海底沉积层中高阶界面波对多掩埋物散射分析

根据界面波散射迭代公式,并利用含多个异质体弹性动力学的散射积分方程,引出海底沉积层中界面波高阶模式对掩埋物的散射情况。根据建立的分层海底模型,对沉积层中传播的界面波进行模式分解。利用界面波传播的Green函数构建界面波散射迭代积分,并根据弹性波对多异质体散射理论,将界面波散射积分推广到对多目标的散射情况。由模式分解的结果,求解出了前三阶界面波对多目标的散射情况,并对界面波高阶模式传播及对不同目标的散射情况做了分析。     

海底沉积物声波现场测试和有限差分数值模拟研究

利用基于Biot理论的高阶交错网格有限差分算法,模拟了声源在不同位置条件下激发声波的传播,重点讨论了纵波和各种转换波的声场,并与我国某海域地区海底沉积层的现场测试数据进行了对比。研究结果表明,海底沉积层中的纵波具有明显的频散现象,这种频散现象与理论预期结果相吻合,而且由Biot理论得到的纵波频散规律和现场测试结果有较好的一致性,但对于纵波的衰减,二者符合得不好,这是因为Biot理论模型的假设条件和海底沉积层的差别较大造成的。计算声场显示,当声源放置在海水中时,在沉积层中产生三种转换波;当声源放置在沉积层中时,在沉积层中产生了多种类型的波。在某些条件下,海底沉积层中的横波速度低于慢纵波速度。     

关于浅海沉积层对声场影响的讨论

一、引言 水声学通常将海底对声场产生巨大作用的海域称为浅海.在我国南海海域存在着一类比较特殊的浅海海域,如图1所示.它的海深大约2km,象深海一样存在着声道,海面处海水的声速一般比海底处的大.海底有一沉积层,沉积层下面是半无限基底.海面处的海水声速与沉积层的声速接近.在这类浅海中,海水表面附近的声场对海底参数的变化非常敏感.对于这一类的浅海声道,最有效的计算模型是简正波模型.目前流行的简正波模型非常多,每一种模型都有各自的优缺点.这类海域的声场计算需要尤其慎重,必须选择合适的简正波模型.本文首先讨论简正波模型的选择问题,然后利用数值模型的方法研究了沉积层的声学特性对声场的影响.     

珊瑚型环礁斜坡地形水下声传播特性分析

从岛礁斜坡地形条件下的声信号衰减和地形阻断效应分析出发,重点针对水下声场分布规律及其对声传播造成的影响开展研究。利用水声模型理论,结合某礁实测地形以及水文数据,建立岛礁斜坡地形下的多途声信道模型,基于Bellhop与RAM声学仿真方法,对不同地形下的声线轨迹、声传播损失以及信号时延等声场特性进行仿真分析,得出岛礁斜坡地形下的声场分布特征。研究结果表明：（1）岛礁斜坡地形是影响其声传播模式的关键因素;（2）斜坡外缘浅海区域的目标不易被岛礁斜坡顶端的声呐所探测; （3）陡坡地形对浅海声源的声传播有利,当声源深度足够大时,缓坡地形下的本征声线数目能够达到在陡坡地形下的5倍,对声传播有利。以上研究结果可为岛礁区水下声场的特性分析以及水下声学对抗等实践应用提供理论基础和技术参考。     

倾斜弹性分层海底声场分布仿真研究

利用弹性抛物方程理论中的单向散射近似法处理倾斜弹性体-弹性体的边界条件,建立了不同地形条件下具有弹性分层海底的海洋环境中声场计算的新方法;经仿真分析等深度均匀海水层覆盖下具有倾斜弹性体-弹性体边界的弹性分层海洋环境中低频声场的空间分布结构,表明当弹性沉积层中纵波波速不大于1.5倍水中声速时,声压传播损失完全由海水介质参数和海底底质决定,与沉积层下界面地形关系不大,且弹性界面处有明显的界面波(Stoneley波)存在,即弹性体-弹性体界面的地形变化,基本不影响海水中的声场分布。与能量守恒方法相比较,仿真验证了文中计算方法的准确性。     

浅海单模入射声场目标回波特性研究

浅海环境中,目标回波受到入射和散射双向过程的信道多途影响,具有复杂的多途结构。基于相控垂直阵的单模发射技术能激发出指定的单个简正波声场,可以降低海底混响干扰,简化目标回波多途结构,为主动探测提供了一种有效手段。在信道中点声源目标回波模型基础上,采用简正波本征函数加权研究了单模声场入射下球形目标散射问题,建立了浅海单模入射声场目标回波预报模型,并利用模型对刚性球回波进行了数值计算。结果表明:总声源级相同条件下,单模发射声场与传统的点声源发射声场相比具有一定的阵发射增益,目标回波具有较高的声压级;单模入射能够消除单程多途的影响,回波结构相对简单,有利于目标的探测和识别。     

薄板结构中声场的计算研究

研究薄板结构中声场的算法对薄壁管中声波导的模拟仿真软件设计有重要理论参考价值。首先,通过弹性应变计算出激励不同分量方向上薄板二维空间质点的运动方程。其次,为了直观显示薄板中超声波的传播,对质点的运动方程进行交错网格有限差分并结合MATLAB数值分析软件对声波传播进行模拟。结合波场快照图,可使薄板中声场分布情况得到清楚显示,数值仿真结果与实际相符。模拟结果表明,该计算方法能正确模拟仿真声波的传播过程,可为软件设计提供可靠理论基础。     

波束位移射线简正波理论在双轴海洋声道中的应用

由于表面声道与深海声道间的耦合效应，声波在双轴海洋声道中的传播比较复杂，因此求解双轴海洋声道中的声场就比较困难。文章将浅海声传播的波束位移射线简正波（BDRM）理论应用于计算双轴海洋声道中的声场，进行了数值模拟并与传统简正波方法进行比较，结果表明应用BDRM理论计算的传播损失具有很高的精度和速度，可以对双轴海洋声道内声传播问题进行分析和预报。     

掩埋物体的小掠角散射声场与回波信混比

探测掩埋物体的水声系统,通常是在一定距离之外以小掠角声波扫描海底的方式工作,现有的射线声学模型已不能进行系统性能预报。采用波数积分模型和等效源建模方法,将声波在介质中的传播和掩埋物体的散射场计算融为一体,可以高精度地进行各种形状物体、不同声波束和任意掩埋状态下的三维散射场计算。利用开发的声场模型,计算出了典型球壳的等效目标强度和信号混响比随频率、掠射角及掩埋深度的关系曲线。在此基础上,提出一种基于时反聚焦发射、水下移动平台接收的双基地探测设想,并采用开发的声场模型验证了其正确性。     

浅海负跃层海底单参数模型研究

海底反射特性在浅海声传播中起着决定性的影响。用小掠射角下海底反射损失随掠射角变化率Fd B构建单参数模型可很好地描述浅海海底特性。对单参数模型进行深入研究和改进,建立了在浅海负跃层条件下描述水下声场的海底单参数模型,并利用数值仿真和海上实验数据,对浅海负跃层海底单参数模型水下声场预报的正确性进行了验证。     

Finite element simulation and visualization of leaky Rayleigh wavesfor ultrasonic NDE

The generation and propagation properties of transient leaky Rayleigh waves are characterized by a two-dimensional finite element model. The displacement vector is used as the primary variable for the solid medium and a potential scalar, which is a replacement for the pressure, is taken as the fundamental variable for the fluid medium. The coupled system of finite element equations are solved in the time domain by direct integration through the central difference scheme. Three configurations are considered: the conversion of a Rayleigh surface wave into a leaky Rayleigh wave, a focused beam probing a fluid/solid interface at the Rayleigh angle, and the interaction of a defocused wave with the interface. The wave velocity in the fluid (water) is lower than the Rayleigh wave velocity in the solid (aluminum). The wave propagation profile in each case is predicted by the model. The finite element model proves to be an effective tool for surface acoustic device design and ultrasonic NDE     

Longitudinal elastic wave propagation in laminated composites with bonds

We extended the finite element displacement method to study the propagation of longitudinal elastic waves in laminated composite with bonds. The geometric arrangement of the composite model considered in this paper is treated as a special type of a trilaminated composite in which each of its major constituents is sandwiched between two bonding layers. The dispersion characteristics of this model are presented here and compared with some exact results. The exact dispersion relation for the trilaminated composite is formally obtained by solving the field equations subjected to continuity conditions at materials' interfaces. Also included in the comparisons are the results obtained with continuum theory with microstructure. It is found that numerical results of finite element analysis, continuum theory and exact analysis corelate well especially for the lower modes of wave propagation.     

Constitutive model and wave propagation in dissipative electroelastic crystals

We propose a constitutive model for dissipative ionic crystals within the classical continuum theory of thermoelectroelasticity which includes polarization gradients as independent variables. According to a previous work, dissipation is modeled by suitable evolution equations for a set of internal variables. The compatibility with the second law of thermodynamics is required to obtain the pertinent constitutive equations. The problem is then linearized about an unstrained and unpolarized state, and the nature of dissipative contributions is pointed out. We apply this model to the study of wave propagation by deriving the eigenvector equation for a two-dimensional problem in the quasi-static hypothesis. The dispersion relation of bulk waves is obtained and discussed for crystals with a centrosymmetric structure.     

弱粘接界面固-固界面波特性及与界面粘接性质关系数值研究

根据两固体粘接结构在不同粘接强度下的弹簧模型边界条件,通过傅里叶积分变换方法进行波动方程求解,理论分析和数值计算了相近横波速度的两种固体间界面波的频散及衰减特性。计算结果表明,当切向弹簧劲度系数从滑移粘接界面向完好粘接界面逐渐变化时,界面波的频散特性随之变化。在此基础上进一步计算了不同界面粘接条件下法向线源脉冲激发的界面波瞬态信号,可以更直观地观察不同粘接界面条件时界面波的信号特征与传播特性,包括频散特征、幅度变化等。采用完全在固-固界面激发、同时在界面接收的新方式,研究了不同界面特性玻璃-钢界面波的频散特性及瞬态波形,发现了不同于其他研究的界面波的独特特性,这些研究结果为进一步基于界面波传播特性进行粘接界面特性的定量和定性超声检测与评价提供了理论依据以及检测和表征的手段。     

Elastic guided wave propagation in electrical cables

This article analyzes the propagation modes of ultrasound waves inside an electrical cable in order to assess its behavior as an acoustic transmission channel. A theoretical model for propagation of elastic waves in electric power cables is presented. The power cables are represented as viscoelastic-layered cylindrical structures with a copper core and a dielectric cover. The model equations then have been applied and numerically resolved for this and other known structures such as solid and hollow cylinders. The results are compared with available data from other models. Several experimental measures were carried out and were compared with results from the numerical simulations. Experimental and simulated results showed a significant difference between elastic wave attenuation inside standard versus bare, low-voltage power cables.     

SBP-SAT方法及其在波动领域的应用

基于分部求和(Summation By Parts)方法和同时逼近项(Simultaneous Approximation Terms)技术建立的有限差分方法,具有更高的精度和稳定性。同时在介质几何不连续、参数突变条件具有较大的优势。国内对SBP-SAT方法的相关研究目前较少,论文对该方法的研究背景,方法发展过程进行了介绍并基于SBP-SAT方法和弹性波动理论,结合初边值条件,推导出曲线网格条件下的弹性波动SBP-SAT离散方程。最后,通过数值模拟实现地震波传播过程,介绍该方法在地震数值模拟领域中的应用价值和前景。     

Interface profile optimization for planar stress wave attenuation in bi-layered plates

Stress waves scatter upon entering a new medium. This occurs due to the reflection and transmission of the waves, which depends on the impedance mismatch between the two materials and the angle of incidence. For a bi-layered structure with finite dimensions and constant impedance ratio, the scattering and intensity of the stress waves may be varied by changing the interface profile between the two layers. In this paper, a methodology is proposed for optimizing the interface profile between the layers of a finite bi-layered plate for the objective of planar stress wave attenuation. The bi-layered plates are subjected at one end to highly impulsive loadings with various durations, and the geometry of the internal interface is optimized for the purpose of minimizing the amplitude of the maximum reaction force at the opposite fixed end. The optimization methodology is based on a genetic algorithm, which is coupled with a finite element method for analyzing the wave propagation behavior of the plates. It is observed that the interface profile and the amount of stress wave attenuation depend on the duration of the applied impulsive loading, with higher amounts of attenuation obtained when the wavelength associated with the impulsive load is small compared to the dimensions of the bi-layered plates.     

Propagation of thickness-twist waves in an inhomogeneous piezoelectric plate with an imperfectly bonded interface

The propagation of thickness-twist waves in an inhomogeneous piezoelectric plate with an imperfectly bonded interface is investigated. Based on the spring-type relation, the imperfectly bonded interface is dealt with, and the exact solution is obtained from the equations of the linear theory of piezoelectricity. The amplitude ratio between the incident wave and the reflected wave, the displacement component and the stress component are all obtained and plotted. Both theoretical analysis and numerical examples show that the effect of the mechanical imperfection on the wave propagation is more evident than that of the electrical imperfection. When the incident wave frequency and the mechanical imperfect parameter meet some particular relation, no reflected waves can appear in the piezoelectric plate. The results are of fundamental importance to the design of resonators and other devices when imperfect joints are considered.     

On a simple wave approximation to a non-linear problem

Summary The range of validity of a simple wave approximation to a non-linear set of two dissipative wave equations has been studied. The non-linear set is, when the dissipative terms are omitted, totally exceptional. It describes the propagation of longitudinal waves in an ideal elastic bar with some viscous stress. Upon a non-linear transformation, the equations become linear. These linear equations have been studied first. The results for the non-linear equations are then easily obtained by transforming backwards. It turns out that, if the non-linearity is small enough, they are similar to those obtained for the linear equations.Detached by Philips Research Laboratories