Numerical acoustic characterisation of a Kelvin cell structure under normal and grazing incidence

Over the last decade, metamaterials based on periodic structures have been largely studied to deeply control and manipulate an acoustic field, exploiting the exotic acoustic behaviours that can be achieved, such as band gaps, hyperfocusing and noise trapping capabilities. In this class of materials, the acoustic response depends on the geometric characteristics, i.e. shape, size, orientation and arrangements of meta-atoms, rather than on the mechanical properties of the base material. Metabehaviours are thus related to constructive and disruptive interactions between incident and reflected waves within the periodic lattice. The present paper deals with the numerical acoustic characterisation of a Kelvin-cell-based layer under normal and grazing incidence. Due to its intricate periodic structure, the Kelvin cell can exhibit interesting acoustic meta-properties, while its high-porosity lattice makes it a good candidate for aeronautical applications, where weight limitation is urgent. Despite an extensive literature on acoustic metamaterials and periodic structures, limited applications on the aeroacoustic responses of these type of structures have been presented so far. The influence of angle of incidence is key to disclose the enormous potential of acoustic metamaterials in aeroacoustics, and possibly open the path to the analysis of the response in more complex situations such as those of interest to the aeronautics community. Uiltimately this could make possible the exploitation of unconventional metabehaviours to mitigate the aviation community noise. In this paper, a numerical parametric analysis of the acoustic response of the Kelvin-cell-based specimen is presented, focusing on the search for transmission band-gaps, directivity pattern modifications and energy absorption.