基于坐标变换方法与梯度声子晶体的宽带声隐身关键技术研究

Research on novel acoustic devices has become a hot topic in the fields of material science, physics and mechanics, and it turns out that coordinate transformation method and gradient-index phononic crystals are keys to the success of these devices. Based on transformation method, the powerful functional devices can be designed to manipulate acoustic waves at will so as to possess profound scientific significance and great practical potential. But as a new subdiscipline within physics, there are many unresolved problems from basic to applied research..On the one hand, it is well known that under the general coordinate transformation, the form invariance property of the acoustic equation can be guaranteed when the mass density of the material is assumed to be an anisotropic tensor. This property can be found in acoustic metamaterials in the sense of local resonance. However, this project tries to give the transformation condition of form invariance even using the conventional scalar mass density, thus facilitating its physical implementations. .On the other hand, the versatility of phononic crystals in controlling acoustic wave propagation can be further enhanced by introduction the concept of gradient-index phononic crystals. In this project,the gradient-index phononic crystals based on the viscoelastic solid matrix, as one kind of multiscale material, are studied for their sound absorption properties and effective theory for sound propagation. Especially the micro-macro property correlation is discussed. This can help us to make macroscopic predictions based on microscopic properties, including shape/distribution/materials of microstructure. Based on the previous studies, a feasible scheme for achieving transformation media using gradient-index phononic crystals has been proposed so as to help to pave the way for future experiment research. As an important application of our theories, we present the design of one kind of reduced acoustic cloak. Owning to the combination of "absorption of high frequency sound" and "bending of low frequency sound", such device can be in effect over the desired frequency super-broadband range of interest and has potential application in submarine stealth and noise control project.

人工材料的波动控制研究是目前国际上材料学、物理学和力学的新课题，其中坐标变换方法与梯度声子晶体理论由于为新型声波复合材料/结构提供了全新的设计手段和物质基础而尤为受到关注。变换方法能直观给出适当的材料本构参数（变换介质）达到任意调控声波的目的，研究取得较快进展，但仍面临许多挑战。课题组在声波方程"形式不变性"的讨论中，引入特殊变换形式，建立完整的各向同性变换理论，避免已有方法对于材料的苛刻要求（如各向异性密度）；而梯度声子晶体是随着声波调控理论发展而提出的一种新颖的功能材料，可设计性强，本项目研究梯度声子晶体的等效介质方法，揭示其声吸收与声传播特性，重点阐明宏观等效参数与微结构单元特性的关系，在此基础上提出变换介质的实现方案。最后提出"高频吸波"与"低频绕波"相结合的宽带声隐身新概念。本项目的完成将为新型声波结构概念设计与实现提供技术支撑，在潜艇隐身与噪声控制工程等具有重要潜在应用价值。

人工材料的波动控制研究是目前国际上材料学、物理学和力学的新课题，其中坐标变换方法与梯度声子晶体理论由于为新型声波复合材料/结构提供了全新的设计手段和物质基础而尤为受到关注。变换方法能直观给出适当的材料本构参数（变换介质）达到任意调控声波的目的，研究取得较快进展，但仍面临许多挑战。课题组在声波方程"形式不变性"的讨论中，引入特殊变换形式，建立完整的各向同性变换理论，避免已有方法对于材料的苛刻要求（如各向异性密度）；而梯度声子晶体是随着声波调控理论发展而提出的一种新颖的功能材料，可设计性强，本项目研究梯度声子晶体的等效介质方法，揭示其声吸收与声传播特性，重点阐明宏观等效参数与微结构单元特性的关系，在此基础上提出变换介质的实现方案。基于变换方法和新型梯度声子晶体开展应用研究，包括声隐身现象的研究。在应用方面，我们提出了声波伪装结构的设计思路，利用保角变换声学理论推导了“声波移位”与“声波合并”两种声波伪装结构的材料参数（非均匀各向同性），为了便于工程实现，我们还进一步对声波结构进行了分层简化处理。在新型超单元构型与表征方面，我们开展了新型迷宫型声波超材料的研究，利用等效介质方法阐明了该种超材料单元的宏观等效参数（折射率与声阻抗等）与微结构单元特性（几何构型与材料属性等）的关系，在此基础上提出声波调控结构的梯度设计方案，基于迷宫超材料首次实现了紧致型声波天线设计（厚度≈0.4入射波波长）。本项目的完成将为新型声波结构概念设计与实现提供技术支撑，在潜艇隐身与噪声控制工程等具有重要潜在应用价值。期间指导博士、硕士研究生2人，发表SCI论文2篇，并参加学术交流活动3次。