半空间内二维圆柱声散射理论解析解

采用理论解析方法研究半空间内二维非紧致圆柱的声散射。齐次Helmholtz方程的解在柱坐标系内用自由空间格林函数的级数展开式表示。基于镜像源方法,利用刚性半空间边界反射圆柱散射声波来解决半空间边界和圆柱之间的多重散射。结合等效源原理,处理半空间边界质量型阻抗特性和刚度型阻抗特性对声传播的影响,推导单位强度简谐单极子点声源产生声场的理论表达式。总声场可以表示为四个分量的总和:入射声场、反射波以及圆柱和镜像圆柱的散射声场。采用边界积分方法对声散射进行计算,以验证理论公式的正确性。点声源模型的理论解析值与边界积分方法数值解在研究的波数和观察点角度范围内一致。     

二维圆柱对旋转单极子点源的声散射

采用理论分析和数值计算方法研究二维刚性圆柱对旋转单极子点源入射声波的散射效应。齐次Helmholtz方程基本解在极坐标系内用自由空间格林函数的级数展开式表示。对任意形状的刚性散射边界,通过求解线性声学波动方程建立声场边界积分表达式。数值解可在频域下采用边界元方法获取。进一步利用贝塞尔函数的加法定理,分别推导点源与圆柱同轴、异轴旋转状态下的声场理论解析表达式。边界元方法的数值解与理论解析解吻合一致。点源向外辐射声波的频率由点源谐振频率、旋转频率和谐波阶次共同决定。点源与圆柱同轴旋转时,谐波阶次为0 的声波以同心圆状向外辐射,不为0 的声波则以螺旋状向外辐射,螺旋瓣的数量由谐波阶次的绝对值相同。点源与圆柱异轴旋转时,圆柱的声散射呈现显著的偶极子特征,且声场的指向性复杂。     

充水透声结构声目标强度预报方法研究

充水透声结构是水下航行体重要的结构类型之一,主动声呐作用于目标时,声波可以透射充水透声结构外表面进入结构内部,在充水透声壳体结构内表面产生内部散射,从而影响目标的目标强度.文章提出了一种适用于透声结构目标强度预报的考虑透声的修正板块元方法,此方法假设散射声场近似为内表面板块散射声场和与之对应的外表面板块散射声场叠加,基...     

一种基于流固耦合问题的低频散射声场预报方法EI北大核心CSCD

提出了一种流固耦合作用下的低频散射声场预报方法。从边界积分方程出发,推导弹性散射中流固耦合方程,通过引入附加质量、附加阻尼、附加压力概念实现解耦。将弹性散射表述为纯刚性散射项与二次辐射项的叠加,建立了刚性散射与弹性散射的联系。与辐射问题不同,散射中流体对结构的作用不仅表现为附加质量和附加阻尼,还存在一个与结构响应无关的压力项,且该压力项是二次辐射项的激励源。采用Fortran编写了边界元算法程序,用DMAP语言实现与Nastran的对接,形成了完整的散射声场预报方法,通过与理论结果对比,验证了预报方法的正确性。圆柱壳散射的计算结果表明:低频散射时弹性不可忽略;圆柱壳厚度对弹性散射强度和指向性有明显影响;肋骨对柱壳散射的影响与振动形式有关,环肋骨对以弯曲模态为主的弹性散射影响很小。     

复杂声场环境下目标声源声场重建误差分析

针对目标声源在复杂声场环境下进行声场重建易受到其它声源干扰的问题,提出存在干扰源声场、散射声场及目标声源声场的混合声场环境下基于单面声场分离技术的目标声源声场分离及重建方法。首先,通过理论分析和公式推导,清除干扰源声场及散射声场的影响;然后,利用传递函数性质对声场分离及重建误差进行分析,并对计算公式中存在的奇异性问题予以消除,得到高分辨率的声场重建图像。数值仿真表明:利用常规的单面声场分离技术很难重建混合声场环境下目标声源的声场,而利用本文研究的声场分离及重建方法能够很好反映目标声源声场的实际情况,提高声场重建的精度。     

浅海海底模型对低频声传播的影响

海底参数特性是影响浅海低频水声传播的重要因素。理论上需要知道海底所有深度的参数特性才能确定水声传播,这在实践中很不现实。如果在误差的允许范围内只需要知道某个深度以上的海底参数特性,而这个深度以下的海底参数特性可以忽略,这个深度就定义为海底最大深度。低频声波能够穿透海底更深,最大深度能够达到几十米深,对比较高频率只有几米深。海水声速剖面也能影响声波穿透海底深度,声速剖面为负梯度时海底最大深度比等声速要深。在低频声传播过程中,海底横波对声场影响也很大,尤其是在超低频100Hz以下,2002年5月的南海试验也证实了这一点。     

基于水平集的二维半空间声屏障优化设计

针对二维半空间声散射问题，采用水平集方法对声屏障截面进行优化设计。以Γ 型声屏障为例，给定水平集函数初始配置后，通过求解反应扩散方程来更新结构边界。目标函数设定为观测区域上声压的积分，并由伴随变量法计算出拓扑导数作为优化的梯度。声场与伴随场均采用边界元法进行求解，同时将半空间格林函数作为基本解，以模拟地面对声波的反射效果。在每次迭代中对零水平集基于结构面积进行过滤，仅保留主体部分作为声屏障的截面外形，实现基于拓扑优化的高效形状优化处理。数值算例给出过滤前后的优化结果，并利用商业软件COMSOL进行验证。     

平顶有肋柱壳低频散射特性研究

应用边界元(BEM)和有限元(FEM)耦合方法对平顶有肋柱壳在声波轴向入射时的低频反向散射特性和振动位移状态进行了计算,对结果作了分析。并且实验测量了与计算相同的三个目标的低频反向散射特性,同计算结果相比较,结果基本一致,理论和实验结果均证明了肋骨存在对回波强度的影响,改变了原有振动系统的振动分布,使得散射谐振发生变化。     

球形障板对矢量水听器接收性能的影响

矢量水听器可以同步共点测量声场空间一点处的声压和质点振速的各个分量,因此其被广泛应用于水声测量、探测、通信等领域。矢量水听器在水声测量的应用中会受到安装载体散射声场的影响,而这种载体可以看作为障板,由于障板的存在使散射声场的分布发生改变,以及矢量水听器的接收性能下降。仅以绝对硬边界为例,对球形障板的散射声场进行了理论计算和仿真分析。并且对带障板条件下矢量水听器的接收指向性的变化进行了仿真研究。结果显示,从散射声场分布图可以直观看出声场发生的变化,矢量水听器的接收性能变化与障板的尺寸、与障板距离远近、声波发射频率等因素有关。     

应用边界元法模拟厅堂中座席低谷效应

座席低谷效应是坐席中声波为掠入射时声场在低频段出现的选择性吸收的现象，它会影响厅堂中低频声的听感。文章用边界元法模拟了坐席低谷效应，考查影响座席低谷效应的因素。计算结果表明了坐席低谷效应在座席不同位置处的变化情况。最后列举了一些降低座席低谷效应的办法。     

A numerical reconstruction method in inverse elastic scattering

In this paper a new numerical method for the shape reconstruction of obstacles in elastic scattering is proposed. Initially, the direct scattering problem for a rigid body and the mathematical setting for the corresponding inverse one are presented. Inverse uniqueness issues for the general case of mixed boundary conditions on the boundary of our obstacle, which are valid for a rigid body as well are established. The inversion algorithm based on the factorization method is presented into a suitable form and a new numerical scheme for the reconstruction of the shape of the scatterer, using far-field measurements, is given. In particular, an efficient Tikhonov parameter choice technique, called Improved Maximum Product Criterion (IMPC) and its linchpin within the framework of the factorization method is exploited. Our regularization parameter is computed via a fast iterative algorithm which requires no a priori knowledge of the noise level in the far-field data. Finally, the effectiveness of IMPC is illustrated with various numerical examples involving a kite, an acorn, and a peanut-shaped object.     

Absorbing boundary condition for floating two-dimensional objects in current and waves

In this paper an absorbing boundary condition for floating two-dimensional objects in current and waves is studied. A numerical algorithm has been developed, which computes the velocity potential in the physical time domain, by using an artificial boundary to split the infinite fluid domain into a computational part and a residual part. A special Green's function has been developed in the residual part. The condition on the artificial boundary is independent of wave frequency, hence not restricted to harmonic waves. Because of the smaller computational domain and the independence of frequency, the time to compute the hydrodynamic coefficients of floating objects decreases.     

Noniterative two-dimensional phase-retrieval method from two Fourier intensities by use of an exponential filter

A noniterative method of retrieving the two dimensional phase of a wave field from two intensity measurements is proposed. In the measurements, one records two far-field intensities of the wave field modulated and unmodulated with an exponential filter. The phase retrieval method is based on the solution of the simultaneous equations with unknown coefficients of the two-dimensional discrete Fourier transform for the phase. Then there is no need for the information about the wave field, which is used in iterative phase-retrieval methods. The usefulness of this method is shown in computer-simulated examples of the reconstruction of two-dimensional complex amplitude objects.     

A singular-value method for reconstruction of nonradial and lossy objects

Efficient inverse scattering algorithms for nonradial lossy objects are presented using singular-value decomposition to form reduced-rank representations of the scattering operator. These algorithms extend eigenfunction methods that are not applicable to nonradial lossy scattering objects because the scattering operators for these objects do not have orthonormal eigenfunction decompositions. A method of local reconstruction by segregation of scattering contributions from different local regions is also presented. Scattering from each region is isolated by forming a reduced-rank representation of the scattering operator that has domain and range spaces comprised of far-field patterns with retransmitted fields that focus on the local region. Methods for the estimation of the boundary, average sound speed, and average attenuation slope of the scattering object are also given. These methods yielded approximations of scattering objects that were sufficiently accurate to allow residual variations to be reconstructed in a single iteration. Calculated scattering from a lossy elliptical object with a random background, internal features, and white noise is used to evaluate the proposed methods. Local reconstruction yielded images with spatial resolution that is finer than a half wavelength of the center frequency and reproduces sound speed and attenuation slope with relative root-mean-square errors of 1.09% and 11.45%, respectively.     

Microwave imaging: Reconstructions from experimental data using conjugate gradient and enhancement by edge-preserving regularization

A method for reconstructing the complex permittivity profile of lossy dielectric objects from measured scattered far-field data is presented. An iterative reconstruction algorithm based on a conjugate gradient method is derived from an integral representation of the electric field and applying a moment method solution. Results obtained from experimental data are shown for both known and unknown targets. Then, a new regularization procedure is developed to enhance the quality of the reconstruction. This method, based on a Markov random fields approach, models the object to be reconstructed by homogeneous areas separated by borderlike discontinuities. Finally, the enhancement is shown by the reconstruction of a polystyrene square cylinder. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 337–342, 1997     

Dynamically adaptive mesh refinement technique for image reconstruction in optical tomography

A novel adaptive mesh technique is introduced for problems of image reconstruction in luminescence optical tomography. A dynamical adaptation of the three-dimensional scheme based on the finite-volume formulation reduces computational time and balances the ill-posed nature of the inverse problem. The arbitrary shape of the bounding surface is handled by an additional refinement of computational cells on the boundary. Dynamical shrinking of the search volume is introduced to improve computational performance and accuracy while locating the luminescence target. Light propagation in the medium is modeled by the telegraph equation, and the image-reconstruction algorithm is derived from the Fredholm integral equation of the first kind. Stability and computational efficiency of the introduced method are demonstrated for image reconstruction of one and two spherical luminescent objects embedded within a breastlike tissue phantom. Experimental measurements are simulated by the solution of the forward problem on a grid of 5x5 light guides attached to the surface of the phantom.     

Time-resolved Fourier optical diffuse tomography.

Time-resolved Fourier optical diffuse tomography is a novel approach for imaging of objects in a highly scattering turbid medium with use of an incident (near) plane wave. The theory of the propagation of spatial Fourier components of the scattered wave field is presented, along with a fast algorithm for three-dimensional reconstruction in a parallel planar geometry. Examples of successful reconstructions of simulated hidden absorptive or scattering objects embedded inside a human-tissue-like semi-infinite turbid medium are provided.     

Conoscopic holography: toward three-dimensional reconstructions of opaque objects

Conoscopic holography is an interferometric technique that permits the recording of three-dimensional objects. A two-step scheme is presented to recover an opaque object's shape from its conoscopic hologram, consisting of a reconstruction algorithm to give a first estimate of the shape and an iterative restoration procedure that uses the object's support information to make the reconstruction more robust. The existence, uniqueness, and stability of the solution, as well as the convergence of the restoration algorithm, are studied. A preliminary experimental result is presented.     

Numerical algorithm for defect reconstruction in elastic media with a circular ultrasonic scanning

A numerical reconstruction method is proposed, which is applied to image identification of defects detected in elastic solid samples, in the case when a circular Ultrasonic scanning provides a measurement of the scattering pattern over full interval of the incident polar angle. The problem is first formulated as a system of respective boundary integral equations whose solution is used to calculate the far-field scattering diagram. Then the inverse reconstruction problem is reduced to a minimization of a certain strongly nonlinear functional. The proposed numerical algorithm is tested on some examples of volumetric flaw. It is also evaluated the influence of the error in the input data on precision of the reconstruction.     

基于小波分解的水中电爆炸冲击波波形重建方法研究

水中金属丝电爆炸产生的冲击波,上升时间仅有数十纳秒,脉冲宽度仅为十几微秒,远小于化学炸药产生的冲击波,采用现有传感器对其进行精确测量非常困难。分析了冲击波波形形成过程,基于帕塞瓦尔时频域能量守恒定律,采用多尺度小波分解的方法,给出了一种冲击波波形重建方法。采用该方法对PCB138传感器实测的压力信号进行了重建,并与Müller-plate针式压力传感器得到的波形进行了比对。结果表明:重建后的信号更加接近真实波形,基于多尺度小波分解的波形重建算法,较基于傅立叶变换的重建算法,稳定性更好,准确度更高。