临界雷诺数区光滑圆柱振动与气动力研究

圆柱在转动时会形成非对称的流场,导致圆柱气动力发生改变,旋转圆柱的气动力在风力发电或航海领域有很广的应用前景,气动力特性的研究对这些应用有重要的意义。通过刚性模型测力风洞试验,对不同转速下旋转圆柱的气动力特性及其变化规律进行研究,总结得到转速比和升阻力系数的关系。结果表明：随着转速的提高,旋转圆柱的平均升力系数增加,平均阻力系数减小;在不同的转速比区间内平均升阻力系数的变化规律有所差异,转速提高时升力系数增加幅度有所变化,而阻力系数受到影响很小。     

A note on impulsive sphere motion beneath a free-surface

A general framework is presented for solving the impulsive oblique motion of a spherical body in close proximity and below a free-surface. The fluid is considered to be impulsive and the flow as incompressible. The irrotational flow field is deduced from a velocity potential. The full nonlinear problem is reduced to a sequence of boundary-value problems by employing a small-time expansion technique. The mixed boundary conditions are of a Dirichlet type on the undisturbed free-surface and of a Neumann type on the equilibrium spherical shape. The solution is obtained by employing a Green's function and the method of multipoles expansions. General expressions, correct to each order in the small-time, are given for the free-surface deflections and the pressure force experienced by the moving sphere.     

A note on a motion control problem for a placement machine

Assembling printed circuit boards efficiently using automated placement machines is a challenging task. Here, we focus on a motion control problem for a specific type of placement machines. More specifically, the problem is to establish movement patterns for the robot arm, the feeder rack, and—when appropriate—the worktable, of a sequential pick-and-place machine. In this note we show that a (popular) greedy strategy may not always yield an optimum solution. However, under the relevant Tchebychev metric, we can model the problem as a linear program, thereby establishing the existence of a polynomial time algorithm for this motion control problem. Finally, we give experimental evidence that computing optimal solutions to this motion control problem can yield significantly better solutions than those found by a greedy method.     

纳米银溶胶稳定性的影响机制研究

用柠檬酸钠还原AgNO3制备纳米银溶胶，考察了反应温度、反应时间、搅拌速度对溶胶粒径和zeta电位的影响规律。研究发现即随着温度的升高银溶胶中最终颗粒的粒径变小，zeta电位的绝对值在升高，胶体稳定性上升；随着反应时间增加，粒径先增加后减小，zeta电位绝对值随时间增长有降低的趋势，胶体稳定性下降；搅拌速度升高，粒径先增加后减小，zeta电位的绝对值有升高趋势，胶体稳定性上升。     

Global asymptotic stability in a model of networks

Global asymptotic stability is of importance from a theoretical as well as an application point of view in several fields. We study a system of cubic polynomials that models biological networks. We classify the equilibria and show that the property that the interconnection matrix is Lyapunov diagonally stable is a key feature that determines convergence to a single equilibrium. The results are applied to chains of negative edges, cycles, and to interconnected graphs. We give numerical examples and study network graphs obtained from a model of the Drosophila circadian clock.     

A fresh look at the free flight dynamics of a spinning projectile

In the first part of the paper, several equations of motion by different groups of workers are critically examined. Exact equations of motion are then developed. This form is then quasi-linearised into a ballistic formulation. The geometrical motion of a spinning projectile, in the presence of a cubic restoring moment is analysed in the cross-flow plane. It is observed that the trajectories assume a cuspidal form at the lower and upper range of the travel, when the angular momentum parameter coincides with these points. In general, both steady and unsteady precessional motions are presented.     

Role of surfactants on the stability of nano-zinc oxide dispersions

Nowadays, stable colloidal dispersions with ultra-fine or nanosized particles are getting importance due to their higher activity. In this article, methods for the preparation of stable aqueous dispersions of zinc oxide (ZnO) were discussed. The quality of the dispersion was improved by capping with different types of surfactants say non-ionic, cationic, and anionic. Accordingly, Triton X 100, polyethylene glycol-6000 (both non-ionic), cetyltrimethylammonium bromide (cationic), and sodium dodecyl sulfate (anionic) were selected for the study. Effect of these surfactants on particle size of ZnO was followed through dynamic light scattering (DLS) studies and zeta potential measurements. Particle size analysis and zeta potential measurement indicated that ZnO dispersions stabilized with anionic surfactants (sodium dodecyl sulfate) showed better stability. Further, the effect of ultrasonication on particle size distribution was examined and optimized.     

Damage-potential comparison of spherical and cylindrical projectiles impacting on a system of bumper plates

This paper describes a series of smooth particle hydrodynamics (SPH) simulations of tungsten cylindrical and spherical projectiles impacting steel bumper plates at different impact configurations. Comparisons of the destructive capabilities of the spherical and cylindrical projectiles for each impact configuration are presented. Cylindrical geometries were spatially rotated to cover a wide range of impact configurations. The incidence angle of the velocity vector and its magnitude were varied. A rational approach is proposed to quantify the level of damage inflicted on the bumper plates on each impact event. This damage measure is used to create a set of comparative damage matrixes for cylindrical and spherical projectiles that identify under which impact configurations a given geometry is more damaged than the other.     

A note on stochastically acceptable quality

A random variable of quality, characterized by the quality distribution function, is considered both for continuous and discrete cases. The acceptable quality distribution function is introduced and different types of stochastic comparison are discussed: comparison in the mean, ordering in distribution, variability ordering and hazard‐rate ordering. Acceptable, unacceptable and intermediate regions for levels of quality are determined by the discretization procedure. It is assumed that the quality of a product is revealed through usage. A simple reliability setting is considered when the quality of a product is directly dependent on its reliability characteristics in usage. Possible generalizations are discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

输电塔线体系在摇摆地震动作用下的动力稳定性分析

为探究摇摆分量对输电塔线体系动力稳定性能的影响,以一实际输电塔为原型进行了动力时程分析,其中摇摆地震动采用改进的谱比法由地震平动中获取;根据增量动力分析(incremental dynamic analysis, IDA)方法结合B-R准则,分别对塔线体系在水平地震作用、水平-摇摆耦合地震作用和水平-竖向-摇摆耦合地震作用下的动力稳定性能进行分析。研究结果表明：摇摆分量及其引起的附加P-Δ效应会使输电塔线体系产生非对称振动,结构发生偏离平衡位置的单向偏移,从而导致塔线体系较仅考虑水平地震作用,更易动力失稳;重力和竖向地震响应下共同的二阶效应,放大了摇摆分量对结构动力稳定性能的影响,加剧输电塔线体系动力失稳破坏;发生动力失稳破坏时,薄弱区域主要集中于塔身中下部,杆件失效使塔身局部变形过大,导致塔线体系发生整体动力失稳。     

A Different Theoretical Approach to the Spin Polaron Problem

Using the finite temperature (Matsubara) Green's function method, the spin polaron problem is investigated in the strong and weak coupling limits. The holon entropy and specific heat are calculated from the thermodynamic potential in the linked-cluster expansion method.     

A note on discontinuous numerical solutions of the kinematic wave motion

This note discusses the numerical solution of the kinematic wave equation under those conditions when the solution contains a discontinuous shock. A finite element solution is described in which shocks are represented by discrete nodal discontinuities. The implementation of the method follows conventional finite element practice over the shockless regions of the solution domain which are coupled by frontal constraints. The basis of the method and examples of its application to the solution of the kinematic wave equation in one and two dimensions are presented.     

水基氧化石墨烯纳米流体的制备及稳定性研究

以去离子水为基液,在无任何分散剂的情况下将氧化石墨烯悬浮于水中,用超声波处理器进行超声振荡,制备出稳定无聚沉的水基氧化石墨烯纳米流体。测试了纳米流体的STEM图像,分析了其粒径分布。对纳米流体的表面Zeta电位进行了测量,并结合DLVO悬浮理论和双电层结构进行稳定性分析,阐明了氧化石墨烯纳米流体能够长期保持稳定性的机理。     

基于相平面方法的车辆稳定性控制

针对车辆的稳定性问题,基于相平面理论,通过建立二自由度车辆的扩展模型以及运用简化的魔术公式对实际的轮胎侧向力进行拟合,获得了相平面图并划分稳定的区域;研究路面附着系数和车速对相平面边界的影响,确定质心侧偏角与质心侧偏角速度β-β相平面及质心侧偏角与横摆角速度β-r相平面的边界函数,提出一种车辆稳定性控制方法.在建立的Simulink与Carsim联合仿真平台上,对所设计的控制器进行验证,当系统相轨迹超出稳定区域时,以稳定边界为控制目标,控制器集成2种相平面图计算所需要的横摆力矩,将不稳定的状态拉到稳定区域,从而实现了车辆稳定性的控制.最后,与受2种单一相平面控制、横摆角速度控制和未控制的车辆进行对比,结果表明:所设计的集成控制器有一定的优势,能够显著提高极限工况下车辆的稳定性.     

Experimental-numerical investigation of the dynamic stability of flexural-torsional vibrations of compressor blades under conditions of attached and separated flow. Part 1. Procedure and equipment for the determination of nonstationary aerodynamic characteristics

We present the results of investigations of the aerodynamic stability of flexural-torsional vibrations of compressor blades under conditions of attached and separated flow with regard for the cross and mutual aerodynamic links of the blades in a broad range of variations of the phase shift, the ratio of the amplitudes of translational and angular components of their displacements, and the angle of attack. A procedure and equipment used for the experimental determination of the nonstationary aerodynamic characteristics of the blades are described on the basis of the model of flat-plate airfoil cascades. __________ Translated from Problemy Prochnosti, No. 2, pp. 129–142, March–April, 2007.     

A note on modeling road accident frequency: a flexible elasticity model

Count data models and their variants have been widely applied in accident modeling. The traditional log-linear function is used to represent the relationship between explanatory variables and the dependent variable (accident frequency). However, this function assumes constant elasticity for the estimation parameters, which is a limitation in the analysis of the effects of explanatory variables on accident risk. Although interaction effects between explanatory variables have been studied in the road safety context (where they are normally assessed by logistic regression), no one has yet examined the possibility of using a flexible function form allowing non-constant elasticity values. This paper seeks to explore the use of the translog function usually used in the economics context to allow the elasticity to vary with the values of other explanatory variables. Therefore, the objective of this study was to evaluate the application of the translog function to accident modeling and to compare the results with those of the traditional log-linear function negative binomial (NB) model. The results show that, in terms of goodness-of-fit statistics and residual analysis, the NB model with the translog function performs better than the traditional NB model. Additional evaluations in terms of predictive performance, hotspot identification and uncertainty associated with the estimated values were taken into account. Although this study is exploratory in nature, it suggests that the translog function has considerable potential for modeling accident observations. It is hoped that this novel accident modeling methodology will open the door to the reliable interpretation and evaluation of the influence of explanatory variables on accident frequency.     

A note on the robust interpretation of regression coefficients

It is known that ratios of regression coefficients have an interpretation more stable under model perturbation than regression coefficients themselves. This is explored in more detail for a special case of linear and log linear regression.     

A note on the turbulent energy as a parameter of turbulent flow

The art of modeling turbulence is a needed tool in the construction of computer codes for turbulent flows. The state to which this art has been developed is inadequate, and quotations from authoritative sources support this point of view. The energy contained in the turbulent fluctuations, i.e., the turbulent energy, is often used as a parameter in the modeling process. The present article attempts to examine this quantity as it is being created, transported, and dissipated. For this purpose experimental evidence from the author's own experiments (free jets), as well as theoretical conclusions from the elementary deductions of the basic equations, the concept of turbulent potential flow, and a general solution to the Navier-Stokes-Reynolds equations, is drawn to attention. Recirculating flow is given special attention. The paper concludes with recommendations for principles that must be satisfied if improved modeling is to be achieved. These principles are necessary; whether they are also sufficient is open to question.Nomenclature A ₀ Constant - b _1/2 Jet's half-width - b ₁ ^2/(0) Jet's half-width at z=z(in0) - E _z Kinetic energy contained in the jet's axial velocity at a given profile - E _r Kinetic energy contained in the jet's radial velocity at a given profile - f() Dimensionless velocity profile [f(0)=1] - F(), H() Defined functions - L _char Jet's characteristic length - m, n Exponents - p Pressure - q Kinetic energy in the turbulent fluctuations - Heat flux - q ² - r, , z Cylindrical coordinates - t Time - û Internal energy - u, v, w Velocity components - Mean velocity components - Mean velocity components - U ₀ Constant - U _plate Plate's velocity - Uskc/(0) Centerline velocity at z=z₀ - X, Y, Z Components of body force - W Total work done by surface stresses - W ₁ Recoverable work done by surface stresses - W ₂ Dissipated work - z ₀ Downstream distance from the nozzle beyond which self-similar velocity profiles occur - Fluid's kinematic viscosity - Fluid's density - Normal stresses - Shear stresses - Normal stresses with the pressure removed - Dimensionless Crossflow coordinate - ₀ Constant - Stress functions - Stress potential Paper dedicated to Professor Joseph Kestin.Definitions of symbols are given under Nomenclature.