限速值对信号交叉口交通安全的影响分析

为研究交叉口进口道车辆限速值对交叉口的安全和运行产生的影响,通过微观仿真平台,分析车辆在交叉口限速分别为30,38和46 km/h时的车均延误、车均行程时间,并将车辆行驶轨迹输入安全辅助评价(SSAM)软件中展开分析。结果表明,车辆限速值的提高对车均延误的影响不大,但会减小车辆在进口道的车均行程时间。另外,限速值从30 km/h提高到46 km/h,引起的碰撞时间(TTC)、后侵占时间(PET)数值变化不显著,但使总冲突数从42个增加到59个。     

随机振动的描述及其试验与仿真

阐述了随机振动的描述方法,随机振动试验条件和相关参数的计算等。并结合实例,介绍了随机振动有限元仿真的流程及其工程意义,以便在深入了解随机振动原理的基础上,指导随机振动试验和有限元仿真的开展。     

A Comparison of Bayesian Methods for Uncertainty Analysis in Hydraulic and Hydrodynamic Modeling

We evaluate and compare the performance of Bayesian Monte Carlo (BMC), Markov chain Monte Carlo (MCMC), and the Generalized Likelihood Uncertainty Estimation (GLUE) for uncertainty analysis in hydraulic and hydrodynamic modeling (HHM) studies. The methods are evaluated in a synthetic 1D wave routing exercise based on the diffusion wave model, and in a multidimensional hydrodynamic study based on the Environmental Fluid Dynamics Code to simulate estuarine circulation processes in Weeks Bay, Alabama. Results show that BMC and MCMC provide similar estimates of uncertainty. The posterior parameter densities computed by both methods are highly consistent, as well as the calibrated parameter estimates and uncertainty bounds. Although some studies suggest that MCMC is more efficient than BMC, our results did not show a clear difference between the performance of the two methods. This seems to be due to the low number of model parameters typically involved in HHM studies, and the use of the same likelihood function. In fact, for these studies, the implementation of BMC results simpler and provides similar results to MCMC. The results of GLUE are, on the other hand, less consistent to the results of BMC and MCMC in both applications. The posterior probability densities tend to be flat and similar to the uniform priors, which can result in calibrated parameter estimates centered in the parametric space.     

Development of Sediment and Nutrient Export Coefficients for U.S. Ecoregions

Water quality impairment due to excessive nutrients and sediment is a major problem in the United States (U.S.). An important step in the mitigation of impairment in any given water body is determination of pollutant sources and amount. The sheer number of impaired waters and limited resources makes simplistic load estimation methods such as export coefficient (EC) methods attractive. Unfortunately ECs are typically based on small watershed monitoring data, which are very limited and/or often based on data collected from distant watersheds with drastically different conditions. In this research, we seek to improve the accuracy of these nutrient export estimation methods by developing a national database of localized EC for each ecoregion in the U.S. A stochastic sampling methodology loosely based on the Monte‐Carlo technique was used to construct a database of 45 million Soil and Water Assessment Tool (SWAT) simulations. These simulations consider a variety of climate, topography, soils, weather, land use, management, and conservation implementation conditions. SWAT model simulations were successfully validated with edge‐of‐field monitoring data. Simulated nutrient ECs compared favorably with previously published studies. These ECs may be used to rapidly estimate nutrient loading for any small catchment in the U.S. provided the location, area, and land‐use distribution are known.     

京津冀地区重污染天气过程的污染气象条件数值模拟研究

通过采用全球再分析格点资料的统计分析和WRF中尺度数值模拟,从天气学和大气边界层气象学角度分析了2013年12月和2014年2月两次重污染过程中京津冀地区天气尺度大气停滞气象条件和大气污染扩散气象条件的特征及其作用,并根据WRF模式精细化模拟结果分析了太行山和燕山对京津冀地区城市大气污染形成的作用.研究结果表明,两次重污染天气过程中京津冀地区500 h Pa等压面上的平均风速均表现为明显的气候异常特征,500 h Pa平均风速较近10年同期分别下降了约30.8%和50.4%,大气停滞系数较近5年同期分别偏高10%和20%以上;京津冀地区发生严重污染时,WRF模式模拟的日平均混合层高度低于200 m,日平均地面10 m风速低于2 m·s-1,日平均通风量可降低到1000 m2·s-1以下,空气质量指数与日平均通风量成负相关,重污染期间的平均通风量比近5年同期平均通风量偏低29.3%~52.8%,这些不利于污染扩散的天气条件持续数日,导致了重污染天气的发生.此外,太行山对西风气流的阻挡是河北中南部地区大气污染加剧的一个重要原因,而当主导风向为偏南风时,偏南气流遇燕山后或转向回流、或爬坡,导致近地面风速减小,不利于污染物扩散,亦加剧了京津冀地区中南部城市的大气污染.     

基于MonteCarlo模拟的土壤重金属综合风险评价与案例分析

基于不确定性理论,将Monte Carlo模拟计算应用到土壤重金属风险评价中,构建了生态风险和健康风险的随机不确定模型,结合多元统计方法进行实例研究.结果表明,研究区土壤Cd、Co和Cr污染水平较高,主要富集在建矿时间较长的工业园区;综合单因子指数和综合风险因子指数评价显示土壤重金属主要为轻度污染(相应概率分别为53.2%和55.6%);手-口摄入是研究区土壤重金属暴露的主要途径,Cr为主要致癌因子;儿童承担的健康风险较高,其非致癌风险指数分别为成人的5.0倍(手-口摄入途径)和8.2倍(皮肤暴露途径),两种暴露途径下其致癌风险指数均高于USEPA推荐的安全水平.     

Analysis of gas dispersion and ventilation within a comprehensive CAD model of an offshore platform via computational fluid dynamics

Maintaining an adequate air flow with a desired air quality that is free from hazardous gases is among the most important actions taken toward the improvement of safety in any process plant. Due to the increased focus on the consequences of existing hazardous material on safety, health, and the environment, air quality and sufficient ventilation within a plant has been increasingly considered in the design stage. This paper investigates and analyzes methane and hydrogen sulfite dispersion and the effect of air ventilation within a CAD model of an offshore platform using computation fluids dynamics (CFD). In addition, this method and its principals could be utilized in any other hazardous environment. Simulations of possible hazardous events along with solutions for preventing or reducing their probability are presented to better assess the data. These investigations are performed by considering hypothetical hazardous scenarios which consist of gas leakages from pipes and process equipment under different conditions. After drafting a precise and highly detailed CAD model of the plant and performing CFD simulations on this model, the results of gas behaviors, dispersion, distribution, accumulation, and its possible hazards are investigated and analyzed. The larger amount of details of the actual plant model in CFD simulation are obtained by using a combination of different methods and software. These include PDMS for 3-D drawing of the plan, Rinoceros for geometrical integration of the process equipment and facilities, and Sharc Harpoon which meshes the model. Moreover, the probability of inducing ignitable or toxic concentration of gases within the atmosphere and air ventilation of the unit is considered by these investigations.     

Numerical study on dust movement and dust distribution for hybrid ventilation system in a laneway of coal mine

Coal dust disaster is the most serious problem in a laneway of coal mine. Dust movement regularity for comprehensive mechanized heading face is the key scientific issues for the principle and technology of dust prevention. The special topic on systematic study of the variation regularity of dust movement and dust distribution is presented with hybrid ventilation for the comprehensive mechanized heading face: Euler–Euler method was firstly established on the numerical platform for gas–solid two phase flow in a laneway. And the forces and the dynamic model of dust particles were performed in three-dimensional flow field. Then based on the visible simulations, the movement characteristics of diffusion, sedimentation and accumulation of dust particles were investigated under the action of the complex air flow, and the spatiotemporal variation of dust distribution was studied with hybrid ventilation system. Meanwhile, the obtained dust distribution regularities were compared with the obtainable experimental results. Finally, selected method on different ventilation patterns for dust control was brought out for the heading face according to the gained regularity. The research results is helpful for further understanding of the essence of dust movement with air flow, which could provide more suitable guidance for the principle of dust control and technology of ventilation.