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The fast development of Urban-Air-Mobility as well as the constant growth of the air transport have made the acoustic pollution abatement a crucial requirement for the aviation industries in order to comply with the increasingly demanding constraints for the community acceptance. The aeroacoustic characterization of arrays of electrically-powered propellers is one of the most challenging issues. The vast majority of the UAM concepts under development adopt propulsion systems based on multiple propellers, for which reliable and cost-efficient aeroacoustic models are still lacking. The present paper proposes the development of surrogate models for the description of acoustic emission of multi-propeller configurations. The numerical investigation focuses on surrogate models able to take into account the effects of the propeller blade geometry (e.g., chord and twist distributions) and global propeller-array geometric parameters (e.g., propellers clearance) on acoustic performances of the whole system. An innovative Artificial Neural Network adaptive metamodelling technique is applied on a numerical database obtained through a boundary integral formulation for the solution of incompressible potential flows around lifting/thrusting bodies, followed by the application of the Farassat 1A boundary integral formulation for the noise field evaluation.

Despite the increased computational resources, the simulation-based design optimization (SBDO) procedure can be very expensive from a computational viewpoint, especially if high-fidelity solvers are required. Multi-fidelity metamodels have been successfully applied to reduce the computational cost of the SBDO process. In this context, the paper presents the performance assessment of an adaptive multi-fidelity metamodel based on a Gaussian process regression (MF-GPR) for noisy data. The MF-GPR is developed to: (i) manage an arbitrary number of fidelity levels, (ii) deal with objective function evaluations affected by noise, and (iii) improve its fitting accuracy by adaptive sampling. Multi-fidelity is achieved by bridging a low-fidelity metamodel with metamodels of the error between successive fidelity levels. The MF-GPR handles the numerical noise through regression. The adaptive sampling method is based on the maximum prediction uncertainty and includes rules to automatically select the fidelity to sample. The MF-GPR performance are assessed on a set of five analytical benchmark problems affected by noisy objective function evaluations. Since the noise introduces randomness in the evaluation of the objective function, a statistical analysis approach is adopted to assess the performance and the robustness of the MF-GPR. The paper discusses the efficiency and effectiveness of the MF-GPR in globally approximating the objective function and identifying the global minimum. One, two, and three fidelity levels are used. The results of the statistical analysis show that the use of three fidelity levels achieves a more accurate global representation of the noise-free objective function compared to the use of one or two fidelities., 12 pages, 6 figures, to be published in conference proceedings of AIAA AVIATION 2021 FORUM

This paper deals with the innovative development of surrogate models suitable for the simulation of aerodynamic performance and acoustic emission in terms of tonal components, of multi-propeller systems like those applicable in urban air-mobility vehicles. These can be of great help particularly when designing distributed-electric-propulsion configurations, as they provide an agile tool that avoids the need for computationally expensive CFD/CAA predictions. Without losing the generality of the conclusions that can be drawn about the capability of the proposed surrogate models to accurately describe multi-propeller aerodynamic and aeroacoustic responses, applications to a single propeller configuration are presented. Focusing on the simulation of the effects due to the spanwise distribution of blade twist and chord length, two surrogate modelling techniques are examined: one based on Artificial Neural Networks and one based on Genetic Programming. The numerical database for the identification of these models is determined by the combined application of a boundary integral formulation suitable for the potential aerodynamics solution around lifting/thrusting bodies, and the Farassat 1A formulation for the evaluation of the noise field. The numerical investigation demonstrates that both metamodelling techniques are able to reproduce propeller aerodynamic performance and radiated noise with a very good level of accuracy, certainly suitable for preliminary design applications.

Modern turbofans with high bypass ratios, low blade passage frequencies and short nacelles require continuous development of acoustic linings to achieve the noise reductions expected by the international aviation authorities. Metamaterials and metafluids have been recently proposed as promising technologies for designing innovative acoustic treatments dedicated to reducing aeronautic turbofan noise emissions. In this work, a phase-gradient metasurface treatment is investigated as a way to tackle the noise radiation from an axially symmetric nacelle. This paper aims to study the potential benefits of the mentioned technology, and is not an attempt to design a complete new liner or nacelle. The metasurface is modelled through an equivalent metafluid, and a simulation-based optimisation is used in defining the design parameters. The tonal contribution of the blade passage frequency is considered, and the numerical results with the metafluid optimised on one azimuthal mode at a time show a significant effect in terms of acoustic levels and directivity over an arc of virtual receivers.

The current work deals with multi-objective optimisations including financial merit factors in the multi- disciplinary conceptual design of a commercial aircraft with a specific focus on noise social cost. The results of optimisations using financial and performance-oriented objective functions have been analysed and combined in a multi-objective optimisation problem using a three-dimensional objective vector. The main novelty of the present work is the analysis of social cost noise fees effectiveness integrated in financial models embracing a wider group of stakeholders’ interests. The foundations of this research lie on the air traffic increase envisaged in the next decades and the contextual growth of the urban areas nearby airport facilities. This scenario requires the aeronautical industry to handle the environment challenge taking into account all the social and economic effects. Indeed, the consolidated technology has approached a saturation point, therefore breakthrough concepts and configurations are mandatory. This matter is addressed as a highly multi-disciplinary problem within conceptual design framework. The financial model proposed is shown to highlight the need of appropriate noise fees as instruments to push all stakeholders towards a technology radical renewal and encourage the aeronautical industry towards a shared strategy in the development of an eco-friendly commercial aviation system.

This paper presents an investigation of the aerodynamic and aeroacoustic interaction of propellers for distributed electric propulsion applications. The rationale underlying the research is related to the key role that aeroacoustics plays in the establishment of the future commercial aviation scenario. The sustainable development of airborne transportation system is currently constrained by community noise, which limits the operations of existing airports and prevents the building of new ones. In addition, the substantial saturation of the existing noise abatement technologies inhibits the further development of the existing fleet, and imposes the adoption of disruptive configurations in terms of airframe layout and propulsion technology. Simulation-based data may help in clarifying many aspects related to the acoustic impact of such innovative concepts. Blended-wing-body equipped with distributed electric propulsion is one of the most promising, due to the beneficial effect of the substantial shielding induced by its geometry. Nevertheless, the novelty of the layout requires a thorough investigation of specific aspect for which no previous experience is available. Herein, the interaction between propellers is analysed for a fixed propeller geometry, as a function of their mutual distance and compared to the acoustic pattern of the isolated one. The aerodynamic results have been obtained using a boundary integral formulation for unsteady, incompressible, potential flows which accounts for the interaction between free wakes and propellers. For the aeroacoustic analyses, the Farassat 1A boundary integral formulation for the solution of the Ffowcs Williams and Hawkings equation has been used. These results provide an insight into the minimum distance between propellers to avoid aerodynamic/aeroacoustic interaction effects, which is an important starting point for the development of distributed propulsion systems.

Metamaterials, man-made composites scaled smaller than the wavelength, have demonstrated a huge potential in their applications in acoustics, opening up for sub--wavelength acoustic absorbers, acoustic invisibility, perfect acoustic mirrors and acoustic lenses for hyper focusing, acoustic illusions and enabling new degrees of freedom in the control of the acoustic field. The zero, or even negative, refractive sound index of metamaterials offers possibilities in control of the acoustic pattern and sound at sub--wavelength scales. Despite the tremendous growth of the research on acoustic metamaterials during the last decade, the potential of metamaterial-based technologies in aeronautics is still not fully explored and its utilization is still in its infancy. Thus the principal concepts mentioned above could very well provide means to develop devices that would allow the mitigation of the impact of the civil aviation noise on the community. This paper gives a review of the state of the art of the most relevant works on acoustic metamaterials, analyzing them against their potential applicability in aeronautics, and in this process identifying possible implementation areas and interesting metabehaviors. It also identifies some technical challenges and possible future directions for research with the goal of unveiling the potential of metamaterials technologies in aeronautics.

This research is part of a FP7 project supported by the European Commission called “Community oriented solutions to minimize aircraft noise annoyance” (COSMA). The present paper focuses on the development and the application of a methodology for the assessment of changes in airport noise exposure in home-like environments, particularly the design of sound scenarios and of experiments. Two airport noise changes are assessed and compared to a reference scenario: a halving of the number of aircraft, and an improvement in aircraft sound quality. For that purpose, sound perception, annoyance and activity disturbance as well as performances to cognitive tasks (Stroop and Word list Memory) are considered in the experiments involving 142 participants in 3 laboratories. Despite a similar LAeq, overall annoyance significantly decreases when reducing the number of aircraft, which reinforces the interest in a noise event index (NNE-NA) in addition to energetic noise indices for regulatory purposes. Results to a change in aircraft sound quality are largely inconclusive. Further work should address the human response to future airport noise scenarios based on new aircraft designs, including innovative concepts.

International audience; The paper presents a novel approach to include community noise considerations based on sound quality in Multidisciplinary Conceptual Design Optimization (MCDO) of civil transportation aircraft. The novelty stems from the use of an unconventional objective function, defined as a measure of the distance between the noise emission of the aircraft under analysis and a reference "weakly annoying" noise, the target sound. The minimization of such a merit factor yields an aircraft concept with a noise signature as close as possible to the given target. The reference sound is one of the outcomes of the European Research Project SEFA (Sound Engineering For Aircraft, VI Framework Programme, 2004-2007), and used here as an external input. The aim of the present work is to address the definition ad the inclusion of the sound-matching-based objective function in the MCDO of aircraft.

Turbulent boundary layersgenerate broadband noise as the effect of vortical‐disturbances scattering into acoustic waves. The paper presents a formulation for evaluating of acoustic pressure in the field in terms of the transpiration velocity, here defined in terms of vorticity and closely related to Lighthill equivalent source. Specifically, the formulation used allows one to obtain, in the frequency domain, a matrix relationship between the transpiration velocity at a number of points on the body surface (those arising from boundary‐element discretization) and the pressure at given points in the irrotational region. From this, the relationship between the corresponding PDF is easily obtained using the Wiener‐Khintchine theorem. The paper will include the general formulation, validating numerical results, and comparison with experimental data. The inverse problem, that is, determining the source intensity on the body surface from field noise (in particular, the invertibility of the operator), will be also addressed.

The paper presents the authors’ most recent advances in the development of a reliable and eective algorithm for the inclusion of community noise consideration within a Multidisciplinary Design Optimization (MDO) framework. The attention is here focused on the possibility to include sound quality issues as an alternative to the classical sound-level-based approach. The improvement of the sound quality could be seen as an additional “degree of freedom” available to the designer to reduce the impact of the air trac on residents’ life. One of the primary objectives of the European Research Project SEFA (Sound Engineering For Aircraft) is the definition of those characteristics that make the aircraft acoustic emissions less annoying. This is done by means of an extensive campaign of psychometric tests, supported by a careful sound engineering work. One of the outcomes of this activity is the synthesis of a target sound, having all the characteristics of “pleasantness” defined. The role of the authors within the project is the development of an algorithm capable to evaluate the feasibility of the target sound matching since the conceptual phase of the design of the aircraft. To accomplish this, a method for the quantitative evaluation of the dierence between the target sound and the acoustic emissions of the aircraft is needed. A careful definition of the “distance” between two sounds is introduced and validated, in order to identify a metric useful to properly build an objective function capable to drive the optimization process toward the target-sound-matching configurations. In this work, the evaluation of the sounds distance is extended to the analysis of non-stationary sounds and applied to match the emissions of recorded sounds in the final approach procedure. Specifically, the distance is evaluated as the L p -norm of the dierence between the current spectrum and target spectrum over the frequency-time domain. In order to evaluate the current spectrum, the computation of airframe noise, fan and compressor noise, buzz-saw and jet noise is performed. Atmospheric attenuation, ground reflection and doppler eect are also taken into account. In this work, an L 2 -distance between current and target spectra is taken as the objective function of an evolutionary algorithm. The static equilibrium of the aircraft is used as a constraint, whereas the variables space used includes both design and procedural parameters. Preliminary numerical results show that the method is capable to eectively drive the optimization process towards those configurations satisfying the sound-matching criterion.

The paper presents an extension of a formulation for life-cycle-costs-based MDO-CD (Multi-Disciplinary Optimization for Conceptual Design) of aircraft configurations for civil aviation, to take into account the stochastic variation of probabilistic parameters. Specifically, in the formulation used, the community noise is included in the optimization process as a cost. This cost depends a lot, for instance, upon government regulation and community perception, and hence is not necessarily determined when one is performing all the conceptual design computations. Thus, in the formulation presented, the cost of noise is used as a probabilistic design parameter and the optimal design is evaluated as that which minimize the expected value of the aircraft life-cycle-cost. In this preliminary work, the noise produced by the aircraft is evaluated using a simple Noise-Power-Distance model and included in the optimization process. A description of the technique used to estimate the community-noise impact on the life-cycle cost of the aircraft is presented. Applications to the optimization of an aircraft of the A320 category are shown.

In the present work we address the problem of the dynamic response of fluid-filled shell structure in uniform translation within a compressible medium in which an acoustic source is present. This represents an important engineering problem (as for instance the noise inside the fuselage generated by an outer source of noise), which involves the interaction of the structure with both the internal and the external fluid. Specifically, a unified finite-state formulation for the integrated analysis of the complete acoustoaeroelastic problem is presented; for the lack of space, only the theoretical formulation is presented. The formulation used here represents a blend of classical modal analysis with BEM. An eigenfunction expansion is introduced for the solution of the internal acoustics and the structural dynamics. The application of BEM is twofold. First, a boundary integral formulation for the external potential flow is given. Such a formulation is based on that of Morino [9], and is solved numerically by means of a boundary element method. On the other hand, the boundary element method is used also for the determination of the acoustic natural modes of vibration of the resonant cavity, for all those cases in which no analytical expressions of the eigenfunctions are avalaible. This methodology makes possible the analysis of cavities with arbitrary geometries.

The paper proposes an approach for MDO/PD (Multi‐Disciplinary Optimization for Preliminary Design) of innovative aircraft configurations in the presence of aeroacoustics considerations. The specific innovative configuration of interest here is a box‐wing aircraft denoted as the Prandtl-Plane, which has, as a distinguishing feature, a low induced drag. Thus, one of the advantages is less noise at take-o. Hence here, as a very first step towards our long range goal, we apply the algorithm to the optimization of the Prandtl-Plane with the objective function modified by adding an empirical term representative of the take-o noise. The fact that the configurations are innovative requires that the formulation be first‐principle based, since in this case the designer cannot rely upon past experience. The mathematical model used is reviewed. The formulation is based upon an integrated modeling of structures, aerodynamics, aeroelasticity and flight mechanics developed primarily by the authors. The methodology is geared specifically towards MDO/PD for civil aviation. The emphasis here is on wing design ‐ the fuselage is assumed as given. The stress analysis is based on finite elements for beams, whereas the structural dynamics is based upon natural modes, which are evaluated by the same finite‐ element algorithm. For the aerodynamic analysis, a boundary‐element quasi‐potential‐flow method is used for both steady and unsteady aerodynamics. An elementary boundary layer model is used to include the steady viscous eects and estimate the drag. A reduced order model (ROM) for the unsteady‐aerodynamics forces is used in dynamic aeroelasticity. Numerical results on the optimized Prandtl-Plane configuration are included. It is shown how the addition of the community-noise term in the objective function aects the configuration and produces noise reduction.

The paper presents recent work to include community noise considerations based on costs in an existing framework for MDO-CD (Multi-Disciplinary Optimization for Conceptual Design) of highly innovative configurations for civil aviation. The paper presents an extension of the formulation used in the code MAGIC (Multidisciplinary Analysis and desiGn of Innovative Configurations), developed by the authors and their collaborators, which comprises models for structures, aerodynamics, aeroelasticity, flight mechanics, and propulsion, along with the recent addition of aeroacoustics and life-cycle costs models. The community noise is treated here as a cost and included in the objective. It should be noted that the cost of noise depends a lot upon government regulation and community perception, and hence is not known during the conceptual design phase. Thus, the paper presents a parametric study of conceptual design based on life-cycle costs, with the cost of noise used as a varying parameter. The paper includes applications to the optimization of innovative configurations, such as the box-wing configuration (Prantl-Plane) proposed by Frediani. The box-wing configuration is particularly interesting because of the greatly reduced induced drag. This implies a reduction of the power required, and hence a reduction of community noise.