The effects of daylight saving time on vehicle crashes in Minnesota

Introduction: Daylight saving time (DST), implemented as an energy saving policy, impacts many other aspects of life; one is road safety. Based on vehicle crash data in Minnesota from 2001 to 2007, this paper evaluates long- and short-term effects of DST on daily vehicle crashes. Method: To provide evidence to explain the causes of more/fewer crashes in DST, we examine the impact of DST on crashes in four periods of a day: 3 a.m.-9 a.m., 9 a.m.-3 p.m., 3 p.m.-9 p.m., 9 p.m.-midnight. The effects of risk and exposure to traffic are also separated. Our statistical models not only include weather conditions and dummy variables for days in DST as independent variables, but also consider traffic volumes on major roads in different periods of a day. Our major finding is that the short-term effect of DST on crashes on the morning of the first DST is not statistically significant. Moreover, it is interesting to notice that while DST per se is associated with fewer crashes during dusk, this is in part offset because it is also associated with more traffic on roads (and hence more crashes). Our path analysis shows that overall DST reduces crashes. Impact on industry: Daylight saving time can lead to fewer crashes on roads by providing better visibility for drivers.     

云南省近500年旱涝灾害时间序列的分形研究

根据云南省近500年的旱涝灾害时间分布序列,应用R/S分析,预测了未来旱涝灾害的发生时间,但预测时间尺度不同,结果也不同,因此仅从旱涝灾害发生的时间序列本身来预测,需确定其能够预测的时间范围;应用Smalley算法分别对云南省干旱和洪涝灾害的时间序列特征进行分析,干旱年发生时间序列的分形维数为0.5498,洪涝年发生时间序列的分形维数为0.4923,具有明显的分形特征,均小于Cantor集合的0.6309,可见云南省旱涝灾害发生的自组织程度可能会提高。分形研究的结果反映的仅是云南省近500年旱涝灾害时间序列的平均状况,由于旱涝灾害的分布具有空间差异,因此需分别研究不同程度灾害区旱涝时间序列的分形及多重分形特征。     

地铁车站人员疏散离散时间模型研究

疏散时间对于保证地铁车站这类复杂建筑结构内人群的安全至关重要,本文借鉴群集流动理论建立了用于计算地铁车站人群移动时间的疏散离散时间模型(EDTM),模型中的人群流动系数取值不同于传统计算公式中的常量,依据的是经典的车站人群密度与速度函数关系式。将此模型用于某地铁车站站台层,求解楼梯出口疏散人数与时间的关系,并与Building EXODUS疏散软件的模拟结果以及传统地铁车站疏散时间计算公式的计算结果进行了对比,分析表明,EDTM计算结果与EXODUS疏散模拟结果非常接近,且比传统公式计算更为精确和符合实际情况。该计算模型可以为地铁车站人群疏散时间计算、建筑出口性能化设计,以及地铁车站事故应急预案的制定提供更有效的工具。     

我国曝气塘水力停留时间和比功率选用范围分析

在总结和分析国外曝气塘水力停留时间和比功率选用状况的基础上,阐述了盲目应用国外曝气塘有关设计参数进行我国曝气塘设计可能出现的问题,并计算和提出了我国曝气塘设计水力停留时间和比功率的选用范围。      

两级完全混合发酵工艺开发碳源试验研究

采用两级完全混合发酵工艺进行碳源开发试验,研究了水力停留时间(HRT)和污泥停留时间(SRT)对工艺开发碳源效果的影响.研究表明,固定SRT为4d,污泥回流比为1,HRT在32~36h,系统出水的溶解性COD(SCOD)可维持在1090~1180mg/L,且浓缩池泥水分离效果较好,出水的SS保持在50~80mg/L.保持系统的HRT为32h,污泥回流比为1,SRT在4~7d时,可实现厌氧系统中产酸菌与产甲烷菌的分离,系统能够保持良好的产酸效果,此时系统出水的SCOD值基本稳定于980~1179mg/L.碱度可以作为衡量水解酸化系统是否有效运行的重要参数.HRT为32h,SRT为4d的工况下,初沉污泥酸化发酵系统对氨氮和磷的释放率分别为3.88 mgN/gVSS和0.27mgP/gVSS.     

基于多时相NDVI数据的复种指数提取方法研究

多熟种植是中国重要的种植制度,在农业生产中具有重要的地位。复种指数反映了耕地实际利用强度,及时获取其空间分布信息是国家农业决策的基础。论文在重新理解和定义复种指数的基础上,提出了一种基于NDVI曲线迭代修正的复种指数遥感提取方法,并以SPOT/VGT多时相NDVI数据为基础,提取了1999~2004年中国北方17省市区农用地的复种指数。结果表明:提取的复种指数空间分布符合我国耕作制度区划,基于目视解译的样点检验总体精度为95.24%,Kappa系数为0.9057,与根据统计数据得出的复种指数结果的线性回归斜率为0.9288（R2=0.9159,P<0.001）,表明该方法能够准确快捷地提取复种指数,及时地为农业和土地管理部门提供耕作制度的空间信息。     

沙尘暴对呼吸及循环系统疾病日门诊量的影响

对沙尘天气频发区甘肃省武威市的7所大、中型医院在2004、2005年沙尘天气高发时段(3月1日～5月31日)的呼吸系统和循环系统每日门诊人数进行了调查,采用半参数广义相加模型(GAM),在排除了医院日门诊量的长期趋势、大气污染因素、气象因素、日历效应等因素之后,分别对每年进行分析.结果表明,沙尘暴和扬沙天气与呼吸、循环系统疾病日门诊人数的增加有联系,且均表现为滞后效应,其影响程度为沙尘暴大于扬沙天气,沙尘暴和扬沙天气对健康的影响因不同年份而不同,表明沙尘天气对健康的影响受多种因素的影响.沙尘暴对健康的影响有性别差异.     

不同泥龄下活性污泥絮体性状的研究

采用序批式反应器对不同泥龄下污泥絮体的化学性状(胞外聚合物成分及含量)、物理性状(表面电荷)、形态性状(粒度分布、分形维数)等进行了对比研究.结果表明:泥龄对胞外聚合物总量及各组分含量的影响规律并不明显;多糖/蛋白质愈大,污泥絮体表面电负性愈强;污泥絮体的平均粒径随泥龄的延长呈逐渐减小的趋势,且粒度分布愈来愈均匀;不同泥龄下,污泥絮体形态结构亦不相同,泥龄短时,絮体表面粗糙,结构开放疏松;泥龄长时,絮体表面平滑,结构紧凑;随着泥龄的增大,絮体分形维数逐渐增加.由于不同泥龄下所表现出的污泥絮体性状的差异,直接影响了污泥的絮凝和沉降性能.     

The effect of migration on the viability,dynamics and structure of two coexisting metapopulations

Two species of butterflies, Euphydryas aurinia and Melitaea phoebe, coexist as two metapopulations in a 38-patch network in Hebei Province, China. A Markovian model, whose transition matrix is the product of two matrices which represent the local extinction and recolonization process respectively, is used to describe the metapopulation dynamics. The application of this model to the metapopulation, consisting of 12 local populations in the northern subregion, shows that the expected life times of E. aurinia and M. phoebe are 160 and 121 years respectively and usually nearly half of the patches are occupied by E. aurinia, while only 1–3 patches are occupied by M. phoebe. We claim that E. aurinia can persist for a long time while M. phoebe faces relatively big extinction risk. By comparing the population dynamics with and without migration, we find M. phoebe benefits much more from migration than E. aurinia. Most patches are occupied mainly by local populations for E. aurinia, while by immigrants from the 8th patch for M. phoebe, meaning that E. aurinia has a classical metapopulation structure while M. phoebe has a source–sink metapopulation structure.     

Experimental investigation of bubbly flow and turbulence in hydraulic jumps

Many environmental problems are linked to multiphase flows encompassing ecological issues, chemical processes and mixing or diffusion, with applications in different engineering fields. The transition from a supercritical flow to a subcritical motion constitutes a hydraulic jump. This flow regime is characterised by strong interactions between turbulence, free surface and air–water mixing. Although a hydraulic jump contributes to some dissipation of the flow kinetic energy, it is also associated with increases of turbulent shear stresses and the development of turbulent eddies with implications in terms of scour, erosion and sediment transport. Despite a number of experimental, theoretical and numerical studies, there is a lack of knowledge concerning the physical mechanisms involved in the diffusion and air–water mixing processes within hydraulic jumps, as well as on the interaction between the free-surface and turbulence. New experimental investigations were undertaken in hydraulic jumps with Froude numbers up to Fr = 8.3. Two-phase flow measurements were performed with phase-detection conductivity probes. Basic results related to the distributions of void fraction, bubble frequency and mean bubble chord length are presented. New developments are discussed for the interfacial bubble velocities and their fluctuations, characterizing the turbulence level and integral time scales of turbulence representing a “lifetime” of the longitudinal bubbly flow structures. The analyses show good agreement with previous studies in terms of the vertical profiles of void fraction, bubble frequency and mean bubble chord length. The dimensionless distributions of interfacial velocities compared favourably with wall-jet equations. Measurements showed high turbulence levels. Turbulence time scales were found to be dependent on the distance downstream of the toe as well as on the distance to the bottom showing the importance of the lower (channel bed) and upper (free surface) boundary conditions on the turbulence structure.