轴流泵参数化设计及应力与模态特征分析

以船用轴流式喷水推进泵为对象,探索了轴流泵参数化设计、水动力性能、静强度和结构声学特征分析的数值途径。轴流泵叶轮采用升力法设计,导叶采用流线法设计,叶片三维造型在NUMECA参数化设计平台中完成。轴流泵水动力性能校核由粘性CFD计算完成,CFD计算同时提取得到叶片分布式水动力载荷。叶片静强度校核由ANSYS有限元计算叶片应力和应变特征完成,应力分析时同时考虑水动力载荷、重力载荷和离心力载荷。叶片结构声学特征分析由NASTRAN有限元计算叶片模态振型和振型频率完成。计算结果表明,轴流泵扬程和功率满足设计指标,效率达87.13%;叶轮叶片形变相对于叶顶间隙来说为极小量,可忽略不计,叶片存在局部应力集中现象,最大应力小于许用应力,满足静强度要求;叶片前四阶振型特征与分析经验一致,且振型频率远离轴频、叶频及其谐频特征频率,能够避免共振产生。     

汽轮机动叶片的可靠性设计方法

提出了汽轮机叶片可靠性设计方法,介绍了叶片可靠性的含义和计算方法。该方法以概率论和统计学为基础,把汽轮机叶片的静应力、动应力、叶片疲劳强度、叶片安全倍率、叶片振动频率和激振力频率处理为随机变量,通过试验数据的统计分析和计算,确定有关随机变量的分布参数。使用概率设计法、应力与强度干涉模型确定汽轮机叶片疲劳强度和振动设计的可靠度。文中给出了叶片疲劳强度的动应力设计法和安全倍率设计法以及第一种调频叶片、第二种调频叶片和整圈连接叶片组的振动可靠性设计的计算公式和一些应用实例。使用这些方法,可以在设计阶段确定汽轮机叶片设计的可靠度,为汽轮机叶片的可靠性设计提供了科学的依据。     

结构动态应力计算方法研究

复杂结构动态应力的准确计算是一个没有圆满解决的问题。本文以最小余能原理为基础，提出了计算结构动态应力的最小伤痛有法。该方法采用二次分析思想，首先采用常规有限元对结构进行适当离散，计算输出结构所需应力区域的有限元结点位移和加速度动力时程反应，再应用最小余能法计算所求部位的动态应力值。这种方法的优点是它可以与现有的有限元程序有机结合，方便使用；动应力在区域内的分布规律可以由计算者根据具体情况而确定，一般情况下，可以选用二次曲线来逼近动应力在区域内的实际分布，避免了常规有限元法计算结构动应力时必须对单元形函数求导的做法，从而提高了动应力计算精度。计算结果表明：本文方法计算结构动态应力结果较常规有限元法的计算结果有明显改进，特别是当结构变化剧烈时，改进效果更为明显。     

叶盆积灰对TRT叶片静力学性能的影响分析

高炉煤气余压发电透平(即TRT)是利用高炉煤气所具有的热能及压力能进行发电的一种机械装置。叶片是TRT装置中能量转化的核心部件,工作过程中在其表面形成的积灰对叶片的力学性能会产生严重的影响。对此本文建立了积灰后叶片的几何模型和有限元分析模型,计算了在同等工况下不同积灰厚度叶片的静态应力和应变,并与未积灰状态的叶片的应力和应变进行了对比分析,获得了积灰状态下叶片的应力、应变分布规律。对积灰后的叶片进行了模态分析,得出积灰厚度与叶片固有频率之间的关系曲线。分析结果对实际生产具有指导意义。     

离心式叶轮流体激振响应及动态疲劳的研究

研究离心式叶轮的动力特性及流体激振下的稳态响应。讨论结构动态疲劳理论,认为流体激振激起了叶轮的共振,是动态疲劳,特点是激励频率高,幅值低,能量小,激起结构多个固有模态,主要是局部模态的共振。通过一个工程实际叶轮算例,研究动态疲劳破坏的机理。采用循环对称理论,计算叶轮的模态及离心力作用下的应力分布,对流体的压力脉动进行了频谱分析。将叶片表面的静压脉动作为载荷施加于叶轮上进行频率响应分析,得到叶轮在各频率成分激振力作用下的响应,最后根据频率响应的结果计算了叶轮的疲劳寿命,为叶轮疲劳破坏提供了分析依据和计算方法。     

惯性系统三轴综合测试台结构静动态特性计算及实验研究

本文建立了惯性系统三轴综合测试台的结构分析模型，用有限元法对其进行了结构静动态特性计算。给出了外框架在实际载荷作用下的静动态特性和整机的动态特性，并在此基础上，对该三轴台外框架进行了静态变形的实际测量。实测表明，理论分析和实测结果相吻合。     

板中正交静应力与任意方向兰姆波波速的关系

运用部分波分析法(或子波分析)理论推导了在正交静应力下,板中任意方向的Lamb波的频散方程,给出了任意方向Lamb波波速、正交静应力和频率之间的关系. 进行了数值计算并讨论了对称方向的Lamb波波速对单向静应力的依赖关系,为将Lamb波用于应力测量提供了理论依据.      

混凝土三维细观模型的建模方法与力学特性分析

根据混凝土材料的细观组成和结构特点，基于三维Voronoi图形提出了一种简单高效的混凝土细观模型生成方法，利用塑性损伤模型对该细观模型进行了单、多轴应力状态下的准静态分析以及SHPB动态有限元分析。结果表明，数值模拟得到的应力应变曲线和破坏模式与实验结果基本吻合，本文中提出的混凝土三维细观模型可较好地模拟混凝土的静、动态力学特性，为进一步从细观力学角度研究混凝土损伤演化规律和破坏机理提供了模型基础。     

航空发动机叶片-机匣碰摩热-结构耦合仿真

针对转静子碰摩过程中的摩擦热效应现象,建立了叶片-机匣的碰摩热-结构耦合模型。采用热-结构耦合单元,对航空发动机叶片-机匣进行了碰摩热效应特性研究。考虑摩擦因数及旋转过程中的离心力作用,对瞬态-热结构耦合场进行了有限元分析,研究了结构在温度场作用下的热-结构耦合应力和温度分布。与纯机械载荷下的应力分布对比,发现了碰摩热效应在叶片与机匣上的扩散规律。研究结果表明,考虑摩擦热效应时由于热应力的作用,导致结构的总应力水平升高进而产生热变形,从而使故障进一步恶化。由此可见,碰摩热效应的影响在实际航空发动机振动分析中不能忽视。     

冲击速度和轴向静载对红砂岩破碎及能耗的影响

地下岩体工程爆破开挖中，距爆源不同距离处岩体承受的地应力和动载荷大小不同，从动载荷的角度表征岩石动态破坏结果与工程实际更吻合。为研究动载荷和地应力大小对岩体破碎和能量耗散特性的影响，利用动静组合加载试验装置，分别设置7个冲击速度和轴向静应力等级，对红砂岩试件进行冲击试验。根据试件的破碎状况，分析不同静应力工况下冲击速度对岩石破坏模式和机理的影响。计算不同工况下的应力波能量值，研究冲击速度和轴向静应力对岩石能耗特性的影响。对破坏试件进行筛分试验，研究岩石破碎分形维数随冲击速度和轴向静应力的变化关系。结果表明，随着冲击速度的增大，试件的破坏程度逐渐加大。无轴压时岩石试件破坏后整体仍是一个圆柱体，属于张拉破坏；有轴压时岩石试件宏观破坏后呈沙漏状，属于拉剪破坏。岩石耗散能随冲击速度的升高呈二次函数关系递增；轴向静应力越高，递增幅度越小。随着冲击速度的升高，岩石分形维数由零逐渐增加；随着轴向静应力的升高，分形维数由零转为大于零的临界冲击速度先升高后降低。     

Preface: Current research progress on mechanics of high speed rail

The construction and operation of high-speed rail（HSR）grid within the past two decades in China,in terms of the scale,may be possibly comparable to any national-wide construction in the history of China,even the Great Wall.By counting railways with commercial train service at the speed of200 km/h,China has the world’s longest HSR network with over 19 369.8 km of track in service today and this number     

ACTA MECHANICA SINICA 2014 ANNUAL SUBJECT INDEX

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人造地震动合成中的位移误差分析

三角函数级数法是合成人工地震动常用算法之一,但是通过对加速度积分求取位移时,却存在与零线漂移相类似的位移漂移现象。     

Rainwater rivulets on a cable subject to windFilets d'eau de pluie sur un câble soumis au vent

Rainwater rivulets appear on inclined cables of cable-stayed bridges when wind and rain occur simultaneously. In a restricted range of parameters this is known to cause vibrations of high amplitudes on the cable. The mechanism underlying this effect is still under debate but the role of rainwater rivulets is certain. We use a standard lubrication model to analyse the dynamics of a water film on a cylinder under the effect of gravity and wind load. A simple criterion is then proposed for the appearance and position of rivulets, where the Froude number is the control parameter. Experiments with several geometries of cylinder covered with water in a wind tunnel show the evolution of the rivulets with the Froude number. Comparison of the prediction by the model with these experimental data shows that the main mechanism of rivulet formation and positioning is captured. To cite this article: C. Lemaitre et al., C. R. Mecanique 334 (2006).     

Hermitian space-time finite elements for estuarine mass transport

Space-time finite element solutions of the convection–dispersion equation using higher-order nodal continuity and Hermitian polynomial shape functions are described. Five separate elements ranging from a complete linear element with C^0,0 nodal continuity to a complete first-order Hermitian element with C^1,1 nodal continuity are subjected to detailed analysis. Wave deformation analyses identify the source of leading or trailing edge oscillations, trailing edge oscillations being the major source of difficulty. These observations are confirmed by numerical experiments which further demonstrate the potential of higher-order nodal continuity. The performance of the complete first-order Hermitian element is quite satisfactory and measurably superior to the linear element.     

Cristallisation par onde acoustique : le cas de l'héliumCrystallization by acoustic waves: the case of helium

We establish a theoretical model to explain the nucleation of a crystal of helium by an acoustic over-pressure. We explain the interfacial laws for this ultra-fast cristallization, close to the sound speed. Assuming spherical symmetry and taking into account the experimental data, we recover the dynamics of the growth and melting during an over-pressure impulse. To cite this article: M. Ben Amar et al., C. R. Mecanique 331 (2003).     

Structure initiale et propriétés de liquéfaction statique d'un sableInitial structure and static liquefaction properties of sand.

Based on the use of two different preparation procedures for reconstituting triaxial samples of sand, i.e. wet tamping and dry pluviation, significant differences in associated mechanical behaviour are observed on a reference sand with respect to the phenomenon of ‘static’ liquefaction. Wet tamping favours the initiation of liquefaction instability, whereas dry pluviation favours a more stable behaviour, less susceptible to liquefaction. Microscopic observation of corresponding sand specimens allows us to identify two well differentiated structures, i.e., for wet tamping, an irregular structure with predominance of aggregates (aggregated grains) and macropores, very contractant and unstable and, for dry pluviation, a more regular structure, without macropores, more dilatant and more stable. These observations show the importance of further characterization, based on the introduction of appropriate parameters, of the initial structure of sandy materials, strongly dependant upon their mode of formation (natural or artificial). To cite this article: N. Benahmed et al., C. R. Mecanique 332 (2004).     

Sources of Complexity in Human Systems

Complex is a special attribute we can give to many kinds of systems. Although it is used often as a synonym of difficult, it has a specific epistemological meaning, which is going to be shared by the incoming science of complexity. Difficult is an object which, by means of an adequate computational power, can be deterministically or stochastically predictable. On the contrary complex is an object which can not be predictable because of logical impossibility or because its predictability would require a computational power far beyond any physical feasibility, now and forever. For complexity refers to some observing system, it is always subjective, and thus it is defined as observed irreducible complexity. Human systems are affected by several sources of complexity, belonging to three classes, in order of descending restrictivity. Systems belonging to the first class are not predictable at all, those belonging to the second class are predictable only through an infinite computational capacity, and those belonging to the third class are predictable only through a trans-computational capacity. The first class has two sources of complexity: logical complexity, directly deriving from self-reference and Gödel's incompleteness theorems, and relational complexity, resulting in a sort of indeterminacy principle occurring in social systems. The second class has three sources of complexity: gnosiological complexity, which consists of the variety of possible perceptions; semiotic complexity, which represents the infinite possible interpretations of signs and facts; and chaotic complexity, which characterizes phenomena of nonlinear dynamic systems. The third class coincides with computational complexity, which basically coincides with the mathematical concept of intractability. Artificial, natural, biological and human systems are characterized by the influence of different sources of complexity, and the latter appear to be the most complex.