Thermo-visco-acoustic metamaterials for underwater applications

几十年来，控制水下噪声的能力一直是人们实际关注的问题。这种噪音，例如来自海上风力发电场、涡轮机和商船的辐射，通常需要人为地减弱，因为它的产生距离敏感的海洋环境很近。当然，国防应用的兴趣在于使船只尽可能安静。在上个世纪，许多材料被设计用于帮助水下噪声衰减。然而，最近人们对声学超材料和超表面的研究兴趣激增。这种介质具有特殊的微观结构，旨在提供天然材料永远无法获得的整体（动态）材料特性，并导致看似相当奇怪的负折射，波重定向，隐身的终极目标。制造这些人造材料的许多机制都依赖于共振的概念。这就提供了低频声衰减的可能性，这是极难实现与普通材料。经典低频谐振器的一个例子是亥姆霍兹谐振器。对于正在研究的许多超材料，特别是在水下环境下，声音衰减的机制，即热的和粘性的，还没有得到适当的理解。该项目的目的是通过数学分析来研究这方面，然后优化设计，以便将超材料用于水下降噪应用。尽管在过去的几年里，人们对“空中”环境产生了一些初步的兴趣，但水下的参数状态会产生新的影响，需要深入探索和理解。

The ability to control underwater noise has been of practical interest for decades. Such noise, radiating from e.g. offshore wind farms, turbines, and merchant vessels, frequently needs to be attenuated artificially given the close proximity of its generation to sensitive marine environments for example. The interest in defence applications is, of course to render a vessel as quiet as possible. Over the last century a number of materials have been designed to assist with underwater noise attenuation. However, recently there has been an explosion of interest in the topic of acoustic metamaterials and metasurfaces. Such media have special microstructures, designed to provide overall (dynamic) material properties that natural materials can never hope to attain and lead to the seemingly rather strange notions of negative refraction, wave redirection, the holy grail of cloaking. Many of the mechanisms to create these artificial materials rely on the notion of resonance. This then provides the possibility of low frequency sound attenuation which is extremely difficult to achieve with normal materials. An example of a classical low frequency resonator is the Helmholtz resonator. The mechanisms of sound attenuation, i.e. thermal and viscous, have not yet been properly understood for the many metamaterials under study, particularly in an underwater context. The aim of this project is to study this aspect via mathematical analysis and then to optimize designs in order to employ metamaterials for use in underwater noise reduction applications. Although there has been some initial interest over the last few years in the "in-air" context, the parameter regime underwater gives rise to new effects that need to be explored and understood thoroughly.