are quite different from, and to some extent independent of the characteristics of the flow in the upper part of the Urban Boundary Layer. In the RS, in fact, the flow is influenced more by the local geometry, than by a homogeneous energy transfer between horizontal layers. In this paper, the results of wind tunnel flow measurements in and above a regular urban geometry pattern, with street canyons parallel and orthogonal to the oncoming wind, are reported. The statistics and the spectral characteristics of the flow in the RS are discussed. The influence of the oncoming turbulence on the RS flow is analysed, and the differences between the flow in transversal and longitudinal street canyons are evidenced.     

湿热地区传统骑楼街区夏季热环境研究

城市传统聚落蕴含丰富的地域气候回应策略,并对指导现代城市的气候设计有重要意义。通过对湿热地区中广泛分布的传统骑楼街区进行热环境实测与模拟,在人体热舒适评价的基础上,分析城市传统聚落的气候适应性特征。以厦门的开元路街区以及广州的恩宁路街区为例,比较不同城市形态以及不同街谷中的热环境分布情况。发现有机形态布局下的厦门街区可以在不同朝向的窄巷中实现较好的热环境,而规整条状布局的广州街区中的东西走向的窄巷则很难形成有效的建筑自遮阳;同时骑楼覆盖下的人行区内可以维持相对稳定的热环境,但其对热环境优化的效果与街谷朝向有较大关联性。     

Thermal conditions and ventilation in an ideal city model of Hong Kong

Urban heat island can significantly increase the demand for cooling of buildings in cities. This paper investigates one of the main causes of the urban heat island phenomenon, i.e. reduced city ventilation. Two simple Hong Kong city models with relatively complex terrain were considered here under different atmospheric conditions. A 3D RNG k-? turbulence model was used for modeling turbulence effects. The simulation results showed that the influence of thermal stratification can be significant on city ventilation driven partially by thermal buoyancy. When the wind speed is relatively large, the impact of thermal stratification on air flow in city street canyons is minor. When the wind speed is small relative to the buoyancy force, the airflow in the street canyons is dependent on thermal stratification. When there is an adverse vertical temperature gradient, the greater the instability, the stronger the vertical mixing and the greater the flow rate caused by turbulence. The heat and pollutants can easily accumulate under stable atmospheric conditions when there is only a weak background wind or none at all.     

Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Five sets of large-eddy simulations (LES) were performed to examine the characteristics of flows and pollutant dispersion in two-dimensional (2D) urban street canyons of unity building-height-to-street-width ratio in neutral, unstable, and stable thermal stratifications. The characteristic flows fall into the skimming flow regime for all the cases tested. The mean wind speed is increased and decreased, respectively, in unstable and stable conditions. Turbulence is enhanced in unstable conditions. Whereas, in stable conditions, the low-level temperature inversion weakens the recirculating flows forming another layer of stagnant air in the vicinity of the ground level. Unexpectedly, an increase in turbulence is found in the street canyon core in the slightly stable condition (Richardson number Rb=0.18). The turbulence promotion could be caused by the unique geometry of 2D street canyon in which the stable stratification slows down the primary recirculation. The rather stagnant flows in turn sharpen the roof-level vertical velocity gradient and deter the entrainment penetrating down to the ground level, leading to a substantial pollutant accumulation. While the pollutant tends to be well mixed in the street canyons in neutral and unstable conditions, a mildly improved pollutant removal in unstable conditions is observed because of the enhanced roof-level buoyancy-driven turbulence.     

A numerical study of reactive pollutant dispersion in street canyons with green roofs

Roof greening is a new technique for improvement of outdoor thermal environment which influences air quality through its impacts on thermal and flow field. In order to examine effects of green roofs on reactive pollutant dispersion within urban street canyons, a computational fluid dynamics (CFD) model was employed which contained NO-NO₂-O₃ photochemistry and energy balance models. Simulations were performed for street canyons with different aspect ratios (H/W) of 0.5, 1.0, and 2.0 such that leaf area density (LAD) of green roofs changed. It was found that roof greening led distribution of pollutants to alter for H/W = 0.5 and 1.0 cases in such a manner that their averaged concentrations had small variations as LAD changed. However, by increasing LAD in H/W = 2.0, ventilation efficiency of nitrogen oxides increased since the flow was enhanced within the canyon. Additionally, averaged concentration of ozone in H/W = 2.0 increased with increasing LAD, owing to downward flux of ozone at roof level. Results show that roof greening is a good strategy which can be used in order to improve air quality and thermal environment, especially within deep street canyons.     

Simulation of the urban thermal comfort in a high density tropical city: Analysis of the proposed urban construction rules for Dhaka, Bangladesh

The thermal comfort in urban canyons of a high density city is a very challenging issue for urban planners and designers, especially in hot humid tropical zone. The present study aims to evaluate the effects of a newly promulgated building construction rules in respect of thermal climate for Dhaka, Bangladesh, a region characterized by high density area with tropical climate. Three different urban canyons from three areas were considered which almost represents the whole city. Measurements were carried out to assess the existing thermal climate in the city canyons. Further, three model canyons were configured according to the new rules of ground coverage, floor area ratio (FAR) and site setback. A three-dimensional numerical model with high spatial and temporal resolution was used to investigate the microclimatic changes within the urban environments. Model calculations were run for a typical summer day. The thermal climate was evaluated based on air temperature T _a, surface temperature T _s, relative humidity RH, and wind speed in the existing and model canyons. For thermal comfort assessment the temperature humidity index (THI) was considered in this study. The results showed that the outdoor spaces of the study areas are thermally uncomfortable and the new building construction rules hardly improve the conditions in general. However, THI decreases in a canyon where sky view factor (SVF) decreases in the model canyon than the existing one. In addition, the surface temperature T _s and air temperature T _a do not show the same picture. Furthermore, the wind speed increases in all the model canyons as the site setback allows wind flow uniformly inside the canyons.     

Urban environment influence on natural ventilation potential

Natural ventilation is less effective in urban than in rural environment, especially in street canyons. Reduced wind velocity, urban heat island, noise and pollution, are considered to be important barriers to its application in urban environment. To quantify their effect on ventilation, wind, temperature, noise attenuation and outdoor–indoor pollution transfer were measured for a large variation range and for various types of urban configuration. The models obtained can be used in the initial stages of building design in order to assess the viability of natural ventilation in urban environment, especially in street canyons.     

The effect of Urban Heat Island mitigation strategies on outdoor human thermal comfort in the city of Baghdad

Todays, most Iraqi cities suffer from extremely hot-dry climate for long periods throughout the year. However, most urban patterns that exist inside these cities are not suitable for this harsh conditions and lead to an increase in the value of the Urban Heat Island (UHI) index. Consequently, this will increase outdoor human thermal discomfort as well as energy consumption and air pollution in cities. This study attempts to evaluate the effect of UHI mitigation strategies on outdoor human thermal comfort in three different common types of urban patterns in the biggest and most populated city in Iraq, Baghdad. Three different mitigation strategies are used here – vegetation, cool materials, and urban geometry – to build 18 different scenarios. Three-dimensional numerical software ENVI-met 4.2 is utilised to analyse and assess the studied parameters. The input data for simulations process are based on two meteorological stations in Baghdad: Iraqi Meteorological Organization & Seismology, and Iraqi Agrometeorological Network. All measurements are taken in a pedestrian walkway. The results of different scenarios are compared based on their effect on human thermal comfort. Outdoor thermal comfort is assessed according to Predicted Mean Vote index, as mentioned in ISO 7730 standard. This study provides a better understanding of the role of UHI mitigation strategies on human thermal comfort in the outdoor spaces of Baghdad's residential neighbourhoods. This can help generate guidelines of urban design and planning practices for better thermal performance in hot and dry cities.     

面向城市高密度街区舒适风环境的设计策略研究——以北京三条商业街的实地数据分析为例

城市建筑布局与城市外部风资源的相互作用决定了城市建成环境内部风环境的特征,对城市热岛效应、建筑能耗、建筑室内外的热舒适、空气质量以及居民健康都具有一定的影响作用。本研究试图避免复杂且高成本的计算机模拟,而基于常年大量现场测试的物理环境指标,采用数据统计分析的方法,通过相对简单的城市建成环境的形态参数和外部城市风环境获取步行街区行人高度下的微环境风速,而同时又保证风环境关键信息的可接受精度和准确性。研究数据的获取主要基于北京王府井步行街、前门大街、大栅栏商业街三处街区环境,并同步采集了城市外部的风向与风速数据。提出并建立的城市行人尺度下的风环境模型能够更加有效地应用于高密度城市区域规划与设计实践的初步设计阶段。