水下目标弹性声散射信号分离

水下目标弹性声散射与其他声散射成分在时域和频域上均存在混叠, 现有信号处理方法受分辨力限制无法在混叠状态下识别目标弹性声散射特征. 针对这个问题, 提出了一种目标弹性声散射信号分离方法. 以目标回波亮点模型为基础, 分析了线性调频信号入射时目标声散射成分的信号特性, 提出了一种目标声散射成分向单频信号的映射方法, 并理论推导出了目标声散射结构与映射结果之间的线性对应关系, 实现了通过窄带滤波分离出目标弹性声散射成分. 仿真与消声水池实验数据处理结果表明, 该方法基本可以完全分离出目标回波信号中的弹性声散射成分, 分离出的弹性声散射具有与理论一致的信号特征, 验证了该分离方法的有效性.     

近周期超晶格中的声学声子及其光散射特性

利用转移矩阵方法计算了近周期超晶格体系（包括有限周期数的超晶格、耦合超晶格、层厚起伏和层厚渐变的超晶格）中声学声子的喇曼散射谱．结果表明上述近周期超晶格的光散射特性与理想严格周期的体系和完全无序的体系都不相同，呈现出许多独特的性质．对有限周期数的超晶格，由于边界的存在，喇曼谱中除了理想超晶格中存在的折叠声学声子峰（主峰）外，还会出现一系列等间距分布的卫星峰．对耦合超晶格体系，理想超晶格中存在的主峰将出现分裂．对层厚起伏变化的超晶格，主峰呈现非对称展宽，展宽主要出现在高波数端．计算结果和实验测得的谱线作了比 关键词：     

含空腔的功能梯度声学覆盖层水下吸声特性

针对传统的声学覆盖层吸声频带窄的问题,基于功能梯度材料的特点提出了一种含空腔结构的水下功能梯度声学覆盖层结构,引入梯度有限元法建立了功能梯度型声学覆盖层的水下声学计算模型,研究了功能梯度声学覆盖层结构的水下吸声特性。与传统的功能梯度结构声学建模方法相比,在保证计算精度和计算效率的基础上,文中所建立的功能梯度结构声学计算模型可适用于更复杂的功能梯度声学结构声学性能评估。研究结果表明,功能梯度声学覆盖层能够有效改善中高频段的吸声性能,获得较好的宽频吸声效果。此外,空腔形状采用锥型空腔结构或者组合型空腔结构可以有效地拓宽功能梯度声学覆盖层的吸声频带。     

内嵌不同形状散射子的局域共振型黏弹性覆盖层低频吸声性能研究

最近研究表明,将声子晶体中的局域共振现象引入到水下吸声材料的设计中,可以观察到由局域共振引起的低频声吸收现象.为了进一步揭示局域共振低频吸声机理并获得更优的水下低频声吸收性能,研究了内嵌不同形状散射子的黏弹性声学覆盖层.利用晶格和散射子在空间排布的对称性,传统有限元方法得到简化,从而节约了计算时间和存储空间,并通过将简化有限元法计算得到的结果与传统有限元法计算的结果进行对比,验证了简化有限元方法的正确性.结合位移云纹图,考察了特定频率点上振动模态与相应的局域共振吸声峰之间的关系,揭示了内嵌圆柱形散射子的黏弹性覆盖层的吸声机理.进一步讨论了相同体积下不同形状的圆柱形散射子对黏弹性覆盖层吸声性能的影响,给出了提升覆盖层低频吸声性能的优化思路.通过讨论不同芯体材质对内嵌圆柱形散射子的黏弹性覆盖层吸声性能的影响,找到了改变第一共振峰位置的方法,从而可以通过调整第一共振峰来实现特定频率范围内的宽带吸声.     

近岸4种鱼的声散射特征提取及融合实验研究*

为了有效地提取表征鱼类间差异的声散射特征参数,该文通过绳系法实验研究了近岸4种经济鱼类的声散射信号特征提取及融合方法。首先,通过自研双频鱼探仪采集花鲈、许氏平鲉、黑鲷和斑石鲷的个体鱼声散射信号;然后,分别测定200 kHz和450 kHz换能器下鱼体的目标强度,同时提取鱼声散射信号的时频域统计特征;最后,将降维后的时频特征与频差特征融合组成新的特征向量。该文通过实验验证了该方法的有效性,基于组合特征的支持向量机识别准确率达93%。结果表明,鱼的频率响应特性和鱼声散射信号的时频域统计特征能一定程度上反映鱼的固有属性,有效地增加判别依据能显著提高以上4种鱼类的识别准确率。     

基于主动声学超材料的圆柱声隐身斗篷设计研究

基于多层复合材料结构的二维声隐身斗篷设计思想, 利用主动隔膜声学空腔有效密度可以任意控制这一特性, 设计了主动声学超材料下的无限长圆柱声隐身斗篷. 给出了主动隔膜声学空腔单元的声电元件类比模拟电路图和具体的有效密度控制方法. 进行了主动声学超材料声隐身斗篷的结构建模, 并对平面入射波入射下此圆柱隐身斗篷周围声压分布场进行仿真计算. 结果表明, 平面波在一定频率范围内可以毫无阻碍地透过圆柱斗篷, 似乎不存在这种障碍物, 达到声隐身效果. 同时, 计算了主动声材料斗篷下总散射截面随频率变化曲线, 研究了此斗篷隐身效果随频率的变化特性. 本文从主动控制角度探讨实验实现隐身斗篷的技术问题, 有望给声隐身斗篷实验设计提供一条新的技术途径.     

柱状分层固体媒质的声散射

本文评述了各向同性和横向各向同性柱状分层固体声散射理论和实验研究进展 ,介绍了描述圆柱状界面薄层特性的弹簧模型 ,也讨论了该领域中有待进一步研究的一些问题。     

Research on the acoustic scattering function and coherence properties from rough seafloor based on finite element model

Acoustic scattering from a rough sea bottom is recognized as a main source of reverberation. In this study, scattering properties from a layered bottom were exploited based on the finite element model. The scattering strength and loss from the layered rough seabed were investigated by ensembling the realizations of rough interface. They were found to be dependent on the thickness of sediment, and interference was significant in the case of thin sediment. Through verification of the finite element model, the scattering loss could be evaluated using the Eckart model with a proper sound speed in the thick sediment. The multiple scattering effect on the sound field was also exploited. It revealed that the effect depended strongly on the bottom type.     

Numerical simulation of laser-generated surface acoustic waves in the transparent coating on a substrate by the finite element method

The optimum finite element model in the system consisting of a transparent coating and an opaque substrate is established based on the analysis of two important parameters: meshing size and time step, and the stability of solution. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field are obtained. According to the thermoelastic theory, this temperature gradient field can be taken as a buried bulk source to generate ultrasonic wave. The surface acoustic waves (SAWs) are obtained. The influence of the coating thickness on the SAWs is analyzed. The model provides a useful tool for the determination of modes which are generated by a laser source in transparent coating on opaque substrate. The surface skimming longitudinal wave exists for the multiple oscillations and it charges from unipolar waveforms to dipolar.     

激光辐照下旋转柱壳温度场的数值模拟

采用有限元方法数值模拟在连续激光辐照下旋转柱壳温度场的变化和分布情况,并分析了热性能参数对温度场造成的影响,同时还比较分析了不同旋转频率对柱壳温度场分布的影响。结果表明,激光作用下旋转柱壳的温升大大低于静止柱壳的温升, 外表面温度呈现出与旋转频率相符的周期性上升过程,而内表面温升由于热传导的原因在较小频率下才表现出这种周期性,当频率增大到一定值时,内表面温升不出现周期性的台阶而是曲线上升。     

Mechanical properties an undercut evaluated resonance test and of silicon nanobeams with by combining the dynamic finite element analysis

Mechanical properties of silicon nanobeams are of prime importance in nanoelectromechanical system applications. A numerical experimental method of determining resonant frequencies and Young＇s modulus of nanobeams by combining finite element analysis and frequency response tests based on an electrostatic excitation and visual detection by using a laser Doppler vibrometer is presented in this paper. Silicon nanobeam test structures are fabricated from silicon-oninsulator wafers by using a standard lithography and anisotropic wet etching release process, which inevitably generates the undercut of the nanobeam clamping. In conjunction with three-dimensional finite element numerical simulations incorporating the geometric undercut, dynamic resonance tests reveal that the undercut significantly reduces resonant frequencies of nanobeams due to the fact that it effectively increases the nanobeam length by a correct value △L, which is a key parameter that is correlated with deviations in the resonant frequencies predicted from the ideal Euler-Bernoulli beam theory and experimentally measured data. By using a least-square fit expression including △L, we finally extract Young＇s modulus from the measured resonance frequency versus effective length dependency and find that Young＇s modulus of a silicon nanobeam with 200-nm thickness is close to that of bulk silicon. This result supports that the finite size effect due to the surface effect does not play a role in the mechanical elastic behaviour of silicon nanobeams with thickness larger than 200 nm.     

Mechanical properties of silicon nanobeams with an undercut evaluated by combining the dynamic resonance test and finite element analysis

Mechanical properties of silicon nanobeams are of prime importance in nanoelectromechanical system applications.A numerical experimental method of determining resonant frequencies and Young’s modulus of nanobeams by combining finite element analysis and frequency response tests based on an electrostatic excitation and visual detection by using a laser Doppler vibrometer is presented in this paper.Silicon nanobeam test structures are fabricated from silicon-oninsulator wafers by using a standard lithography and anisotropic wet etching release process,which inevitably generates the undercut of the nanobeam clamping.In conjunction with three-dimensional finite element numerical simulations incorporating the geometric undercut,dynamic resonance tests reveal that the undercut significantly reduces resonant frequencies of nanobeams due to the fact that it effectively increases the nanobeam length by a correct value △L,which is a key parameter that is correlated with deviations in the resonant frequencies predicted from the ideal Euler-Bernoulli beam theory and experimentally measured data.By using a least-square fit expression including △L,we finally extract Young’s modulus from the measured resonance frequency versus effective length dependency and find that Young’s modulus of a silicon nanobeam with 200-nm thickness is close to that of bulk silicon.This result supports that the finite size effect due to the surface effect does not play a role in the mechanical elastic behaviour of silicon nanobeams with thickness larger than 200 nm.     

Response of thermal source in a transversely isotropic thermoelastic half-space with mass diffusion by finite element method

The two-dimensional problem of generalized thermoelastic diffusion material with thermal and diffusion relaxation times is investigated in the context of Lord-Shulman theory. As an application of the problem, a particular type of thermal source is considered and the problem is solved numerically by using a finite element method. The components of displacement, stress, temperature distribution, chemical potential, and mass concentration are obtained. The resulting quantities are depicted graphically for a special model. Appreciable effect of relaxation times is observed on various resulting quantities.     

Response of thermal source in a transversely isotropic thermoelastic half-space with mass diffusion by using a finite element method

The two-dimensional problem of generalized thermoelastic diffusion material with thermal and diffusion relaxation times is investigated in the context of the Lord-Shulman theory.As an application of the problem,a particular type of thermal source is considered and the problem is solved numerically by using a finite element method.The components of displacement,stress,temperature distribution,chemical potential,and mass concentration are obtained.The resulting quantities are depicted graphically for a special model.An appreciable effect of relaxation times is observed on various resulting quantities.     

Investigation on periodic cracking of elastic film/substrate system by the extended finite element method

Multiple cracking behavior in a thin elastic film bonded to a thick elastic substrate is investigated by the extended finite element method. Stress and stress intensity factor are obtained using a periodic finite element model for the cracked film/substrate system. The influences of various parameters including crack length, film thickness, periodic crack spacing, and relative stiffness of the substrate on the stress and stress intensity factor are discussed in detail. It is demonstrated that the effects of geometric parameters are more sensitive than that of material property. In particular, the crack spacing has a saturation value due to interactions of neighboring cracks and relief of tensile stress in the film. The film/substrate couple with multiple periodic cracks can exhibit a positive potential in improving the durability of the film/substrate system.     

对于具有大范围运动和非线性变形的柔性梁的有限元动力学建模

研究具有大范围运动和非线性变形的柔性梁的有限元动力学建模.采用有限元方法对梁结构进行离散,利用Lagrange方程建立系统的精确动力学方程.该方程不仅增加了新的表征纵向、横向、侧向弯曲变形,以及扭转变形的耦合项,同时包含了变形运动与大范围运动之间的相互耦合项.     

Numerical simulation of the EM scattering and Doppler spectrum of a low flying target above dynamic oceanic surface by using the FEM-DDM method

The Doppler (DP) spectrum of a moving target such as a ship, an iceberg or an air-plane above dynamic oceanic surfaces[1—3] is one of the most important subjects for ra-dar oceanic surveillance, target tracking and oceanic remote sensing[4—7]. With the ad-vancement of oceanic remote sensing and radar surveillance, the experimental observa-tion and theoretical modeling of oceanic clutter have been extensively studied, e.g. by using oceanic field measurement, wave tank experiment and some stud…