多层次轨道交通网络与多尺度空间协同优化——以上海都市圈为例

大城市空间与交通网络互动、优化的关键在于把握出行活动的时空约束关系及基本规律。针对不同空间尺度界定骨干交通模式的服务目标,即构建适应圈层、轴带、珠链等不同空间形态的轨道交通系统结构及模式。首先,分析上海市轨道交通网络与城镇空间的发展历程及问题,以轨道交通对都市圈不同区位和不同增长模式的适配程度为切入点,提炼空间—交通互动作用在出行时间与活动空间、时间尺度与空间形态、新城发展与区域城镇格局、交通设施与增长形态四个方面的规律。根据上海市2040年总体规划的目标,针对都市圈核心功能区、通勤圈等不同空间尺度范围的布局特征,确定多模式轨道交通层次结构、功能定位、网络形式等系统设计准则和策略导向。最后,提出一小时时间约束的上海都市圈交通廊道识别、节点选择和枢纽体系设计方法和方案。     

Energy dissipation in high-energy ship-offshore jacket platform collisions

Ship collisions with offshore structures may be characterized by large amounts of kinetic energy that can be dissipated as strain energy in either the ship, or the installation, or shared by both. In this paper a series of FE numerical simulations are performed with the aim of providing a clearer understanding on the strain energy dissipation phenomenon, particularly upon the ship-structure interaction. Ships of different dimensions and layouts are modelled for impact simulations. Likewise, three platform jacket models of different sizes and configurations are considered. The collision cases involve joints, legs, and braces and are simulated for several kinetic energy amounts of the vessels and different impact orientations. An overview of the plastic deformation mechanisms that can occur in both ship and jacket structure is also given. The results from the various models with different collision scenarios are compared in terms of the strain energy dissipation with respect to the different ship/installation strength ratios. From the FEA simplified approaches are also derived in terms of the relative stiffness of the two structures for assessing the responses and energy absorptions of the two structures. The conclusions drawn from this study can be applied to a broader range of collision assessment of offshore steel jacket platforms subjected to high-energy ship impacts.     

How much should gasoline be taxed when electric vehicles conquer the market? An analysis of the mismatch between efficient and existing gasoline taxes under emerging electric mobility

The taxation of gasoline is characterized by large variability across countries and recent research has analyzed existing gasoline tax levels from an economic efficiency point of view focusing on conventional internal combustion engine vehicles. Most studies find that existing fuel tax rates do not coincide with economically efficient levels. As long as policymakers do not take action to reduce the resulting efficiency gap, there will be an ongoing welfare loss to the economy. However, the composition of passenger car fleets will probably be subject to fundamental changes in the (near) future due to the emergence of electric mobility. This raises the question of whether the mismatch between current and efficient fuel taxation will persist, shrink, or even exacerbate under emerging electric mobility. This paper aims at answering this question by determining the structure and level of optimal gasoline taxes in the presence of electric vehicles (EVs). First, the optimal (nationwide) gasoline tax is analytically derived employing a general equilibrium approach. It is shown that differences in traffic related marginal external costs among fuel powered cars and EVs affect the corrective Pigouvian component of the optimal gasoline tax while a differential tax treatment influences the fiscal rational of the tax. Second, the model is applied to Germany using differentiated data on e.g. external costs and behavioral responses. Under a wide range of scenarios, the present analyses indicate a strong relationship between optimal gasoline taxes and electric mobility, calling for a downward adjustment of efficient gasoline taxes. The effect is mainly driven by financial incentives for purchasing and using EVs. Since fuel is likely to be undertaxed in many countries, the emergence of electric mobility will therefore close the gap between gasoline taxes in place and economically efficient taxes. On the other side, it will increase the efficiency gap in those countries where gasoline is overtaxed. This also has important implications for policy concerned with environmental objectives. Pushing electric mobility seriously and at the same time taxing gasoline efficiently could actually prevent sufficient CO₂ emission savings. However, at least in the case of Germany, even a downward adjusted optimal gasoline tax under electric mobility is likely to be higher than the current (non-optimal) tax.     

坡度和曲线线路对弓网受流性能影响规律研究

在高速列车关键力学问题的研究中,坡度和曲线线路对弓网受流性能的影响不可忽视,为此,采用有限元技术和多体动力学理论,搭建了全空间范围内的车弓网动力学仿真模型,对定坡度、竖曲线和平曲线区段的弓网受流性能进行了研究。研究结果表明:接触线在坡度变化率为零时,线路对弓网动态响应影响并不明显;坡度变化率为正时,线路会使弓网机械磨耗降低,受流稳定性也会降低,但弓网电气损耗增加;而坡度变化率为负时影响正好相反;平曲线和竖曲线半径越小,弓网受流稳定性越差,仿真时不可忽略,且应在平曲线和竖曲线区段的线路设计、施工、区间限速中加以考虑。     

Influence of seabed trench formation on fatigue performance of steel catenary risers in touchdown zone

The subsea survey results using remote operating vehicles (ROV) show that trenches with a depth of several riser diameters can be developed underneath the steel catenary risers (SCR). Therefore, an important question in respect of the riser–seabed interaction is, how the trench formation beneath the riser affects the riser fatigue performance in the touchdown zone. A common methodology reported in literature to study the impact of trench formation on riser fatigue life is the insertion of an artificial mathematical expression of the riser profile into the seabed. This study shows that such methodology can be inconsistent and leading to contradictory results. The current paper has employed ABAQUS finite element software and coded a non-linear soil hysteretic model to automatically simulate the variable seabed stiffness and the gradual trench development through the touchdown zone. In this method, the seabed model parameters are initially adjusted to extreme values allowing trench with desired depth to be developed over a moderate number of displacement cycles of the SCR. The design wave scatter diagram is then applied, simulating a generic Spar system, after switching the model parameters to values with normal range. The paper presents the impact of trenches of different depths on the fatigue performance of SCRs in the touchdown zone.     

Prediction of slamming pressure considering fluid-structure interaction. Part II: Derivation of empirical formulations

This is Part II in a series of papers. Part I [1] investigated the slamming responses of flexible flat stiffened steel and aluminum plates using the nonlinear explicit finite element code LS-Dyna with the Multi-Material Arbitrary Lagrangian-Eulerian (MMALE) solver. Subsequently, a simplified finite element FSI model of water hitting structures that is realistically close to the slamming phenomenon occurring on the bottom part of offshore structures was proposed. The proposed FSI methodology presented in Part I was verified by comparison with the relevant test data. It was evident that the use of the proposed numerical method presented in Part I was very effective for a benchmarking investigation of slamming load considering the hydroelastic effect. However, the method required much effort in terms of computation time and power analysis resources. The present study, Part II, aimed, as an alternative to the FSI analysis approach, to develop empirical formulae for prediction of slamming loads acting on deformable flat stiffened plates used in marine applications. This paper begins by describing the limitations of the existing approaches based on theoretical, experimental and even numerical studies conducted in the past for estimation of slamming loads. Next, it presents, based on the simulation methodology developed in Part I, rigorous parametric studies that had been performed on actual scantlings of marine-seagoing structures. The effects of structural geometry and water impact velocity on slamming pressure are then investigated in detail. Subsequently, the parametric results are analyzed and utilized to derive empirical formulations for the prediction of slamming loads acting on flat stiffened plates of marine structures. The accuracy and reliability of the proposed formulations are established by comparison with the results of the test and other existing formulations. The proposed formulations are expected to be used for the purposes of the design without any time-consuming FSI analysis of advanced and optimal structures that are robust to slamming.     

跨域无人平台水面垂直起飞动态特性数值模拟

[目的]旨在研究跨域无人平台从水面垂直起飞过程中平台的运动及动力学特性.[方法]采用黏流CFD方法结合重叠网格技术和多自由度DFBI(dynamic fluid body interaction)运动模型,针对跨域无人平台在水面垂直起飞至空中的跨域过程的动态特性,开展数值模拟研究.[结果]模拟结果显示:在垂直起飞过程中...     

基于任务驱动的交互式网络学习模式的探索

本文分析了目前远程教育存在的问题,提出了基于任务驱动的交互式网络学习模式,阐述了其意义、任务设计的原则及其教学过程,旨在激发学生网络学习的主动性,提高网络学习平台的使用率和交互率.