火灾模型对钢交错桁架结构性能化抗火设计的适用性分析

对目前性能化防火设计中常用的4种火灾作用模型进行了分析和总结,并运用这4种模型对钢结构交错桁架在火灾时的热环境进行了模拟,分析不同火灾作用模型下该体系的结构响应,研究不同火灾作用模型对交错桁架结构性能化抗火设计的适用性。研究表明:火源功率、火灾载及火灾持续时间等因素对钢结构交错桁架的抗火分析具有重要影响。国际标准升温曲线和参数化室内升温曲线不考虑火源功率、建筑通风口等因素,缺乏针对性,分析结果偏于保守;区域模型所得温度偏低,不宜用于交错桁架结构的性能化抗火设计;场模型考虑了火源功率及环境温度分布的不均匀性,和实际火灾较接近,计算结果相对较为准确,可用于结构的性能化防火设计。     

日光照射对街道峡谷污染物扩散影响的研究

为了分析日光照射对城市街道峡谷机动车污染物扩散的影响．对街道峡谷日光照射的物理模型进行了简化．采用数值模拟技术对日光照射下的城市街道峡谷内气体流动和机动车污染物扩散规律进行了研究。结果表明．在一定条件下。日光照射是研究城市街道峡谷内污染物扩散必须考虑的因素。在污染物扩散受日光照射影响较大的街道峡谷内部．当街道地面或迎风面受日光照射时．街道峡谷内部将出现2个方向相反的漩涡，并导致迎风面建筑物一侧的污染物浓度升高．这与不计日光照射的特征有显著的不同。     

上海城市街道峡谷道路绿化模式研究

街道峡谷在上海城市中普遍存在,街道峡谷中污染气体的扩散受到屋顶风向、风速、高宽比、峡谷两侧建筑物的对称性、高度分布和街区形状等因素的影响。文章对适合于不同街道峡谷形式的、较为典型的道路绿化模式进行了研究。结果表明,上海城市街道峡谷中的道路绿化模式可归纳为:梯级递进式、斑块复合式、疏朗开敞式、模纹花镜式和独立行道树式5种。当街道峡谷与主导风垂直:(1)街道峡谷的W/H>5且为平行型峡谷时,绿化模式应采用梯级递进式或疏朗开敞式;(2)街道峡谷的W/H为0.6～5且为平行型峡谷时,绿化模式在迎风侧采用流朗开敞式、背风侧采用模纹花镜式,或者两侧都采用模纹花镜式;(3)街道峡谷的W/H<0.6且为平行型峡谷时,绿化模式应采用模纹花镜式或独立行道树式;(4)街道峡谷的W/H<0.6且为递升型峡谷时,绿化模式在迎风侧采用斑块复合式或模纹花镜式,背风侧采用模纹花镜式或独立行道树式;(5)街道峡谷的W/H<0.6且为递降型峡谷时,绿化模式可采用模纹花镜式或独立行道树式;当街道峡谷与主风向平行:宽、高比大的街道绿化模式应采用梯级递进式或斑块复合式,当街道峡谷的高度是宽度的倍数时,道路绿化宜采用疏朗开敞式或独立行道树式。     

孤立与非孤立城市街道峡谷内污染物扩散

通过求解二维不可压N-S方程、k-ε方程及污染物对流扩散方程,模拟了孤立街道峡谷与非孤立街道峡谷内的流场及交通污染物浓度场.计算结果与风洞试验结果总体趋势一致.非孤立街道峡谷内污染物壁面浓度要大于孤立街道峡谷内的壁面浓度.通过计算街道峡谷建筑屋顶高度处的垂直方向污染物通量,说明了湍流扩散是污染物扩散出街道峡谷的主要原因,其污染物通量总为正,而平均流通量可以为负.非孤立街道峡谷由于平均流流动和湍流流动的总扩散通量减少,造成污染物在街道峡谷内集聚,从而理论上解释了非孤立街道峡谷与孤立街道峡谷污染扩散的差别.     

The Modelling of Turbulence from Traffic in Urban Dispersion Models — Part I: Theoretical Considerations

The modelling of pollutant dispersion at the street scale in an urban environment requires the knowledge of turbulence generated by the traffic motion in streets. In this paper, a theoretical framework to estimate mechanical turbulence induced by traffic in street canyons at low wind speed conditions is established. The standard deviation of the velocity fluctuations is adopted as a measure of traffic-produced turbulence (TPT). Based on the balance between turbulent kinetic energy production and dissipation, three different parameterisations for TPT suitable for different traffic flow conditions are derived and discussed. These formulae rely on the calculations of constants that need to be estimated on the basis of experimental data. One such estimate has been made with the help of a wind tunnel data set corresponding to intermediate traffic densities, which is the most common regime, with interacting vehicle wakes.     

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.     

A field study of factors influencing the concentrations of a traffic-related pollutant in the vicinity of a complex urban junction

The paper describes a field study focused on the dispersion of a traffic-related pollutant within an area close to a busy intersection between two street canyons in Central London. Simultaneous measurements of airflow, traffic flow and carbon monoxide concentrations ([CO]) are used to explore the causes of spatial variability in [CO] over a full range of background wind directions. Depending on the roof-top wind direction, evidence of both flow channelling and recirculation regimes were identified from data collected within the main canyon and the intersection. However, at the intersection, the merging of channelled flows from the canyons increased the flow complexity and turbulence intensity. These features, coupled with the close proximity of nearby queuing traffic in several directions, led to the highest overall time-average measured [CO] occurring at the intersection. Within the main street canyon, the data supported the presence of a helical flow regime for oblique roof-top flows, leading to increased [CO] on the canyon leeward side. Predominant wind directions led to some locations having significantly higher diurnal average [CO] due to being mostly on the canyon leeward side during the study period. For all locations, small changes in the background wind direction could cause large changes in the in-street mean wind angle and local turbulence intensity, implying that dispersion mechanisms would be highly sensitive to small changes in above roof flows. During peak traffic flow periods, concentrations within parallel side streets were approximately four times lower than within the main canyon and intersection which has implications for controlling personal exposure. Overall, the results illustrate that pollutant concentrations can be highly spatially variable over even short distances within complex urban geometries, and that synoptic wind patterns, traffic queue location and building topologies all play a role in determining where pollutant hot spots occur.     

Fractionation and Biotoxicity of Heavy Metals in Urban Dusts Collected from Hong Kong and London

Samples of urban dusts, road site dusts and car park dusts, were collected at two selected sites each in Hong Kong and London. Sequential extraction was used to characterise the chemical compositions of these urban dusts. Copper, lead, zinc, pH, electrical conductivity and organic content were measured. Biotoxicity tests of urban dusts were conducted on higher plants (Brassica chinensis and Lolium perenne), a dinoflagellate green alga (Dunaliella tertiolecta) and luminescent bacteria (Photobacterium phosphoreum). A significant correlation was found between total lead (r =; – 0.70, p < 0.01) and zinc (r =; – 0.74, p < 0.05), and the 20 min – EC50 using P. phosphoreum. In addition, there was a significant correlation (r = – 0.72, p < 0.01) between the exchangeable lead content in dust and the 48 h- EC30 using D. tertiolecta. No specific trend was obtained for higher plants. Total lead and zinc contents were higher in dusts from London while the percentage of exchangeable fraction of metal contents was higher in those from Hong Kong.     

交错齿槛型泥石流排导槽研究初探

排导槽是一种常见的泥石流防治措施,提出了交错齿槛槽定义,即该类槽是指由两束流侧墙及置于其间交错排列的梯级齿槛群构成的一种新型泥石流排导槽,根据流域面积与沟床纵比降可分为3种类型;详细分析了交错槽的结构特征,并根据泥石流的不同阶段阐述了泥石流在交错槽中的流态形式;最后从消能机制、冲刷磨蚀、生态效益等方面将交错槽与东川槽等常规阶梯深潭系统做了比较,指出交错齿槛槽具有明显的优势,为进一步研究泥石流与交错齿槛的作用机制、泥石流在其中的排导特征等打下基础。     

The underutilization of street markets as a source of food security indicators in famine early warning systems: a case study of Ethiopia

Famine Early Warning Systems (EWS) are reliant on data aggregated from multiple sources. Consequently, they are often insensitive to localized changes in food security status, leading to delayed response or interventions. While price and infrastructural data are often gathered, this case study suggests that local street markets and vendor knowledge are underutilized. Few efforts have been made to monitor systematically the street markets as an indicator of local stressors. Findings from Ethiopia show that knowledge generated by expanding food security indicators in this sector can be used in combination with EWS to facilitate earlier intervention in, or to monitor more effectively, on-going humanitarian crises. Indicators developed from this study are accurate, cost effective, and sensitive to local climatic and food stressors.