基于组合超表面的跨水空声波调控研究

现有声学超表面的研究多是针对空气或水等单一介质,有关跨介质声波调控的研究相对较少.文章主要基于组合超表面研究跨水空界面声波的调控特性.首先,利用遗传算法设计了可以调控波前的高透射空气声超表面.接着,通过复合离散型跨水空高透射超表面设计组合超表面实现了对跨水空界面声波的波前调控,并讨论了组合超表面间距对波控性能的影响.最后加工制作组合超表面试样,并对跨水空声波聚焦功能进行了实验验证.结果表明通过复合两种独立设计的超表面可以实现跨水空界面声波的异常折射和聚焦功能.组合超表面的间距会影响其对声波的调控效果.当间距较小时,组合超表面虽然可以实现对跨水空界面声波的调控,但是调控效果较差;随着间距的增加,组合超表面对跨水空声波的调控效果会迅速增强并趋于稳定.在实验中观察到了跨水空界面的声聚焦现象以及组合超表面间距对跨水空界面声波聚焦效果的影响,实验结果与仿真结果基本一致.研究工作为跨水空声学器件的设计提供了一定的数值和实验基础.     

声学/弹性相位梯度超表面设计: 原理、功能基元、可调和编码

声学/弹性超表面是一类亚波长厚度的二维超材料, 具有很强的声波/弹性波操控能力, 在声学成像、通信、隐身、伪装、振动/噪声控制、能量收集、无损检测等领域具有潜在的应用. 本文主要综述了声学/弹性相位梯度超表面的最新发展, 包括设计原理、功能基元设计、波场操控及其应用、可调超表面设计, 以及新兴的数字编码超表面. 最后, 展望了该领域未来的研究方向.      

弹性波斜入射声子晶体的传输特性

引入四维波矢的概念,推导出弹性波斜入射到多层介质系统的转移矩阵以及透射系数和反射系数公式.利用这些公式计算了横波和纵波斜入射一维声子晶体时横渡和纵波的透射系数,得出横波斜入射,透射波中的横波和纵波在一定频率范围内都会出现禁带,纵波斜入射时也有类似的结果.透射波中横波与纵波的相互转型随着入射角的增大而越加明显.     

橡胶弹簧非线性刚度的有限元解

本文探讨了用双重非线性有限元手段，求解橡胶弹簧的非线性刚度。橡胶的本构关系采用了广义虎克定律和基于应变能的超弹性材料的本构关系联合使用的方案，用单元内的静水压力ｐ，来修正广义虎克定律的弹性常数Ｅ和Ｇ。还提出了带刚性域的超单元和波前凝聚法两种新的办法，获得橡胶弹簧的刚度，使内存用量和ＣＰＵ时间大幅度下降。因而几何、物理双重非线性橡胶弹簧的计算，能在微机上实现。文内选用英国ＤＵＮＬＯＰ橡胶公司橡胶元件     

基于Lord-Shulman理论的热弹性波在偶应力三明治固体中的反射和透射

在温度急剧变化、短时间极速加热等极端情况下，基于Fourier定律的热流矢量与温度梯度成正比关系的经典热传导理论不能准确描述其物理过程．经典热弹性理论的热传导方程是抛物型的，而广义热弹性理论包含双曲型方程，热将以具有有限传播速度的波动形式传播．本文基于Lord-Shulman广义热弹性理论和修正偶应力弹性理论，得到在偶应力热弹性固体中四种色散波，研究热弹性波的传播和在偶应力固体三明治结构中的反射透射问题，重点研究横波入射时偶应力参数和热弛豫时间对各种热弹性耦合波反射透射系数的影响．     

SH0导波在粘接结构中传播时的相位变化1）

对粘接结构进行超声导波无损检测与评估是一个有挑战性的前沿性课题.针对此问题,研究了SH₀导波在界面为理想连接的三层板状粘接结构中传播时的相位变化情况.首先基于波传播的控制方程,建立了粘接结构中反射和透射SH₀导波相对于入射SH₀导波的相位差解析模型.然后利用数值模拟计算了铝/环氧树脂/铝粘接结构中反射和透射SH₀导波的相位差曲线.最后分析了入射角度和频厚积的变化对反射和透射SH₀导波相位差的影响.结果表明,对于具体的粘接结构,反射和透射SH₀导波在其中传播时的相位差变化主要依赖于入射角度、频率等参数.在特定的频厚积下,当声波水平入射时,反射和入射SH₀导波同相位.无论入射角度为多大,随着频厚积的增大,反射SH₀导波的相位差曲线均会产生周期性谐振.对于透射SH₀导波,当声波垂直入射时,其相位差曲线的改变无规律可循;但是随着入射角度逐渐增大,透射SH₀导波的相位差曲线逐渐变规则.所得结果可为实验时研究板状粘接结构中SH₀导波的传播特性以及提取SH₀导波回波中的有用信息和定位提供一定的理论指导.     

波涡相互作用研究的某些进展(Ⅰ)

波涡相互作用是当代非定常非线性流体力学的一个重要领域.本文评述了迄今人们对微扰纵波(声波,不包括激波)和横波(涡波)与层状和轴状涡相互作用的研究进展,包括涡声理论、锐缘及光滑表面涡层分离以及自由涡层的2维感受性和非定常Kutta条件,波产生时均涡流的非线性Eulcr整流和广义Lagrange整流,波涡共振等课题的进展.还介绍了用非定常激发实施涡控制这一重要应用领域的一些新进展.      

声波超材料设计的力学原理与进展

声波超材料是一种特殊的人工复合材料,它基于连续介质力学理论描述,通过对微观结构的精心设计,在宏观上获得反常规的动态材料属性,如负动态质量,负动态模量,负折射率等.超材料的出现为实现波动控制提供了可能,相关研究在工程领域具有潜在的应用价值.本文着重阐述了弹性固体超材料、声波超材料、超表面、主动超材料的设计原理与相关研究进展, 所述原理对于波控结构的设计与制备实现具有指导意义.     

基于Lamb波频散特性的薄板声发射源定位方法研究

声发射源时差定位方法中波速的确定是定位准确的关键问题,根据模态声发射理论,声发射信号在薄板内的传播过程中具有频散现象和多模态特性.不同频率不同模态的声波其传播速度不同.基于Lamb波频散特性和声发射检测技术,采取时频联合分析方法对铝薄板中声波模式进行识别;利用低频段频散不明显的扩展波的波速和同一频率同一模态波到达两传感器的时间差来实现声发射源的准确定位;通过铝薄板中AE源定位实验表明,采用S0模式,即扩展波的波速进行定位计算,可以比较准确地确定声发射源的位置,而且理论值与实际值相差很小.     

无限长圆柱体中广义电磁热弹耦合问题研究

基于L-S广义热弹性理论,研究了处于磁场中无限长理想圆柱导体在边界受热冲击作用时的电磁热弹耦合问题.建立了广义电磁热弹耦合的有限元方程,为避免积分变换方法求解带来的精度丟失.采用将有限元方程直接在时间域求解的方法,得到了圆柱体中的温度、位移、应力、感应磁场和感应电场的分布规律,反映了热的波动性及电磁热弹的耦合效应.结果表明,将有限元方程直接在时间域求解,可以获得各物理量的准确分布.得到温度在热波波前处的阶跃,准确地反应热波的波动效应.     

Focusing and Scattering of a Nonstationary Wave by the Free Surface of an Anisotropic Elastic Medium

The interaction of a plane discontinuous wave with the free curvilinear surface of an anisotropic elastic medium is studied. The ray-path method is used to describe the wavefront. The nonlinear Snell's equations are solved by combining Newton's method and the parameter continuation algorithm. The cases of focusing and scattering of wave rays are studied     

Asymptotic modeling of Helmholtz resonators including thermoviscous effects

We systematically employ the method of matched asymptotic expansions to model Helmholtz resonators, with thermoviscous effects incorporated starting from first principles and with the lumped parameters characterizing the neck and cavity geometries precisely defined and provided explicitly for a wide range of geometries. With an eye towards modeling acoustic metasurfaces, we consider resonators embedded in a rigid surface, each resonator consisting of an arbitrarily shaped cavity connected to the external half-space by a small cylindrical neck. The bulk of the analysis is devoted to the problem where a single resonator is subjected to a normally incident plane wave; the model is then extended using “Foldy’s method” to the case of multiple resonators subjected to an arbitrary incident field. As an illustration, we derive critical-coupling conditions for optimal and perfect absorption by a single resonator and a model metasurface, respectively.     

A novel design scheme for acoustic cloaking of complex shape based on region partitioning and multi-origin coordinate transformation

Acoustic cloaking is an important application of acoustic metamaterials. This article proposes a novel design scheme for acoustic cloaking based on the region partitioning and multi-origin coordinate transformation. The cloaked region is partitioned into multiple narrow strips. For each strip, a local coordinate system is established with the local origin located at the strip center, and a coordinate transformation in the local coordinate system is conducted to squeeze the material along the strip length direction to form the cloaked region. To facilitate the implementation of the acoustic cloak, the multilayer effective medium is used to approximate the non-uniform anisotropic material parameters. The effectiveness of the proposed coordinate transformation method is verified by comparing the results from our method with those in the literature. Firstly, the results of a circular acoustic cloak in the literature are reproduced by using our finite element (FE) simulations for validation. Then, a comparison is made between the traditional coordinate transformation scheme and our new scheme for simulating an elliptical acoustic cloak. The results indicate that the proposed multi-origin coordinate transformation method has a better cloaking effect on the incident wave along the ellipse minor axis direction than the traditional method. This means that for the same object, an appropriate transformation scheme can be selected for different incident wave directions to achieve the optimal control effect. The validated scheme is further used to design an arch-shaped cloak composed of an upper semicircular area and a lower rectangular area, by combining the traditional single-centered coordinate transformation method for the semicircular area and the proposed multi-origin method for the rectangular area. The results show that the designed cloak can effectively control the wave propagation with significantly reduced acoustic pressure level. This work provides a flexible acoustic cloak design method applicable for arbitrary shapes and different wave incident directions, enriching the theory of acoustic cloaking based on coordinate transformation.     

Design of elliptical underwater acoustic cloak with truss-latticed pentamode materials

Pentamode acoustic cloak is promising for underwater sound control due to its solid nature and broadband efficiency, however its realization is only limited to simple cylindrical shape. In this work, we established a set of techniques for the microstructure design of elliptical pentamode acoustic cloak based on truss lattice model, including the inverse design of unit cell and algorithms for latticed cloak assembly. The designed cloak was numerically validated by the well wave concealing performance. The work proves that more general pentamode acoustic wave devices beyond simple cylindrical geometry are theoretically feasible, and sheds light on more practical design for waterborne sound manipulation.     

环形激波聚焦流场特性的数值研究

针对环形激波聚焦过程产生的高温、高压特性,采用间断有限元方法模拟了环形激波在同轴圆柱 形激波管内的聚焦流场特性。计算结果表明,采用间断有限元方法能够有效地捕捉激波聚焦过程形成的二次 激波、涡环、三波交点和球面双马赫反射等主要流动特征。此外,通过改变环形管道内外半径对聚焦流场进行 模拟发现,环形管道外径对中心轴线上聚焦峰值压力的大小和位置影响较小,环形管道内径对中心轴线上聚 焦峰值压力的大小和位置影响较大。计算结果可以为工程应用提供一定的理论指导。     

Acoustic radiation force on a rigid cylinder in an off-axis Gaussian beam near an impedance boundary

The exact equations of the axial and transverse acoustic radiation force functions of a Gaussian beam arbitrarily incident on an infinite rigid cylinder close to an impedance boundary and immersed in an ideal fluid are deduced by expressing the incident wave, the scattering wave and the boundary reflected wave in terms of the cylindrical wave function. The effects of the beam waist, the sound reflection coefficient, the cylinder position and the distance from the impedance boundary on the acoustic radiation force are studied using numerical simulations. The simulation results show that the amplitude of the acoustic radiation force function increases with beam width. Moreover, the values of the acoustic radiation force in both the axial and transverse directions reach those of a plane wave when the beam width is considerably larger than the wavelength of the Gaussian beam. The properties of the impedance boundary and the position of the cylinder in the Gaussian beam have a considerable effect on the magnitude and direction of the force. The simulation results, particularly in the case of a transverse force, indicate the presence of a negative acoustic radiation force that is related to the nondimensional frequency and position of the cylinder in the Gaussian beam.     

Design and analysis of nonlinear-transformation-based broadband cloaking for acoustic wave propagation

A coordinate-transformation method can be used to design invisibility cloaks for many types of waves, including acoustic waves. The traditional method for designing a cloak depends on a transformation from a virtual space to a physical space. Previous acoustic cloaks that are mainly designed with linear-transformation-based acoustics have drawbacks that acoustic wave trajectories in the cloaks cannot be controlled and tuned. This work uses a nonlinear mapping from a ray trajectory perspective to construct acoustic cloaks with tunable non-singular material properties. Use of a ray trajectory equation is a straightforward and alternate way to study propagation characteristics of different types of waves, which allows more flexibility in controlling the waves. A broadband cylindrical cloak for acoustic waves in an inviscid fluid is realized with layered non-singular, homogeneous, and isotropic materials based on a nonlinear transformation. Some advantages and improvements of the invisibility nonlinear-transformation cloak over a traditional linear-transformation cloak are analyzed. The invisibility capability of the nonlinear-transformation cloak can be tuned by adjusting a design parameter that is shown to have influence on the acoustic wave energy flowing into the region inside the cloak. Numerical examples show that the nonlinear-transformation cloak is more effective for making a domain undetectable by acoustic waves in an inviscid fluid and shielding acoustic waves from outside the cloak than the linear-transformation cloak in a broad frequency range. The methodology developed here can be used to design nonlinear-transformation cloaks for other types of waves.