柱塞副微观表面对间隙油膜压力的影响

根据分形理论中的W-M函数建立柱塞与缸体孔表面三维形貌数学模型,并利用MATLAB软件编写程序,分析分形维数对表面微观形貌的影响。将柱塞副间隙油膜沿轴向展成平面,建立油膜厚度及压力分布计算模型,采用有限体积法求解二维雷诺方程,分析粗糙度对柱塞副间隙油膜压力分布的影响。结果表明:随着分形维数的增加,柱塞表面粗糙度呈增加趋势,且表面粗糙度越大,压力峰值越大。     

甲烷在钯基催化剂上脱氢的分子模拟

为探明甲烷在钯基二聚体催化剂上脱氢反应过程的微观机理,对甲烷燃烧催化剂的设计与使用提供指导。在M06L/6-311++G(d,p)+SDD//M06L/6-311G(d,p)+LANL2DZ基组水平上,采用密度泛函理论(DFT)对甲烷在钯基二聚体催化剂(Pd_2、PdPt和PdNi)上的脱氢过程进行了研究。对比了甲烷在催化剂Pd_2、PdPt和PdNi上反应的能垒(E_b)、活化能(E_a)及反应速率常数(k),结果表明:CH_2→CH是甲烷在二聚体Pd_2反应的速率控制步骤(RDS),而CH_3→CH_2是催化剂PdPt和PdNi反应的RDS;钯基二聚体催化剂对甲烷脱氢的催化活性顺序为PdPtPd_2PdNi;抗积炭性能顺序为PdNiPd_2PdPt。PdPt适用于要求催化效率较高的项目,而抗积炭性能较好的PdNi催化剂可用于大型工业催化。     

A variational approach for free vibrating micro-rods with classical and non-classical new boundary conditions accounting for nonlocal strengthening and temperature effects

Abstract

Transverse vibration of a circular cross sectional micro-rod subjected to a new kind of boundary constraints with elastic torsional springs is presented based on nonlocal elasticity. A nonlocal strengthening beam model is utilized and the effect of temperature changing is taken into consideration. The variational method and Hamilton’s principle are applied to derive the governing equation of motion and corresponding boundary conditions. A higher-order partial differential equation that is a typical characteristic of nonlocal strengthening model is developed, and the boundary conditions contain not only classical conditions but also non-classical higher-order conditions. Unlike previous studies which were only concerned with some conventional boundary constraints, we consider more general boundary conditions named elastic torsional spring supports. Such boundary conditions are between the simply supported and clamped ones, and they are closer to the actual constraints of existing engineering structures. Natural frequencies of micro-rods with new boundary constraints are determined via an eigenvalue method and compared with other results in the literature. It is shown that the nonlocal scale factor, thermal parameter, rigidity parameter and torsional spring coefficient play significant roles in free vibration of micro-rods. The research can provide a reference for a large class of boundary conditions ranging from simply supported to clamped micro-rods.     

考虑速度滑移的多孔质静压气体轴承静特性

对于多孔质材料内的气体流动,基于Darcy定律建立其理论模型,并建立气膜间隙流场的雷诺方程,考虑速度滑移修正方程;将上述2个区域的压力分布方程进行耦合,通过有限元方法对耦合后的压力分布方程进行离散化,用超松弛迭代求解出气膜内各节点的压力分布,分析速度滑移对多孔质静压气体轴承静特性的影响。结果表明,考虑速度滑移所计算的气膜压力分布变化平稳过渡,没有较大的突变。计算轴承的承载力及刚度,结果显示在气密间隙小于15μm时,随着气膜厚度的增大偏心导致的压差增大使承载力不断增大;且当速度滑移系数小于0.1时,速度滑移对轴承承载力及刚度有较大的影响。     

A framework for understanding institutional factors affecting the success and failure of offshoring models in India

The objective of this paper is to identify and analyse the factors that influence offshore location decisions from an institutional theory perspective. In the years, emerging economies have shown great growth potential and have attracted significant foreign direct investment, especially from companies willing to set up operations. Despite such a seemingly favourable trade environment, not all companies that entered an emerging economy have been successful. Thus, there is a need to understand the factors that influence the success of any offshore location decision. It is also important to understand the relative importance of these factors, in order to develop novel insights useful to practitioners. There is a lack of literature addressing this issue, which this study aims to fill. We conducted a multi-case analysis with examples of European companies entering India. The findings indicate that successful companies employed locally focused business strategies that enabled them to acquire important relationships and knowledge, and also to adapt to formal institutions, including governmental and regulatory procedures. It was found that, irrespective of size, disregarding collaborative strategies was the main reason for exiting India. These findings are highly relevant for managers and policy makers.     

Component spectroscopic properties of light-harvesting complexes with DFT calculations

Photosynthesis is a fundamental process in biosciences and biotechnology that influences profoundly the research in other disciplines. In this paper, we focus on the characterization of fundamental components, present in pigment-protein complexes, in terms of their spectroscopic properties such as infrared spectra, nuclear magnetic resonance, as well as nuclear quadrupole resonance, which are of critical importance for many applications. Such components include chlorophylls and bacteriochlorophylls. Based on the density functional theory method, we calculate the main spectroscopic characteristics of these components for the Fenna-Matthews-Olson light-harvesting complex, analyze them and compare them with available experimental results. Future outlook is discussed in the context of current and potential applications of the presented results.     

Comparative analysis of a Sisko nanofluid over a stretching cylinder through a porous medium

The present article examines the Sisko nanofluid flow and heat transfer through a porous medium due to a stretching cylinder using Buongiorno's model for nanofluids. Suitable similarity transformations are used to transform the governing boundary layer equations of fluid flow into nonlinear ordinary differential equations. The finite difference method is used to solve coupled nonlinear differential equations with MATLAB software. The impact of different parameters viz., the Sisko material parameter, porosity parameter, curvature parameter, thermophoresis parameter, and Brownian diffusion parameter on the velocity and temperature distribution are presented graphically. Moreover, the effect of the involved parameters on the heat transfer rate is also studied and presented through table values. It is noticed from the numerical values that the porosity parameter reduces the velocity while enhancing the temperature. The curvature parameter enhances the velocity throughout the fluid regime and reduces the temperature near the surface while enhancing the temperature far away from the surface. The study reveals that the thermophoresis and Brownian diffusion parameters that characterize the nanofluid flow reduce the wall heat transfer rate, while the curvature parameter enhances it. This investigation of wall heating/cooling has essential applications in solar porous water absorber systems, chemical engineering, metallurgy, material processing, and so forth.     

煤巷高效快速掘进技术优化浅析

煤巷掘进效率低容易造成采掘接替紧张,以柠条塔煤矿S12002工作面胶运顺槽和辅运顺槽为工程背景,采用理论分析和现场工程对比试验相结合的方法,分析了制约巷道快速掘进各环节的因素,提出了优化控制对策。在减少顶板锚杆的支护数量,改良锚固方式并增大锚固长度和排距后,支护效率提高了20%,支护密度降低了30%,极大降低了顶板支护环节的时间;对掘进工艺进行优化后,将日单进水平从35 m左右提升到50~60 m左右,月单进水平从900 m提升到1500 m以上,创出了单日掘进75 m新水平,实现了高效快速掘进。实践表明,该方式现场成巷效果良好,可以满足煤巷高效快速掘进的需要。     

Statistics of ceramic strength: Use ordinary Weibull distribution function or Weibull statistical fracture theory?

“Weibull statistics” for strength distribution analysis refers to either the ordinary Weibull distribution function or the Weibull statistical fracture theory. The ordinary Weibull distribution function is an empirical distribution function on an equal footing with other type of classical empirical distributions such as normal and log-normal distributions for fitting the statistical data of various random variables nonexclusive to materials strength. It has no explicit physical meaning and cannot be used for size scaling and prediction of strength. The Weibull statistical fracture theory is a weakest-link statistical fracture model for a solid with the strength distribution of an elemental volume being described by the ordinary Weibull distribution function. It has the capability of size scaling and prediction of strength for specimens with different geometries and different loading modes. The three-parameter Weibull statistical fracture theory in uniaxial flexure of prismatic beams is reformulated and validated by both numerical and real strength experiments of different ceramics.     

Experimental and numerical assessment of grain boundary energies in polycrystalline uranium dioxide

Uranium dioxide ceramics are widely used as nuclear fuels. Thus, it is important to understand the role of the grain boundaries (GBs) which decisively govern the properties of these polycrystalline materials and subsequently determine their performances. Here, we report a coupled numerical - experimental approach enabling to assess GB energies. Firstly, GB formation energies (γ_gb) were computed for 34 symmetric tilt GBs in UO₂ with molecular dynamics simulations at 1700 K. The surface energies (γ_S) relative to the respective planes of these GBs were calculated as well. The Herring relation was then used to assess the dihedral angles Ψ of the corresponding GB grooves. Secondly, a UO₂ ceramic sample was annealed at 1673 K to obtain GB grooves. The CSL GBs of interest were identified by EBSD and their Ψ angles determined by AFM. Computed and measured Ψ values were found to be very close.