Tailoring acoustic metamaterials to aeroacoustic applications

The advent of metamaterials in acoustics is shaking-up the research community. The theoretical models and the realization technologies developed during the last decade have suddenly made possible applications that were barely conceivable before. Acoustic invisibility, perfect acoustic mirrors and lenses or ideal wave guides are now realizable with devices exploiting the potential of acoustic metamaterials. The present paper aims to investigate the applicability of such technologies in the aeroacoustic domain. The final goal of the research is the disclosure of the tremendous potential of metamaterial technologies in the abatement of civil aviation noise. The acoustic metamaterial theory developed so far is based on the assumption that the compressible medium and the scattering obstacles are at rest. This is clearly a limitation not compatible with aeroacoustic applications, where aerodynamic convection plays a fundamental role in the propagation and scattering patterns. The paper presents an original approach to extend the existing theory to the analysis of moving media and obstacles. Attention is focused on the design of a cloaking device, i.e. a metamaterial cover able to cancel the scattering of the cloaked object. Two formulations are presented, both based on the extension of the classical coordinate transformation approach to consider the existence of the mean flow. Preliminary numerical results are presented to demonstrate the effectiveness of both the approaches.