The Uniaxial Tensile Deformation of Ni Nanowire: Atomic-Scale Computer Simulations

The mechanical deformations of nickel nanowire subjected to uniaxial tensile strain at 300 K are simulated by using molecular dynamics with the quantum corrected Sutten-Chen many-body force field. We have used common neighbor analysis method to investigate the structural evolution of Ni nanowire during the elongation process. For the strain rate of 0.1%/ps, the elastic limit is up to about 11% strain with the yield stress of 8.6 GPa. At the elastic stage, the deformation is carried mainly through the uniform elongation of the distances between the layers (perpendicular to the Z-axis) while the atomic structure remains basically unchanged. With further strain, the slips in the {111} planes start to take place in order to accommodate the applied strain to carry the deformation partially, and subsequently the neck forms. The atomic rearrangements in the neck region result in a zigzag change in the stress-strain curve; the atomic structures beyond the region, however, have no significant changes. With the strain close to the point of the breaking, we observe the formation of a one-atom thick necklace in Ni nanowire. The strain rates have no significant effect on the deformation mechanism, but have some influence on the yield stress, the elastic limit, and the fracture strain of the nanowire.

凝析天然气管道分层流动相间水力摩阻系数计算式评价

前人已经得出了许多计算分层流动相间水力摩阻系数的经验、半经验半理论的公式.然而,由于它们多是以室内小型实验架上得到的数据为基础,虽然与各自的实验结果符合较好,但是普遍存在适用范围窄、精度低的缺陷.获得了大型多相流环道上的实验数据,并利用这些数据对部分常用的经验公式进行了评价.依据评价结果认为,当气相速度低于15 m/s时,依据Chen等人的关联式的计算值与实验结果比较接近,误差不超过13.2％;当气相速度大于15 m/s时,Hart等人的模型预测值与实验结果符合较好,误差普遍介于0～15.94％之间.因此

Instability Analysis of Nonlinear Surface Waves in a Circular Cylindrical Container Subjected to a Vertical Excitation

Singular perturbation theory of two-time scale expansions was developed both in inviscid and weak viscous fluids to investigate the motion of single surface standing wave in a liquid-filled circular cylindrical vessel, which is subject to a vertical periodical oscillation. Firstly, it is assumed that the fluid in the circular cylindrical vessel is inviscid, incompressible and the motion is irrotational, a nonlinear evolution equation of slowly varying complex amplitude, which incorporates cubic nonlinear term, external excitation and the influence of surface tension, was derived from solvability condition of high-order approximation. It shows that when forced frequency is low, the effect of surface tension on mode selection of surface wave is not important. However, when forced frequency is high, the influence of surface tension is significant, and can not be neglected. This proved that the surface tension has the function, which causes free surface returning to equilibrium location. Theoretical results much close to experimental results when the surface tension is considered. In fact, the damping will appear in actual physical system due to dissipation of viscosity of fluid. Based upon weakly viscous fluids assumption, the fluid field was divided into an outer potential flow region and an inner boundary layer region. A linear amplitude equation of slowly varying complex amplitude, which incorporates damping term and external excitation, was derived from linearized Navier-Stokes equation. The analytical expression of damping coefficient was determined and the relation between damping and other related parameters (such as viscosity, forced amplitude and depth of fluid) was presented. The nonlinear amplitude equation and a dispersion, which had been derived from the inviscid fluid approximation, were modified by adding linear damping. It was found that the modified results much reasonably close to experimental results. Moreover, the influence both of the surface tension and the weak viscosity on the mode formation was described by comparing theoretical and experimental results. The results show that when the forcing frequency is low, the viscosity of the fluid is prominent for the mode selection. However, when the forcing frequency is high, the surface tension of the fluid is prominent. Finally, instability of the surface wave is analyzed and properties of the solutions of the modified amplitude equation are determined together with phase-plane trajectories. A necessary condition of forming stable surface wave is obtained and unstable regions are illustrated. (c) 2005 Elsevier SAS. All rights reserved.

粘弹性力学的对应原理及其数值反演方法

结合国内外的研究成果，就粘弹性力学中存在的各种对应原理及数值反演方法进行了归类和总结。结合在求解粘弹性边值问题中的应用，对各类方法的特点进行了评述，并指出存在的问题及发展新的数值方法的研究重点。

The Flow Theory of Mechanism-Based Strain Gradient Plasticity

The flow theory of mechanism-based strain gradient (MSG) plasticity is established in this paper following the same multiscale, hierarchical framework for the deformation theory of MSG plasticity in order to connect with the Taylor model in dislocation mechanics. We have used the flow theory of MSG plasticity to study micro-indentation hardness experiments. The difference between deformation and flow theories is vanishingly small, and both agree well with experimental hardness data. We have also used the flow theory of MSG plasticity to investigate stress fields around a stationary mode-I crack tip as well as around a steady state, quasi-statically growing crack tip. At a distance to crack tip much larger than dislocation spacings such that continuum plasticity still applies, the stress level around a stationary crack tip in MSG plasticity is significantly higher than that in classical plasticity. The same conclusion is also established for a steady state, quasi-statically growing crack tip, though only the flow theory can be used because of unloading during crack propagation. This significant stress increase due to strain gradient effect provides a means to explain the experimentally observed cleavage fracture in ductile materials [J. Mater. Res. 9 (1994) 1734, Scripta Metall. Mater. 31 (1994) 1037; Interface Sci. 3(1996) 169].

Richtmyer-Meshkov失稳的数值模拟

利用五阶迎风紧致差分格式加群速度控制方法对激波与轻气体柱形界面的干扰问题进行直接数值模拟，并研究了涡量的产生及其发展和界面的变形，分析了柱形界面的失稳过程，得知在线性发展阶段，重气体以恒定的速度进入到轻气体中，并形成尖钉结构；在非线性发展阶段，在尖钉头部的两侧形成一对旋转方向相反的涡结构。同时证实了激波与界面的相互干扰有助于增加混合。并将所得的数值解与实验结果进行了比较。

Tailoring of dendritic microstructure in solidification processing by crucible vibration

We analyzed the effects of both natural convection and forced flows on solid–liquid interface morphology during upward Bridgman solidification of metallic alloys. Experiments were carried out on Al–3.5wt% Ni alloy, for a cylindrical sample. The influence of natural convection induced by radial thermal gradient on solidified microstructure was first analyzed as a function of the pulling rate. Then, the influence of axial vibration on solidification microstructure was experimentally investigated by varying vibration parameters (frequency and amplitude). Experimental results demonstrated that vibrations could be used to either attenuate fluid flow in the melt and obtain a uniform dendritic pattern or to promote a fragmented dendritic microstructure. However, no marked effect was observed for cellular growth. This pointed out the critical role of the mushy zone in the interaction between fluid flow and solidification microstructure.

轴对称近似等熵压缩流场的乘波前体优化设计

以升阻比为优化目标，在来流马赫数Ma＝2～4及飞行高度H＝20km～24km条件下，进行了轴对称近似等熵压缩流场的乘波前体优化设计，通过CFD验证M_∞＝4优化乘波体的气动特性，并研究了Ma＝3优化乘波前体在非设计条件下的气动特性。结果表明：近似等熵压缩下表面的乘波前体在设计条件下具有良好的气流压缩效果，可满足机体／发动机一体化设计的需要；乘波前体升阻比在1.5～1.9之间，纵向压心位置靠后；非设计条件下，压缩波不聚焦，小于设计马赫数升阻比时降低，大于设计马赫数时升阻比略大。

气相爆轰波贯穿胞格变化研究

根据气相爆轰波胞格结构的规则特性建立一种以子单元分解胞格结构的新方法。根据该子单元的性质和斜冲击波关系，求解胞格结构中的三波点对撞问题，并推导贯穿胞格时，爆轰速度的相对波动幅度仅依赖于胞格几何形状的关系。然后，采用爆炸波衰减模型，计算气相爆轰波贯穿胞格时的衰减过程，计算结果与实验符合得较好。

Instability of Two-Layer Rayleigh-Benard Convection with Interfacial Thermocapillary Effect

The linear instability analysis of the Rayleigh-Allarangoni-Benard convection in a two-layer system of silicon oil 10cS and fluorinert FC70 liquids are performed in a larger range of two-layer depth ratios H, from 0.2 to 5.0 for different total depth H less than or equal to 12 mm. Our results are different from the previous study on the Rayleigh-Benard instability and show strong effects of thermocapillary force at the interface on the time-dependent oscillations arising from the onset of instability convection.

Scour of the seabed under a pipeline in oscillating flow

The scour of the seabed under a pipeline is studied experimentally in this paper. Tests are carried out in a U-shaped oscillatory water tunnel with a box imbedded in the bottom of the test section. By use of the standard sand, clay and plastic grain as the seabed material, the influence of the bed material on the scour is studied. The relationship between the critical initial gap-to-diameter ratio above which no scour occurs and the parameters of the oscillating flow is obtained. The self-burial phenomenon. which occurs for the pipeline not fixed to two sidewalls of the test section, is not observed for the Bred pipeline. The effect of the pipe on sand wave formation is discussed. The maximum equilibrium scour depths For different initial gap-to-diameter ratios, different Kc numbers and different bed sands are also given in this paper.

带有两个非共线裂纹载流薄板的温度场

对于带有两个非共线直线裂纹的无限大载流薄板，当在无穷远处通人均布电流时，由于在裂纹尖端出现绕流和焦耳热集中效应，使裂纹尖端熔化形成焊口，而达到止裂的目的。寻找电流密度与裂纹尖端处热源功率之间的关系，板内温度场的分布状态、裂纹前缘温度值等等，是推广应用电磁热效应对导电体实施裂纹止裂的关键问题。文中采用“伪电流”和“伪热流”的方法，分别得到通电后域内的电流密度分布及温度分布，给出上述问题的解析解，并给出算例。

电阻点焊薄片应变传感器的传递及疲劳性能实验研究

对于许多工程实际中的应变测量,现场环境可能不适合应变片的养护,需要密封而造成打磨粘贴困难以及工期要求比较紧而无法直接在现场粘贴养护,可以将应变粘贴养护及部分密封工作提前到实验室进行.电阻点焊技术可以提供将薄片应变传感器焊接在母材上的手段,并且能够保证焊接过程中不产生引起应变片损坏的热量.本文对电阻点焊薄片应变传感器技术的传递性能和疲劳性能进行了一系列试验研究,为薄片传感器的实际应用提供了实验依据.

利用PIV方法测量半浮区液桥热毛细对流的速度场

利用PIV法对硅油液桥热毛细对流的定常速度场进行了实时测量。为了便于测量，液桥上桥端面采取了铜环中嵌透明材料的方法，从液桥的顶部进行观测。当液桥上下桥有温差时，热毛细对流出现；本实验对于不同上下桥的温差，对液桥横剖面内的速度场分别进行了测量，研究外加温差对于流场速度分布的影响；并且在液桥中取了几个典型横截面进行测量，以期对大Pr数液桥的定常速度场有比较全面的定量测量。此外，实验结果也可作为数值模拟计算结果的验证。