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Aeroacoustics of innovative aircraft cannot disregard the development of low-order models for the jet noise source, which are essential to assess the propulsion–airframe interactions from the conceptual design stage. The main scope of this work is to provide a noise source model that can be coupled with relatively low computational cost methods for aeroacoustic scattering. To this end, this paper presents for the first time a multiobjective optimization of the zeroth-mode wave-packet in the jet near field. The importance of calibrating the model with near-field pressure data stems from the fact that in new aircraft, the engine nacelles are typically positioned at a few diameters from the wing or fuselage. In this work, the near field of a high subsonic jet at a Mach number of 0.9 is represented as a cylindrical surface radiating the pressure disturbances of a wave-packet source, which is optimized using large-eddy simulation data from three lines at different radial distances. The optimized model has been tested at Strouhal numbers between 0.25 and one, and the optimized solutions have been chosen using a Pareto-ranking criterion considering the wave-packet prediction over an extra line. A good agreement is achieved between the reference data and model predictions for multiple near-field radial distances.

The present work reports an investigation on the use of adaptive metamodels based on radial basis functions (RBFs) for aeroacoustic applications of highly innovative configurations. The relevance of the topic lies on the paramount importance of metamodelling techniques within the design optimisation process of disruptive aircraft layouts. Indeed, the air traffic growth, consequently the hard environmental constraints imposed by regulations, will make a technological breakthrough, an imperative need within few years. As a consequence, the engineering community is paying particular attention to the development of innovative techniques for the design of unconventional configurations. For this class of applications, the designer cannot successfully rely on historical data or low-fidelity models, and the expensive direct simulations remain the only valuable design strategy. In this regard, it can be demonstrated that the use of surrogate models, i.e., metamodels, significantly reduces the computing costs, especially in view of a robust approach to the optimised design. In order to further improve the efficiency of metamodel-based techniques, dynamic approaches based on hyperparameter optimisation and adaptive sampling procedures have been recently developed. The case study presented here pertains the exploiting of dynamic RBF-based metamodels for noise shielding applications. The analysis of the metamodel performances and its convergence properties shows how the final number of direct simulations is significantly reduced by the hyperparameter optimisation algorithm, still strongly depending on the choice of the RBF kernel function.

This work deals with the investigation and modelling of wall pressure fluctuations induced by a supersonic jet over a tangential flat plate. The analysis is performed at several nozzle pressure ratios around the nozzle design Mach number, including slightly over-expanded and under-expanded conditions, and for different radial positions of the rigid plate. Pitot measurements and flow visualizations through the background oriented schlieren technique provided a general overview of the aerodynamic interactions between the jet flow and the plate at the different regimes and configurations. Wall pressure fluctuations were measured using a couple of piezoelectric pressure transducers flush mounted over the plate surface. The spectral analysis has been carried out to clarify the effect of the plate position on the single and multivariate wall pressure statistics, including the screech tone amplitude. The experimental dataset is used to assess and validate a surrogate model based on artificial neural networks. Sound pressure levels and coherence functions are modelled by means of a single fully connected network, built on the basis of a recently implemented fully deterministic topology optimization algorithm. The metamodel uncertainty is also quantified using the spatial correlation function. It is shown that the flow behaviour as well as the screech and broadband noise signatures are significantly influenced by the presence of the plate, and the effects on spectral quantities are correctly reproduced by the proposed data-driven model that provides predictions in agreement with the available data.

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.

The project AERIALIST (AdvancEd aicRaft-noIse-AlLeviation devIceS using meTamaterials), funded within the Breakthrough Innovation topic of the H2020 program, has closed its activity on May 2020. The objective of the project was the disclosure of the potential of metamaterials in developing disruptive devices for the mitigation of aircraft noise, in order to contribute to the identification of the breakthrough technologies targeted at the achievement of the noise reduction targets foreseen by the ACARE Flightpath 2050. Although targeted to low TRL, AERIALIST has been focused on the development of an integrated toolchain capable to address the entire design loop, from the early conception to the numerical and experimental proof of concept, up to the final design and manufacturing. The toolchain was founded onto four pillars: i) the extension of the acoustic metamaterial theory to aeroacoustics; ii) the exploitation of the latest additive manufacturing technologies; iii) the wind-tunnel assessment of the selected concepts; iv) the identification of a development roadmap towards higher TRL. After three years of activity, the project has attained all its objectives. The present paper is a review of the main outcomes of the project, their application potential and relevance to the ACARE objectives.

This article deals with the validation of two analytical spacetime corrections for the extension of the acoustic metafluid model to aeroacoustics. The need for convective metafluid corrections is due to the performance decay of a statically-designed acoustic metafluid device in presence of a mean flow in the hosting fluid. Aeronautical applications cannot disregard the presence of a Mach number which causes the distortion of acoustic propagation pattern due to the coupling between acoustic propagation and fluid dynamics phenomena. Hence, analytical corrections represent one of the possible strategies to overcome such performance decay. For a more comfortable manipulation and the introduction of the analytical corrections, a spacetime reinterpretation of the characteristic equations is needed. Two analytical corrections, derived from Taylor’s and Prantdl-Glauert’s transformations, and several Mach numbers up to 0.3 are considered during the validation process. The test case considers a rectangular domain occupied by a metafluid embedded in a conventional medium domain, characterized by a uniform background flow and where a monopole source emits acoustic perturbations. The numerical setup is chosen to eliminate any spurious effects introduced by the analytical approximations and to limit, as much as possible, those related to the numerical implementation. Moreover, it legitimates the comparison between the corrections used. To ensure the continuity of the acoustic propagation between the two media, a crucial role is covered by the correct imposition of the boundary conditions. The importance of using primitive variables in doing so is underlined. Numerical simulations are implemented in the frequency domain using the commercial FEM solver COMSOL Multiphysics.

The expected increase of the air traffic and the emergence of new paradigms of air transportation induced by the evolution of the market demand are going to significantly worsen the community noise scenario of our cities. An accurate and timely prediction of the noise emission is, therefore, a mandatory aspect of the design process of any innovative air transport solution. Unfortunately, aeroacoustic simulations are extremely expensive and a popular approach to circumvent the problem is the development of surrogate models equally accurate yet computationally cheaper for the evaluation of the objective functions. This work deals with a multi-fidelity approach to surrogate modeling for the identification of optimal engine installation to maximise noise shielding. The paper explores the possibility to couple reinforcement learning techniques with a trust-region-based multi fidelity approach to meta modelling. Specifically, an accelerated variant of the well known Q-learning algorithm, namely the Evolutionary Q-learning (EVQL) is used to train a group of agents, each one deployed in one of a set of trust subregions, defined around the high-accuracy samples. The subregions of confidence are updated according to the exit directions of each agent. These directions are also used to enrich the set of high-accuracy samples by evaluating the meta model uncertainties along them in the untrusted regions of the domain.

The use of surrogate models in recent times plays a key role in the design process and, overthe past few years, machine learning algorithms are adapting to the use of active metamodellingtechniques. In the present study, Artificial Neural Networks formulation is used as a data–driven nonlinear model aimed at describing the dynamics of an experimental test campaignrelated to the noise emitted by a single–stream jet in under–expanded conditions. The architectureof the neural network is selected employing a deterministic optimization algorithm,coupled with data informed tuning of the input parameters. The data set explored here wasacquired in the state–of–the–art aeroacoustic facility at the University of Bristol. Both the nearandfar–field acoustic measurements were carried out for a cold under–expanded jet for Machnumbers ranging from 1.1 to 1.4. The predictions by the metamodel are in good agreementwith the experimental data, and the results demonstrate the capability of metamodels as areliable tool to estimate jet noise in under-expanded flow conditions for a wide range of Machnumbers and near–field locations.

Two major issues affecting the solution of Boundary Integral Equations via collocation methods (such as the Boundary Element Method) are the high memory requirement/usage for large problems resulting, \textit{e.g.}, in high frequency acoustics, and the need for regularisation due to the non-uniqueness of the solution for the so-called spurious frequencies for exterior problems. The first problem can be effectively mitigated introducing methods such as the Adaptive-Cross-Approximation technique, which builds a hierarchical block-wise approximation of the matrix arising from the discretisation of the BIE, with a combination of low-rank approximated and fully populated blocks, reducing the storage requirement. Typical strategies to address the regularisation of the solution are the CHIEF points and the Burton \& Miller methods. More recently, the Dual Surface method has been introduced and adapted for acoustics. The Dual Surface method is a regularization technique that does not increase the problem size and avoids the evaluation of hyper--singular integrals, combining the advantages of the Burton \& Miller and CHIEF methods, requiring, however, the evaluation of an increased number of integrals for each matrix entry. This drawback is partially compensated when using the ACA partial pivoting algorithm, which limits the number of entries to be evaluated when building the system matrix. In this paper, the implementation of the Dual Surface method will be discussed for the classic Kirchhoff-Helmholtz integral equation and for a BIE for the sound scattered by moving bodies based on the Ffowcs Williams \& Hawkings equation.

Aeroacoustic analyses of new generation and highly innovative aircraft configurations such as Hybrid and Blended Wing Body cannot disregard the development of low-order models for the jet noise source, essential to assess the propulsion-airframe interactions since the conceptual/preliminary design stage. The use of wave-packets to model jet noise is based on the widely accepted hypothesis that the large-scale turbulent structures, responsible for noise peaks emitted by subsonic and supersonic jets, can be modelled as instability waves that grow and then decay with axial distance. In this study, a M=0.9 high-subsonic jet is represented as a cylindrical surface radiating the pressure disturbances of a wave-packet source, whose parameters are optimized using near-field information from LES simulations. The importance of calibrating the model with near-field pressure data stems from the fact that innovative aircraft configurations have engines nacelles typically positioned at a few diameters from the wing or fuselage. The main scope of this analysis is to provide a noise source model that can be coupled with Boundary Elements Method (BEM) codes for aeroacoustic scattering evaluations. A quite good agreement is achieved at multiple near-field radial distances between the simulation data and model prediction for the dominating 0th azimuthal mode at the selected Strouhal numbers up to 1.

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

(RBF) in the development of dynamically adaptive meta–models for aeroacoustic applications. The analysis focuses on the influence of the RBF kernel and the chosen stochastic parameters on the modelling of target functions of interest in the aeroacoustics of aircraft. The rationale underlying the research is related to the key role that aeroacoustics plays in the establishment of the commercial aviation scenario foreseen for the next three decades. Indeed, the sustainable development for the airborne transportation system is strongly constrained by community noise, which, nowadays, limits the increase of the capacity of the existing airports and prevents the building of new ones. In such a situation, the design of the next generation of aircraft must take into account the impact of noise on the population since the early conceptual phase of the design. This causes a substantial increase of the required computational resources, especially for unconventional, breakthrough concepts for which simple semi–empirical models are not available and the only viable strategy is computational aeroacoustics. The availability of reliable meta–models can give a significant contribution in two ways: i) in a process of multiobjective, multidisciplinary design optimization a dynamic adaptive stochastic meta– model can reduce significantly the calls to the computationally expansive tools and enhance the effectiveness of the design space exploration; ii) the versatility and applicability range of end–user tools for the estimate community noise impact can be greatly improved by fast yet accurate models of the noise signature of novel concepts. Two target functions are analysed here: the total acoustic field induced by a point source co–moving with a scattering profile, and the shielding factor along a line of observation points below the scatterer. The performance of RBF meta–models based on tailored kernels is compared to the most commonly used kernels in terms of accuracy and convergence rate. The effectiveness of the dynamic meta–model update based on uncertainty quantification is assessed for different choices of the stochastic parameter

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.

To give residents better understanding of the impact of future airport scenarios, a virtual reality application with sound simulation will be tested in the frame of the European ANIMA project. The set-up has been evaluated in laboratory before being used in real situation. This paper presents the laboratory experiment whose aim is to assess the application’s relevance for simulating flyovers. Although the perceptual experiment is designed to test the influence of aircraft vision crossed with aircraft sound, this paper focusses only on the impact of the landscape where the flyovers are observed. Two landscapes (park and buildings) are presented to 60 participants, in balanced order, with 12 audio-visual stimuli in both landscapes. Participants had to rate four differential semanticscales. Globally there is no influence of the landscape on the overall pleasantness, but when looking at the individual answers, it appears that three groups of participants can be discriminated. The majority of people do not change their pleasantness ratings in both landscapes, but some participants prefer experiencing the flyovers in the park landscape because it is visually more pleasant, and others prefer the oppositebecause it is more annoying to be submitted to aircraft noise in a green park landscape

The present work deals with the synthesis of sounds produced by brass instruments through the direct physical modelling. The purpose is the development of an integrated methodology for the evaluation of the response of a wind instrument taking into account the properties of the surrounding environment. The identification of the frequency response of the resonator and the performing environment is obtained by means of a Boundary Integral Equation approach. The formulation produces the matrix transfer function between the inflow at the input section of the instrument bore and the signal evaluated at an arbitrary location, and can account for the response of any boundary and object present in the surroundings. The reflection function obtained from the above model is coupled to a simplified model of valve, used to represent the excitation mechanism behaviour. The exploited algorithm has demonstrated to be accurate and efficient in offline calculation, and the observed performance discloses the possibility to implement real–time applications.

The present work deals with the multiobjective, multidisciplinary optimisation of takeoff and approach operations of a commercial aircraft aimed at the mitigation of the impact of aviation noise on the population. The innovative approach used here couples the minimisation of the aircraft noise level at the certification points with the improvement of the sound quality. The latter objective represents the main novelty of the present work and is addressed using a spectral–matching approach to make the aircraft noise as close as possible to a target sound. The rationale underlying the research is the development of a community–oriented approach to the assessment airport operations in view of the complete redefinition of the future airport scenarios. Indeed, the air traffic growth, the rapid expansion of urban areas around airports, and the expected advent of urban air mobility, are transforming the aviation noise into a serious hazard to the sustainable development of society. The sound–quality–based objective imposes a comprehensive multidisciplinary approach also in the procedural optimisation, due to the detail required to estimate the noise spectrum composition. Two merit factors are minimised, specifically the EPNL at the noise certification points and the Lp –norm of the difference between the noise produced by the configuration under analysis and a target sound. The target sounds are obtained by using sound engineering techniques aimed at the sound quality improvement, on the basis of the results of the psychometric tests campaigns performed within the projects SEFA and COSMA. The minimisation is achieved adopting a global evolution method, and the results are presented in terms of approximated Pareto frontiers for a single–aisle aircraft in both takeoff and landing conditions.

The effect of background flows on the response of acoustic metamaterials is a key aspect that prevented the full disclosure of their potential in those applications where an aerodynamic velocity field strongly influences the propagation of acoustic disturbances. Indeed, the classic approaches for metamaterial design do not consider the aeroacoustic interaction, and the resulting metamaterials cannot preserve their response when operating in flows. So far, only few authors have addressed the problem, mostly focusing on understanding the phenomenon or identifying corrective techniques with limited usability in practical applications. The present study proposes a general method for the modification of the mechanical properties of acoustic metacontinua to preserve their response in presence of a background flow. The method is based on the application of spacetime coordinate transformations exploiting the spacetime formal invariance of the generalised d’Alembertian. This methodology applies to the equation governing the propagation of acoustic disturbances in a metamaterial having arbitrary constitutive equations independently on the method used for its original design. The approach is validated through numerical simulations, using as a benchmark the problem of the acoustic cloaking of a cylinder impinged by a perturbation generated by an isotropic point source within a flowing medium. Numerical results are obtained for an asymptotic Mach number $$M_\infty \le 0.35$$ M ∞ ≤ 0.35 .

In laboratory-scale burner it has been observed that the acoustic excitations change the flame topology inducing asymmetry and oscillations. Hence, an acoustic and aeroacoustic study in non reactive condition is of primary importance during the design stage of a new burner in order to avoid the development of standing waves which can force the flame. So wall pressure fluctuations inside and outside of a novel slot burner have been studied experimentally and numerically for a broad range of geometrical parameters and mass flow rates. Wall pressure fluctuations have been measured through cavity-mounted microphones, providing uni- and multi-variate pressure statistics in both the time and frequency domains. Furthermore, since the onset of combustion-driven oscillations is always presaged by intermittent bursts of high amplitude, a wavelet-based conditional sampling procedure was applied to the database in order to detect coherent signatures embedded in the pressure time signals. Since for a particular case the coherent structures identified have a multi-scale signature, a wavelet-based decomposition technique was proposed as well to separate the contribution of the large- and small-scale flow structures to the pressure fluctuation field. As a main outcome of the activity no coupling between standing waves and velocity fluctuations was observed, but only well localized pressure signatures with shape strongly affected by the neighbouring flow physics.

Space-coiling metamaterials that exploit the so-called generalized Snell's law are suitable for the subwavelength manipulation of acoustic waves, leading to extraordinary refracting and reflecting metasurfaces. Typically, space-coiling metasurfaces are characterized by a narrow operating range in frequency due to the intrinsic connection between the design wavelength and the characteristic dimensions of the metasurface. The main goal of the present work is to design a broadband and modular space-coiling based metasurface, extending the range of frequency where the effective metabehaviour is achieved. The metasurface building set is composed by eight different unit cells, each introducing a tailored phase shift in the field, that can be combined side by side to produce the desired acoustic effect. The design parameters of each cell are selected as the result of an optimization process that minimizes the dependency on the operating frequency of the metabehaviour, keeping the overall thickness below a quarter of the design wavelength. Significant results are obtained for benchmark problems.

This paper presents uncertainty quantification (UQ) benchmarking activities within the NATO AVT-252 Task Group on "Stochastic Design Optimization for Naval and Aero Military Vehicles." UQ methods are assessed and compared as an essential part of stochastic optimization procedures. Several metamodel (dynamic radial basis functions, dynamic Kriging, Gaussian process), Monte Carlo (multi-level Monte Carlo) and collocation (non-intrusive stochastic collocation) methods are applied to 2D and 15D UQ problems (that is with 2 or 15 uncertainties) of a NACA 2412 airfoil subject to operational (Mach number) and geometrical (design variables) uncertainties, both in the subsonic and transonic regimes. The flow is solved by MSES, XFoil, and the FINE TM /Open suite. A pure Monte Carlo simulation based on more than 300,000 samples is used as a benchmark to assess the accuracy and efficiency of UQ methods. These can provide satisfactory results with errors for the expected value and standard deviation of C D smaller than 0.5 and 5% respectively.

The application of global/local hybrid DIRECT algorithms to the sim- ulation-based hull form optimization of a military vessel is presented, aimed at the reduction of the resistance in calm water. The specific features of the black-box-type objective function make the problem suitable for the application of DIRECT-type algorithms. The objective function is given by numerical iterative procedures, which could lead to inaccurate derivative calculations. In addition, the presence of local minima cannot be excluded a priori. The algorithms proposed (namely DIRMIN and DIRMIN-2) are hybridizations of the classic DIRECT algorithm, with deterministic derivative-free local searches. The algorithms’ performances are first assessed on a set of test problems, and then applied to the ship optimization application. The numerical results show that the local hybridization of the DIRECT algorithm has beneficial effects on the overall computational cost and on the efficiency of the simulation-based optimization procedure.

A multi-objective deterministic hybrid algorithm (MODHA) is introduced for efficient simulation-based design optimization. The global exploration capability of multi-objective deterministic particle swarm optimization (MODPSO) is combined with the local search accuracy of a derivative-free multi-objective (DFMO) line search method. Six MODHA formulations are discussed, based on two MODPSO formulations and three DFMO activation criteria. Forty-five analytical test problems are solved, with two/three objectives and one to twelve variables. The performance is evaluated by two multi-objective metrics. The most promising formulations are finally applied to the hull-form optimization of a high-speed catamaran in realistic ocean conditions and compared to MODPSO and DFMO, showing promising results.

The paper presents a method to modify the mechanical properties of a statically--designed metamaterial device in order to preserve its performance when operating in a non--uniform aerodynamic flow. The objective of the research is to contribute to the disclosure of the acoustic metamaterial potential in aeroacoustic applications, where acoustic propagation and scattering are deeply affected by aerodynamic convection. The emphasis is on aeronautical applications aiming at the development of methods for the design of breakthrough solutions for aircraft noise mitigation. The present approach is based on the application of the inverse Taylor transformation to the static design space in order to obtain the properties of an equivalent metamaterial in the convected space. The metamaterial so--obtained is capable to guarantee the target acoustic response in presence of a non--uniform background flow at low Mach number and with negligible vorticity. Numerical results obtained through finite--element simulations are presented for a free stream Mach number $\leq 0.35$, which is compatible with the take--off and landing operation of a commercial airplane. The benchmark used is the widely assessed problem of the scattering cancellation (cloaking) of a circular obstacle. The acoustic disturbance is here assumed to be generated by an isotropic point source located in the vicinity of the scatterer and co--moving with it. The numerical results reveal that the effect of the proposed correction strongly depends on the relative position of the source and the metamaterial device, with the worst performance obtained when they are aligned with the free stream. The reason of this dependence is analytically explained and verified numerically. When the alignment of the source and the treated object is orthogonal to the flow, the Taylor--corrected metamaterial recovers almost completely its expected behaviour. This result makes the approach appealing for all those engineering applications where the relative position of sources and moving boundaries is fixed and compatible with the most favourable conditions.

This paper deals with the development of an integrated framework for the development of acoustic metamaterials tailored to aeroacoustic applications. Indeed, the contributions available in the literature targeted are still limited (although constantly increasing) with respect to the gigantic amount of works on standard acoustic metamaterials. The research is driven by the urgent need to substantially reduce the noise produced by the civil aviation to guarantee the sustainable evolution of the system in a constantly growing market. The target reductions required to mitigate the impact on population will be achieved only through the introduction of breakthrough technologies. The present research aims at the complete disclosure of the acoustic metamaterials potential to contribute to this target. The approach starts from the formulation of the fundamental equations governing the propagation of an acoustic disturbance within a generic metacontinuum operating in the presence of a background flow. The model is coupled with an original homogenization scheme, to identify, through numerical optimization, the basic properties of a periodic structure matching the target metacontinuum response. Preliminary numerical results are obtained using dedicated finite and boundary element methods.This paper deals with the development of an integrated framework for the development of acoustic metamaterials tailored to aeroacoustic applications. Indeed, the contributions available in the literature targeted are still limited (although constantly increasing) with respect to the gigantic amount of works on standard acoustic metamaterials. The research is driven by the urgent need to substantially reduce the noise produced by the civil aviation to guarantee the sustainable evolution of the system in a constantly growing market. The target reductions required to mitigate the impact on population will be achieved only through the introduction of breakthrough technologies. The present research aims at the complete disclosure of the acoustic metamaterials potential to contribute to this target. The approach starts from the formulation of the fundamental equations governing the propagation of an acoustic disturbance within a generic metacontinuum operating in the presence of a background flow. The model is ...

An overview is provided of stochastic uncertainty quantification (UQ) and validation methods for application to realistic problems in ship hydrodynamics. The paper summarizes the research activities conducted by the hydrodynamics team within the NATO Task Group AVT-191 "Application of Sensitivity Analysis and Uncertainty Quantification to Military Vehicle Design." UQ methods assess the expected value (EV), standard deviation (SD) and cumulative distribution function or probability density function (CDF/PDF) of the hydrodynamic performance of interest. The hydrodynamic solvers include unsteady Reynolds-averaged Navier-Stokes and potential flow. Three UQ problems are presented. The first is a unit study, addressing the effects of stochastic Reynolds number on the lift and drag of a NACA 0012 2D hydrofoil at constant angle of attack. The second and the third are industrial problems of a high-speed catamaran, advancing in calm water and waves respectively, with stochastic operating conditions and geometry. Numerical validation benchmarks include the deterministic V&V for selected conditions, along with converged (quasi) Monte Carlo (MC) samples. Non-intrusive UQ methods are applied and discussed. MC simulations with direct CFD simulations and with metamodels are shown. Metamodels include linear and Hermite-cubic interpolation, inverse distance weighting, quadratic and cubic response surface, Hermite and Legendre polynomial expansions, least-square support vector machine, thin plate spline, radial basis functions network with multiquadric kernels, polyharmonic splines, Kriging using linear and exponential covariance functions, dynamic radial basis functions and dynamic Kriging. Polynomial chaos method and quadrature formulas (trapezoidal, Simpson's rule, Gaussian quadrature) are also assessed and compared to metamodel-based methods. Convergence criteria for UQ methods include deterministic and stochastic convergence criteria for MC simulation, along with convergence criteria for MC with metamodels, and quadrature formulas. UQ methods have shown their maturity for application to realistic stochastic design optimization problems. MC with dynamic metamodels is found the most promising method overall, due to its function-adaptation capability and high computational efficiency, which make the method also recommended for stochastic optimization.

The paper deals with the use of sound quality as a criterion for the ranking of the non-dominated solutions of a multi-objective design optimization of aircraft oriented to environmental sustainability. The solutions belonging to the approximated Pareto front of the bi-objective minimization of the fuel burned and the noise level at certification points are ranked with respect to the matching of the emitted noise spectrum against a reference target sound. The latter is a synthesized aircraft sound satisfying sound-quality specifications coming from psychometric tests conducted in the EC-funded projects (Sound Engineering for Aircraft) and COSMA (Community Oriented Solutions to Minimize aircraft noise Annoyance). Indeed, the environmental impact of civil aviation is nowadays the most urgent concerns in the development a sustainable air transportation system since, during the last three decades, the air traffic growth has generated a remarkable increase of the operative airports. In parallel, the urban development has led to the inclusion of secondary airports within the city areas. As a consequence, the number of people directly exposed to aircraft noise has sensibly increased. In such a social context, the designers can hardly attain further substantial technical improvements as the standard concepts and technologies have reached a development saturation, thus imposing the identification of breakthrough technologies and innovative concepts, with a plausible date of entry into service not early than 2050. The research initiated in SEFA and further developed in its follow-up COSMA aims at the alleviation of the noise impact on the population, switching the attention on noise quality rather than noise level, in order to provide to designers additional tools to mitigate the effects of noise. The present work is the evolution within a multi-objective context of the early attempts of sound-quality-oriented MDO developed during those projects.

The EC-funded project COSMA (Community Oriented Solutions to Minimize aircraft noise Annoyance, 7th Framework Programme) started in June 2009 with an ambitious, twofold goal: improve the understanding of the annoyance induced by aircraft noise on the population and identify the engineering guidelines to establish appropriate design strategies and operational procedure to reduce these effects. The project was conceived within the context of the X-Noise Collaborative Network, a worldwide network of experts and institutions committed to the commercial aviation noise challenge. The COSMA objectives were addressed using a highly multi-disciplinary approach, integrating competences pertaining to psychoacoustics, sound engineering, and aeronautical engineering. The work of twenty-three research groups from nine European countries was structured in six, strongly interconnected work packages. Aim of the present paper is the review of this cross-disciplinary research from the point of view of the aircraft designer. The attention is focused on the Optimisation of Airport Noise Scenarios work package, where the aeronautical technology contextwas established in detail and where, eventually, the design criteria and operational recommendation were defined on the basis of the outcomes of the annoyance examination experimental campaign.