非理想爆源产生的爆炸场数值模拟

本文采用有限差分方法TVD(Total Variation Diminishing,A. Harten,1983)格式对非理想爆源在地面上形成的二维轴对称爆炸场进行了数值模拟,数值计算值与试验结果吻合得相当好。     

泄爆过程中二次爆炸的动力学机理研究

在容积为0.00814m3的柱形泄爆容器中，对泄爆现象进行实验研究. 容器内充满当量比为1的甲烷-空气预混气，采用底端中心点火，泄爆压力为230±15kPa. 基于k-ε湍流模型和EBU燃烧模型，利用同位网格的SIMPLE算法，对该现象进行了数值模拟. 实验和计算获得的外轴线上4个测压点的压力曲线和外流场的阴影和数值照片，形象地描述了高压泄爆时外部流场的变化. 数值结果与实验结果基本一致. 根据实验和数值结果，详细地讨论了泄爆过程中二次爆炸产生的动力学机理. 泄爆的初始阶段，在破膜激波的引导下，泄出的未燃气体因欠膨胀在外流场形成稀疏波低压区和悬激波高压区. 高压区可燃气体密度和温度上升，成为高密度的预热区域. 随后，火焰以射流形式从泄爆口泄出，点燃可燃气云. 受湍流等因素的影响，特别在高密度的预热区域，燃烧速率可能迅速增大，从而导致二次爆炸.     

水下爆炸作用下基于声学的爆源子结构输入方法

无限域吸收边界和爆炸波输入是水下爆炸载荷数值模拟的两个关键问题.本文借鉴基于内部子结构的地震波动输入方法和爆源子结构多尺度分析方法,考虑水下爆炸载荷与地震载荷同属于波动问题,提出一种水下爆炸作用下爆源子结构的爆炸波输入方法,该方法首先通过对爆源区域进行水下爆炸波自由场的波场分解,并利用地震波的内部子结构输入方法,将该自由波场运动转换为等效爆炸波输入载荷,从而实现了水下爆炸问题中冲击波的输入;本文采用圆形爆源子结构区域,并采用AUTODYN软件中一维自由场模型计算水下爆炸作用下该区域的自由波场压力时程.进一步,基于连分式近似方法提出了一种模拟无限域水体辐射效应的高精度时域人工边界条件,该吸收边界可设置于离结构和爆源子结构较近处.本文提出的分析方法通过圆形爆源子结构进行水下爆炸载荷的转化,并采用高精度吸收边界大大减少计算区域,既保证了计算精度,又降低了单元数量,具有较高的计算效率和较强的实用性.通过数值算例分析,验证了本文模型与方法的准确性,模拟了水下爆炸作用下的冲击波和气泡脉动阶段测点的压力时程曲线,并研究了圆形结构对水下近场爆炸波散射效应的影响规律.     

空中强爆炸冲击波地面反射规律数值模拟研究

为准确预测空中强爆炸产生的冲击波载荷分布，基于欧拉坐标系建立了能够模拟具有高密度比、高压力比的强激波问题的二维多介质流体数值方法。结合网格自适应技术，对1 kt TNT当量的空中强爆炸在不同爆炸高度下的冲击波地面反射过程进行了数值模拟，并考虑了真实气体状态方程和空气随高度不均匀分布的影响。计算得到了地面上距爆心投影点大尺度范围内的反射超压和冲量等冲击波载荷分布，并给出了冲击波载荷随爆高的变化规律。     

铁电体在当代高技术中的应用

回顾了铁电体问世以来，在军事，工业和科研应用领域的发展，着重介绍了目前国际，国内在高技术领域研究和应用趋势，特别在粒子加速器领域。铁电体作为高品质新型脉冲强流电子束源，展示的一系列独特优点，正受到高功率粒子加速器界和阴极电子学界的关注。近几年,随着高功率微波技术的发展，人们利用铁电体的高重复率激励能力，研制出一种触发可靠的高重复率，超低抖动（约70ps)的先进开关，这种开关将能满足超宽带（UWB）及高功率瞬变阵列等技术的精密要求。对铁电体这种新型功能材料的探索和开发应用将会给当代高技术的诸多方面注入新的活力，获得使用传统技术不可能达到的新进展。     

泄爆外流场的可视化

通过实验和数值方法,对泄爆外流场进行了可视化研究。实验中采用YA-16高速阴影系统,拍摄了泄爆外流场的时序阴影照片。基于K-湍流模型和漩涡破碎（eddy dissipation）燃烧模型,利用同位网格SIMPLE算法,对泄爆过程进行了数值模拟。根据计算结果,由计算光学获得泄爆外流场的时序计算阴影图。实验阴影图与计算阴影图较一致,都形象地揭示了泄爆后湍流火焰的发展及二次爆炸的产生和变化过程。     

有限长柱形药包土中爆腔特征尺寸的计算方法

为计算柱形药包土中爆腔尺寸，提出了一种有限长柱形药包在土中爆炸的特征尺寸近似计算方法，该方法利用球形药包爆腔膨胀准静态模型叠加的方式，给出了长径比较大情况下柱形药包爆腔特征尺寸及塑性区半径。与数值模拟对比表明，该方法的误差随长径比的增大而减小，当球形药包数量N=n、长径比在10及以上时，误差在12.2%以内，表明该方法能够较准确地预测有限长柱形药包爆腔的特征尺寸。     

多爆源云雾爆炸波相互作用的三维数值研究

从三维全N S方程出发,用二阶迎风TVD格式,针对贴地和近地三种不同的爆源位置,研究了两团云雾爆炸后爆炸场的相互作用。计算结果表明:本文的方法可模拟多爆源爆炸后冲击波的传播和爆炸场复杂的波系结构。压力分布可确定爆炸场任意位置的压力时间变化历程。     

黄土中爆炸挤密实验与数值模拟

为研究爆炸挤密加固技术在黄土中的应用,规避在既有公路上进行大规模爆炸挤密现场实验的风险,验证借助计算机软件进行数值模拟的可行性和可靠性,先设计实施了小型爆炸挤密室外实验,再利用室外实验的材料参数和几何尺寸建立与各实验工况相对应的有限元模型,利用ANSYS/LS-DYNA进行数值模拟,通过将爆腔体积、爆后土壤密度和作用于土壤的峰值压应力3个方面对数值模拟结果和实测结果进行对比,验证了将ANSYS/LS-DYNA用于数值模拟爆炸挤密技术加固黄土的可行性和可靠性,并得出上述三个方面的变化规律,可为根据现场路基状况和几何尺寸进行数值模拟提供借鉴和参考。     

一维轴对称杆件爆源模型及其在台阶爆破模拟中的应用

为了解决在实体炮孔建模时需要加密网格、计算量大等问题,提出了一种一维轴对称爆源模型:利用一维线状杆件表述炮孔及炸药,实体单元表述周围岩体,通过杆件节点与实体单元的拓扑关系,将杆件节点上的爆生气体压力施加至周围实体单元上,并根据实体单元的体应变计算出杆件节点处的横截面变化情况,从而实现模拟炸药与周围岩体的相互作用。通过与实体炮孔模型的数值对比分析,发现压力衰减指数为1.25时,一维轴对称爆源模型获得的径向质点峰值振动速度（peak particle velocity, PPV）衰减规律及振动速度时程曲线与实体炮孔模型基本一致,证明了该模型在模拟爆破问题中的精确性。针对混凝土块动态爆破破坏特性的研究,通过与文献对比分析,进一步验证了该模型的正确性。为验证该一维轴对称爆源模型在台阶爆破模拟中的应用,以鞍钢露天铁矿台阶爆破开采为研究对象,建立了5排50炮孔三维台阶爆破概化模型,模拟了爆区内露天边坡的损伤破坏状态。数值计算结果表明,爆区内拉伸破坏为主,并且除了离爆源较近的第一个测点外,其余测点处的振动速度峰值大小及其随距离的变化规律与现场实测数据基本一致,证明了所提爆源模型在三维台阶爆破远场模拟的可行性。     

Viscoelastic properties of polymerlike reverse micelles

A dramatic increase in the viscosity of reverse micellar solutions of lecithin in a variety of organic solvents of up to a factor of 10⁶ upon the addition of a small amount of water can be observed. The formation of viscoelastic solutions can be explained by a water-induced aggregation of lecithin molecules into flexible cylindrical reverse micelles and the subsequent formation of a transient network of entangled micelles. The viscoelastic properties of these solutions are characterized as a function of water content and temperature for different organic solvents by means of dynamic shear viscosity measurements. The results are interpreted by making analogies to the behavior of semidilute polymer solutions and living polymers.Dedicated to Prof. Dr. J. Meissner on the occasion of his 60th birthday.     

Rainwater rivulets on a cable subject to windFilets d'eau de pluie sur un câble soumis au vent

Rainwater rivulets appear on inclined cables of cable-stayed bridges when wind and rain occur simultaneously. In a restricted range of parameters this is known to cause vibrations of high amplitudes on the cable. The mechanism underlying this effect is still under debate but the role of rainwater rivulets is certain. We use a standard lubrication model to analyse the dynamics of a water film on a cylinder under the effect of gravity and wind load. A simple criterion is then proposed for the appearance and position of rivulets, where the Froude number is the control parameter. Experiments with several geometries of cylinder covered with water in a wind tunnel show the evolution of the rivulets with the Froude number. Comparison of the prediction by the model with these experimental data shows that the main mechanism of rivulet formation and positioning is captured. To cite this article: C. Lemaitre et al., C. R. Mecanique 334 (2006).     

Effect of secondary zone dimensions on melt fracture manifestations of high density polyethylene

An experimental study was performed on melt fracture phenomena in extrusion of high density polyethylene. The purpose of the work was to study the sensitivity of melt fracture driven roughness to the size of recirculation zones, viz. secondary zones. The experimental apparatus consists of a right angle die and a hypodermic needle used as a capillary. The position of the needle relative to the die was adjusted using a special fixture. The roughness of the extrudate was studied as a function of penetration depth. A developed procedure provides a comparison between profile lengths of extruded strands. The computed mean, median, and mode values for roughness were presented as a function of capillary position. A qualitative analysis was conducted for the force oscillations during extrusion with a separate set of dies, equipped with the fixed capillaries of identical lengths and different depths of penetration. It was observed that the oscillatory pattern is sensitive to the sizes of the secondary zones. This qualitative observation supports the conclusions from the quantitative analysis that the roughness of the extrudate can be controlled through an adjustment of the secondary zone sizes.Partly presented at the 58th Annual Meeting of the Society of Rheology, Tulsa, Oklahoma, October 20–23, 1986     

The effect of dissolved polymers on the rheological properties of coating colours

Coating colours used for the coating of paper and board consist mainly of a mineral pigment, which is very often clay, a synthetic binder such as a styrenebutadiene latex, dispersion agents and water retention aids. The latter are often water soluble polymers. These polymers have a very strong influence on the rheological properties of the coating colours, both on the strain rate dependence of the apparent viscosity and on the viscoelasticity. The effects of two different grades of carboxymethylcellulose (CMC) and one grade of hydroxyethylcellulose (HEC), on the rheological properties at room temperature of a clay-based coating colour at pH 8, have been investigated. It is concluded that the high values of the dynamic modulus of the colours are due to interactions between the cellulose derivatives and the solid particles, i.e. mainly the clay particles. For HEC this interaction is associated with adsorption of the polymeric molecules on the clay particles. In the case of CMC, the adsorption is strongly retarded by the presence of the dispersant (a polyacrylate salt). It is suggested that the marked elasticity of the CMC-containing colour in addition to a possible polymer adsorption may be due to charge interactions and/or depletion flocculation. The effect of CMC and HEC on the water-retention properties of the colour is also discussed.     

A physico-mechanical approach to modeling of metal forming processes—part I: theoretical framework

A combined physico-mechanical approach to research and modeling of forming processes for metals with predictable properties is developed. The constitutive equations describing large plastic deformations under complex loading are based on both plastic flow theory and continuum damage mechanics. The model which is developed in order to study strongly plastically deformed materials represents their mechanical behavior by taking micro-structural damage induced by strain micro-defects into account. The symmetric second-rank order tensor of damage is applied for the estimation of the material damage connected with volume, shape, and orientation of micro-defects. The definition offered for this tensor is physically motivated since its hydrostatic and deviatoric parts describe the evolution of damage connected with a change in volume and shape of micro-defects, respectively. Such a representation of damage kinetics allows us to use two integral measures for the calculation of damage in deformed materials. The first measure determines plastic dilatation related to an increase in void volume. A critical amount of plastic dilatation enables a quantitative assessment of the risk of fracture of the deformed metal. By means of an experimental analysis we can determine the function of plastic dilatation which depends on the strain accumulated by material particles under various stress and temperature-rate conditions of forming. The second measure accounts for the deviatoric strain of voids which is connected with a change in their shape. The critical deformation of ellipsoidal voids corresponds to their intense coalescence and to formation of large cavernous defects. These two damage measures are important for the prediction of the meso-structure quality of metalware produced by metal forming techniques. Experimental results of various previous investigations are used during modeling of the damage process.      

Bag breakup of nonturbulent liquid jets in crossflow

An experimental investigation of the bag breakup of round nonturbulent liquid jets in gaseous crossflow at room temperature and pressure is described. Pulsed photography, pulsed shadowgraphy, and high-speed imaging were used to observe the column and surface waves along the liquid jet and the formation and breakup of bags. Measurements included: wavelengths of column and surface waves, jet velocities, the number of bags along the liquid jet, the number of nodes per bag, droplets sizes and velocities, and trajectories of droplets. Present results show that the column waves of a nonturbulent liquid jet in crossflow within bag breakup regime can be explained based on Rayleigh–Taylor instability. The number of nodes per bag affected the breakup mechanism of the bags. Three distinctive sizes of droplets were produced due the breakup of the bag membrane, the ring strings and the ring nodes. The size of the droplets resulting from the breakup of the bag membrane was constant independent of the crossflow Weber number. Finally different trajectories were observed for the three groups of droplets.     

弹性体摩擦的黏滞疲劳理论及实验研究

为了更好地解决弹性体摩擦学方面的问题,建立了弹性体摩擦的黏滞疲劳理论,阐明了弹性体摩擦是粘附的表面拉伸效应和迟滞的体积变形效应的综合作用,揭示了弹性体磨损的本质机理是周期性疲劳破坏,解释了弹性体在摩擦过程中的各种特征现象.实验论证了黏滞疲劳理论在摩擦和磨损方面的可行性和精确性.通过建立的公式计算了弹性体的摩擦系数和摩擦力,并控制误差在6%的范围内;计算了弹性体的表观单位磨损率,并控制误差在7%的范围内.