Unconventional principles of underwater wave control in the sub-wavelength regime

Project Information

POSEIDON

Grant agreement ID: 101039576

DOI

10.3030/101039576

EC signature date 1 March 2022

Start date 1 September 2022

End date 31 August 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 482 750,00

EU contribution

€ 1 482 750,00

Coordinated by

POLITECNICO DI TORINO
Italy

Periodic Reporting for period 1 - POSEIDON (Unconventional principles of underwater wave control in the sub-wavelength regime)

Reporting period: 2022-09-01 to 2025-02-28

Summary of the context and overall objectives of the project

The growing interest in marine renewable energy and ocean-related human activities are the main causes of an alarming increase in the overall noise level in the oceans and seas, with dramatic consequences on the marine ecosystems.

The current mostly used underwater sound mitigation strategies derive from (i) materials with viscoelastic micro/macro-inclusions and (ii) coatings with periodic air inclusions, where the sound insulation performance derives from the deformation / resonance of the inclusions themselves. Other approaches (iii) include hydro sound dampers (nets with air filled elastic balloons) and (iv) curtains of air bubbles. Despite vast optimization studies conducted on the geometry and material composition of the inclusions, as well as introducing heavy backings or multi-layered structures, the performance of underwater noise mitigation systems is still limited at very low frequencies, where the wavelength in play is in the order of meters, and, thus, requiring massive thicknesses to ordinary barriers to be effective. Consequently, a viable solution to attenuate underwater waves at very low frequencies and over a rather broadband frequency range does not exist, yet.

POSEIDON aims to fill this gap and to develop a new class of meta-screens allowing extremely low transmission / radiation over a broadband frequency range and exhibiting practical structural requirements, such as being compact, lightweight, and efficient under hydrostatic pressure. To meet these goals, POSEIDON is exploring (i) the intimate relationship between the micro-structure and the macroscopic vibrational properties of a multi-scale metamaterial, (ii) topologically protected and impedance adapted meta-barriers. Both approaches are supported by the innovative assumption that (3) wave control through periodic structures is already exploited by Nature.

If successful, POSEIDON will have an important impact in extending the knowledge of architectured (meta-)materials (with applications to underwater acoustics) and significantly advance the frontiers of bottom-up material design.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Qualitative and quantitative imaging as well as dynamic / mechanical studies of some biological systems, including diatoms, seashells, and insect wings allowed us to identify key features for inspiring the design of a new class of metamaterials valuable for underwater noise reduction applications. These features include, but not limited to, hierarchical and graded organizations of structural elements across multiple length scales, elastic anisotropy, coupling of elastic waves with different polarization, quasi-periodicity, and broken symmetries.

Starting from these observations, we have shown that introducing the concept of structural hierarchy in elastic metamaterials and phononic crystals can lead to a new class of periodic structures capable of generating multiple, highly attenuative and broadband bandgaps at multi-scale frequencies, including the lowest, where the size of the unit cells becomes extremely small (deep sub-wavelength) with respect to the wavelength of the waves in play. The concept of hierarchy is intended in the sense that a representative unit cell of a periodic structure comprises multiple arrangements of non-self-similar inhomogeneities (cavities introduced into a continuous matrix, for instance) at various length scales activating diverse types of scattering mechanisms (Bragg scattering, local resonances, and / or inertial amplification).

We have proposed a couple of preliminary designs of metamaterial unit cell: the first, concerning quasi-Helmholtz resonators with a hierarchical architecture inspired by the multilevel organization of pores typical of the « Coscinodiscus genus frustule ». Preliminary numerical results (with no hierarchical levels, yet) have shown high potential for absorption performance (with the possibility of quasi-perfect absorption under specific assumptions). The second design, coupling shear and longitudinal motion to achieve sound transmission loss through reflection in highly sub-wavelength unit cells. The idea here is that a unit cell mixed polarization will allow us to convert the motion excited at one end of the meta-barrier impinged by an underwater wave (thus, mainly longitudinal) into (partially) shear motion at the other end, limiting, thus, the propagation of sound in water. Preliminary analytical / numerical models predicted a lattice parameter to the maximum wavelength ratio up to 1 / 80 and 20 – 40 dB reduction.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

POSEIDON’s outcomes obtained so far strongly contribute to the perspective of « elastic metamaterial principles already exploited by Nature » and / or « metamaterial design inspired by Nature ». This is not a trivial outcome because elastic metamaterials are currently often associated with « man-made » or « artificially engineered materials » possessing « properties typically not available in Nature ». In this sense, POSEIDON provides an important contribution for a step-change in the vision of these materials, bridging the gap with respect to photonics, where several examples of bandgap behavior observed in biological systems have already been reported (for instance, « structural coloring »).

The design principles of the unit cells proposed in POSEIDON could significantly advance the current state-of-the-art in bottom-up (meta-)material design (with applications to underwater acoustics and beyond). For instance, the experimental confirmation of a quasi-Helmholtz underwater resonator could open the route to a « transposition of the physical principles exploited in airborne acoustics into underwater acoustics », where a considerable technological delay is present due to the heavy-fluid-structure interaction. Also, a metamaterial unit cell with strong longitudinal-shear coupling could inspire new designing strategies towards lightweight deep-subwavelength meta-barriers. Finally, POSEIDON’s findings may represent a pivot for pushing topological protection into the realm of engineering applications, providing a practical solution to the problem of wave redirection without backscattering and / or localization in elastic and underwater applications.

POSEIDON could also lead to unexpected outcomes in the field of wave control at the micro- and nanoscale. In fact, in the attempt of replicating very small biological systems (such as diatoms) for a more controlled characterization, intriguing potential for energy guiding / harvesting applications at these scales (micro- and nano), where precise control of elastic waves in frequency and space (via bandgaps and topological protection) could play a pivotal role due to its immunity to defects, has emerged.

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