层状不均匀介质的反射系数

本文讨论介质的间断面和介质参数变化同时存在时,层系的反射系数问题.在地球物性参数反问题中,这样的模型更切合实际.第一部分给出层系反射系数的表达式;第二部分讨论有关的数学理论.     

Switching Control of Nonlinear Systems Based on the Quasi-ARX Model and the SVR Algorithm北大核心CSCD

该文基于改进的含有外部输入项的准线性自回归(准ARX)径向基函数(RBF)网络模型和支持向量回归(SVR)算法,提出了一种非线性切换控制方法.改进的准ARX模型非线性部分采用RBF网络.控制系统设计过程分为三个部分:首先,利用聚类方法确定模型的非线性参数;然后,采用线性SVR算法来解决控制系统的鲁棒性问题;接下来,基于控制误差给出切换判定函数,确定切换律给出控制序列.最后通过数值仿真验证了该方法的有效性.     

平面P波在弹性介质和非饱和多孔弹性介质分界面上的传播

使用线性粘滞的多孔弹性介质模型,解决在弹性介质和非饱和多孔弹性介质分界面上平面P波的反射与透射问题,这里的非饱和多孔介质中固体骨架被两种相互耦合的流体（液体和气体）所充满．通过势函数的方法得到了振幅反射系数与振幅透射系数．然后推导得到入射波与反射波、透射波之间能量转换情况．研究发现:用振幅比和能量比所表示的反射系数与透射系数是与入射角度、饱和度、入射频率以及上下层介质的弹性常数有关的方程式．数值计算通过图形的形式表达出来,而且入射角度、频率及饱和度对振幅和能量的反射与透射系数的影响分别进行了讨论．证明了在整个波的传播过程中分界处并没有发生能量的耗散．     

尖劈吸波体的研究和微波暗室的模拟

对尖劈形状吸波体的吸波性能进行了研究,并对导弹导引仿真实验用的微波暗室的性能进行分析和仿真.首先对尖劈体的二维反射性能进行了研究,从单条波线反射的原理出发,得到波束平面反射的统计模型.单条波线的反射通过数值模拟得到;波束反射模型则通过对数值模拟的结果进行统计和拟合得到,最终用多项式表示.对于一些简单或特殊的情况,也给出了解析解.通过分析发现,三维反射和二维反射之间有明确的关系.这种关系可以由三维入射角和反射次数决定,而反射次数可以通过二维模型得到.据此将平面反射模型扩展为三维反射模型,从而得到尖劈形状吸波体的三维反射模型.无回波暗室用于模拟没有背景微波辐射的环境,其关键在于选择合适的吸波材料.基于微波反射通量平衡原理,建立了考虑暗室墙面各点之间的相互影响的耦合模型,从而可以求解出在指定的发射源照射之下墙面各点的辐射强度分布.对模型的求解精度和收敛性进行了验证.基于此模型,对一个导弹引导试验进行了数值模拟,推算出了使用两种不同吸波材料时静区接收到的微波信号的信噪比.     

浅水中度振幅孤立波解的分支

利用平面动力系统分支方法研究浅水中度振幅方程的定性行为和孤立波解.给出了系统在不同参数条件下的相图.获得了光滑孤立波、cuspon解和周期波解的隐式表达式.对方程的光滑孤立波解、cuspon解和周期波解进行了数值模拟.获得的结果完善了相关文献已有的结果.     

各向异性介质中带衬砌的圆形结构与SH波相互作用

本文利用复变函数方法求解各向异性介质中带衬砌的圆形结构与ＳＨ波的相互作用问题。各向异性介质可以用来模拟地质条件。利用文献［５］中的方法则可决定介质中的散射波，再利用圆形结构上给出的边界条件，则可对问题进行求解，作为算例，本文给出了在一种各向异性介质中，二种不同材料组成的圆形带衬砌的结构与ＳＨ波相互作用问题的数值分析结果，并予以讨论。     

波动方程的边界单元法

本文从三维波动方程的Kirchhoff积分公式出发,首先经离散化给出无限均匀介质中的边界单元法公式.其后,在引入了波在不同介质面和自由面上的反射、折射系数以后,给出了三维波动方程在分区均匀介质中的边界单元法公式.     

弹性波方程有限元模拟的曲边地表处理

本文对比研究了关于弹性波模拟中的曲边地表形状处理的两种方法,一种是用给定的实际介质数值划定的地表形状,另一种是用样条插值逼近地表形状.本文采用有限元方法进行弹性波数值模拟,给出了基于这两种方法计算的数值例子,并对结果进行了分析比较.结果表明使用后一种方法对地表进行处理时,地表人工离散产生的干扰明显减少,优于前一种方法.     

介质反射系数的反演方法及其计算机实现

1 引言 本文考虑横向均匀的声波介质,利用脉冲平面波垂直入射,根据表面测量数据来直接反演介质的反射系数. 由于介质反射系数与介质声阻抗有着相互依赖的关系,人们以前的注意力都主要集中在声阻抗的反演问题上.如Syms讨论了解的存在性、唯一性等理论问题.张关泉从一维波动方程出发,研究了由阻抗反演声速的问题.顾桂定和张关泉就声阻抗的反演做过数值实验.Bube等人也对声阻抗反演问题进行过探讨. 本文从声波方程和应力—应变方程的联立方程组出发,导出一种直接反演介质反射系数的数值方法,不需要先求声阻抗,再由声阻抗求反射系数.值得指出的是利用这种方法可以导出一种非常有效的同时反演介质声速和密度的计算方法.详见文献[4]. 就本文提出的算法,利用SGI工作站做了大量数值实验,结果表明本算法稳定性好、精确度高.同时给出了由反射系数求声阻抗的例子.     

微极热弹性无限板的轴对称自由振动

研究了在应力自由和刚性固定边界条件下,无能量耗散的均匀、各向同性微极热弹性无限板的轴对称自由振动波的传播,导出了相应的对称和斜对称模态波传播的闭合式特征方程和不同区域的特征方程.对短波的情况,应力自由热绝缘和等温板中对称和斜对称模态波传播的特征方程退化为Rayleigh表面波频率方程.根据导出的特征方程得到了热弹性、微极弹性和弹性板的结果.在对称和斜对称运动中计算了板的位移分量幅值、微转动幅值和温度分布,给出了对称和斜对称模式的频散曲线,并示出了位移分量和微转动幅值和温度分布的曲线.能够发现理论分析和数值结论是非常一致的.     

Reflection of magneto-thermoelastic waves with two relaxation times and temperature dependent elastic moduli

The model of the equations of generalized magneto-thermoelasticity with two relaxation times in an isotropic elastic medium under the effect of reference temperature on the modulus of elasticity is established. The modulus of elasticity is taken as a linear function of reference temperature. Reflection of magneto-thermoelastic waves under generalized thermoelasticity theory is employed to study the reflection of plane harmonic waves from a semi-infinite elastic solid in a vacuum. The expressions for the reflection coefficients, which are the relations of the amplitudes of the reflected waves to the amplitude of the incident waves, are obtained. Similarly, the reflection coefficients ratios variations with the angle of incident under different conditions are shown graphically. A comparison is made with the results predicted by the coupled theory and with the case where the modulus of elasticity is independent of temperature.     

Wave propagation in an initially stressed transversely isotropic thermoelastic solid half-space

The governing equations of thermoelasticity of transversely isotropic solid with initial stresses are formulated at uniform temperature. These equations are solved analytically in two-dimensions to show the existence of three plane quasi waves, namely, Quasi-Longitudinal (QL), Thermal (T-mode) and Quasi-Transverse (QT) waves. Reflection from a thermally insulated stress free surface of an initial stressed transversely isotropic thermoelastic solid half-space is studied. A particular model is chosen for the numerical computations of the propagation speeds, attenuation coefficients and reflection coefficients. Effects of initial stress parameter and thermal disturbances are observed on speeds of propagation, attenuation coefficients and reflection coefficients.     

Reflection at the free surface of fiber-reinforced thermoelastic rotating medium with two-temperature and phase-lag

The present investigation is concerned with the reflection of plane waves from the free surface of a homogeneous, anisotropic, fiber-reinforced thermoelastic rotating medium under two-temperature and dual-phase-lag model. It has been observed that three coupled plane waves travel in the medium with distinct speeds. Using appropriate boundary conditions, the amplitude and energy ratios for the reflected waves are derived and the numerical computations have been carried out with the help of MATLAB programming. The numerical values of the modulus of reflection coefficients are presented graphically to exhibit the two-temperature, phase lag and rotation parameter effects. The expressions of energy ratios have also been obtained in explicit form and are shown graphically as functions of angle of incidence. It has been verified that during reflection phenomena, the sum of energy ratios is equal to unity at each angle of incidence.     

具有两个驰豫时间的热弹性立方晶体材料中平面波的传播

研究具有两个驰豫时间的、两个不同弹性和热性质的、广义传热立方晶体固体半空间的有缺陷结合面上,热弹性平面波的反射和折射问题.具有两个驰豫时间的广义热弹性理论,是1972年由Green和Lindsay提出并应用于问题的研究.对有缺陷边界,给出了反射系数和折射系数(即反射波和折射波振幅与入射波振幅之比)的表达式,并推演了法向刚性边界、横向刚性边界、接触传热边界、滑动边界和结合面边界时的表达式.给出了在不同边界条件及出射角时,不同的反射波和折射波的振幅比,在不同的入射波时的变化图.发现反射波和折射波振幅比受到介质刚性和热性质的影响.     

Formation of nonlinear magnetization oscillations by spin waves transmission through the boundary of two uniaxial ferromagnets

In this paper, the possibility of nonlinear spin wave formation at transmission of a bulk spin wave through the boundary of two ferromagnets with uniaxial magnetic anisotropy in the external magnetic field was shown at the certain conditions. The bulk spin waves with different values of precession angles of magnetizations can propagate in the second ferromagnet so both linear and nonlinear states can be excited. Also, expressions for coefficients of reflection and transmission of spin waves were obtained depending on value of a precession angle of magnetization in the second ferromagnet.     

具有微极弹性构架的流体-饱和多孔固体中波在边界面处的传播

通过研究匀质非黏性液体半空间和微极流体饱和多孔固体半空间交界面处平面波的反射与透射,得到不同角度入射波的反射和透射系数.计算出反射波与透射波的振幅比数值,并用图表示出微极性和多孔性对反射和透射的影响.最后根据公式对一些特例进行了推导.     

Reflection and transmission of couple longitudinal waves at a plane interface between two dissimilar half-spaces of thermo-elastic materials with voids

The problem of reflection and transmission of elastic waves due to incident plane couple longitudinal waves at a plane interface between two dissimilar half-spaces of thermo-elastic materials with voids has been investigated. Using the theory of Iesan (1986), the propagation of couple longitudinal waves in the thermo-elastic materials with voids has been explained. The expressions of the reflection and transmission coefficients and energy ratios corresponding to reflected and transmitted waves are obtained. These coefficients and energy ratios of the various reflected and transmitted waves are computed numerically for a specific model.     

On conditions of the dynamic equivalence of waves in multilayer elastic media

Necessary and sufficient conditions for the dynamic equivalence of two waves propagating in an elastic p-layer medium and undergoing all kinds of exchanges on the medium interfaces are formulated. These conditions are represented in the form of equalities involving powers of the reflection and refraction coefficients occurring in the expressions for the intensities of these waves. Bibliography: 5 titles. Translated fromZapiski Nauchnykh Seminarov POMI, Vol. 218, 1994, pp. 166–175. This work was supported by the Russian Foundation of Fundamental Research (Grant 93-011-16148). Translated by N. S. Zabavnikova.     

Scattering of surface gravity waves over a pair of trenches

A numerical model based on boundary element method is developed to study the scattering of surface gravity waves over a pair of trenches of varied configurations under the assumption of small amplitude water wave theory. Both the cases of symmetric and asymmetric trenches are considered in the present study. The accuracy of the numerical results is validated by comparing the reflection and transmission coefficients with energy identity, and the known results associated with single trench available in the literature. The study reveals that wave reflection decreases in an oscillatory manner with an increase in trench width. Moreover, Bragg resonance in wave reflection is observed for wave number corresponding to waves in shallow and intermediate depths in the case of a pair of trenches. Further, Bragg reflection increases with an increase in the number of trenches. In the case of multiple trenches, subharmonic peaks in Bragg reflection are depicted and the number of subharmonic peaks between two harmonic peaks is found to be two less than the number of trenches. However, for triangular trenches, the occurrence of the subharmonic peak is invariant of the number of trenches and the same vanishes for larger trench depth. Irrespective of trench configurations, wave reflection follows certain uniform oscillatory pattern with an increase in the gap between the trenches in case of deep water.