Effects of Street-Bottom and Building-Roof Heating on Flow in Three-Dimensional Street Canyons

Using a computational fluid dynamics （CFD） model, the effects of street-bottom and building-roof heating on flow in three-dimensional street canyons are investigated. The building and street-canyon aspect ratios are one. In the presence of street-bottom heating, as the street-bottom heating intensity increases, the mean kinetic energy increases in the spanwise street canyon formed by the upwind and downwind buildings but decreases in the lower region of the streamwise street canyon. The increase in momentum due to buoyancy force intensifies mechanically induced flow in the spanwise street canyon. The vorticity in the spanwise street canyon strengthens. The temperature increase is not large because relatively cold above-roof-level air comes into the spanwise street canyon. In the presence of both street-bottom and building-roof heating, the mean kinetic energy rather decreases in the spanwise street canyon. This is caused by the decrease in horizontal flow speed at the roof level, which results in the weakening of the mean flow circulation in the spanwise street canyon. It is found that the vorticity in the spanwise street canyon weakens. The temperature increase is relatively large compared with that in the street-bottom heating case, because relatively warm above-roof-level air comes into the spanwise street canyon.

Numerical Studies on Flow Fields Around Buildings in an Urban Street Canyon and Cross-Road

The questions on how vortices are constructed and on the relationship between the flow patterns and concentration distributions in real street canyons are the most pressing questions in pollution control studies. In this paper, the very large eddy simulation (VLES) and large eddy simulation (LES) are applied to calculate the flow and pollutant concentration fields in an urban street canyon and a cross-road respectively. It is found that the flow separations are not only related to the canyon aspect ratios, but also with the flow velocities and wall temperatures. And the turbulent dispersions are so strongly affected by the flow fields that the pollutant concentration distributions can be distinguished from the different aspect ratios, flow velocities and wall temperatures.

A Laboratory Model for the Flow in Urban Street Canyons Induced by Bottom Heating

Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind, the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However, in a narrow street canyon, just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building.

Numerical Study on the Impact of Ground Heating and Ambient Wind Speed on Flow Fields in Street Canyons

The impact of ground heating on flow fields in street canyons under different ambient wind speed conditions was studied based on numerical methods. A series of numerical tests were performed, and three factors including height-to-width （H/W） ratio, ambient wind speed and ground heating intensity were taken into account. Three types of street canyon with H/W ratios of 0.5, 1.0 and 2.0, respectively, were used in the simulation and seven speed values ranging from 0.0 to 3.0 m s-1 were set for the ambient wind speed. The ground heating intensity, which was defined as the difference b-ween the ground temperature and air temperature, ranged from 10 to 40 K with an increase of 10 K in the tests. The results showed that under calm conditions, ground heating could induce circulation with a wind speed of around 1.0 m s-i, which is enough to disperse pollutants in a street canyon. It was also found that an ambient wind speed threshold may exist for street canyons with a fixed H/W ratio. When ambient wind speed was lower than the threshold identified in this study, the impact of the thermal effect on the flow field was obvious, and there existed a multi-vortex flow pattern in the street canyon. When the ambient wind speed was higher than the threshold, the circulation pattern was basically determined by dynamic effects. The tests on the impact of heating intensity showed that a higher ground heating intensity could strengthen the vortical flow within the street canyon, which would help improve pollutant diffusion capability in street canyons.

Using the OSPM Model on Pollutant Dispersion in an Urban Street Canyon

An observational campaign was conducted in the street canyon of Zhujiang Road in Nanjing city in 2007. Hourly mean concentrations of PM10 were measured at street and roof levels. The Operational Street Pollution Model （OSPM） street canyon dispersion model was used to calculate the street concentrations and the results were compared with the measurements. The results show that there is good agreement between measured and predicted concentrations. The correlation coefficient R2 values （R2 is a measure of the correlation of the predicted and measured time series of concentrations） are 0.5319, 0.8044, and 0.6630 for the scatter plots of PM10 corresponding to light wind speed conditions, higher wind speed conditions, and all wind speed conditions, respectively. PM10 concentrations tend to be smaller for the higher wind speed cases and decrease rapidly with increasing wind speed. The presentations of measured and modelled concentration dependence on wind direction show fairly good agreement. PM10 concentrations measured on the windward side are relatively smaller, compared with the corresponding results for the leeward side. This study demonstrates that it is possible to use the OSPM to model PM10 dispersion rules for an urban street canyon.

Numerical and Experimental Studies on Flow and Pollutant Dispersion in Urban Street Canyons

In this study numerical simulations and water tank experiments were used to investigate the flow and pollutant dispersion in an urban street canyon. Two types of canyon geometry were tested. The studies indicate that in a step-up notch canyon （higher buildings on the downstream side of the canyon）, the height and shape of the upstream lower buildings plays an important role in flow pattern and pollutant dispersion, while in a step-down notch canyon （lower buildings on the downstream side）, the downstream lower buildings have little influence. The studies also show that the substitution of tall towers for parailelepiped buildings on one side of the canyon may enhance the street ventilation and decrease the pollutant concentration emitted by motor vehicles.

Physical Experiments to Investigate the Effects of Street Bottom Heating and Inflow Turbulence on Urban Street-Canyon Flow

The effects of street bottom heating and inflow turbulence on urbanstreet-canyon flow are experimentally investigated using a circulating water channel. Threeexperiments are carried out for a street canyon with a street aspect ratio of 1. Results from eachexperiment with bottom heating or inflow turbulence are compared with those without bottom heatingand appreciable inflow turbulence. It is demonstrated that street bottom heating or inflowturbulence increases the intensity of the canyon vortex. A possible explanation on how street bottomheating or inflow turbulence intensifies the canyon vortex is given from a fluid dynamicalviewpoint.

Evaluation of Local Scale PM Pollution Levels in Typical Street Canyon in Riga

The present study describes long term PM10 and PM2.5 changes in typical street canyon with particular emphasis on seasonal, diurnal variations in context with meteorological data. In order to understand PM10 pollution sources during 28 April 2007-31 December 2007, chemical composition measurements were done with particular emphasis on heavy metals (As, Cd, Ni, and Pb), crustal material (Ca, Mg, Na, and K) and anions (sulphates, nitrates, chlorides). Meteorological data used for this evolutional analysis were measured close to traffic related stations and several meteorological parameters were analyzed in relation to particulate measurements. Keep in mind that atmospheric aerosols are generally hydroscopic. Relative humidity which plays very important role in rain/snow and humidity impact are analyzed.

Pollution Dispersion in Urban Street Canyons with Green Belts

In this study,numerical simulations were used to explore the effects of roadside green belt,urban street spatial layout,and wind speed on vehicle exhaust emission diffusion in street canyon.The diffusion of different sized particles in the street canyon and the influence of wind speed were investigated.The individual daily average pollutant intake was used to evaluate the exposure level in a street canyon microenvironment.The central and leeward green belts of the road were the most conducive to the diffusion of pollutants,while the positioning of the green belts both sides of a road was least conducive to the diffusion of pollutants.Pollutant levels increased with increasing canopy height,canopy width,and decreasing tree spacing,with optimal values of 12 m,7 m,and 0.4 H,respectively.This provides protection from pollution for low-rise residents and pedestrians.The results presented here can be used to improve the air quality of the street microenvironment and provide a basis for the renovation of old street buildings.

Coupling Relationship Between Summer Microclimate and Spatial Layout of Street Canyons in the White Stupa Temple Area of Beijing

With the rapid urbanization, many high-rise buildings and new districts have been built continuously. However, the old town of cities has gradually been forgotten by people and its environment is becoming increasingly harsh. The old town usually has diversified space and function. As an important part of the old town, the street canyon that is closely related to human settlements is a public space where people communicate and do activities. Therefore, research on microclimate-based improvement of the street canyon in the old town is of great significance. Six typical street canyons in the White Stupa Temple area of Beijing were selected for research to explore spatial characteristics of the street canyon. Microclimate factors such as temperature, humidity, wind speed, and solar radiation were measured on the spot in summer. These factors were combined with two microclimate assessment indexes of physiological equivalent temperature(PET) and wet bulb globe temperature(WBGT) to evaluate the microclimate of the street canyon. In the analysis of the measured data, the mean value comparison method was used to analyze the average values of the microclimate factors in different time periods. Spatial layout of microclimate included the orientation of the canyon, the ratio of the canyon height to canyon width, and green coverage of the canyon, and an in-depth study was made on the coupling relationship between the spatial layout and the microclimate of the canyon. Research results can provide an optimization strategy for the transformation design of the street canyon in White Stupa Temple area, and provide a scientific reference for the research on spatial layout and microclimate improvement in the old town, so as to improve the living quality of residents in the old town.

基于全景图的城市街谷平均辐射温度计算模型研究

在评估城市街谷热环境对居民生活质量的影响时,明确街谷的微气候条件,尤其是平均辐射温度(Mean Radiant Temperature,MRT)对于改善城市热环境具有重要意义。现阶段,利用基于鱼眼图的天空视域因子(Sky View Factor,SVF)计算方法较为繁琐且难以实现大范围街谷MRT的时空分布评估。因此,本研究旨在通过全景图像技术,提出一种快速大量计算城市街谷MRT时空分布的新方法,并进一步考虑城市街谷中树木的影响,以改进传统模型。本研究首先基于全景图像批量获取城市街谷中的SVF,并结合城市街谷中的几何特征和植被视域因子,通过改进的平均辐射温度计算模型对单点的MRT进行计算,同时采用定点实测数据对该模型进行精确度验证,并将其应用于西安市街谷MRT的实际计算中。研究结果显示,经过模型验证,本研究方法具有较高的精度,相对误差大多数情况下在20%以内,RMSE在2.85~4.66℃。同时,模型与实测数据的一致性较好,能够清晰地反映MRT的变化趋势,R2大于0.74,IA大于0.80。与仅考虑单一不透水面的计算模型相比,考虑树木植被后的模型精度有明显提升,RMSE从5.15℃降至3.87℃,模型的R2由0.72提升至0.74,表明改进模型与观测结果具有更好的一致性。提出的基于全景图的MRT计算方法不仅提高了评估城市街谷热环境的效率和精度,而且通过考虑树木植被的影响,为城市规划和绿化管理提供了更加科学的指导。此外,本研究的方法和结论能够为城市热岛效应的缓解和城市生活环境的改善提供理论依据和技术支持。通过实例分析,研究成功地应用于西安市,展示了2021年7月14日上午9:00的街谷MRT分布图,为后续的城市热环境评估和改善工作奠定了基础。

典型浅街峡谷布局及其植物配置模式春季花粉飞散特征模拟研究

城市中的气传花粉已成为春秋季节性过敏的主要过敏原之一,研究典型建筑布局及其植物配置模式下花粉飞散特征能够为合理优化城市绿地配置提供数据支撑,为花粉飞散研究提供补充。基于北京市海淀区校园绿地春季观测数据,提取3个采样地街谷布局和植物特征参数,借助CFD平台构建15个典型浅街峡谷布局及其植物配置模式参数化场景,探究不同场景下花粉飞散特征和距离阈值。研究结果表明:①花粉高浓度区位于风速较低、空气流动性较差的区域,花粉低浓度区位于风速高、空气流动性较好的地方。②4种植物配置模式的花粉飞散特征表现为行道树式>散点式>行道树+组团种植>组团式,花粉沉降特征表现为组团式>行道树+组团种植>行道树式>散点式。错列式布局结合行道树式植物配置花粉浓度降低率最高为24.89%,飞散效果相对更好。围合式布局结合组团式植物配置花粉浓度降低率最低为7.31%,聚积效果相对较好。③受不同建筑、植物和风场的影响,3类建筑布局花粉飞散距离远近依次为行列式>围合式>错列式,行列式需要约35—75m的飞散距离,围合式需要约28—60m的飞散距离,错列式需要约20—53m的飞散距离。风速低且花粉浓度高、飞散距离近的植物配置模式易于聚积,风速高且花粉浓度低、飞散距离远的植物配置模式易于飞散。

环境风条件下非对称街道峡谷的火灾烟气扩散与回卷机制

城市中常见多种街道峡谷(简称“街谷”)结构,一旦发生火灾,易形成大量火灾烟气等有害气体。受城市冠层环境风的影响,烟气向下风向倾斜,有可能被下风向建筑物阻挡而无法顺利向自然环境中扩散,导致出现街谷内烟气回卷现象,对身处其中人员的安全疏散和身体健康十分不利。运用火灾计算流体模拟软件FDS,重点考虑街谷非对称性对火灾烟气扩散的影响效应构建了街谷模型;选取典型的街谷条件,通过改变上、下风向建筑物高度和环境风速,揭示了烟气扩散与流场结构演化特征,获得了各非对称街谷条件下的临界回卷风速;引入街谷非对称因子m(上、下风向建筑物高度之比),建立了临界回卷风速与非对称因子m之间的预测模型。结果表明:保持下风向建筑物高度不变而增加上风向建筑物高度时,烟气扩散呈现完全回卷、半回卷和不回卷三阶段控制,反之,保持上风向建筑高度不变而增加下风向建筑物高度时,烟气扩散呈现不回卷和完全回卷两阶段控制;m值越大,即上风向建筑物越高或下风向建筑物越低时,越难形成烟气回卷现象,所对应的临界回卷风速也越大,而m值相近时街谷火灾烟气回卷行为相似;最终提出了临界回卷风速vc与街谷非对称因子m之间的分段线性预测模型。

树干高度耦合壁面热效应对城市街谷内污染扩散的影响研究

在半封闭的街道峡谷内,交通排放和二次污染物容易在通风不良的区域积聚,严重威胁人们的健康。在影响街道峡谷流场和污染物扩散特性的诸多因素中,太阳辐射引起的壁面热浮力以及不同树干高度对空气动力学的影响一直没有得到足够的重视。通过设置5种树干高度(0.18H、0.40H、0.62H、0.84H、1.06H,H为建筑高度)耦合4种壁面加热配置,研究不同树干高度(耦合树木遮阴效应)和墙体加热条件对城市街道峡谷内气流流动和污染物扩散的影响。结果表明,不同树干高度及壁面热效应对城市街道峡谷内气流流动和污染物扩散有显著影响。当树干高度低于建筑物高度时,壁面加热产生的热浮力作用会降低街谷内污染物浓度并增强通风性能;当树干高度超过建筑物高度时,迎风面加热所产生的热浮力会对污染物扩散造成阻碍。采用全壁面加热能够实现更低的污染物积累。研究结果可为城市绿色设施的优化设计,实现对局部微气候环境和空气质量精准调控提供技术指导。

灌木篱墙对高架桥街道峡谷内CO扩散的影响

为探究灌木篱墙布局对高架桥街道峡谷内部流场及污染物扩散的影响,将灌木篱墙假设为多孔介质,采用标准k-ε模型结合组分运输方程对布置中央灌木篱墙和两侧灌木篱墙的高架桥街谷的底部和桥面道路机动车排放的CO扩散过程进行模拟,并与风洞试验结果比较。结果表明:数值模拟方法可靠,且与标准街谷不同,高架街谷中的灌木篱墙不仅不会阻碍流场运动,还能加强底部受阻风场与上部风场的对流,有利于底部道路排放污染物的扩散,能够有效降低街谷的整体污染物浓度,改善污染物在背风侧、桥体上方和底部集聚的问题。两侧均设置篱墙时风场的上下对流更强,背风面壁面处CO质量分数降低60%,增设灌木篱墙可作为改善其内部空气污染程度的有效措施。

树种及其树冠尺寸对街谷内污染物分布特征的影响

本研究运用数值模拟方法分析了树冠尺寸、树种的变化对街道峡谷内气流结构及污染物扩散的影响。采用平均无量纲浓度(K)和空气交换率(ACH)对街道峡谷内通风能力与污染状况进行评估。结果表明:树冠尺寸和树种压力损失系数的变化均会影响街谷内的气流流动结构和污染物扩散规律。其中,树冠尺寸和树种压力损失系数的增大都会使树冠对气流的阻碍作用增强,不利于街谷内通风及污染物扩散,导致街谷内特别是背风墙附近污染物浓度明显上升,并且树冠尺寸变化对污染物扩散的影响相较于树冠压损系数的变化更为显著。

绿化空间布局及壁面加热条件对街道峡谷内污染物扩散的影响

采用经风洞实验验证的标准k−ε数值模型,模拟了绿化空间布局及壁面加热条件对街道峡谷内流场和污染物扩散的影响。考虑了3种绿化组合方式与4种壁面加热条件,并采用无量纲浓度K与空气交换率RACH分别对街道峡谷内污染情况与通风性能进行评估。结果表明,不同的绿化空间布局和壁面加热条件会改变街道峡谷内流场和污染物浓度分布。选择绿化结合布局时,应尽量选择绿墙和绿篱的组合以减弱绿化阻塞作用对污染物扩散的不利影响。背风面加热和迎风面加热对街道峡谷的通风能力提升作用并不明显,而三壁加热能明显改善街道峡谷的通风性能。

Effects of increasing the degree of building height asymmetry on ventilation and pollutant dispersion within street canyons

Rational urban design helps to build sustainable cities with high ventilation capacity and pollutant removal ca-pacity,but the effect of building height on ventilation and pollutant dispersion inside asymmetric canyons has not been fully studied.In this paper,we studied the effect of increasing the degree of building height asym-metry(DBHA)on canyon ventilation and pollutant diffusion in shallow and deep asymmetric street canyons by considering six different building height ratios(BHR=3/4,1/2,1/3,4/3,2/1 and 3/1).The results show that increasing the DBHA in asymmetric canyons can improve the ventilation and pollutant removal capacity.For step-up canyons,increasing the downwind building height is very useful to improve ventilation and pollutant re-moval.For shallow/deep step-up canyons with BHR=1/3,the air exchange rate(ACH)increased to 211.2%and 380.1%of the flat canyons,respectively.The spatially-average pollutant concentration in the pedestrian zones(leeward Kavg∗ang windward Kavg∗)decreases significantly with the increase of DBHA,especially for the deep step-up canyon with BHR=1/3,the leeward Kavg∗and windward Kavg∗decrease to 15.3%and 3%,respectively.Also,increasing the upwind building height can also improve the ventilation capacity in the step-down canyons.For the deep step-down canyon with BHR=3/1,the leeward Kavg∗and windward Kavg∗decreased to 40.6%and 24.1%of the deep flat canyon,respectively.Notably,the ventilation capacity is very low for step-down canyons with BHR=4/3,and for step-down canyons with BHR≥2/1,the ventilation capacity and pollutant removal capacity increase significantly with the increase of DBHA.Therefore,in urban planning,step-down canyons with BHR=4/3 should be avoided and designed to satisfy the condition of BHR≥2/1.These findings will be a valuable reference for urban designers to build sustainable cities with high ventilation capacity.

临街建筑开窗下街谷气流及污染物扩散的研究

【目的】为了探究城市典型街谷-临街建筑污染的街谷-开窗耦合作用。【方法】基于机动车现场实测和CFD(Computational Fluid Dynamics)数值模拟的方法,研究临街建筑开窗情况下街谷内及室内空气流动和污染物的分布规律。【结果】结果表明:临街建筑外壁面开窗因其对周围气流产生扰动而使街谷内涡旋流动有所增强,在建筑背风面和迎风面的窗口附近形成了多个不规则的气流小涡旋,各层局部风向和风速改变各异;背风侧临街建筑湍流强度受开窗情况的影响比迎风侧建筑更加明显,1-4层比未开窗增加达20.6%;室内污染物浓度逐层变化与室外街谷污染物分布大体一致,背风建筑室内污染物浓度远高于迎风面建筑;背风建筑室内污染物浓度随楼层增高不断降低,1-4层污染物占到总污染的43.7%,迎风建筑室内污染物浓度沿楼高并未呈现逐层降低的单调趋势,而是先减小,在建筑中位层附近最小,随后在窗口扰动和街谷环流漩涡的共同作用下,污染物浓度再逐层增大。街谷-开窗建筑整场求解克服了模拟临街建筑的入流水平设置与实际工况的不符,较好地解决了开窗位置复杂涡旋边界设置及湍流逐层各异的难题。