圆弧形超表面对透射声波的可调控制与功能转换

基于“螺丝-螺母”的工作原理, 设计了可调的透射型三通道螺旋单元,通过调节螺丝的旋拧深度来改变声通道的长度, 从而实现对透射声波相位的调节.利用有限元方法计算了单元的透射波相位差和透射系数随频率和旋拧深度的变化规律.在平面广义Snell定律基础上推导了适用于圆弧形曲面的广义Snell定律.设计了圆弧形超表面, 包括弧状和圆环状两种, 实现了对透射声波波前的可调控制.根据所要实现的声学功能和给定的工作频率,利用单元的透射波相位差随旋拧深度的变化规律和圆弧形表面的广义Snell定律,确定超表面上所需的相位分布梯度及每个单元的旋拧深度,并同时考虑透射系数随旋拧深度的变化规律来对单胞旋拧深度进行适当的调整,以保证超表面具有较高的透射率.利用圆弧形超表面实现了宽频范围内声波的定向折射、波束分离和声束聚焦等声学功能的转换;利用圆环形超表面则实现了三向分波、波场螺旋化及源位置虚拟移动等声学功能的转换.同时针对上述功能进行了全波场的有限元数值模拟和相应的声学实验,实验结果与有限元模拟结果吻合良好, 验证了所设计超表面对声波波前调控的有效性.研究结果将为不规则非平面可调声学器件的设计提供理论指导.      

低频弹性波超材料的若干进展

超材料是一类新兴的具有超常物理性质的人造周期/拟周期材料, 能够改变电磁波、声波以及弹性波等在介质中的传播特性. 因在航天、国防以及民用科学等方面的巨大应用潜力, 超材料自被提出后便受到极大的关注并引发研究热潮. 弹性波超材料是超材料的一种, 能够基于弹性波与超材料结构的相互耦合作用实现对弹性波的操控. 带隙是评估弹性波超材料实现弹性波操控的重要工具, 其性质与超材料的材料参数、晶格常数以及局域振子的固有频率相关. 受制于超材料的承载能力、外观尺寸以及局域振子结构等因素, 利用传统超材料开启低频(约100 Hz)弹性波带隙依然存在较大困难. 文章首先简要介绍超材料开启弹性波带隙的基本原理, 然后从低频弹性波超材料基本结构与低频带隙实现方法、低频带隙优化与调控策略、低频带隙潜在应用等三个方面详细总结低频弹性波超材料的研究工作. 其中, 低频带隙超材料的基本结构主要包括布拉格散射型超材料、传统局域共振型超材料以及准零刚度局域共振超材料. 文章通过总结低频弹性波超材料的研究进展, 分析了目前研究中的不足并对未来低频弹性波的研究方向进行了展望.      

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.     

超细长弹性杆动力学仿真的曲面处理算法

超细长弹性杆动力学研究在DNA的平衡、稳定性等问题的研究中有重要的应用。为了便于超细长弹性杆动力学研究中数值结果图形后处理以及研究表面接触等问题的需要,需要建立弹性杆的表面模型和相应算法。本文利用Kirchhoff弹性杆模型的动力学比拟技巧,建立了描述超细长弹性杆曲面的常微分/积分方程组,利用Adames方法和递推方法设计了方程的数值解法,并给出了超长弹性杆的数值仿真结果的图形处理的计算实例。     

弹性波传播理论一些问题的研究现状和展望

本文简要地阐述了弹性波传播的一些基本理论,如位移势,表现定理及特征线理论等,并评述了两个领域内的问题。一为无限介质,半无限介质及层状介质中波的传播问题,重点在于地震波传播问题的解法。另一个领域为弹性波的绕射和散射。这一问题在动断裂、无损探伤及地震学中有重要的应用。评迷的重点是解题方法。 对本学科将来的发展也作了展望。认为反问题,随机波理论和在新技术领域中的应用是发展方向。在不少工程领域中的应用将越来越多地不能局限在弹性波范围内     

弹性波超材料板中拓扑交界态与缺陷保护的主动控制研究

在波动周期结构中,六边形单胞是常见的具有动态拓扑性质的重复单元。本文设计了具有主动控制特性的六边形单胞,通过粘贴压电片并连接负电容电路对材料参数进行调控,实现了弯曲波拓扑交界态传输与缺陷保护特性的主动控制,并发现了该保护行为具有方向选择特性。上述工作将物理学与声学中拓扑态概念引入到弹性波超材料板中,从而实现了其中弯曲波拓扑传输的主动控制功能。     

考虑阻尼的无限长小垂度缆索弹性波的传播

缆索结构被广泛应用于电气、土木、海洋和航空工程等领域,随着缆索在工程中的应用长度越来越长,高阶振动越来越明显,研究时应该考虑扰动沿着缆索的传播.现有对缆索弹性波传播的研究中,通常不考虑阻尼项,然而阻尼对于波的传播有着重要影响.文章考虑阻尼的影响,发展了包含阻尼项的三维弹性缆索运动方程.通过求解上述含阻尼项的运动方程,分别考察了面内面外弹性波的频率关系、相速度和群速度等自由传播特性,进而通过计算无限长弹性缆索在初始余弦型脉冲作用下的位移响应,分析扰动沿着该缆索的传播规律,考察波的色散现象以及阻尼对于缆索弹性波传播的影响.结果表明,考虑阻尼后,面内波和面外波均为色散波,面内波在曲率的作用下,为高度色散波.此外,在阻尼的影响下,波的峰值在传播过程不断减小,且波的后缘端点响应总是高于前缘端点响应.     

Plotnikov——Toland板模型中水弹性孤立波的迎撞

通过奇异摄动方法研究了在薄冰层覆盖的不可压缩理想流体表面上传播的两个水弹性孤立波之间的迎面碰撞.借助特殊的 Cosserat 超弹性壳 理论以及Kirchhoff--Love 板理论,冰层由 Plotnikov--Toland板模型描述.流体运动采用浅水假设和Boussinesq 近似. 应用Poincaré--Lighthill--Kuo 方法进行坐标变形,进而渐近求解控制方程及边界条件, 给出了三阶解的显式表达. 可以观察到碰撞后的孤立波不会改变它们的形状和振幅. 波浪轮廓在碰撞之前是对称的, 而在碰撞之后变成不对称的并且在波传播方向上向后倾斜. 弹性板和流体表面张力减小了波幅. 图示比 较了本文与已有结果可知线性板模型可作为本文的一个特例.      

弹性波与力学超材料设计与应用专题序

超材料是一类由人工微结构单元构筑的复合材料, 具有天然材料所不具备的超常物理性质, 经过多年发展, 超材料已经从电磁领域逐渐拓展到声学、力学、热学、材料学等诸多学科领域, 国内外学者在概念与内涵、理论与设计、制备与应用等各方面都开展了深入探究与应用研究, 产生了深远影响. 相较于传统均质材料的半经验式研究, 超材料从微结构基元精确设计出发, 是未来先进功能材料的创新研究新范式. 弹性波与力学超材料是整个超材料家族的重要组成部分, 近年来受到国内外学者的广泛关注, 已经在结构设计、数学模型建立以及应用等方面取得了丰硕的研究成果.      

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.     

基于扭曲Kagome点阵结构的一模材料设计

零能模式超材料指弹性矩阵的特征值中有若干为零的弹性材料,根据零特征值的个数可将其分类为一模至五模材料。当前,针对五模材料已有较深入研究,并在水声和弹性波调控方面获得重要应用,而对其他类型零能模式材料的研究尚未展开。本文对扭曲Kagome周期桁架这样一类欠约束点阵材料的有效弹性性质进行了研究,结果表明通过调节点阵材料的微观几何构型和杆件刚度,该类结构能够涵盖一系列一模材料谱系。针对给定一模弹性张量,发展了软-硬模式分离的微结构逆向优化设计策略。通过特定一模材料中的波传播现象对有效性质预测和微结构设计进行了数值验证。     

Particle manipulation via integration of electroosmotic flow of power-law fluids with standing surface acoustic waves (SSAW)

Standing surface acoustic wave (SSAW) based microfluidic devices have shown great promise toward fluid and particle manipulation applications in medicine, chemistry, and biotechnology. In this article, we present an analytical model for investigating continuous manipulation of particles (both synthetic and biological) within electroosmotic flow of non-Newtonian bio-fluids in a microfluidic channel under the influence of standing surface acoustic waves (SSAW). The particles are injected along the center of channel into the electroosmotically driven flow of power-law fluids, wherein their transport through the SSAW region is dictated by the hydrodynamic, electrophoretic, and acoustic forces. We first present a mathematical model to analyze the characteristics of electroosmotic flow of non-Newtonian power-law fluids in a hydrophobic slit microchannel. Next, we investigate the trajectories of particles in the flow field due to the combined effect of electroosmotic, electrophoretic, and acoustophoretic forcing mechanisms. The effect of key parameters such as particle size, their physical properties, input power, flow rate, and flow behavior index on the particle trajectories is examined while including the effect of the channel walls. The presented model delineates the methodologies of improving SSAW-based particle separation technology by considering the fluid rheology as well as the surface properties of the channel walls. Therefore, we believe that this model can serve as an efficient tool for device design and quick optimizations to explore novel applications concerning the integration of electroosmotic flows with acoustofluidic technologies.     

Pentamode material for underwater acoustic wave control

五模材料是一种具有固体特征的复杂流体,可通过超材料技术由固体材料经过微结构精心设计近似得到.可调的模量各向异性和固体特征赋予五模材料优越的水声调控能力,在降低水下物体目标强度等领域有着重要潜在应用,因此受到了国内外工程和学术界关注.本文就五模材料基本概念、微结构设计、声波调控、加工制备等方面对该类材料的研究进展进行详细介绍,并对五模材料在工程中应用存在的问题进行了讨论,以期为后续相关研究者提供参考.     

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.     

Analysis of nonlinear acoustoelastic effect of surface acoustic waves in laminated structures by transfer matrix method

The propagation of surface acoustic waves in a layered half-space is investigated in this paper, where a thin cubic Ge film is perfectly bonded to an isotropic elastic Si half-space. Application of the transfer matrix and by solving the coupled field equations, solutions to the mechanical displacements are obtained for the film and elastic substrate, respectively. The phase velocity equations for surface acoustic waves are obtained. Effects of the homogeneous initial stresses induced by the mismatch of the film and substrate are discussed in detail. The results are useful for the design of acoustic surface wave devices.     

Acoustic scattering from baffled membranes that are backed by elastic cavities

An elastic membrane backed by a fluid-filled cavity in an elastic body is set into an infinite plane baffle. A time harmonic wave propagating in the acoustic fluid in the upper half-space is incident on the plane. It is assumed that the densities of this fluid and the fluid inside the cavity are small compared with the densities of the membrane and of the elastic walls of the cavity, thus defining a small parameter . Asymptotic expansions of the solution of this scattering problem as →0, that are uniform in the wave number k of the incident wave, are obtained using the method of matched asymptotic expansions. When the frequency of the incident wave is bounded away from the resonant frequencies of the membrane, the cavity fluid, and the elastic body, the resultant wave is a small perturbation (the “outer expansion”) of the specularly reflected wave from a completely rigid plane. However, when the incident wave frequency is near a resonant frequency (the “inner expansion”) then the scattered wave results from the interaction of the acoustic fluid with the membrane, the membrane with the cavity fluid, and finally the cavity fluid with the elastic body, and the resulting scattered field may be “large”. The cavity backed membrane (CBM) was previously analyzed for a rigid cavity wall. In this paper, we study the effects of the elastic cavity walls on modifying the response of the CBM. For incident frequencies near the membrane resonant frequencies, the elasticity of the cavity gives only a higher order (in ) correction to the scattered field. However, near a cavity fluid resonant frequency, and, of course, near an elastic body resonant frequency the elasticity contributes to the scattered field. The method is applied to the two dimensional problem of an infinite strip membrane backed by an infinitely long rectangular cavity. The cavity is formed by two infinitely long rectangular elastic solids. We speculate on the possible significance of the results with respect to viscoelastic membranes and viscoelastic instead of elastic cavity walls for surface sound absorbers.     

Acoustic radiation force on an elastic cylinder in a Gaussian beam near an impedance boundary

Acoustic radiation force (ARF) is studied by considering an infinite elastic cylinder near an impedance boundary when the cylinder is illuminated by a Gaussian beam. The surrounding fluid is an ideal fluid. Using the method of images and the translation-addition theorem for the cylindrical Bessel function, the resulting sound field including the incident wave, its reflection from the boundary, the scattered wave from the elastic cylinder, and its image are expressed in terms of the cylindrical wave function. Then, we deduce the exact equations of the axial and transverse ARFs. The solutions depend on the cylinder position, cylinder material, beam waist, reflection coefficient, distance from the impedance boundary, and absorption in the cylinder. To analyze the effects of the various factors intuitively, we simulate the radiation force for non-absorbing elastic cylinders made of stainless steel, gold, and beryllium as well as for an absorbing elastic cylinder made of polyethylene, which is a well-known biomedical polymer. The results show that the impedance boundary, cylinder material, absorption in the cylinder, and cylinder position in the Gaussian beam significantly affect the magnitude and direction of the force. Both stable and unstable equilibrium regions are found. Moreover, a larger beam waist broadens the beam domain, corresponding to non-zero axial and transverse ARFs. More importantly, negative ARFs are produced depending on the choice of the various factors. These results are particularly important for designing acoustic manipulation devices operating with Gaussian beams.