Thermo-visco-acoustic metamaterials for underwater applications

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.