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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SmartAnswer (Smart mitigation of flow-induced acoustic radiation and transmission for reduced aircraft, surface transport, workplaces and wind energy noise)

Teaser

Summary

A step change in our noise mitigation strategies is required in order to meet the environmental targets defined for a number of sectors of activity affecting people through noise exposure, such as air / ground transportation and wind energy production. The constant growth in air and ground traffic, the desired increase of the share of wind energy production, etc., require very substantial reductions based on new technologies. Radically new concepts for flow and acoustic control, such as meta-materials, porous treatment of airframe surfaces, airfoil leading-edge or trailing-edge serrations, â€¦ appear nowadays promising, however still largely as the result of extensive trial-and-error tests conducted at laboratory scale. It now appears that the development and maturation of novel noise reduction technologies is hindered by several factors:

1) An insufficient understanding of the physical mechanisms responsible for the alteration of the flow or acoustic fields. In absence of a phenomenological understanding, theoretical models and numerical simulations can hardly be very successful.

2) Very tight design constraints are imposed to any novel noise mitigation strategy making its way to the full-scale application. This is nowadays the case in all industrial sectors, where safety, robustness, weight, maintainability, small volume, ecological disposal and naturally cost are as important concerns, if not more important, as acoustic comfort and compliance with regulations.

3) There is an insufficient knowledge about the possibilities that are nowadays offered by new materials, new manufacturing techniques such as 3D-printing, recent advances in control systems, etc.

SmartAnswer implements a research and training platform focused on innovative flow and noise control and optimization approaches addressing the above shortcomings. It has the following objectives:

- Foster a training-through-research network of young researchers, where the ESR fellows investigate promising emerging technologies for noise reduction, by means of laboratory experiments, simplified theoretical models and numerical simulations.

- Bring in a coordinated research environment industrial stakeholders picked from the aeronautical, automotive, wind turbine and cooling/ventilation sectors.

- Offer a training infrastructure where the young researchers will be confronted with the design, manufacturing and economical constraints by strong interactions with the industrial final users.

Work performed

The research carried out during the first reporting period addressed the noise generation and propagation mechanisms, tackling already some mitigation strategies. The applications include an automotive cooling fan, a wind turbine blade, an aeroengine and a building ventilation fan. A workshop was held where the ESRs assessed the potential of a range of noise mitigation techniques to reach the market in the above-mentioned sectors, accounting for their performance, complexity of integration, space, weight, durability, costs, etc.

The complex phenomena associated with the production of noise due to the interaction of an airfoil or blade with incoming or self-generated turbulence have been explored experimentally. Porous materials and leading-edge serrations were tested for the reduction of the noise resulting from the interaction of incoming turbulence with an isolated airfoil and in a centrifugal blower, showing encouraging preliminary results. The first measurements of the aerodynamic and acoustic behaviors of advanced passive/active liners, metamaterials and micro-perforate panels have been conducted as well.

On the side of modelling, simulation and optimization, the project has seen the elaboration of simplified models for the prediction of turbulence-interaction noise and trailing-edge noise, and to describe the effects of leading-edge serrations. First simulations of over-the-tip liners in aeroengines, of wind turbine blade vortex generators, of an automotive cooling fan and of acoustic meta-materials were obtained. A domain decomposition method for a FEM-based resolution of the Helmholtz equation, and an adjoint aeroacoustic simulation chain are being implemented.

A number of trainings were attended by all ESRs, covering an introduction to fluid mechanics, acoustics and structural dynamics, the aeroacoustics of rotating machines, and the development of advanced liner technologies. The secondment program is on track with a number of ESRs visiting Beneficiaries and Partners of the consortium in line with their Personal Career Development Plans.

https://www.youtube.com/channel/UCjQ4OV0piRlp7-CIkUf80SQ

Final results

\"IIn the aeronautical field, porous materials have demonstrated a good potential for the mitigation of the noise produced by turbulence-surface interactions. The preliminary experimental results and theoretical predictions obtained by the ESRs have brought some light onto those mechanisms and will support a better understanding of the dominant parameters.

Another innovative use of acoustic treatments to reduce noise at the source is to locate liners over the fan rotor tip on turbofan engines. There is a great potential for reducing fan noise sources but only preliminary tests have been performed so far. The first numerical simulations carried in the project provide a good basis to assess the potential of the technique.

In wind turbines, there remains a significant discrepancy between the actual noise reduction observed in field tests, the noise measured in wind tunnels, that predicted from theoretical models and that obtained from more advanced numerical calculations (e.g. CAA). SmartAnswer will help to elucidate the reasons for such discrepancies and eventually provide truly predictive models.

The availability of new manufacturing techniques for producing anisotropic light micro-perforates with large steady pressure drops and still significant acoustic absorption properties opens interesting perspectives of integration in ventilation ducts and low-speed cooling/ventilation fans. But high level complex multi-frequency acoustic excitation yields insufficiently understood interaction effects. The combination of high level multi-frequency acoustic excitation and grazing flow or bias flow in the experiments has already provided useful data.

Metamaterials and micro-electro-mechanical devices (MEMs) must be studied not only in terms of theoretical absorption but also, in a more engineering way, to evaluate their sensitivities to sound pressure levels and to grazing flow effects. These innovative solutions have started being explored in that line.

Likewise, non-local passive and active MDOF liners are currently designed with the assumption that they are locally reacting, however several advanced liner concepts involve non-local impedance (periodic inclusions, or bulk reacting liners like metallic foam). The necessary physical modelling efforts is being carried out jointly with the development of advanced numerical methods.

Finally, the next generation of composites or \"\"metacomposites\"\", which should realize, among others, high-performance characteristics for treating acoustical or vibrational transmission problems, are being investigated numerically, which should eventually promote the advent of low-transmission intelligent lightweight materials and architected non-linear materials with application in many engineering fields.
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