掠过双板流动引起的旋涡脱落特性数值模拟

板状燃料组件在先进核反应堆中得到重要应用.流体以一定的流速轴掠板状组件时会导致板后产生旋涡脱落现象.旋涡脱落有可能引发板状燃料组件的流致振动.使用BELIEF程序,通过改变方柱间距模拟了Re=200情况下刚性矩形通道内不同节距条件下双平行方柱的旋涡脱落现象,得出了双柱间节距对双平行方柱旋涡脱落特性的影响,并进一步对双平...     

圆柱空腔内旋转流动中轴对称涡破裂现象的数值模拟

对于上端盖旋转圆柱空腔内旋转流动来说,当流动达到一定条件时,就会出现旋涡破裂现象。目前针对这种现象已经进行了大量的稳态和非稳态实验测量,获得了精确的实验数据。采用数值模拟方法,重现了涡破裂现象及其发生发展过程,同已经发表的相关实验数据进行了比较     

区熔Si单晶中旋涡缺陷的研究进展

本文简要地评述了区熔Si单晶中旋涡缺陷的观察方法,旋涡缺陷的结构,形成机理以及消除它们的可能方法。     

区熔Si单晶中旋涡缺陷的研究进展

本文简要地评述了区熔Si单晶中旋涡缺陷的观察方法,旋涡缺陷的结构,形成机理以及消除它们的可能方法。     

激波冲击V形界面重气体导致的壁面与旋涡作用及其对湍流混合的影响

基于多组分混合物质量分数模型,采用色散最小耗散可控的高分辨率有限体积方法,数值模拟了弱激波冲击V形空气/SF_6界面后,界面不稳定性生成的旋涡与固体壁面作用问题.激波冲击V形界面之后,因斜压效应诱导涡量沉积在界面附近,形成沿界面规则排列的多个涡对结构.旋涡的诱导作用使界面不断变形和卷起,同时旋涡之间不断发生相互并对,诱导更多更小尺度的旋涡产生.旋涡诱导作用的叠加效应,使界面尖端处的初始涡对向上下壁面发展.随后,涡结构开始与壁面发生复杂的相互作用.旋涡与壁面作用后沿壁面加速,使得物质界面沿壁面伸展,随后,旋涡从壁面回弹,并诱导二次旋涡产生.旋涡与壁面相互作用的过程,能够明显加剧物质混合.本文从物质混合的角度研究了该过程的机理,分析了旋涡与壁面作用对物质混合的影响.     

关于旋涡局部稳定性的研究

以线性近似方法研究了不可压流中任意形状旋涡局部稳定问题,考虑了流线曲率及螺线流动对旋涡稳定性的影响.得出了各类型二维旋涡局部稳定准则,这些准则直观简练,在工程研究中应用方便.研究表明,旋涡稳定存在上、下二种界限,前人的研究尽属下限稳定范围.当上限稳定条件不满足时,在尾涡内部会出现低压气穴并引起旋涡局部脱落或散裂.应用上限稳定准则可以方便地解释Taylor涡及Karman涡街现象产生的机理.     

空调风机叶道内旋涡流动分析及进气口偏心的影响

本文采用CFD方法,详细分析了空调用多翼离心风机叶道中的气流分布以及进气口偏心的影响。研究表明,风机叶道内存在大范围的气流分离现象。在后盘附近,存在分离现象的叶道约占2/3,主要分布在蜗壳内侧;而在“前盘”附近,几乎所有叶道都存在分离现象。在“前盘”附近,蜗舌下方的叶道中气流几乎停滞,蜗舌下游叶道为回流和尾缘旋涡所充满,至临近蜗壳出口侧,前缘旋涡逐步形成、发展并融合尾缘旋涡,最后衰减、消失。风机进气口向蜗壳内侧偏置适当距离, “前盘”附近叶道旋涡分布范围明显减小。     

液固两相流中流体旋涡对固体粒子运动影响的数值研究

基于离散涡方法求得的非定常、不稳定流场和颗粒的Lagrangian运动方程,数值计算了四种St数下的泥沙粒子在圆柱绕流场中的运动。计算结果证明了颗粒运动与流体旋涡存在着明确的相关结构：在钝体（圆柱）表面附着泥沙,当流体流过钝体时产生具有剧烈分离的不稳定流动,带动钝体表面泥沙起浮。对于小St数（0．15～0．59）与中等St数（1．33～2．36）的泥沙颗粒被流体旋涡所带起,井被卷入流体旋涡结构内,被卷入流体旋涡结构内的泥沙颗粒在运动过程中始终分布于旋涡区,即在旋涡区聚集。     

准一维深色纳米ZnO的水热方法生长及其结构和光学性能的调变

本文以高纯度金属锌箔为Zn和衬底,利用传统的水热合成方法制备了深色的纳米结构ZnO. 研究发现,改变制备温度可以调变ZnO样品的颜色和光学性能,尤其是提高温度可以显著地降低可见光波段太阳能的活性. 推测这种现象与ZnO纳米棒顶端的尖锥形貌密切相关,这种结构加剧了光在ZnO纳米棒之间的折射和反应. 改善ZnO的光吸收能力对于提升ZnO基太阳电池的效率有重要意义.     

自由旋涡气动窗口全流场的数值模拟

建立自由旋涡气动窗口全流场仿真模型,对大密封压比气动窗口的全流场展开数值研究,得到自由旋涡气动窗口的流场结构,发现大密封压比气动窗口形成的自由旋涡射流在光束输出通道内无明显的波系结构.根据模拟结果对自由旋涡气动窗口的性能进行优化,对自由旋涡喷管上壁面型线进行二次粘性修正.优化自由旋涡射流场后,激光器输出光束通道内压力分布稳定上升;增加扩压器外端壁吹气1.19MPa、内端壁吹气1.68MPa时,自由旋涡射流总能提高,气动窗口密封压力从37.5torr降低至6torr.该研究结果对自由旋涡气动窗口技术的发展具有参考意义.     

Effect of Eshelby twist on core structure of screw dislocations in molybdenum: atomic structure and electron microscope image simulations

This paper addresses the question as to whether the core structure of screw dislocations in Mo in the bulk can be obtained from high-resolution electron microscopy (HREM) images of such dislocations viewed end-on in a thin foil. Atomistic simulations of the core structure of screw dislocations in elastically anisotropic Mo were carried out using bond order potentials. These simulations take account automatically of the effects of the surface relaxation displacements (anisotropic Eshelby twist). They show that the differential displacements of the atoms at the surface are different with components perpendicular to the Burgers vector about five times larger than those in the middle of the foil, the latter being characteristic of the bulk. Nye tensor plots show that the surface relaxation stresses strongly affect the incompatible distortions. HREM simulations of the computed structure reflect the displacements at the exit surface, modified by interband scattering and the microscope transfer function. Nye tensor plots obtained from the HREM images show that interband scattering also affects the incompatible distortions. It is concluded that it would be very difficult to obtain information on the core structure of screw dislocations in the bulk Mo from HREM images, even under ideal experimental conditions, and that quantitative comparisons between experimental and simulated images from assumed model structures would be essential.     

Iron nanoparticles produced by high-energy ball milling

In this investigation, the chemical and structural characteristics of Fe nanoparticles synthesized by high-energy ball milling have been explored. After the milling process the nanoparticles were collected using a magnetic field. The structure, morphology and composition of the powders were obtained using high-resolution electron microscopy. HREM images confirmed the nanoparticles’ presence with approximately 2–4 nm in size. It was found that using this method allowed the formation of nanoparticles in a smaller size range than other synthesis methods. Also, it was confirmed by HREM images that the obtained nanoparticles were mainly of the fcc nature and some of them of the MTP type.     

High-resolution electron microscopy of grain boundaries

Our ability to observe atomic-scale features of grain boundaries has tremendously improved during the past decade. In this paper we give, aided by a number of examples, a select overview, on progress in the field of grain boundary research directly related to the advent of modern high-resolution electron microscopy (HREM) instruments (point-to-point resolution better than 0.2 nm). Examples of grain boundary issues addressed by atomic structure observations of grain boundaries in oxides and metals will be given with emphasis on systematic investigations of the role of macroscopic and microscopic grain boundary parameters. Since comparisons between observed interface structures and atomistic computer modeling results are quite important, considerable efforts towards quantification have been undertaken recently by a number of authors. Most valuable insights have been obtained by the systematic examination of a range of grain boundary structures, using a combination of experimental observations and computer modeling results. In this manner HREM observations have been invaluable not only as a test of theoretical models, but also by exposing common atomic-scale features of high-angle grain boundaries. This has brought us closer to the goal of generating a general understanding of the interface structure and its connection to properties. Such studies have given valuable insights regarding the correlations between macroscopic grain boundary geometry, interfacial energy, and atomic relaxation modes.Work supported by the U.S. Department of Energy, Basic Energy Sciences, under contract W-31-109-Eng-38.     

一种适用于非周期结构高分辨象模拟计算的新方法

通常的FFT多片层法(FFTMS)动力学衍射计算中,由于采用了边界采样点周期性延伸的假设,从而导致非周期结构高分辨象模拟的严重失真。在仔细研究这种误差成因的基础上,本文提出了一种适用于非周期结构的高分辨象模拟计算新方法,它基于实空间多片层(RSMS)象模拟计算方法,但在势投影及传播因数计算中充分考虑了非周期结构的特点,从而得到与实验符合较好的结果。 关键词：     

HRTEM Studies of the Structures and the Defects of Exact and Near ∑ = 11 {332} Tilt Grain Boundaries in Ni

A = 11 {332} Ni bicrystal was grown by solidification. The Grain Boundary (GB) atomic structure studied by high resolution electron microscopy (HREM) has been shown to depend upon the position in the bicrystal of the extracted sample. Strongly correlated to the atomic structure, (GB) defects (GBD) have been characterized: all their Burgers vectors belong to the Displacement Shift Complete (DSC) lattice and the height of the step associated with each GBD varies up to a few nanometers. In the symmetrical and almost symmetrical GBs, at the head of the bicrystal, the dislocation cores can be well localized, whereas in the asymmetrical GBs, at the end of the bicrystal, their cores are much more difficult to localize on the HREM image. The influence on the GB structures of the impurity content which varies along the bicrystal is also discussed.     

Atomic structure of boron nitride nanotubes with an armchair-type structure studied by HREM

Hexagonal networks of boron nitride (BN) nanotubes were investigated by high-resolution electron microscopy (HREM) and image simulation. From HREM images, lattice planes of {002} and hexagonal rings of a BN nanotube were confirmed. Asymmetrical layer arrangements were found, and a structure model for double-walled BN nanotube with an armchair-type structure has been proposed.     

Solving modulated structures by X-ray and electron crystallography

X-ray diffraction can be used for accurately determining not only classical, ordinary structures, but also modulated ones. For structures with weak modulations, the modulation induced satellite reflections are often hard to be observed by X-ray diffraction, but they appear clearly in electron diffraction. In these cases, X-ray diffraction will give only average structures whereas electron diffraction will yield information about the modulations. Sr(1.4)Ta(0.6)O(2.9) is a complex modulated compound with weak modulation and small modulated domains. Here we demonstrate the power of combining X-ray and electron crystallography for studying modulated structures on powders. The modulations of Sr(1.4)Ta(0.6)O(2.9) were determined from electron diffraction (SAED) and high resolution electron microscopy (HREM) images. With specially developed image processing techniques, the weak modulations were enhanced, facilitating the interpretation of HREM images in terms of atomic structure.     

In-Situ HREM Studies of Grain Boundary Migration

Thermally activated grain boundary migration (GB) is important in grain growth and thermo-mechanical processing of materials, but very little is known about the atomic-scale mechanisms involved. The purpose of this paper is to identify atomic-scale GB migration mechanism and investigate their dependence on GB structure. High-angle tilt and general GBs are investigated at elevated temperatures by high-resolution transmission electron microscopy (HREM) in Au and Al bicrystalline thin films. Digital analysis of HREM video recordings is used to detect atomic-scale structural changes at migrating GBs. GB motion typically is not smooth, but involves sharply varying speeds and thermally activated spatial fluctuations. Collective effects in GB migration are shown to exist and several different migration mechanisms are identified. Atomic-scale GB migration is found to depend on the macroscopic GB geometry as well as details of the interatomic interactions.     

结合会聚束电子衍射和高分辨电子显微术来测定十四铌酸锂的结构

我们在一台有0.4nm点分辨率的分析透射电子显微镜上,结合会聚束电子衍射(CBED)和高分辨电子显微术(HREM)研究了十四铌酸锂的结构。CBED结果证明十四铌酸锂的空间群是C2/m,HREM结构象表明十四铌酸锂是由4×4氧八面体方块组成,方块中3×3通道及八面体中心的金属原子可以清楚地看到,其距离是0.38nm。结合CBED和HREM来做结构分析是发展电子晶体学的重要的一步。 关键词：     

Structural transformation from the AlCuFe icosahedral phase to the 1/1 cubic (AlSi)CuFe approximant phase; three dimensional models of translation defects

Unusual results are obtained by High Resolution Electron Microscopy (HREM) studies of the 1/1 cubic approximant phase of the (AlSi)CuFe icosahedral phase. We study, in a 3D model of AlCuFe icosahedral phase, the possibilities of transformation of this structure into a periodic cubic structure. We propose a model of transformation coherent with the experimental results. Received 31 May 1999 and Received in final form 23 December 1999