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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HARPOCRATES (Smart phononic crystals for aircraft noise reduction)

Teaser

Summary

After the introduction of commercial jets in 1958, aircraft noise pollution has gradually become a major public issue widespread worldwide. Thus, starting from the 60\'s, noise reduction programs have been introduced with the objective of decreasing the aircraft noise emissions, especially in the vicinity of airports and approach routes. Since then, and in particular over the last 30 years, proper measures and technology innovation allowed individual aircraft to be over 75% less noisy. Nevertheless, the incessant growth of the global air traffic implies that many citizens are still exposed to high noise levels. This means that aircraft noise pollution is and will continue to be a serious environmental problem to deal with in the next years.

In this context, HARPOCRATES proposed a new device capable to propitiously alter noise emission spectra exploiting phononic crystals and acoustic metamaterials. The project shed light (via numerical and experimental approaches on scaled and full-scale models) on the possibility of achieving special dynamic properties, such as the opening of frequency band gaps (BGs), i.e. frequencies in which the propagation of elastic waves is inhibited, mode shifting, topological protection, selective frequency filtering, focusing and wave guiding of elastic waves. This may open new avenues for the practical realization of compact, passive and cost-effective devices for wave manipulation.

Work performed

\"HARPOCRATES has begun December, 1st 2015 and ended November, 30th 2017.

The work performed in this period can be divided as follows:

1. Training activities of the beneficiary Dr. Marco Miniaci. The training has included: (i) theoretical tutoring on phononic crystals, metamaterials, piezoelectricity and (ii) extensive laboratory training. The first part of the training has been carried out by Prof. B. Morvan and MdC R. Sainidou while the second one mainly by Prof. B. Morvan.

Specifically, the first part of the training concerned the acquisition of the basics of wave propagation phenomena in periodic structures made of identical building blocks (named unit cells) capable of spectral and spatial control of waves due to two fundamental properties: frequency-dependent directionality and band gap (BG) effect.
The second part of the training has concerned the acquisition of preparatory competencies in view of the experimental phase. This included ad-hoc training on: (i) a vast range of piezoelectric (contact or air-coupled) transducers to excite and detect elastic waves in complex configurations; (ii) Scanning Laser Doppler Vibrometer (SLDV) for contactless measurements; (iii) anechoic chamber for the \"\"intrinsic\"\" acoustic material characterization and the assessment of the transmitted and reflected sound properties; (iv) a tube of impedance for sonic range characterization of scaled specimen and single building block (unit cell); (v) power amplifier and generator, dedicated frame for motorized displacements, portable anechoic chamber, etc.
In parallel, Dr. Marco Miniaci has been supervised by Prof. B. Morvan in developing complementary soft skills for the independent management of the project.

2. The investigation of BG nucleation with respect to the material properties and geometrical arrangement and the implementation of numerical models in order to investigate the device dispersive behaviour.

In this phase Dr. Marco Miniaci, assisted by Prof. Morvan, explored different geometries capable to exhibit special dynamic behaviour. In parallel, he learned how to implement periodic boundary conditions within the FE software Comsol Multiphysics. Assisted by MdC A. Madeo, a semi-analytical model based on relaxed micromorphic continuum model with weighted free and gradient micro-inertia has been implemented and used to describe the dynamical behaviour of a real two-dimensional phononic crystal for a wide range of wavelengths.

3. Investigation of the abilities of filtering / control of waves of the proposed device. This phase of the project has been accomplished following both numerical and experimental approaches, subsequently. In particular, the implementation of numerical models aimed at predicting the multi-physics behaviour of the phononic device at the frequencies of interest has been carried out.

4. The \"\"Synthesis and Dissemination\"\" of the main results resulting from actions 1-3 has been brought together in order to provide a consistent synthesis level in which theories, methods and experimental validations have been presented at a national and international levels.
Results have been published in high quality international peer-reviewed journals such as (Physical Review Letters, Applied Physics Letters, New Journal of Physics, etc.) and presented at national and international meetings, as well as at regular seminars.

Also, during the first phase, the kick-off meeting with Aircelle industry which supports the project has also been organized and held at the LOMC on February, 24th 2016. From that discussion important issues related to industrial current technological breakthrough needs emerged and a plan for the next regular meetings (1 each 8 months) has been discussed.

Dissemination activities included the participation to national and international conferences, research-periods in foreign laboratories (Georgia Institute of technology, California Institute of Technology) and the organization of an international workshop (10th -\"

Final results

In the framework of aircrafts noise emission, the jet and fan contributions are the most significant to the production of the overall sound level. Over the years, the most common noise management techniques arose from a combination of changes to the engine design and active / passive low-noise design features. Among the latter, while active approaches are still at a laboratory scale, passive techniques represent the current state-of-the-art in jet noise reduction technology.

HARPOCRATES aimed to go beyond this state-of-the-art by means of an interdisciplinary approach. It has proposed a device capable of propitiously alter wave emission spectra making use of a new class of materials, namely phononic crystals (PCs) and acoustic metamaterials (MMs). Scaled and full-scale demonstrators of the ability of this device to manipulate waves within specific target frequencies have been provided.

The potentials of HARPOCRATES are to contribute to the reduction of social costs impact and hazard to human life related to aircraft noise pollution. The project allowed to shed light on the realization of a ground-breaking insulation device capable to be properly tailored to cut noise within specific frequencies, allowing air (and light) to pass through. Such versatility makes research outcomes suitable not only for aircraft noise insulation purposes but useful also for a vast array of industrial applications.