基于准二维各向异性亚波长结构的宽带声隐身机理与特性研究

According to effective medium theory and acoustic parameters of materials already known, the ideal acoustic parameters of invisible cloak required by coordinate transformation or other design approaches are difficult to realize with broadband subwavelength structures. For simplifying the fabrication of the broadband cloak, the researchers usually keep the refractive index the same as required by the coordinate transformation and properly modulate the acoustic impedance. This measure will lead to impedance mismatch. And in the design of the free space cloak, extreme parameters are often involved and the operating band width is very limited. In this project, we deal with the problem of impedance mismatch for acoustic invisibility near sound hard boundaries and the problem of operating band width of the free space cloak, use the special acoustic features of three dimensional structures in two dimensional waveguides and multi-physics coupling method to broaden the range of realizable effective parameters with metamaterials, and explore the possibility of obtaining the ideal broadband cloak. The related methods for improving the performance of the invisible cloak will be compared, and the parameter dependency of the designed structures will be analyzed in detail. Combing the corresponding results, we will build optimized coordinate transformation model and acoustic scattering model, improve and supplement the existing cloaking theory, develop more flexible design methods for two dimensional cloak, and provide some inspirations and techniques for realizing ideal three dimensional cloak and other novel devices based on acoustic subwavelength structures.

根据等效介质理论和已知材料参数，用现有的亚波长结构难以实现宽带斗篷设计中坐标变换等方法所要求的理想声学参数。目前相关工作中普遍采取的措施是维持坐标变换所要求的折射率不变，适当调制声阻抗以简化声学参数，这会导致阻抗失配。并且在自由场隐身斗篷的设计中还常常牺牲了工作带宽或采用了难以实现的材料参数。本项目针对刚性边界附近的宽带斗篷介质中阻抗失配以及自由场斗篷的工作带宽等问题，提出利用三维结构在二维波导内的特殊声学性质，并结合热声耦合等多物理耦合方法，拓展声学超材料所能实现的参数范围，在此基础上探索理想宽带斗篷的实现；对改进隐身效果的相关方案展开研究，详细分析所设计结构的参数依赖关系，结合相应的结果建立更优的坐标变换模型以及声散射模型，改进与补充已有理论，探索更灵活的二维斗篷设计方法，为实现理想三维斗篷以及其他基于声亚波长结构的新型器件提供一些思路和方法。

随着近年来的声人工结构领域的快速发展，多种新型声隐身方案被相继提出，由于这些方案提供了能使声波绕开散射体而不产生散射的可能性，在国际上得到了广泛的关注。但目前亚波长人工结构还难以实现这些设计所要求的理想参数，研究者常在实际制备时被迫采用简化参数或牺牲工作带宽的折衷措施，这会弱化装置应对主动探测的能力，限制其应用前景。本项目针对这一问题，通过研究准二维结构在低频极限下声学特性对其几何参数和温度等物理参数的依赖关系，提出一种准二维各向异性材料参数的独立调制方法，拓宽了其所能实现的声学参数范围，设计出各向异性声透明介质，并应用于宽带声隐身。在自由场中分别基于坐标变换和散射消除两种不同的宽带隐身设计方法，设计了特定方向的宽带声隐身和散射消除型的全方向隐身，提出了不同情况下对隐身结构及隐身效果的评估方法与参数指标，建立了相应的实验系统对所制备的装置进行了实验测定与评估。结果表明利用各向异性透明介质可以在给定方向产生较理想的宽带隐身效果，而散射消除型声隐身能对波长尺度的物体实现全方向隐身，两种方案都可在较宽带宽内将散射截面降低一个数量级以上。能在给定路径上引导声波的能力使这两种设计在声波操控以及宽带声隐身等应用中具有重要价值。