东亚经济危机:极限效应

１９９８年刚刚开始，经历了近半年金融风暴袭击的东亚各国再次面对新一轮的本币汇价下泻狂潮。１月６日，ＣＣＴＶ、ＢＢＣ、ＶＯＡ都详细报告了这一次由印尼盾对美元的汇价大幅下跌而掀起的东亚货币贬值震荡。其中泰国、印尼、韩国、日本的货币自去年至今对美元的汇率变...

风浪耦合作用下桥塔-基础体系的极限荷载效应

为了准确地估计风浪耦合作用下桥塔-基础体系的极限荷载效应,基于统计外推法提出多种环境变量联合作用下结构极限荷载效应的统计方法.该方法根据环境变量的联合分布特性和结构的动力特性统计外推出荷载效应极值的分布函数,极限荷载效应则为给定超越概率下的荷载效应极值.利用该方法计算风浪耦合作用下某跨海大桥桥塔-基础体系的基底剪力极限荷载效应,并讨论风浪耦合效应对计算结果的影响.结果表明,该方法能够准确地估计风浪联合随机作用下结构的极限荷载效应,能够为结构设计提供合理依据.

基于EC法的风浪联合作用主塔-基础体系极限荷载效应

针对基于Rosenblatt映射变换的极限状态曲线计算困难的现状,提出了多维随机变量极限状态曲线的简化算法。该算法通过寻找同时满足边缘分布、联合分布和可靠指标的变量估计值,将随机变量条件分布函数及其逆函数的求解转化为边缘分布函数的求逆,从而达到简化极限状态曲线计算的目的。以某跨海大桥桥塔-基础体系为工程实例,以涠洲岛海洋站风浪同步观测资料为环境变量统计资料,通过基于Copula函数的联合分布模型构造了风浪联合分布函数,根据简化算法计算了风浪极限状态曲线,利用EC法估计了桥塔-基础体系的基底剪力极限荷载效应,并与外推法的估计结果进行了比较。结果表明,基于简化算法和EC法的跨海大桥桥塔-基础体系基底剪力极限荷载效应具有较高的准确性。

车辆极限荷载效应外推离散性分析及原因探究

在中国,由于车辆荷载不断加重,很多公路桥梁已出现结构损伤甚至倒塌,因此,对结构运营安全状况进行评定至关重要。其中,在荷载效应计算方面,许多学者采用Rice理论分析效应超越界限率分布,且以Cremona提出的方法确定尾端最优拟合起点,以此计算极限效应值。本文主要针对该方法极限值计算结果存在的离散性问题展开研究,并探究该问题的产生原因,从而为极限荷载效应预测方法的进一步发展提供重要的理论参考。结果表明,该方法的主观性、样本的随机性以及样本量的差异都可以导致较为显著的最优起点以及极限效应计算值的离散问题。深入分析后发现,其主要原因与该方法中不恰当的K-S检验使用方式以及不恰当的效应超越率理论分布假设有关。

西部开发的极限增长效应及对策

用经济学中的经济增长极限理论分析了西部开发中环境问题的现状和潜藏的危机,提出了解决西部环境问题的对策。

极限承载状态排土场柔性地基泄水涵洞稳定性研究

为研究过载排土场下泄水涵洞的稳定性,以某矿山排土场柔性地基的泄水涵洞为例,考虑竖向和侧向载荷极限状态组合效应,构建箱涵力学计算模型,计算顶板及侧壁的轴向力和剪应力,研判其稳定性,并与数值模拟和现场勘察结果进行对比分析。结果表明:箱涵左右侧壁所受轴向力最大,箱涵倒角部位承受剪应力和轴向力最大,均处于极限允许范围内,数值模拟结果与计算结果相符,涵洞整体稳定性良好,现场调查结果验证了力学计算结果的合理性。在实际排土作业过程中,矿山应严禁超限堆土。该研究为排土场下泄水涵洞的稳定性分析提供了重要的理论指导。

基于Copula函数的风浪多方向极限状态曲线

针对基于Rosenblatt变换极限状态曲线构造困难的现状,引入Copula函数提出了二维随机环境变量极限状态曲线构造的改进方法。该方法将随机环境变量的条件分布函数及其逆函数的求解转化为条件Copula函数及其逆函数的求解,提高了极限状态曲线构造的可行性。结合风浪同步观测数据研究了风浪多方向极限状态曲线,并在此基础上估计了某导管架平台的基底剪力极限荷载效应及对应的平均风速和有效波高取值。结果表明:各方向的风浪极限状态曲线呈现较大的差异,而全方向的极限状态曲线处于各条曲线的中间位置;各方向导管架平台的基底剪力极限荷载效应及对应的平均风速和有效波高取值差异较大,根据实测数据分方向进行风浪极限状态曲线的构造和结构极限荷载效应的估计可得到更加精确的结果。

Similarity Criterion and Scale Effect for Ship Distortion Model Under Combined Loads

For the ultimate strength model test evaluation of large ship structures, the distortion model with non-uniform ratio between the main size and the plate thickness size is usually adopted. It is the key to carry out scale model test to establish a distortion model similar to the real ship structure under combined load. A similarity criterion for ship distortion model under the combined action of bending moment and surface pressure was proposed, and the scale effect for the criterion was verified by a se ries of numerical analysis and model tests. The results show that the similarity criterion for ship distor tion model under combined loads has a certain scale effect. For the model tests of ship cabin struc tures, it is suggested that the scale range between the plate thickness scale and the main dimension scale should be controlled within 2:1, which can be used as a reference for distortion model design and ultimate strength test of large-scale ship structures.

实现电子器件散热的微小型平板LHP蒸发器传热特性研究

提出了微小型平板LHP来实现高热流密度电子器件的散热,分析了LHP的工作原理以及运用于电子器件散热的优点.建立了蒸发器多孔芯,金属壁面以及工质汽、液空间区域的耦合数学模型,并运用SIMPLE算法进行求解.数值结果表明,微小型平板LHP蒸发器具有较高热流的散热能力,加热表面的温度水平较低,均温性较好,有利于电子器件的散热.提出微小型平板LHP存在侧壁效应传热极限,由于该极限的存在,系统传热能力在17.5×104W·m-2左右.

带翅片的小型平板CPL蒸发器相变传热的数值模拟

提出一种小型平板毛细抽吸两相流体回路(CPL)系统来实现电子元器件散热,分析了其工作原理及特点。针对小型平板式CPL蒸发器的结构特点,分析了蒸发器侧壁导热所引起的侧壁效应对CPL传热能力的影响,提出小型平板CPL存在侧壁效应传热极限。为了提高系统的传热能力,蒸发器下壁增设了散热翅片。建立了小型平板CPL蒸发器毛细多孔芯的二维气液两相分层饱和多孔介质模型,金属壁面及工质气、液空间区域耦合的数学模型,并用SIMPLE算法对模型进行求解。数值研究表明,蒸发器受热面的温度水平较低,均温性较好。蒸发器采用翅片可以增加多孔芯内的温度梯度,使芯内温度分布更加合理;可以降低加热表面以及下壁温度,提高了侧壁效应传热极限,增强了系统的传热能力。

小型平板CPL蒸发器耦合计算及优化设计研究

本文建立了小型平板CPL蒸发器毛细多孔芯内汽液两相流动与传热的模型以及金属外壁和工质区的导热模型,并进行耦合求解。分析了金属侧壁效应对蒸发器性能的影响,提出小型平板CPL存在着侧壁效应传热极限.数值结果表明,工质蒸发发生在多孔芯加热表面附近,蒸发器采用单一金属外壁时由于侧壁效应导致系统传热极限低,而上壁采用导热系数大,侧壁及下壁采用导热系数小的新型结构能够明显的提高系统的传热能力,同时使加热表面的温度维持在较低的水平.

Stress/strain aging mechanisms in Al alloys from first principles

First-principles based calculations were carried out to explore the possible mechanisms of stress/strain aging in Al alloys. Potential effects of temperature and external stress/strain were evaluated on the solvus boundary of Al3Se in Al-Sc alloy, and the interface energy of Al/θ＂ in Al-Cu alloys. Results show that applying tensile strain/stress during conventional aging can significantly decrease the solubility entropy, by red-shifting the phonon DOS at high states. The resulted solvus boundary would shift up on the phase diagram, suggesting a reduced solubility limit and an increased maximum possible precipitation volume of AlaSc in Al-Sc alloy. Moreover, the applied strain/stress has different impacts on the formation energies of different orientated Al/θ＂ interfaces in Al-Cu alloys, which can be further exaggerated by the Poisson effect, and eventually affect the preferential precipitation orientation in Al-Cu alloy. Both mechanisms are expected to play important roles during stress/strain aging.

Casimir Force on a Piston in Randall-Sundrum Models

The Casimir effect of a piston for massless scalar fields which satisfy Dirichlet boundary conditions in the context of five-dimensional Randall- Sundrum models is studied. In these scenarios we derive and calculate the expression for the Casimir force on the piston. We also discuss the Casimir force in the limit that one outer plate is moved to the remote place to show that the nature of the reduced force between the parallel plates left. In the Randall^undrum model involving two branes the two plates attract each other when they locate very closely, but the reduced Casimir force turns to be repulsive as the plates separation is not extremely tiny, which is against the experimental phenomena, meaning that the RSI model can not be acceptable. In the case of one brane model the shape of the reduced Casimir force is similar to that of the standard two-parallel-system in the four-dimensional fiat spacetimes while the sign of force remains negative.

Non-relativistic Limit of Dirac Equations in Gravitational Field and Quantum Effects of Gravity

Based on unified theory of electromagnetic interactions and gravitational interactions, the non-relativistic limit of the equation of motion of a charged Dirac particle in gravitational field is studied. From the Schroedinger equation obtained from this non-relativistic limit, we can see that the classical Newtonian gravitational potential appears as a part of the potential in the Schroedinger equation, which can explain the gravitational phase effects found in COW experiments. And because of this Newtonian gravitational potential, a quantum particle in the earth＇s gravitational field may form a gravitationally bound quantized state, which has already been detected in experiments. Three different kinds of phase effects related to gravitational interactions are studied in this paper, and these phase effects should be observable in some astrophysical processes. Besides, there exists direct coupling between gravitomagnetic field and quantum spin, and radiation caused by this coupling can be used to directly determine the gravitomagnetic field on the surface of a star.

Design method of pile-slab structure roadbed of ballastless track on soil subgrade

Pile-slab structure roadbed is a new form of ballastless track for high speed railway. Due to lack of corresponding design code, based on the analysis of its structure characteristics and application requirements, it is proposed to carry out load effect combination according to ultimate limit state and serviceability limit state, and the most unfavorable combination of each state is chosen to carry through design calculation for pile-slab structure. Space model of pile-slab structure can be simplified as a plane flame model, by using the orthogonal test method, and the design parameter of pile-slab structure is optimized. Moreover, based on the engineering background of Suining-Chongqing high-speed railway, the dynamic deformation characteristics of pile-slab structure roadbed are further researched by carrying on the indoor dynamic model test. The test results show that the settlement after construction of subgrade satisfies the requirement of settlement control to build ballastless track on soil subgrade for high-speed railway. Slab structure plays the role of arch shell as load is transmitted from slab to pile, and the vertical dynamic stress of subgrade soil is approximately of ＂K＂ form distribution with the depth. The distribution of pile stress is closely related to soil characteristics, which has an upset triangle shape where the large dynamic stress is at the top. Pile compared with soil shares most dynamic stress. Pile structure expands the depth of the dynamic response of subgrade has limited effect on dynamic response. These results can provide subgrade. and improves the stress of subgrade soil, and the speed of train scientific basis for pile-slab structure roadbed used on soil

Light Extraction Efficiency in OLED with Sub-wavelength Structures

By simulating with finite-difference time-domain(FDTD) method, it is proved that two kinds of new photonic crystal slab(PCS) structures could enhance the light extraction efficiency of OLED. By comparing the results, the most effective PCS structure with maximum light extraction efficiency(E_ r =1.99) is got. The optimized geometric parameters and optimized performance parameters of the PCS structures are also obtained.

Creep and Shrinkage Effects on the Bond-Slip Characteristics and Ultimate Strength of Composite Slabs

Composite one-way concrete slabs with profiled steel sheeting as permanent formwork are commonly used in the construction industry. The steel sheeting supports the wet concrete of a cast-in-situ reinforced or post-tensioned concrete slab and, after the concrete sets, acts as external reinforcement. In this type of slab, longitudinal shear failure between the concrete and the steel sheeting is the most common type of failure at the ultimate load stage. Design codes require the experimental evaluation of the longitudinal shear capacity of each type of steel decking using full-scale tests. This paper presents the results of the short-term testing up to failure of two types of profiled steel decking that are commonly used in the construction industry in Australia. Fourteen full-scale, simply-supported slabs were tested in four-point bending with shear spans of either span/4 or span/6. Four slabs were tested at age of 28 days and the other 10 slabs were subjected to drying shrinkage and various levels of sustained loads for a period of at least 6 months prior to testing to failure. The effects of creep and drying shrinkage on the load carrying capacity and deformation of the slabs at ultimate loads are presented and discussed. The bond-slip relationship of each slab is determined from the test data and the values of maximum longitudinal shear stress calculated using different methods are described and compared.

Ultimate bearing capacity of concrete dam involved in geometric and material nonlinearity

To get the actual ultimate bearing capacity of concrete dam, the effect of geometric nonlinearity and strain softening on it, which appears in the failure process of concrete dam, is studied. Overload method is adopted to obtain the bearing capacity of a concrete dam by taking into consideration strain softening in the material constitutive law, geometric nonlinearity in geometric equation and equilibrium differential equation. Arc-length method is used to find the extreme point and descending branch of the load-displacement curve of the dam. The results present that the effect cannot be ignored. And geometric nonlinearity of structure and strain softening of materials should be considered for numerical analysis of ultimate bearing capacity of a concrete dam.

A Sixth-Order Parabolic System in Semiconductors

The authors investigate the global existence and semiclassical limit of weak solutions to a sixth-order parabolic system,which is a quantum-corrected macroscopic model derived recently to simulate the quantum effects in miniaturized semiconductor devices.

The Asymptotic Behavior and the Quasineutral Limit for the Bipolar Euler-Poisson System with Boundary Efects and a Vacuum

In this paper, a one-dimensional bipolar Euler-Poisson system （a hydrodynamic model） from semiconductors or plasmas with boundary effects is considered. This system takes the form of Euler-Poisson with an electric field and frictional damping added to the momentum equations. The large-time behavior of uniformly bounded weak solutions to the initial-boundary value problem for the one-dimensional bipolar Euler-Poisson system is firstly presented. Next, two particle densities and the corresponding current momenta are verified to satisfy the porous medium equation and the classical Darcy＇s law time asymp- totically. Finally, as a by-product, the quasineutral limit of the weak solutions to the initial-boundary value problem is investigated in the sense that the bounded L∞ entropy solution to the one-dimensional bipolar Euler-Poisson system converges to that of the cor- responding one-dimensional compressible Euler equations with damping exponentially fast as t → ＋∞. As far as we know, this is the first result about the asymptotic behavior and the quasineutral limit for the one-dimensional bipolar Euler-Poisson system with boundary effects and a vacuum.