街道峡谷内汽车污染物扩散的模拟研究

对街道峡谷污染问题的研究常采用实地测量、物理风洞试验和数值模拟等方法。本文采用数值模拟对城市街道峡谷汽车污染物的扩散规律进行了研究,从描述街道峡谷内空气流动和污染物扩散的控制方程出发,采用数值模拟的方法对街道内部的空气流动和污染物的扩散进行了模拟分析,并将结果与风洞试验结果进行比较,从而获得了城市街道峡谷汽车污染物的扩散规律。研究结果可以为城市街道大气污染监测、评价以及防治提供科学依据。     

城市街道峡谷空间形态及其污染物扩散研究——以杭州市中山路为例

概述了城市街谷污染物的扩散机制及相关研究方法。选择CFD数值模拟法,对杭州市中山路街谷进行试验分析。根据试验结果,从水平断面、纵断面、横断面等对街谷形态及其内部污染物的扩散、分布进行了三维的分析研究,进而总结了街谷形态及其污染物扩散的基本规律,并提出基于促进空气流通和污染物扩散、稀释的相关规划设计策略。     

交通污染下临街建筑自然通风采风口形式探究

应用计算流体力学对交通污染情况下的街道污染物分布和街道临街建筑自然通风情况进行数值计算。以实测数据作为边界条件建立了街道峡谷污染物扩散数学模型,采用街区模拟结果对临街建筑自然通风情况进行模拟计算,分析了建筑表面南风北风高浓度区域四个工况下室外压力、污染物浓度对室内自然通风的影响,并在其中一种工况下对室内自然通风采风口形式进行优化。     

垂直绿化对街道峡谷内PM2.5的影响研究

近年来细颗粒物PM_(2.5)引起的空气质量污染问题引起了社会的广泛关注,垂直绿化有较好的细颗粒物沉降效果,有望成为改善空气质量的有效办法。用ENVI-met数值模拟的方法,对不同峡谷街道宽高比、不同垂直绿化布置方式,进行多工况的比较分析,总结垂直绿化对PM_(2.5)影响的一般规律。结果表明,垂直绿化对PM_(2.5)的影响有两种,移除颗粒物和影响颗粒物分布,分别对应垂直绿化滞尘量、PM_(2.5)浓度改变量;街道宽度保持不变时,垂直绿化作为建筑外表皮能有效降低PM_(2.5)浓度;垂直绿化滞尘量随高度的升高而降低。     

自然通风下高海拔公路单向隧道污染物扩散影响因素

通过物理与数学模型,从隧道长度、交通量、进口风速、汽车尾气排放量等方面,分析了自然通风下,高海拔公路单向隧道污染物扩散的主要影响因素,最终得出了一些有意义的结论。     

空气温度差异对城市街道峡谷通风及污染物分布特征影响的研究

热力作用是影响城市街谷内风热环境的重要因素之一。为了研究实际日光照射对街道峡谷内通风特征的影响,本文对日光照射下的城市街道峡谷模型进行了简化,采用CFD模拟与城市冠层模型相结合的方法具体分析了不同高宽比下不同热边界条件设置时的街谷内部气流结构及污染物扩散规律。研究结果表明:热浮力作用会明显增强峡谷内部气流流动,由城市冠层模型计算出的壁面温度作为边界条件比以往研究中定壁温或者定热流的设置方式更能反映实际日光照射下峡谷内的风热环境,尤其是在深街道峡谷(H/W=3)时,由于建筑物的遮挡及太阳辐射的变化导致峡谷底部产生一个稳定的分层,导致峡谷底部污染物积累。此外,本文选取通风量(AER)和有效通风量(PFR)作为衡量峡谷通风性能的评价指标,发现仅用通风量作为评价指标是不准确的。     

街道峡谷热力特征对局地臭氧生成传播影响的研究

本文通过城市冠层模型、CFD模型与臭氧光化学反应模型进行耦合,形成了街道峡谷内的热力、动力和光化学反应同步模拟方法。以此为基础,采用数值模拟方法研究了街道峡谷热力特征对局地臭氧生成传播特征的影响。研究结果表明：在夏季典型日内,街道峡谷内空气温度与来流温度的差异对臭氧生成速率的影响并不显著;热力作用对臭氧浓度及分布的影响主要归因于自然对流对街道峡谷内气流结构的改变;热力作用对于臭氧局地分布特征的影响随着街道峡谷内高宽比的增大而增大。     

城市建筑物产生的大气漩涡结构对污染物扩散的影响研究

应用雷诺时间平均N—S方程组的数值模拟方法,利用Realizablek—e模型对圆柱和平板射流的发散比率有更加精确的预测。研究了建筑结构产生的大气漩涡结构对街道峡谷、周围环境污染物扩散的影响。计算结果与RafailidiS等进行的风洞试验结果相近。研究发现,建筑物体的形状、特别是屋顶形状以及建筑物之间的相对位置、建筑高度都会产生不同形式、不同强度的气流漩涡结构,进而影响到街道峡谷、周围环境的污染物扩散。合理设计及规划城市建筑对街道峡谷、周围环境的污染物扩散会产生有益的效应。     

街道峡谷内燃气管道泄露扩散模拟分析

随着城镇燃气建设事业的蓬勃发展,城区管道泄露对环境及人员的威胁越来越引起人们的重视。本文针对湖北省荆州市楚源大道燃气管道,建立物理模型并利用商用CFD软件进行数值计算。计算得到不同环境下的组份分布情况,并分析了不同外界风速对气体泄漏过程的影响及其爆炸极限分布情况。     

街道峡谷几何形态规划要素对太阳能潜力的影响研究

街道峡谷是城市建成环境中重要的空间类型,优化其几何形态有利于高效利用街道峡谷的太阳能潜力。通过对西安建成区街道峡谷几何空间形态对太阳能潜力影响的分析,借助CitySim软件模拟,研究双变量模型分析街道峡谷几何形态规划要素与太阳能潜力的相关性。结果表明:西安典型城市街道峡谷样本逐月太阳辐射量变化情况整体呈"先增后减、再增再减"的特征;街道朝向是影响街道峡谷太阳能潜力的关键几何形态规划要素;街道高宽比与太阳能潜力呈负相关。     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.