小斜率近似方法分析粗糙界面声散射问题

利用小斜率近似方法计算了起伏海面的声散射特性,得到了具有高斯分布粗糙海面的平均反射系数（即镜反射系数）计算公式,并与微扰法和Kirchhoff近似做了比较,结果表明小斜率近似是一种非常有效的分析起伏表面散射特性的近似方法。最后讨论了海面镜反射系数随海面的不平整度（波浪的均方根高度）、声波频率和声波入射方向的变化关系,得出了只有在声波波长和起伏波浪高度可比拟时,才有明显的镜反射的结论。这为分析浅海目标声散射特性时,选择是否需要考虑海面（海底）所引起的多途效应提供了理论依据。     

螺旋结构手性材料的微波介电和反射/透射性能研究

研究了螺旋结构手性材料的排列方式及其螺直径对金属螺旋体介电常数和反射/透射系数的影响。利用矢量网络分析仪测量了螺旋手性材料在8.2～12.4GHz的反射/透射系数和微波介电性能。结果表明,螺旋结构手性材料的排列方式和螺直径与微波性能密切相关。选取适当的排列方式和螺直径,可使手性材料的性质在左手和右手之间转变,并可使反射系数值在X波段内为0.05。     

流体饱和孔隙介质参数反演的模拟退火算法

本文研究了模拟退火算法在流体饱和孔隙介质参数反演中的应用.通过计算响应数据与实测响应数据的拟合将参数反问题归结为最优化问题.由于流体饱和孔隙介质运动方程的复杂性,动力响应与材料参数之间呈复杂的非线性关系,优化目标函数是非凸多峰函数.传统的梯度类优化方法一方面受局部极值的困扰难以搜索到全局最优解; 另一方面确定搜索方向须进行复杂的参数敏度分析.为克服这些困难,本文应用模拟退火算法进行了多参数反演数值模拟,模拟结果表明了模拟退火算法的可行性和稳健性.     

超声在流体和一般各向异性固体界面上的反射

考虑到由流体斜入射到各向异性固体时超声波模式转换的一般情况,给出了超声在流体和各向异性固体任意取向界面上反射和折射问题的求解方法;超声反射和折射系数是媒质弹性参数、界面取向及入射角的函数.给出了超声在流体和不同对称类晶体及纤维增强复合材料界面上反射、折射的数值解例.     

粗糙界面法向接触振动响应与能量耗散特性研究

粗糙界面的法向接触振动与能量耗散特性对描述界面动力学机理具有重要的理论和实际意义。本文通过建立粗糙界面法向接触振动的动力学模型,提出了其法向接触振动响应特征量和振动能量耗散量的计算方法。基于粗糙表面的三维分形模型描述,构造了粗糙界面的接触力-变形关系式,并与Hertz接触模型的力-变形关系进行了对比分析;建立了粗糙界面接触振动系统的动力学方程,计算了不同表面形貌粗糙界面系统每周期的振动能量耗散率和累计能量耗散率;分析了粗糙界面法向接触振动的响应特征与能量耗散特性,从理论上对界面法向微动能量耗散的实验结果进行了解释,为描述接触界面的动力学机理提供了理论依据。     

润滑状态下线接触滑动粗糙界面的动摩擦特性研究

滑动粗糙界面的摩擦润滑特性对界面的润滑设计和润滑状态预测具有重要的理论和实际意义。本文通过建立不同润滑状态下的滑动粗糙界面模型,基于界面的法向载荷由润滑油膜和粗糙体共同承担的载荷分配思想,采用Greenwood-Williamson统计模型描述粗糙表面形貌,考虑界面润滑的时变效应和润滑油的粘-压特性,建立了线接触滑动粗糙界面的油膜厚度方程和粗糙体接触压力方程,获得了整个润滑区的润滑油膜载荷比例因子、油膜厚度和摩擦系数随滑动速度的变化关系,推导了界面由混合润滑过渡为液压润滑的临界速度关系表达式,分析了滑动粗糙界面的润滑承载机理,获得了界面油膜厚度、摩擦系数和临界速度随界面形貌参数、法向载荷、润滑油属性参数的变化规律,为机械结构的界面润滑设计、润滑状态预测和润滑优化提供理论和实验参考。     

波束在液-液界面上反射和透射的研究

波束在液－液界面上反射和透射的研究陈宇（清华大学物理系，中科院声场声信息国家重点实验室北京·１０００８４）黄杰周阳辛（清华大学物理系北京·１０００８４）１引言在自然界中，波在界面上的反射是一种常见的现象。很早就有人提出理论来解释这类现象。现在人们已经...     

平面波与层状多孔介质海底的反射和透射

针对典型的海底介质结构情况,研究了从海水入射到含有非固结沉积层的层状流体饱和多孔介质海底的平面波的反射和透射问题,分析了沉积层和基岩中纵、横波速度和衰减的频散变化特点,在沉积层厚度和频率变化时,对海水-海底界面上的位移势函数反射系数进行了计算和分析。研究结果表明：在沉积层厚度一定的情况下,较高频率时,沉积层对海水中声场的影响较大,而频率较低时,基岩对海水中声场的影响较大。在以不同的掠射角入射时,由于沉积层中质点的法向共振,广义位移势函数反射系数随频厚积的变化曲线会出现一系列的共振峰,随着掠射角的减小,共振峰个数减少,但共振峰的幅度会增加。     

非连续粗糙多界面接触变形和能量损耗特性研究

非连续多层叠加结构的粗糙界面接触形变与能量损耗特性一项研究较少的难题。本文在单一界面能量损耗特性研究的基础上,通过建立“多层粗糙金属板--刚性平面”的多界面模型,采用有限元方法,对加载与卸载过程中,具有不同界面形貌、不同塑性变形行为和界面摩擦的多层叠加模型粗糙多界面的接触力和变形进行计算,研究了多层叠加结构粗糙界面上的接触力-变形关系,以及由塑性变形与界面摩擦引起的能量损耗特性,构造了描述粗糙多界面上接触力-变形关系表达式,计算了多层粗糙界面的能量传递损耗率,分析了多层叠加结构的非连续粗糙多界面接触变形机理和能量损耗特性。     

声波由粘弹性介质向流体介质入射情况下的声反射分析

在研究含腔体的结构的声学特性时,会遇到声波从粘弹性介质向流体介质入射的情况。主要研究了声波从粘弹性介质向流体介质斜入射情况下的声反射特性,分析表明：反射声波中有横波和纵波同时存在,纵波声压反射系数随入射角度的增大逐渐增大,横波的声压反射系数随入射角度的增加先增大后减小,在垂直入射时只有纵波,而没有横波存在。这些结论为进一步研究带空腔结构的声学特性打下了基础。     

Reflection and transmission of plane harmonic waves at an interface between liquid and micropolar viscoelastic solid with stretch

A solution of the field equations governing small motions of a micropolar viscoelastic solid half-space with stretch is employed to study the reflection and transmission at the interface between a liquid and a micropolar viscoelastic solid with stretch. The amplitude ratios for various reflected and refracted waves are computed and depicted graphically. Effects of axial stretch and viscosity on the amplitude ratios are discussed     

超声在流体和一般各向异性固体界面上的反射

考虑到由流体斜入射到各向异性固体时超声波模式转换的一般情况，给出了超声在流体和各向异性固体任意取向界面上反射和折射问题的求解方法；超声反射和折射系数是媒质弹性参数、界面取向及入射角的函数。给出了超声在流体和不同对称类晶体及纤维增强复合材料界面上反射、折射的数值解例。     

Reflection and transmission properties of a wire grid embedded in a SNG or SZ medium

The reflection and transmission properties of a wire grid embedded in lossless Single Negative (SNG) or Single Zero (SZ) medium have been analytically studied. The wires of the grid are taken to be lossless as well as lossy. It is shown that a lossy grid has approximately identical reflection characteristics for the Transverse Magnetic incident wave with Epsilon Negative and Mu Negative background medium. The lossy grid with Epsilon Zero background medium acts as a perfect reflector for the normally incident wave. Likewise, it is further studied that a lossy wire grid embedded in Mu Zero medium can be cloaked for the incoming normally incident wave.     

温度对水饱和孔隙介质压缩波速度的影响

温度作用使孔隙介质密度、弹性模量等参数发生改变,从而影响声波传播性质。文章基于变形体能量守恒定律,考虑热机耦合和两相介质热耦合,推导水饱和孔隙介质的热传导方程;考虑孔隙水和固相孔隙介质存在的三种耦合性,基于热作用下应力应变关系分析,建立了热机耦合声波传播模型;设计温度变化沉积物样品声学测量实验,当温度由2℃增加到27℃时,压缩波速度平均增大88m·s^-1运用热机耦合声波传播模型解释了实验测量结果的上升趋势,获得理论计算值与测量值相对偏差范围为-3．03％～1．19％。     

Reflection of ultrasonic waves from imperfect interfaces: A combined boundary element method and independent scattering model approach

A numerical technique for obtaining interface reflection coefficients for imperfect bonds between similar materials for a wide range of distributed defects is developed. A numerical boundary element method is utilized to find the far-field scattering amplitudes of a single defect for a normally incident plane wave. Then, the normal incidence reflection coefficient for a planar distribution of such defects is obtained from the independent scattering model. As a validation, the reflection coefficients are compared to the quasi-static model results where the latter are available. This establishes the basis for one application of the new model, the determination of spring constants which are not available. Other applications of the model, including studies of the response at frequencies beyond the quasi-static limit, the ratio of longitudinal to transverse wave reflectivities, and the effects of selected multiple scattering are discussed.     

Reflection and transmission of normally incident full-vector X waves on planar interfaces

The reflection and transmission of full-vector X waves normally incident on planar half-spaces and slabs are studied. For this purpose, X waves are expanded in terms of weighted vector Bessel beams; this new decomposition and reconstruction method offers a more lucid and intuitive interpretation of the physical phenomena observed upon the reflection or transmission of X waves when compared to the conventional plane-wave decomposition technique. Using the Bessel beam expansion approach, we have characterized changes in the field shape and the intensity distribution of the transmitted and reflected full-vector X waves. We have also identified a novel longitudinal shift, which is observed when a full-vector X wave is transmitted through a dielectric slab under frustrated total reflection condition. The results of our studies presented here are valuable in understanding the behavior of full-vector X waves when they are utilized in practical applications in electromagnetics, optics, and photonics, such as trap and tweezer setups, optical lithography, and immaterial probing.     

Reflection of plane waves at the free surface of a fibre-reinforced elastic half-space

The propagation of plane waves in fibre-reinforced, anisotropic, elastic media is discussed. The expressions for the phase velocity of quasi-P (qP) and quasi-SV (qSV) waves propagating in a plane containing the reinforcement direction are obtained as functions of the angle between the propagation and reinforcement directions. Closed form expressions for the amplitude ratios for qP and qSV waves reflected at the free surface of a fibre-reinforced, anisotropic, homogeneous, elastic half-space are obtained. These expressions are used to study the variation of amplitude ratios with angle of incidence. It is found that reinforcement has a significant effect on the amplitude ratios and critical angle     

Reflection and transmission of ultrasound by a region of damaged material

Reflection and transmission of ultrasonic waves by a layer-like region of distributed microcracks in a bulk material has been investigated. It has been assumed that for sufficiently low frequencies and far away from the damaged layer, the reflected and transmitted waves are plane waves. By applying the Betti reciprocal theorem to a cell containingN cracks, and by choosing one elastodynamic state as the actual wave state and the other as a suitably chosen auxiliary wave state, the reflection and transmission coefficients have been expressed in terms of integrals over theN cracks. Simple expressions have been obtained for the case that all cracks are identical and parallel to each other. For the case that the cracks do not interact with each other, numerical results for the reflection and transmission coefficients are presented for a distribution of penny-shaped cracks. The variation of these coefficients with frequency, relative layer thickness and angle of incidence has been displayed in graphs.