Analysis of the capillary forces between two small solid spheres binded by a convex liquid bridge

This work deals with a theoretical study of convex liquid bridges, where solid particles are partially wetted by an amount of liquid whose profile is convex. In our analysis, we adopt a two particle model assuming that the solids are spherical and with the same size and properties, and that the liquid meniscus features an arc-of-circumference contour. We analyze different parameters which govern the liquid bridge: inter-particle separation, wetting angle and liquid volume. The results are compiled in liquid volume-wetting angle diagrams in which the regions of existence of the bridge are delimited in terms of maximum and minimal inter-particle distances. The capillary force is found to present both attractive and repulsive characters for wetting angles smaller and higher than 90°. A map delimiting this force character is also presented.     

Inertia force effects in the non-Newtonian couple stress squeeze film between a sphere and a flat plate

On the basis of the Stokes micro-continuum theory together with the method of mean averaged inertia, the influences of fluid inertia forces on the non-Newtonian squeeze film characteristics between a sphere and a flat plate have been presented. Comparing with the case of a non-inertia non-Newtonian lubricant, the consideration of fluid inertia forces provides a longer squeeze film time especially for the squeeze film operating with a lower film height, and a larger non-Newtonian parameter and density parameter. It prolongs the life of squeeze films.     

Electro-elastic fields of piezoelectric materials with an elliptic hole under uniform internal shearing forces

The existing investigations on piezoelectric materials containing an elliptic hole mainly focus on remote uniform tensile loads. In order to have a better understanding of the fracture behavior of piezoelectric materials under different loading conditions, theoretical and numerical solutions are presented for an elliptic hole in transversely isotropic piezoelectric materials subjected to uniform internal shearing forces based on the complex potential approach. By solving ten variable linear equations, the analytical solutions inside and outside the hole satisfying the permeable electric boundary conditions are obtained. Taking PZT-4 ceramic into consideration, numerical results of electro-elastic fields along the edge of the hole and axes, and the electric displacements in the hole are presented. Comparison with stresses in transverse isotropic elastic materials shows that the hoop stress at the ends of major axis in two kinds of material equals zero for the various ratios of major to minor axis lengths; If the ratio is greater than 1, the hoop stress in piezoelectric materials is smaller than that in elastic materials, and if the ratio is smaller than 1, the hoop stress in piezoelectric materials is greater than that in elastic materials; When it is a circle hole, the shearing stress in two materials along axes is the same. The distribution of electric displacement components shows that the vertical electric displacement in the hole and along axes in the material is always zero though under the permeable electric boundary condition; The horizontal and vertical electric displacement components along the edge of the hole are symmetrical and antisymmetrical about horizontal axis, respectively. The stress and electric displacement distribution tends to zero at distances far from the elliptical hole, which conforms to the conclusion usually made on the basis of Saint-Venant’s principle. Unlike the existing work, uniform shearing forces acting on the edge of the hole, and the distribution of electro-elastic fields     

Socio-Economic Demands,Ecological Conditions and the Power of Tradition: Past Woodland Management Decisions in a Central European Landscape

Humans have managed European landscapes, including woodlands, for millennia. Prior to the birth of modern forestry in the eighteenth and nineteenth centuries, there existed two basic management forms for lowland broadleaved woods: coppicing and wood pasture. While the existence and characteristics of these two basic management types are well-known, the reasons why particular woods were coppiced while others pastured are little investigated. As a case-study, we chose two large ancient woods in the southern Moravian region of Central Europe. One was managed as coppice, while the other as wood pasture for most of their histories. The woods are similar in size, location, climate and vegetation. We examined several potential explanations (terrain morphology, soil productivity, abundance of woodland, ownership and economy) for past management decisions in these woods. We found that the links between soil productivity, economic demands and ownership were of key importance. Other factors were less significant, for example woodland abundance played no detectable role in spite of the decreasing availability of woodland resources. We also found that tradition mattered; it took a major shift in ideology to change entire management strategies. In sum, management decisions were driven by a complex network of multiple interactions among the individual factors.     

计算机网络信息安全刍议——以武警部队为例

本文从计算机网络的含义和发展历程着手，分析了武警部队网络信息安全存在的主要危险，并针对这些威胁提出了几点对策。旨在与同行共同切磋探讨。     

4-硝基苯乙烯基吡啶类氢键液晶的合成及表征

该文用4-硝基苯甲醛和4-甲基吡啶合成了质子受体4-硝基苯乙烯基吡啶,用无水乙醇重结晶得黄色固体,其熔点126.8～128.9℃,产率27%。用4-羟基苯甲酸和1-溴代正烷合成了质子供体4-烷氧基苯甲酸,用体积分数为95%的乙醇重结晶,所得4种中间体产率都在20%左右。最后,以上述两个中间体为原料,制备了系列4-硝基苯乙烯基吡啶类氢键型液晶。IR和1H NMR证实了各中间体结构。POM、IR、DSC检测了各复合物。POM拍摄的向列相、近晶相液晶织构表明,3种该氢键复合物均呈现明显的液晶态,且n=3时为向列相,n=5、8时为近晶相;IR图谱3500～2500cm-1处峰的变化及羰基向高波数位移,证明了羧基和吡啶间的分子间氢键代替了羧基间的分子间氢键。各复合物的液晶区间范围比各相应单体增大了1倍,表明羧酸吡啶间氢键起到了稳定液晶态的作用。     

Accurate Effective Potentials of Real Substances from Acoustic Virial Coefficients

Accurate second acoustic virial coefficients have been employed to determine parameters of an effective intermolecular potential for nine pure substances: argon, nitrogen, carbon dioxide, carbon tetrafluoride, and the first five alkanes (methane, ethane, propane, butane, and pentane). The values used for the second acoustic virial coefficients were taken from data reported in the literature. To obtain the form and parameters of the effective potential, we employed an inversion method recently introduced for volumetric second virial coefficients of nonconformal potentials. The potential parameters determined in this way are useful in predicting various gas properties. The model reproduces the experimental second virial data within their uncertainty, including volumetric second virial coefficients that were not used in the potential determination.     

NUMERICAL SIMULATION OF FLOW OVER TWO SIDE-BY-SIDE CIRCULAR CYLINDERS

In the present paper, the unsteady, viscous, incompressible and 2-D flow around two side-by-side circular cylinders was simulated using a Cartesian-staggered grid finite volume based method. A great-source term technique was employed to identify the solid bodies (cylinders) located in the flow field and boundary conditions were enforced by applying the ghost-cell technique. Finally, the characteristics of the flow around two side-by-side cylinders were comprehensively obtained through several computational simulations. The computational simulations were performed for different transverse gap ratios (1.5≤T/D≤4) in laminar (Re=100,200) and turbulent (Re=104) regimes, where T and D are the distance between the centers of cylinders and the diameter of cylinders, respectively. The Reynolds number is based on the diameter of cylinders,D. The pressure field and vorticity distributions along with the associated streamlines and the time histories of hydrodynamic forces were also calculated and analyzed for different gap ratios. Generally, different flow patterns were observed as the gap ratio and Reynolds number varied. Accordingly, the hydrodynamic forces showed irregular variations for small gaps while they took a regular pattern at higher spacing ratios.     

Off-line modelling and planning of optimal clamping forces for an intelligent fixturing system

An intelligent fixturing system (IFS) for machining aims to adaptively adjust the clamping forces to achieve minimum deformation of the workpiece according to the cutter position and the cutting forces. This paper presents the concept, architecture, control scheme, models and methodologies for an IFS. Using off-line simulations and on-line experimental verifications, the performance of the proposed IFS is evaluated and verified. As adaptive clamping forces appropriate to the dynamic machining environment are employed, the IFS offers higher quality of machined parts and greater robustness to disturbances. This system is suitable for application in high-precision machining environment as well as flexible manufacturing systems (FMS).