粘弹性阻尼对弹性杆内纵向孤波运动的影响

本文用微扰论的计算方法, 求出在粘弹性阻尼较小但不可忽略的情况下, 弹性杆内纵向孤波的形状、速度及能量随时间的变化规律. 结果表明, 粘弹性阻尼使孤波逐渐变矮、增宽、速度减漫, 能量逐渐耗散, 最后导致孤波的消失. 关键词：     

粘弹性阻尼对弹性杆内纵向孤波运动的影响

本文用微扰论的计算方法,求出在粘弹性阻尼较小但不可忽略的情况下,弹性杆内纵向孤波的形状、速度及能量随时间的变化规律。结果表明,粘弹性阻尼使孤波逐渐变矮、增宽、速度减慢,能量逐渐耗散,最后导致孤波的消失。 关键词：     

DETERMINATION OF ELASTIC CONSTANTS OF ANISOTROPIC LAMINATED PLATES USING ELASTIC WAVES AND A PROGRESSIVE NEURAL NETWORK

In this paper, a procedure is suggested to inversely determine the elastic constants of anisotropic laminated plates using a progressive neural network (NN). The surface displacement responses are used as the inputs for the NN model. The outputs of the NN are the elastic constants of anisotropic laminated plates. The hybrid numerical method (HNM) is used to calculate the displacement responses of laminated plates to an incident wave for given elastic constants. The NN model is trained using the results from the HNM. A modified back-propagation learning algorithm with a dynamically adjusted learning rate and an additional jump factor is developed to tackle the possible saturation of the sigmoid function and to speed up the training process for the NN model. The concept of orthogonal array was adopted to generate the representative combinations of elastic constants, which reduces significantly the number of training data while maintaining its data completeness. Once trained, the NN model can be used for on-line determination of the elastic constants if the dynamic displacement responses on the surface of the laminated plate can be obtained. The determined elastic constants are then used in the HNM to calculate the displacement responses. The NN model would go through a progressive retraining process until the calculated displacement responses using the determined results are sufficiently close to the actual responses. This procedure is examined for an actual glass/epoxy laminated plate. It is found that the present procedure is very robust and efficient for determining the elastic constants of anisotropic laminated plates.     

轴对称非均匀弹性介质中引力波对声子的作用效应

采用Ｓｅｒｅｂｒｙａｎｙ的复空间中的复坐标法，讨论了轴对称非均匀弹性介质中引力波对声子的作用效应，并给出了由引力波产生的激发力的表达式和相应的声子解。解的形式表明，平行于对称轴方向传播的引力波只对声子场的径向分量、切向分量产生扰动，垂直于对称轴方向传播的引力波则对声子场的径向分量、切向分量和轴向分量均产生扰动，扰动强度取决于切变模量径向分布函数的具体形式和引力波的振幅。此外，本文还将所得结果与含有螺旋位错的拓扑声子空间中引力波的扰动效应作了比较。 关键词：     

晶间界面附近区域中的弹性场——各向异性理论

根据现代关于晶间界面的研究可知,晶界和相界多具有二维周期结构。由此出发,采用Fourier级数展开的表示,在给定了界面结构,在数学上即是形式地给定了边界条件的情况下,求出了两个各向异性物相间相界附近的弹性场。并以Ni中扭转晶界为例,作了应力场的数值计算。本文的方法和方程对于研究各种类型的晶界和相界均普遍可用,为处理这类问题提供了一种有效的简捷手段。 关键词：     

斜入射声场中无限弹性圆柱的共振、表面波及声散射

本文讨论斜入射声波场中无限长弹性圆柱体的声散射特性与柱体的共振和表面波特性之间的关系。由自由柱的三维简正波解,可求得柱的自由共振频率与相应螺旋表面波的相速度以及螺旋角的关系,进而可求得表面波相速度的频散关系与水中声波入射角的关系,通过与Flax等在有关论文中的结果相比较,表明在倾斜入射条件下,无限弹性柱的共振方式与形函数曲线中极小值的频率基本相符,这一现象与正横入射时的现象相类似。     

关于无限薄平板中圆孔的线性声阻

本文提出至今尚未圆满解决的无限薄平板中一个圆孔的线性声阻的理论分析方法,得到了线性声阻率的高频(或大孔径)近似式及低频(或小孔径)极限值;理论计算和实测值符合良好;最后给出了在全频率范围适用的声阻率近似公式。 关键词：     

光能够走多慢?——极慢光速研究若干进展

首先介绍了光波群速度，介质的色散性质，电磁感应透明技术等基本概念和理论，对国外极慢光速研究的进展情况做了概括性介绍，并就几个主要实验及结果进行了较为详细的描述，其中包括L.V.Hau小组于1999年完成的“光速每秒17m”实验和D.F.Phillips小组于2001年发表的“在原子气中储存光”实验，文章最后就极慢光速研究在科学和应用两方面的意义及价值进行了讨论。     

关于"电磁波在非均匀等离子体中的吸收"的评论

王舸等人将非均匀等离子体分层，认为总出射功率等于每层反射和透射功率之和，而实际上，作者认为应是总出射波振幅等于每层反射和透射振幅之和。     

水中有限长弹性圆柱的目标强度研究

本文将有限长钢圆柱当作弹性体处理,用有限元素法计算了频率为27kHz的声波从不同方向入射时的目标强度值,从中得到目标强度随方位变化的曲线。同时对该圆柱的目标强度进行实验测量,并将测量结果与上述计算结果和已知的刚性结果进行比较。结果表明弹性结果与实测结果更为接近。     

COINCIDENCE OF THERMOELASTIC AND THERMOVISCOUS ACOUSTIC WAVES IN FLUID-FILLED ELASTIC TUBES

Thermoelastic and thermoviscous acoustic wave propagation in fluid-filled steel tubes is studied using the exact three-dimensional (3-D) fluid-elastic coupled system equations for the vibration in the n=0 and 1 circumferential modes. Water- and air-filled tubes are examined. The water-filled steel tube shows a strong fluid-elastic coupling effect in the lower frequency range and the air-filled tube shows a strong thermal effect for all frequencies. An 88·9 mm outer diameter tube with 3·05 mm wall thickness is used for the study. Due to the fluid-elastic coupling introduced for air having a specific heat ratio of 1·4 (the solution uncouples when the ratio is 1·0), thermal effects are seen to be very important with the modal attenuation rate being at least 32% underestimated if the thermal effect is not included in the air-steel system. A coincidence phenomenon is accurately found directly from the coupled modes in the fluid-elastic coupled system. When coincidence occurs, the axial modal attenuation rate drops sharply, allowing the exact determination of the coincidence frequency by locating the local minimum of the modal spatial attenuation rate with increasing frequency. In the water-steel system, the coincidence frequency is seen to be 8% in error if methods are employed using the uncoupled theory for the separate fluid and elastic wall.     

电磁波在非均匀等离子体中的吸收

研究了电磁波在非均匀碰撞等离子体中的传播和吸收,提出了一种计算电磁波在非均匀等离子体层中反射和透射功率的模型。对几种典型的非均匀密度剖面,计算了在不同的碰撞频率和层边缘密度下电磁波衰减率与电磁波频率的关系,结果表明,对于给定的密度剖面和有效吸收频宽,存在一个最佳的磁撞频率。     

相速和群速的演示实验

本文介绍了演示相速和群速的演示实验装置和方法。     

浅海中简正波的衰减与群速

本文从均匀浅海的频散方程出发导出了计算简正波衰减与群速的公式,并通过数值例子比较了它们的精度。文中特别讨论了临界简正波的衰减与群速,证明Pekeris浅海中临界简正波的衰减与群速一般小于海底吸收与声速,从而修正了 Kornhauser,Raney,Weston与Tindle关于临界简正波衰减与群速正好等于海底吸收与声速的结论。     

INHOMOGENEOUS WAVES IN ANISOTROPIC POROUS LAYER OVERLYING SOLID BEDROCK

The problem of propagation of inhomogeneous waves in anisotropic porous layered medium is studied using transfer matrix. Firstly, transfer matrix for an anisotropic porous layer is derived. Biot's poro-elastic theory is incorporated to model the acoustics of anisotropic porous layer. The interface between porous layer and elastic half-space is considered as imperfect and modified boundary conditions are applied for this more realistic situation. The theory of transfer matrix is used to derive the analytical expression for the surface impedance. Numerical computation of results is done for different degrees of bonding in the low as well as high-frequency range. In the first case, which is relevant to geophysical studies, the surface impedance is predicted for low-frequency range and surface impedance for second model is computed in high-frequency range. It is observed that loose bondedness is accompanied by the loss of energy at the interface. The technique of transfer matrix is utilized to compute the surface impedance in both cases. The role of surface impedance in seismological studies and in the study of composites is discussed.     

辐射在各向异性等离子体中散射的结构因子

本文导出多组元各向异性等离子体中二体密度关联函数的计算公式,求得双麦克斯韦等离子体中考虑了电子-电子和电子-离子关联效应的电子-电子密度关联函数,最后给出电磁辐射在各向异性等离子体中散射的结构因子。 关键词：     

长声学格波即弹性波的论证

从原子的运动方程出发，把长声学格波的原子运动方程演化成弹性波的波动方程，将长声学格波条件下的微观原子运动与宏观的弹性波波动统一起来，对长声学格波即弹性波作了进一步的证明。     

各向异性等离子体中的静电屏蔽

首先证明在各向异性等离子体中,任意分布函数对于静止试验电荷的静电屏蔽总是完全的,其次用计算机算出双麦克斯韦等离子体中的静电屏蔽势,当距离R充分大时,静电屏蔽势与R^-3成比例地衰减,在一定条件下,还会出现超屏蔽(即正试验电荷周围的电势出现负值)的区域。 关键词：     

色散、群速与群折射率

本文讨论了群折射率存在的物理意义,指出群折射率不仅包含了折射率概念而且还体现了折射率的色散性质,同样能作为材料本身的光学性质参量.接着指出在折射率的白光干涉法测量中存在着忽视色散作用而导致的将群折射率误认为折射率的情况.将群折射率与折射率区别开来,不仅可以消除折射率测量中所存在的错误,而且可以突出色散现象在光传播时间特性上的意义.为此建立了折射率与群折射率的相互变换关系,通过对于典型物质的折射率与群折射率变换关系的计算,说明二者在数值上的差别甚至对于气体介质来说也是不能忽略的.