On the mechanics of the modern working-recurve bow

Characteristic for the bow are the slender elastic arms or limbs. The bow is braced by putting a string shorter than the bow between the tips of the limbs. Additional deformation energy is stored in the elastic limbs by drawing the bow into the fully drawn position. Part of this amount of energy is transformed into kinetic energy of a light arrow.In the 1930's the design of the bow became a subject of scientific research. Experiments were performed in which design parameters were changed more or less systematically. However, the mathematical models were rather simple. Because fast computers are now available the presented model in this paper can be much more advanced. The resulting set of partial differential equations with known initial values and moving boundaries is solved numberically using a finite-difference method.In this paper the design parameters associated with the developed model are charted accurately. Bows used in the past and nowadays on shooting meetings such as the Olympic Games are compared. It turns out that the application of better materials which can store more deformation energy per unit of mass and that this material is used to a larger extent, contribute most to the improvement of the bow. The parameters which fix the mechanical performance of the bow appear to be less important as is often claimed.     

摩擦热弯曲对转子稳定性的影响

动静摩擦产生热弯曲易导致转子失稳。本文利用简单摩擦弯曲模型建立了发生摩擦热弯曲的转子－轴承系统传递左阵,分析了摩擦热弯曲对转子稳定性的影响,同时利用传递矩阵方法计算了特征值对参数的灵敏度。针对一压缩机转子模型进行稳定性分析计算,表明转子的稳定状态随转速及和的变化出现差异,文中对这种现象作了解释。     

Resistance to ship-hull motion through brash ice

A three-metre long model hull of a tanker ship with simplified bow shapes was towed through a simulated brash ice channel in the model-ice towing tank of the Iowa Institute of Hydraulic Research (IHR). The study was to investigate resistance forces encountered by commercial ships, with hulls characterized by a long parallel middlebody, transisting brash ice channels.The model hull was structurally divided into a bow portion and an aft portion, which were coupled and instrumented so that resistance forces exerted on the bow as well as the total towing force could be measured. Two bow forms were used; a wedge bow and a simplified icebreaker bow.From the results of the experiments, it is shown that resistance to hull motion through brash ice may consist of the following components: bow resistance due to internal friction and accumulation of brash ice; bow resistance due to submergence of brash ice beneath hull; bow resistance due to momentum exchange between hull and brash ice; parallel-part resistance due to friction between hull and brash ice; stern thrust due to ascension of brash ice at the hull's stern; and open-water resistance. The ratio of each component to the total resistance depends on bow shape, hull speed, brash-ice characteristics and channel dimensions. The effects of brash-ice layer thickness and channel width on resistance are discussed. Additionally, some insight into the effect of brash-ice refreezing on resistance is also offered.     

On the mechanics of the arrow: Archer's Paradox

In ancient bows the grip of the bow was in the way of the arrow. The arrow needed to get round the bow while being accelerated; this phenomenon is called the Archer's Paradox. In the forties it was observed experimentally with high-speed cameras that the arrow vibrates in a horizontal plane perpendicular to the vertical median plane of the bow. These movements are started and controlled by the movements of the two points of contact with the bow, viz. the middle of the string in contact with the rear end of the arrow and the grip where the arrow slides along the bow. The latter contact imposes a moving-boundary condition. The numerically obtained results are satisfactorily in agreement with experimental data. The model can be used to estimate the drawing force of ancient bows of which only the contemporary arrows are available and also for the design of new archery equipment.     

Viscosity and Surface-Tension Effects on Wave Generation by a Translating Body

Various theoretical and experimental studies have been carried out to examine the generation of waves ahead of a translating body. Not all issues pertaining to this wave-motion problem are, however, fully resolved. In particular, mechanisms pertaining to generation of white-water instability and inception of vortices in the bow region are not fully understood. In this paper, the two-dimensional, unsteady, nonlinear, viscous-flow problem associated with a translating surface-piercing body is solved by means of finite-difference algorithm based on boundary-fitted coordinates. Effects of surface tension and surfactants are examined. Results of this work resolve certain classic issues pertaining to bow flows. A continuous generation of short and steepening bow waves is observed at low (draft) Froude number, a nonlinear phenomenon uncovered recently in the case of inviscid fluid also. This indicates that, steady-state nonlinear bow-flow solution may not exist, even at low speed. It is postulated that these short bow waves are responsible for the white-water instability commonly observed ahead of a full-scale ship. The amplitudes of these short bow waves are suppressed by surface tension, which is, possibly, the reason why white-water instability is not distinctly observed in laboratory-scale experiments. The presence of surfactants on the free surface is found to intensify the generation of free-surface vorticity, thus resulting in the formation of bow vortices. The accumulation of surface-active contaminants at the bow is hence responsible for the generation of bow vortices observed in laboratory experiments at low Froude number. At high Froude number, an impulsive starting motion of the body results in the generation of a jet-like splash at the bow and a gentle start an overturning bow wave, as previously observed in the case of inviscid bow flow.     

On the static deformation of a bow

Summary The storage of deformation energy in a bow with or without recurve is considered. Some numerical examples are discussed. For a simple bow it is shown that theoretically a shooting efficiency of hundred percent is possible.     

Collision energy absorption of ships' bow structures

The energy absorbed during the collapse of ship's bow structures in a collision is largely due to the plastic work done by axial crumpling of the plate elements in the structures. In this paper a method is presented for predicting crushing strength of ships' bow structures during a collision.The method of predicting the crushing strength of a variety of structural elements is described using the kinematic method of plasticity. Effects of strain-rate on the axial of crushung of structural elements are included. The crushing strenght of a ship's bow structure is obtained by summing the energy dissipated in all individual structural elements. The theoretical predictions are compared with experimental results for ships' bow models. The paper concludes with an example of the application of the theoretical method to a ship collision with a concrete pier.     

Shock evolution after shaped charge jet impact and its relevance to explosive initiation

When a shaped charge jet impacts a target containing explosive, the explosive may be initiated by one of three shocks: the impact shock, a bow shock that forms in the inert plate covering the exlosive, or a bow shock that forms in the explosive. In this paper, numerical calculations are used to determine how thick the cover plate must be to prevent initiation by the impact shock and how much time (or distance) is required to form a bow shock in the explosive. The results show that the cover plate must be from 4 to 12 jet diameters (depending on jet velocity) thick to sufficiently attenuate the impact shock so that it will not cause initiation in a common secondary explosive. For a 7-km/s copper jet, a distance of about 8 jet diameters was required to form a bow shock in the explosive. This corresponds well to experimental data reported elsewhere.     

Effect of the electric discharge on nonstationary processes during supersonic air and methane jet flow over a flat obstacle

The features of supersonic air and methane jet flows over a flat obstacle, accompanied by an electric discharge, were studied. The flow under consideration is characterized by a number of unsteady effects appearing as oscillations of shock-wave fronts with various amplitudes and frequencies depending on the nozzle-obstacle system configuration and jet gas-dynamic parameters. The oscillation frequency of the bow shock wave increases as the obstacle becomes closer to the nozzle edge and the relative obstacle diameter and the supersonic jet stagnation pressure increase. Initiation of the energy release region at the supersonic jet edge causes a change in the oscillation frequency of the bow shock wave; under certain conditions, it causes a change in the entire shock-wave flow pattern.     

Diagnosis of the Thermal Bow of a Shaft in a Three Stage Centrifugal Compressor

In practice many turbo-machines driven by motors are started up to perational speed within a very short time,i.e.in less than 20 seconds .For this type of machines the compatibility of thermal deformaiton of the rotor stucture must be taken into account in the machine desing ,or the thermal defromation will be constrained and a huge resultant force can cause the shaft bending and consequently resulting in violent vibrations.In this paper,detection of thermal bow of a shaft in a three stage centrifugal compressor in a petrochemical plant is presented .The diagnostic results show that the thermal bow was induced by the incompatibility of axial thermal deformation of the rotor stucure.A remedial action allowing free axial thermal expansion of the outer parts of the rotor is suggested.     

Flow near the leading edge of a rectangular wing of small aspect ratio with applications to the bow of a ship

Summary A local solution is developed for the square-root singularity at the leading edge of a rectangular flat plate of small aspect ratio. The analysis is based on the assumption that the spanwise load distribution is elliptical, and it follows that the two-dimensional lifting-surface integral equation can be reduced to a one-dimensional integral equation of the Wiener-Hopf form. A solution is then obtained, valid near the leading edge, which yields the strength of the square-root singularity. This solution is used to compute the leading-edge suction force, which differs by 15% from the known value based on the total lift and drag expressions. The local solution is applied to study the flow around the stem of a ship's bow, as a result of a yawing motion, using Lighthill's rule to correct for the finite radius of curvature at the bow.Supported by the Office of Naval Research, Contract N00014-67-A-0204-0023, NR062-411, and by the National Science Foundation, Grant GK 10846. Reproduction in whole or in part is permitted for any purpose of the United States Government.     

Plasmonic coupling of bow tie antennas with Ag nanowire

Ag nanowire with the receiving and transmitting Ag bow tie antenna pairs at its incident and emission ends was patterned on the SiO(2) substrate to realize an enhanced surface plasmon emission with a factor of 45 compared to the single Ag nanowire without antenna pairs. The receiving and transmitting bow tie antenna pairs enhanced the plasmon coupling and emission efficiencies of the Ag nanowire. And the maximum plasmon emission sensitively depended on the length of Ag nanowire, the arm length of bow tie antennas, and the incident angle of optical excitation. This enhanced plasmon emission was confirmed by finite-difference time-domain simulations and explored with analytical calculations using the impedance matching theory at optical frequency.     

Imaging polarimetry of the fogbow: polarization characteristics of white rainbows measured in the high Arctic

The knowledge on the optics of fogbows is scarce, and their polarization characteristics have never been measured to our knowledge. To fill this gap we measured the polarization features of 16 fogbows during the Beringia 2005 Arctic polar research expedition by imaging polarimetry in the red, green and blue spectral ranges. We present here the first polarization patterns of the fogbow. In the patterns of the degree of linear polarization p, fogbows and their supernumerary bows are best visible in the red spectral range due to the least dilution of fogbow light by light scattered in air. In the patterns of the angle of polarization α fogbows are practically not discernible because their α-pattern is the same as that of the sky: the direction of polarization is perpendicular to the plane of scattering and is parallel to the arc of the bow, independently of the wavelength. Fogbows and their supernumeraries were best seen in the patterns of the polarized radiance. In these patterns the angular distance δ between the peaks of the primary and the first supernumerary and the angular width σ of the primary bow were determined along different radii from the center of the bow. δ ranged between 6.08° and 13.41°, while σ changed from 5.25° to 19.47°. Certain fogbows were relatively homogeneous, meaning small variations of δ and σ along their bows. Other fogbows were heterogeneous, possessing quite variable δ- and σ-values along their bows. This variability could be a consequence of the characteristics of the high Arctic with open waters within the ice shield resulting in the spatiotemporal change of the droplet size within the fog.     

对中国清代角弓力学特性的一点见解

本文在前人研究成果基础上对中国清代角弓所具有的力学特性进行分析，阐述了弓稍结构对拉力变化的影响，并对拉力曲线做出可能的推测。     

基于整船整桥模型的船桥碰撞数值仿真

桥梁在船舶碰撞时受到的动力载荷和响应是复杂的动力非线性问题。近代非线性有限元技术为该问题的求解提供了有效的工具。简述了该技术的基本原理,并基于整船整桥模型对一艘4万吨实船与桥梁的碰撞过程进行了计算。仿真结果显示了船艏结构损坏、碰撞力演变、能量传递和桥墩内部应力变化的详细情景,讨论了船-桥碰撞的力学特征。演示的方法比传统的经验公式和简化解析法提供更为精确的结果。所提供的桥墩应力状态对桥梁的设计与碰撞后的损伤评估有重要参数价值。     

A first step towards a simple in-line shape compensation routine for the roll forming of high strength steel

The roll forming process is increasingly used in the automotive industry for the manufacture of structural and crash components from Ultra High Strength Steel (UHSS). Due to the high strength of UHSS (<1GPa) even small and commonly observed material property variations from coil to coil can result in significant changes in material yield and through that affect the final shape of the roll formed component. This requires the re-adjustment of tooling to compensate for shape defects and maintain part geometry resulting in costly downtimes of equipment. This paper presents a first step towards an in-line shape compensation method that based on the monitoring of roll load and torque allows for the estimation of shape defects and the subsequent re-adjustment of tooling for compensation. For this the effect of material property variation on common shape defects observed in the roll forming process as well as measurable process parameters such as roll load and torque needs to be understood. The effect of yield strength and material hardening on roll load and torque as well as longitudinal bow is investigated via experimental trials and numerical analysis. A regression analysis combined with Analysis of Variance (ANOVA) techniques is employed to establish the relationships between the process and material parameters and to determine their percentage influence on longitudinal bow, roll load and torque. The study will show that the level of longitudinal bow, one of the major shape defects observed in roll forming, can be estimated by variations in roll load and torque.     

两类船-桥碰撞力差异及桥梁结构响应分析

 船桥碰撞是跨航道桥梁需考虑的重要问题。本文以美国AAHSTO规范推荐两类船舶为例,研究了驳船和散装货轮撞击桥梁后碰撞力、船艏刚度和碰撞能量的变化过程,讨论了导致两类船舶碰撞力、船艏刚度和碰撞能量变化差异的原因,分析了两类船桥碰撞桥梁结构的主要响应,并将本文动力模型计算响应与已有规范计算得到响应进行了对比。结果表明,两类船舶不同的船艏外形及内部构造会对碰撞力造成较大影响;同等吨位和碰撞速度下,驳船碰撞峰值荷载比散装货轮大,驳船碰撞的墩顶位移比散装货轮小,基底剪力和弯矩比散装货轮大,驳船与散装货轮作用下桥梁结构响应的动力反应系数存在较大差异;不同规范对于碰撞荷载规定差异较大,欧洲规范计算得到响应总体较大,中国公路规范荷载对于内河船舶撞击计算得到的响应最小,中国铁路规范计算得到的响应与其他规范海轮撞击响应进行对比最小。     

Mendel John Diness of Jerusalem and Cincinnati

Abstract

I am 52 years of age. But my youth is still just behind me in that solid world of things that one was able to touch. One looked and touched simultaneously. At times one touched by stretching out one's hands without really seeing them, or by seeing them confusedly as if there were pupils, excessively dilated from emotion, at the tips of one's fingers. The passage of time has not dulled the strong physical emotions I experienced with my playmates; in those days a tiny animal was a bottomless well of marvels, and one stretched one's bow until it broke because that was the right way to test how much it could stand. No ‘wise’ advice on not drawing the bow so far as to destroy it could replace the cognitive pleasure of stretching the elasticity of the wood to breaking point. Too bad if the bow broke; the pain of loss was the inevitable corollary of a necessary audacity. After every game of pleasure a mini-Last Judgement arrived implacably from the outside world, perhaps followed by punishment and then tears, but in the end one had done what one had to do: dare!     

Wonderful Mira

Since being named 'wonderful' in the seventeenth century for its peculiar brightness variability, Mira A has been the subject of extensive research and become the prototype for a whole class of 'Mira' variable stars. The primary star in a binary system, Mira A is reaching the end of its life and currently undergoing an extended period of enhanced mass loss. Recent observations have revealed a surrounding arc-like structure and a stream of material stretching 12 light years away in opposition to the arc. In this article, I review recent modelling of this cometary appearance as a bow shock with an accompanying tail of material ram pressure stripped from the head of the bow shock, place Mira in an evolutionary context, predict its future with reference to the similar star R Hya and planetary nebula Sh 2-188, and speculate some avenues of research both on Mira itself and on other 'Mira-like' stars with bow shocks and tails. I also discuss the implications of this discovery for our own star, the Sun.     

电力机车弓网故障的动态监测系统

目的　实时监测电力机车弓网故障 .方法　利用冲击加速度传感器以及左右限位传感器 ,实现对弓网故障的监测 .结果　设计了一套动态监测系统 ,当电力机车弓网发生故障时 ,系统能及时报警 ;同时系统还有自复位功能 .结论　司机能及时了解列车运行的情况 ,避免造成不必要的损失 .整个系统操作简单、方便灵活