Modelling of Flow and Pollution Dispersion in a Two Dimensional Urban Street Canyon

The wind-driven flow patterns and the dispersion of vehicle exhaust pollutants released at street level has been simulated with the three-dimensional (3-D) dispersion model ADREA-HF (Andronopoulos et al., 1993), for idealised two-dimensional urban fetches occupied by buildings with slanted roofs. The simulation used oncoming atmospheric boundary layer characteristics corresponding to realistic above-town wind characteristics, as measured in reference wind tunnel experiments (Rafailidis, 1997). At that stage, analysis was limited to neutral stability conditions only. Firstly, the quality assurance of the numerical model was investigated in terms of the sensitivity to different grid allocations. The modelling results were corroborated by comparison with wind tunnel measurements in a similar two-dimensional domain (Pavageau et al., 1997). The numerical modelling replicated well the high degree of non-uniformity in the dispersion field in the test street, and the results agreed satisfactorily with the experimental measurements. The reasons for the differences observed have been investigated. With the model thus validated, three different exhaust release scenarios have been tested, keeping the same overall emission rate but different spatial patterns of street-release. The effect of the different street-release scenarios was found to be only marginal, with the dispersion patterns on the sidewalls affected only locally, close to the street level.     

The role of built environment on pedestrian crash frequency

This study investigates (i) the link of land use and road design on pedestrian safety and (ii) the effect of the level of spatial aggregation on the frequency of pedestrian accidents. For this purpose, pedestrian accident frequency models were developed for New York City based on an extensive dataset collected from different sources over a period of 5 years. The assembled dataset provides a rich source of variables (land-use, demographics, transit supply, road network and travel characteristics) and two different crash frequency outcomes: total and fatal-only collision counts. Among other things, it was observed that the census tract analysis (disaggregate data) provides more insightful and consistent results than the analysis at the zip code level. The results indicate that tracts with greater fraction of industrial, commercial, and open land use types have greater likelihood for crashes while tracts with a greater fraction of residential land use have significantly lower likelihood of pedestrian crashes. Moreover, census tracts that have a greater number of schools and transit stops - which are determinants of pedestrian activity - are more likely to have greater crashes. Results also show that the likelihood of pedestrian-vehicle collision increases with the number of lanes and road width. This suggests that retrofitting or narrowing the roads could possibly reduce the risk of pedestrian crashes.     

复杂街区脏弹恐怖袭击下放射性物质扩散模拟

为研究复杂街区“脏弹”恐怖袭击情景下的放射性物质扩散问题,以实体街区为例,基于欧拉模型,对街区内放射性物质扩散进行数值模拟,分析并总结复杂街区结构和不同风场条件下放射性物质的扩散特征。结果发现,同一风向下,当风速小于20 m/s时,风速越大,街区下风向出口处的辐射风险越高;当风速为10 m/s时,街区下风向各出口的总辐射风险最高,风速为6 m/s时风险最低。街区复杂结构对扩散的影响随风速的增大而增大;风速小于2 m/s时,近源处易形成大面积高浓度聚集,污染物整体呈高斯分布。研究表明:发生袭击时,应避免在下风向建筑物邻近区域或密集区域以及环形区域等位置停留,疏散时也应选择远离这些区域的出口。     

规模化城市污水处理厂沉淀池模拟优化

高效沉淀池广泛应用于市政污水处理厂,其表面负荷是沉淀效率的关键指标。为将实际工程表面负荷稳定提升到12 m·h⁻¹,采用计算流体力学方法,模拟优化了高效沉淀池的负荷。结果表明,污泥特性和几何构型的协同关系是提高沉淀池表面负荷的关键。宽下降区、长挡墙有利于提升流场的均匀性,但远端出水堰的污泥流失风险限制了表面负荷提升。优化的挡墙长度、上升区/下降区(1/2.65)之比等几何构型特征,能够减少积泥斗污泥扰动,改善总体流场均匀性,从而将表面负荷提高到11~12 m·h⁻¹。污泥沉淀过程中的流变特性和流动本构认识不足,限制了表面负荷的进一步提升。上述结果提升了高效沉淀池工程负荷,深化了对高效沉淀池流态的认识,亦为污水处理系统的减污降碳协同增效提供运行优化工具及案例参考。     

Morphology Characteristics of Southern Appalachian Wilderness Streams1

Zink, Jason M., Gregory D. Jennings, and G. Alexander Price, 2012. Morphology Characteristics of Southern Appalachian Wilderness Streams. Journal of the American Water Resources Association (JAWRA) 48(4): 762‐773. DOI: 10.1111/j.1752‐1688.2012.00647.x Abstract: Watersheds without urbanization or impacts from logging are rare in the southern Appalachian Mountains. The Joyce Kilmer/Slickrock Wilderness of North Carolina and Tennessee contains 24 km² of old‐growth forest, with the balance of the wilderness in a mature second‐growth forest. The watersheds of Little Santeetlah and Slickrock Creek are located within the wilderness. Morphological information, including channel dimensions and longitudinal profiles, was gathered from 14 alluvial stream reaches in these watersheds. The study sites had drainage areas from 0.25 to 41.6 km² and stream slopes from 0.014 to 0.104 m/m. Bankfull cross‐section dimensions of the study stream reaches were strongly correlated to drainage area across the observed range of slopes and bed morphology. Cross‐section area and width relationships for the streams in this study did not differ significantly from regional curves for the mountain physiographic region of North Carolina. Observations of these reaches did not suggest a definitive rule regarding the proportion of steps and riffles in streams. Pools occupied greater than 50% of the length in all stream reaches with slopes less than 0.07 m/m. Significant correlation existed between step height ratio and slope, suggesting that step height can be approximated as the product of channel width and slope. Riffle length and riffle slope ratios were also significantly correlated with slope, though pool spacing was not.