关键词： Metals   Fluids   Cavities   Flow (Dynamics)   Cycles   Computational fluid dynamics   Thermomechanics   Stators   Engines   Temperature   Geometry   Springs   Finite element analysis  

摘要： The optimization of heat transfer between fluid and metal plays a crucial role in gas turbine design. An accurate prediction of temperature for each metal component can help to minimize the coolant flow requirement, with a direct reduction of the corresponding loss in the thermodynamic cycle. Traditionally, in industry fluid and solid simulations are conducted separately. The prediction of metal stresses and temperatures, generally based on finite element analysis, requires the definition of a thermal model whose reliability is largely dependent on the validity of the boundary conditions prescribed on the solid surface. These boundary conditions are obtained from empirical correlations expressing local conditions as a function of working parameters of the entire system, with validation being supplied by engine testing. However, recent studies have demonstrated the benefits of employing coupling techniques, whereby computational fluid dynamics (CFD) is used to predict the heat flux from the air to the metal, and this is coupled to the thermal analysis predicting metal temperatures. This paper describes an extension of this coupling process, accounting for the thermo-mechanical distortion of the metal through the engine cycle. Two distinct codes, a finite element analysis (FEA) solver for thermo-mechanical analysis and a finite volume solver for CFD, are iteratively coupled to produce temperatures and deformations of the solid part through an engine cycle. At each time step, the CFD mesh is automatically adapted to the FEA prediction of the metal position using efficient spring analogy methods, ensuring the continuity of the coupled process. As an example of this methodology, the cavity flow in a turbine stator well is investigated. In this test case, there is a strong link between the thermo-mechanical distortion, governing the labyrinth seal clearance, and the amount of flow through the stator well, which determines the resulting heat transfer in the stator well. Th

关键词： Graphene   graphite   molecular dynamics   vacancy  

摘要： The mechanical properties of graphene and graphite containing vacancies under tensile loading were investigated using molecular dynamics simulations. Two types of potential functions were used in the simulations: the second-generation reactive empirical bond-order (REBO) potential for covalent C-C bonds, and the Lennard-Jones potential for the interlayer interaction in graphite. The influence of the size and distribution of the vacancies on the mechanical properties of graphene and graphite were studied. It was found that the tensile strengths of graphene and graphite are significantly decreased when they contain randomly distributed vacancies.

关键词： bonds (chemical)   fracture   graphene   molecular dynamics method   polymorphism   Young's modulus  

摘要： Graphyne is the allotrope of graphene. In this letter, four different graphynes (alpha, beta, gamma, and 6,6,12-graphenes) are investigated by molecular dynamics simulations to explore their mechanical properties and failure mechanisms. It is found that the presence of the acetylenic linkages in graphynes leads to a significant reduction in fracture stress and Young's modulus with the degree of reduction being proportional to the percentage of the linkages. This deterioration in mechanical properties stems from the low atom density in graphynes and weak single bonds in the acetylenic linkages where the facture is initiated. (C) 2012 American Institute of Physics. [http://***/10.1063/1.4747719]

关键词： Hydrolysis   Organopalladium   Mechanism   Activation parameters   Density functional theory   Solvent effects  

摘要： In this work, mechanism and kinetics of the hydrolysis of Organopalladium Complex [Pd(CNN)P(OMe)(3)](+) in low acidity range, has been theoretically investigated in detail. Using quantum mechanical approach, different probabilities were investigated and ultimately a model was presented in which an enol form is produced in the rate determining step and in continuation is converted into products. Considering the solvent effects, the activation energy, activation enthalpy and activation Gibbs free energy for the rate determining step were calculated, which are in good agreement with the experimental values. (C) 2012 Elsevier B.V. All rights reserved.

关键词： bicrystals   brittle fracture   chemical vapour deposition   failure (mechanical)   graphene   molecular dynamics method   nanofabrication   nanoribbons   nucleation   slip   surface cracks   tensile strength   tilt boundaries  

摘要： Recent experimental studies have shown that multi-grains are ubiquitously present in graphene grown with chemical vapor deposition method technique. The potential application of the unique two-dimensional material in future nanotechnology demands full understandings of their structure and properties. Using molecular dynamics simulations, we study the mechanical response of various bicrystal graphene consisting of symmetric tilt boundary subject to uniaxial tensile loading. Both bulk graphene and graphene nanoribbons (GNRs) are studied. We revealed that nano-crack initiated at grain boundaries (GBs) leads to brittle failure with no plasticity at room temperature. The mechanism that crack nucleated at the intersection of GB and free surface followed by fast advance of crack, preventing plasticity that involves dislocation slip or GB sliding that is seen in metals. Cleavage along GBs is observed to be dominant fracture behavior in the studied GNRs. Furthermore, the ultimate tensile strength decreases with increasing vacancy concentration, illustrating that the lowered strength of GB interface is primarily due to not well-bonded atoms, shedding light to the structure-properties relationship. Finally, local strain and atomic-level stress have been shown to be able to characterize the onset of crack nucleation and thereby good quantities for predicting the resulting ultimate tensile strength. (C) 2012 American Institute of Physics. [http://***/10.1063/1.4749812]

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摘要： 长期以来叶轮叶盘类零件的高精密加工一直被业内人士公认为是最富有挑战性的任务,这些零件具有曲面极其复杂、叶片截面很薄、材料难切削等特点,不但需要非常专业的加工编程技巧,也需要最优化的刀具解决方案和先进的五轴机床设备。而由香港精品机械集团投资设立的西安万钧航空动力技术团队在这方面积累了10多年的专家经验,可以满足客户对各类叶轮叶盘类零件最苛刻的高精度产品要求。

关键词： 机械手   动力学分析   仿真  

摘要： 针对5自由度机械手,基于Lagrange方程建立了5自由度机械手的动力学模型,通过Solidworks建立实体仿真模型,并且通过CosMotion进行了动力学仿真,得出了各关节力矩和时间的关系曲线,分析了机械手各关节在不同角速度下对各关节力矩的影响,这对改善机械手的设计提供了帮助,并为机械手的驱动控制打下了基础。

关键词： 柔性关节   四自由度   动力学   拉格朗日方程  

摘要： 关节是机械臂的核心部件,在机械臂动力学中起着重要的作用,精确的关节动力学模型是机械臂系统设计、分析和控制的基础.以四自由度机械臂为研究对象,首先给出柔性关节机械臂的简化模型,然后运用La-grange方法建立了考虑关节柔性和电气特性动力学方程.

关键词： 空间机器人   接触碰撞   动力学模型   模型降阶  

摘要： 研究了空间机械臂接触/碰撞动力学仿真模型降阶问题。以刚体近似模型方法建立了空间机械臂的接触动力学模型,并通过线性化将其转化为状态方程形式,根据赫兹接触理论建立了接触/碰撞力的模型,在此基础上根据控制理论中主导极点的概念研究了基于系统主导特征值的接触动力学模型降阶方法。以一个3自由度平面机器人的接触碰撞问题为例进行了计算机仿真试验,结果表明,接触/碰撞动力学仿真模型降阶方法能够在高度逼近原系统的前提下显著提高空间机器人接触碰撞动力学仿真的计算速度。

关键词： 机械密封   CFD   流体基本物理场  

摘要： 以计算流体动力学(CFD)数值计算理论及方法为基础,对处于高速、高温、高压复杂工况的波纹管集装式机械密封装置腔内流场进行了数值模拟分析,得到了密封腔内介质的温度场、压力场、湍动能及流场区域涡量分布,分析了腔内流体的流动及传热动力学特性,为动、静环温度场的热流固耦合数值算法提供依据。