三维街道峡谷机动车尾气时空扩散模拟研究

针对信号控制路段,采用非稳态κ-ε湍流模型、组分输运方程进行非定常三维街道峡谷数值模拟,研究了三维街道峡谷内动态交通流下机动车污染物CO的时空扩散过程,并对比了含信号、不含信号的定常模拟结果.结果表明,(1)受信号控制及峡谷内流场影响,峡谷内污染物浓度呈现显著的时空不均匀性;(2)各路段背风面浓度值要大于迎风面,且背风面和迎风面浓度峰值均位于峡谷中部的人行横道区;(3)信号周期内人行横道区污染物浓度始终远高于峡谷内其他区域.在距离背风面建筑1 m的人行横道处污染物浓度可达24.15 mg/m3,超过国家空气质量二级标准141.50％;(4)受信号控制影响,含信号控制街道峡谷污染物浓度高于不含信号控制路段,人行横道背风面污染物浓度是不含信号控制人行横道的3.5倍.     

T型街道内空气流动与污染物扩散特性研究

为研究T型街道峡谷内空气流动与污染物扩散传质的特性,利用数值模拟研究来流风向角(θ)的变化(θ为45°、90°和135°)对T型街道交叉路口内空气流动与机动车尾气污染物扩散传递的影响,并与风洞实验测量数据进行验证。3种湍流模型中,可实现k—ε模型计算的速度相对偏差小于8%,与风洞实验结果一致性最好。结果表明,来流风向角的变化,会造成从街道顶部或侧面进入街道内的气流方向及通量发生改变,从而显著影响T型街道交叉口内及其附近的流动结构和污染物浓度分布。污染物容易在建筑尾流区等流动不畅的区域产生聚集,造成污染浓度偏高。当θ=135°时,T型街道内通风条件最好,街道内行人呼吸高度和建筑临街立面附近污染物浓度水平均相对较低。由于流动结构的改善,T型街道峡谷内的污染水平低于一般街道峡谷。     

街道峡谷机动车尾气污染扩散模拟与影响因子分析研究

为预测和分析街道峡谷污染物浓度,研究了街道峡谷污染物浓度影响因子.利用重庆市交通干线街道峡谷两侧NOx浓度的监测数据,验证了街道峡谷机动车尾气污染扩散模型--OSPM模型.风速转换系数修正后的OSPM模型的模拟值与实测值的R达0.862 58;风场因子验证了风速转换系数修正后的OSPM模型能较好地模拟重庆市街道峡谷的污染物浓度,一定程度上能满足环境空气质量评价要求.同时,通过分析OSPM模型的影响因子,提出了控制街道峡谷机动车尾气污染状况的建议.     

道路路障对街道峡谷内污染物扩散的影响

采用数值模拟,研究不同风向角α(α=0°、45°、90°)及道路屏障位置(中间单路障和两侧双路障)对街道峡谷内机动车尾气污染物扩散的影响。数值模拟采用标准κ-ε湍流模型且Sc_t选择0.3时,计算结果与风洞实验结果较好吻合。结果表明,2种路障布置方式可有效降低人行道内污染物浓度,特别是,当α=45°时,污染物浓度最多可降低46.23%。同时,风向角α对街道峡谷内污染物扩散影响较大。当α=90°时,空气流通不良使得污染程度最为严重,且污染集中在背风侧近地面。单路障比双路障布置对污染物扩散影响更大,前者使污染物主要集中在街道中心背风侧,其他位置浓度明显降低;双路障时仅在一定范围内改善人行道内空气品质,但对街道整体污染物分布影响不大。     

基于人工神经网络的街道峡谷NOx浓度的数值模型研究

通过对反向传播人工神经网络的算法和网络结构的研究，发现拟牛顿算法训练速度较快，能够较好地接近误差目标值，同时建立了包括输入层、隐含层、输出层的人工神经网络三层拓扑结构。通过对街道峡谷人工神经网络的训练，模拟计算了街道峡谷NOx浓度分布值。结果显示，训练误差和测试误差比为1．11，训练样本的模拟值与实测值的相关系数为0．93，测试样本的模拟值与实测值的相关系数为0．87，模拟值与实测值的相关系数均高于显著水平为α=0．05与α=0．01所对应检验性表的相关系数临界值。该模型能够用于街道峡谷污染物浓度的模拟计算，具有较好的泛化能力。     

主导风向对不规则建筑群街区污染物扩散的影响

以城市道路某段典型的街道峡谷为研究对象,采用ICEM CFD数值模拟技术,分析不同风向对不规则建筑群街区污染物扩散影响。结果表明:(1)北风时,距地10.0m以下范围是污染物高浓度聚集区。与北风工况相比,西北风时街谷内污染物浓度分布变化较大,沿高度截面上升依次呈连续线状→线状和部分团状→断裂团状。(2)随着建筑高度的增加,主干道中的气流绕流作用减弱。通过控制建筑的连续界面诱导街谷中的气流横向绕流,或在临街上游设置合适的开敞空间,以增加来流通风廊道,可有效改善街谷中污染物的扩散。(3)两种风向下每条街道人员停留区内污染物停留时间排序规律相同;不同风向下每条街道人员停留区内污染物停留时间不同,说明风向对每条街道内污染物的影响存在差异,每段街谷内的污染物扩散分布不是孤立系统,而是相互关联的有机整体。     

基于人工神经网络的街道峡谷NO_x浓度的数值模型研究

通过对反向传播人工神经网络的算法和网络结构的研究,发现拟牛顿算法训练速度较快,能够较好地接近误差目标值,同时建立了包括输入层、隐含层、输出层的人工神经网络三层拓扑结构。通过对街道峡谷人工神经网络的训练,模拟计算了街道峡谷NOx浓度分布值。结果显示,训练误差和测试误差比为1.11,训练样本的模拟值与实测值的相关系数为0.93,测试样本的模拟值与实测值的相关系数为0.87,模拟值与实测值的相关系数均高于显著水平为α=0.05与α=0.01所对应检验性表的相关系数临界值。该模型能够用于街道峡谷污染物浓度的模拟计算,具有较好的泛化能力。     

计算流体力学模拟街道峡谷特征和风向对细颗粒物污染扩散的影响

街道峡谷结构和风向会对街道峡谷内的污染物浓度和扩散特征带来一定影响。利用计算流体力学(CFD)软件,针对街道峡谷高宽比、建筑物间隔(建筑物间空隙与街道总长度的比值)和风向对街道峡谷内细颗粒物扩散的影响进行数值模拟。模拟结果表明,建筑物间隔为20%,风向为北风,风速为3m/s,街道峡谷高宽比分别为1∶2、1∶1和2∶1时,街道中心线距地面1.5m高度细颗粒物最大质量浓度分别位于-19.3、-88.0、-19.3m(以与街道中心点的距离计,正值表示在街道中心点以东,负值表示在街道中心点以西,下同)位置,为37.5、46.4、28.4μg/m3。街道峡谷高宽比为1∶1,风向为北风,风速为3m/s,建筑物间隔分别为0、20%和40%时,街道中心线距地面1.5m高度的细颗粒物最大质量浓度分别位于148.0、-92.3、-186.7m位置,为88.1、31.6、33.7μg/m3。街道峡谷高宽比为1∶1,建筑物间隔为20%,风速为3m/s,且分别处于西风、北风和西南风时,街道中心线距地面1.5m高度的细颗粒物最大质量浓度分别位于165.3、58.0、1.5m位置,为10.6、11.2、16.0μg/m3。可见,CFD模拟近地面污染物扩散时应考虑街道峡谷结构和风向的影响。     

高层建筑群对街谷内颗粒物扩散特性的影响

为了获得城市冠层内高层建筑群的高度变化对城市颗粒物污染的作用情况,采用大涡模拟方法研究了不同高层建筑群的街谷形状因子对街谷内空气流动与污染物扩散规律的影响。结果表明:在高层建筑群上方形成一个顺时针旋涡,旋涡中心位于城市峡谷内靠近高层建筑群背风处;随着街谷形状因子的增大,高层建筑群的滞留效应增强,导致高层建筑物上方的剪切层湍动能增强;当形状因子为2.5时,湍动能达到1.9 m~2·s~(-2),此时城市街谷内可吸入颗粒物的稀释扩散条件变差;在涡旋和气流夹带作用下,可吸入颗粒物浓度在垂直方向上分布具有明显的分层现象,大量可吸入颗粒物聚集于低建筑迎风面底部。不同街谷形状因子下街谷内空气流动与污染物扩散规律的探明将为有关部门制定相应规划提供参考。     

温度层结对城市街区流场及污染物扩散影响的数值模拟研究

采用标准k-ε湍流模型研究了温度层结对三维街区流场和污染物扩散的影响.结果表明,温度层结对街区流场和污染物均有一定影响.随着不稳定性的增加,气流涡旋中心向地面靠近.中性温度层结下,污染物随着街区内的涡旋先向背风侧迁移,然后主要随气流向下游迁移,很少向上游街区迁移.而不稳定温度层结下,上游街区污染物浓度也随之增加.根据污...     

On the escape of pollutants from urban street canyons

Pollutant transport from urban street canyons is numerically investigated using a two-dimensional flow and dispersion model. The ambient wind blows perpendicular to the street and passive pollutants are released at the street level. Results from the control experiment with a street aspect ratio of 1 show that at the roof level of the street canyon, the vertical turbulent flux of pollutants is upward everywhere and the vertical flux of pollutants by mean flow is upward or downward. The horizontally integrated vertical flux of pollutants by mean flow at the roof level of the street canyon is downward and its magnitude is much smaller than that by turbulent process. These results indicate that pollutants escape from the street canyon mainly by turbulent process and that the net effect of mean flow is to make some escaped pollutants reenter the street canyon. Further experiments with different inflow turbulence intensities, inflow wind speeds, and street aspect ratio confirm the findings from the control experiment. In the case of two isolated buildings, the horizontally integrated vertical flux of pollutants by mean flow is upward due to flow separation but the other main results are the same as those from the control experiment.     

Numerical investigation of pollutant transport characteristics inside deep urban street canyons

A validated LES model was employed to simulate the street canyons of aspect ratio (AR) 3, 5, and 10. Three, five, and eight vertically aligned primary recirculations were found for the three cases, respectively, which showed decreasing strength with decreasing height. The ground-level wind speeds were found to be very small, making it extremely difficult for the ground-level pollutants to disperse. Local maxima of turbulence intensities were found at the interfaces between the primary recirculations and the shear layer. The pollutant trajectory followed the primary recirculations. High pollutant concentration and variance were found near the buildings where wind flowed upward. Large gradients of pollutant concentration and variance were also observed at the interfaces between the primary recirculations and the shear layer. Detailed analyses of concentration budget showed that the advection terms were responsible for pollutant redistribution within primary recirculations, while the turbulent transport terms were responsible for pollutant penetration between primary recirculations as well as pollutant removal from the street canyon.     

A concentration correction scheme for Lagrangian particle model and its application in street canyon air dispersion modelling

Pollutant dispersion in street canyons with various configurations was simulated by discharging a large number of particles into the computation domain after developing a time-dependent wind field. Trajectory of the released particles was predicted using a Lagrangian particle model developed in an earlier study. A concentration correction scheme, based on the concept of “visibility”, was adopted for the Lagrangian particle model to correct the calculated pollutant concentration field in street canyons. The corrected concentrations compared favourably with those from wind tunnel experiments and a linear relationship between the computed concentrations and wind tunnel data were found. The developed model was then applied to four simulations to test for the suitability of the correction scheme and to study pollutant distribution in street canyons with different configurations. For those cases with obstacles presence in the computation domain, the correction scheme gives more reasonable results compared with the one without using it. Different flow regimes are observed in the street canyons, which depend on building configurations. A counter-clockwise rotating vortex may appear in a two-building case with wind flow from left to right, causing lower pollutant concentration at the leeward side of upstream building and higher concentration at the windward side of downstream building. On the other hand, a stable clockwise rotating vortex is formed in the street canyon with multiple identical buildings, resulting in poor natural ventilation in the street canyon. Moreover, particles emitted in the downstream canyon formed by buildings with large height-to-width ratios will be transported to upstream canyons.     

Impact of building facades and ground heating on wind flow and pollutant transport in street canyons

This paper investigates the impacts of building facades and ground heating on the wind flow and pollutant transport in street canyons using the computational fluid dynamic (CFD) technique. Street canyons of H/W (H representing the building height and W the street width) varied from 0.1 to 2, which covered the basic flow regimes of skimming flow (H/W=1 or 2), wake interference flow (H/W=0.5), and isolated roughness flow (H/W=0.1), were examined in a series of sensitivity tests. Heating that occurred on different surfaces, including ground surface and building façades, posed considerable effects on the street canyon wind flow and pollutant transport compared with those under isothermal conditions. The CFD results showed that the mechanically induced wind flow and pollutant transport were complicated by the buoyancy under temperature stratification. Individual street canyons of different H/W and surface-heating scenarios exhibited their unique wind flow structure and pollutant transport behaviors. Two counter-rotating vortices were calculated in the street canyons of H/W=1, in which the zone of higher pollutant concentration under isothermal conditions was switched from the leeward side to the windward side. In the street canyon of H/W=2, the recirculating wind pattern was perturbed by surface heating that led to the development of either one primary vortex or three closely coupled vortices. Because of the complicated wind structure, the zones of higher pollutant concentration located either on the leeward or windward ground level were subjected to the surface-heating scenarios. Only two vortices were developed inside the street canyon of H/W=0.5. The large primary vortex, centered inside the street canyon, extended above the roof level of the street canyon. Meanwhile, a small secondary vortex was found at the ground-level windward corner whose size results as a function of surface-heating configurations. Finally, in the street canyon of H/W=0.1, an isolated clockwise-rotating vortex was developed beside the leeward building while the wind in the windward side blew in the prevailing wind direction. As a result, air pollutant emitted at the street centerline was unlikely to be carried into the leeward vortex. Instead, it was dispersed rapidly on the windward side before being removed from the street canyon.     

Street canyon ventilation and atmospheric turbulence

Operational models for pollutant dispersion in urban areas require an estimate of the turbulent transfer between the street canyons and the overlying atmospheric flow. To date, the mechanisms that govern this process remain poorly understood. We have studied the mass exchange between a street canyon and the atmospheric flow above it by means of wind tunnel experiments. Fluid velocities were measured with a Particle Image Velocimetry system and passive scalar concentrations were measured using a Flame Ionisation Detector. The mass-transfer velocity between the canyon and the external flow has been estimated by measuring the cavity wash-out time. A two-box model, used to estimate the transfer velocity for varying dynamical conditions of the external flow, has been used to interpret the experimental data. This study sheds new light on the mechanisms which drive the ventilation of a street canyon and illustrates the influence of the external turbulence on the transfer process.     

On the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flow

The ventilation and pollutant transport in a two-dimensional (2D) street canyon of building-height-to-street-width (aspect) ratio h/b = 1 under different unstable stratifications were examined. To characterize the combined wind-buoyancy-driven flow and pollutant transport at different Richardson number Ri, a computational fluid dynamics (CFD) model based on the Reynolds-averaged Navier–Stokes (RANS) equations with the Renormalization Group (RNG) k ? ε turbulence model was adopted. Unlike the isothermal condition, a secondary recirculation is initiated at the ground-level windward corner of the street canyon once the unstable stratification is switched on (Ri < 0). It traps the ground-level pollutant leading to elevated pollutant concentration there. As Ri further decreases, the enlarging secondary recirculation enables direct pollutant removal from its core to the shear layer that offsets the ground-level pollutant accumulation. The ventilation and pollutant removal performance under different unstable stratifications are compared by the air (ACH) and pollutant (PCH) exchange rates, and pollutant retention time (τ). Both the mean and turbulent components of ACH are found to increase with decreasing Ri, suggesting that unstable stratification promotes ventilation in street canyons. Moreover, the CFD results agree well with our theoretical model that ACH² varies linearly with Ri. Turbulent transport originally dominates the pollutant removal under isothermal condition. However, progressive domination of pollutant removal by mean wind can be observed with decreasing stability (decreasing Ri from 0 to ?10.6). The critical value is estimated to be Ri = ?8, below which mean wind is the major pollutant removal carrier. Reduction in τ is also observed with decreasing Ri. Hence, in unstable stratification, pollutant resides shorter time in the street canyon compared with its isothermal counterpart, and the ventilation and pollutant removal are more favorable.     

A note on average vertical profiles of vehicular pollutant concentrations in urban street canyons

Recent observations of air pollutant concentrations measured within and above street canyons were used to study the average vertical profiles of vehicular pollutant concentrations in the urban environment. The idea of an exponential vertical concentration distribution, exp( −Bz^q), resulted from a near ground-level source diffusing over flat terrain, was tentatively extended to the urban street canyons, where the empirical parameters B and q are generally dependent on the atmospheric stability and the aerodynamic characteristics of the canyon.     

A Wind Tunnel Study of Gaseous Pollutants in City Street Canyons

Steady state mean concentrations of tracer gas were measured in a 400:1 scale model of an idealized city with variable geometry placed within a wind tunnel at various orientations to the mean flow for a free stream velocity of 6.8 ft/sec. The tracer gas was released from two parallel line sources to simulate lanes of traffic in an effort to quantify the persistence of pollution as well as the mean values realized at street levels. An aerodynamically rough turbulent boundary layer of neutral thermal stratification was employed to simulate the atmosphere. Values of concentration measured in the model city were converted to prototype concentrations in ppm and compared to National Ambient Air Quality Standards. It was shown that single isolated structures may cause favorable mixing of pollution downwind but very high concentrations exist in the immediate leeward vicinity of the building. Two favorable geometries for city blocks tested were found to reduce pedestrian exposure to pollution both near heavy traffic congestion and downwind. It was concluded that the pollutant dilution was controlled by the mean flow rather than by turbulent diffusion and that the lateral spread of the plume was slight as one proceeded downwind of the line source. The combination of favorable geometry and higher dilution velocities may bring pollution levels down to existing Air Quality Standards. The body of information presented in this paper should interest city planners and air quality monitoring personnel, as well as those researchers attempting to study and model flow in city street canyons. It provides order of magnitude estimates on pedestrian and office worker exposure to pollutants under a wide range of conditions.     

Effects of Strength and Position of Pollutant Source on Pollutant Dispersion Within an Urban Street Canyon

A two-dimensional numerical model for simulating airflow and pollutant dispersion inside an urban street canyon was first developed using the FLUENT code, and then it was validated against a wind tunnel experiment. Then the effects of strength and position of pollutant sources on pollutant dispersion within an urban street canyon were investigated numerically. The numerical results showed that the dimensionless pollutant concentrations within the urban street canyon were independent from the source strength. The results also revealed that the pollutant distributions inside the urban street canyon with a two-lane road were influenced significantly by the positions of the two sources: 1) the closer the two sources were to the street center of the canyon, the lower the pollutant concentrations on the leeward wall and at the human respiration level in the leeward footpath became; 2) the pollutant concentrations on the windward wall and at the human respiration level in the windward footpath were not sensitive to the locations of the two sources as long as the source on the windward lane was situated outside the small recirculation zone at the bottom corner of the canyon windward wall; 3) the pollutant concentrations on the lower parts of the windward and leeward walls as well as in the two footpaths increased greatly when the two sources were moved from outside into the small recirculation zones.     

Modelling dispersion of traffic pollution in a deep street canyon: Application of CFD and operational models

In this study, numerical modelling of the flow and concentration fields has been undertaken for a deep street canyon in Naples (Italy), having aspect ratio (i.e. ratio of the building height H to the street width W) H/W = 5.7. Two different modelling techniques have been employed: computational fluid dynamics (CFD) and operational dispersion modelling. The CFD simulations have been carried out by using the RNG k–? turbulence model included in the commercial suite FLUENT, while operational modelling has been conducted by means of the WinOSPM model. Concentration fields obtained from model simulations have been compared with experimental data of CO concentrations measured at two vertical locations within the canyon. The CFD results are in good agreement with the experimental data, while poor agreement is observed for the WinOSPM results. This is because WinOSPM was originally developed and tested for street canyons with aspect ratio H/W ≌ 1. Large discrepancies in wind profiles simulated within the canyon are observed between CFD and OSPM models. Therefore, a modification of the wind profile within the canyon is introduced in WinOSPM for extending its applicability to deeper canyons, leading to an improved agreement between modelled and experimental data. Further development of the operational dispersion model is required in order to reproduce the distinct air circulation patterns within deep street canyons.