Search

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.

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.

The paper deals with a community-oriented approach to the multiobjective optimisation of sustainable takeoff and landing procedures of commercial aircraft. The objective functions to be minimised are defined as the measure of area surrounding the airport where the Sound Exposure Level (SEL) is higher than 60 dBA, and the amount of fuel burned during the procedure. The first merit factor is a measure of the number of citizens affected by a potentially harmful noise level, whereas the second is proportional to the chemical emissions. The novelty of the present approach is the use of a criterion based on sound quality for the selection of the optimal procedure from the Pareto front set. The spectrum of the noise produced by each non-dominated solution is compared to a reference spectrum, the target sound. This is synthesised to meet the acceptance requirements that emerged by a campaign of psychometric tests. The rationale underlying the research is tightly linked to the expected transformation of civil aviation, with the advent of new air transport solutions in urban and suburban environments. The breakthrough nature of the emerging scenarios requires a drastic renewal of the approaches used in the management of operations, and the present work represents a contribution to this evolution. The optimisation is attained adopting a global, deterministic method, and numerical results are obtained for single- and twin-aisle aircraft.

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 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 .

The present work deals with the development of an innovative approach to the weight estimation in the conceptual design of a Hybrid-Electric-Powered (HEP) Blended Wing Body (BWB) commercial aircraft. In the last few decades, the improvement of the environmental impact of civil aviation has been the major concern of the aeronautical engineering community, in order to guarantee the sustainable development of the system in presence of a constantly growing market demand. The sustained effort in the improvement of the overall efficiency of conventional aircraft has produced a new generation of vehicles with an extremely low level of emissions and noise, capable of covering the community requirements in the short term. Unfortunately, the remarkable improvements achieved represent the asymptotic limit reachable through the incremental enhancement of existing concepts. Any further improvement to conform to the strict future environmental target will be possible only through the introduction of breakthrough concepts. The aeronautical engineering community is thus concentrating the research on unconventional airframes, innovative low-noise technologies, and alternative propulsion systems. The BWB is one of the most promising layouts in terms of noise emissions and chemical pollution. The further reduction of fuel consumption that can be achieved with gas/electric hybridisation of the power-plant is herein addressed in the context of multidisciplinary analyses. In particular, the payload and range limits are assessed in relation to the technological development of the electric components of the propulsion system. The present work explores the potentialities of an energy-based approach for the initial sizing of a HEP unconventional aircraft in the early conceptual phase of the design. A detailed parametric analysis has been carried out to emphasise how payload, range, and degree of hybridisation are strictly connected in terms of feasible mission requirements and related to the reasonable expectations of development of electric components suitable for aeronautical applications.

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.

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.

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.

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 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.

The paper presents an original boundary integral equation (BIE) formulation for the analysis of the acoustic cloaking of a scatterer. The advantage of such an approach is the lower computational burden, especially when the analysis of a large portion of the hosting domain is required. The partial differential equation governing the propagation inside the cloak is recast in the form of non-homogeneous wave equation, with field sources depending on the mechanical properties of the cloak. The boundary integral formulation is derived using the standard procedure. The boundary element method (BEM) is used to derive the matrix transfer function of the cloak. The latter is applied to the incident field at the cloak's outer boundary to obtain the total field at arbitrary locations in the host. The formulation is applied to the simple case of a radially symmetric cloak embedding a circular obstacle. Numerical results are presented for sound-hard and sound-soft obstacles, including a study of the cloaking efficiency as a function of the frequency.

The state of the boundary layer is of high importance since skin friction drag and heat transfer rates in a turbulent boundary layer can be several times higher than those of a laminar one. A lot of different strategies are used to delay or prevent the transition process. One possibility to manipulate the transition is using porous surfaces to influence the growth of the second mode in a passive way. The 2nd or so called Mack mode is dominant for the transition process at hypersonic Mach numbers. Some studies showed that an ultrasonically absorptive coating (UAC) can suppress the 2nd instability and then delay the transition of hypersonic boundary layer. The acoustic scattering problem affects the design of ultrasonic absorptive coatings for hypersonic laminar flow control. To investigate this phenomenon a finite element method (FEM) was employed to formulate 2-D simple model of UAC. The results provided are in strong agreement with existing results achieved by direct N-S solution (DNS) and theoretical models. DNS requires important computational costs while theoretical modelling is not adaptable to any type of geometry. Conversely, the FEM is extremely flexible and computationally affordable, resulting as the best candidate to perform design optimizations of more efficient UAC. Pressure time histories for coating of different porosities, various forcing frequencies and single or double cavity configurations are presented. Overall, the amplitude reflection by the UAC decreases with higher porosity and, in most cases, by reducing the dimensionless wavelength of the forcing. Hence, on the basis of our parametric study of the geometrical factors, we identified the dimensionless wavelength of the forcing λ∗ and the porosity ϕ as the most important parameters for UAC design. Moreover a comparison between UAC based on single-depth and double-depth rectangular cavity has been performed suggesting an interesting strategy to design a novel generation of UAC.

This paper presents a methodology for the identification of a reduced-order model (ROM) for the perturbation aeroelastic analysis of fixed wings in transonic flight. It is based on a linearized, frequency-domain, boundary-field integral equation for the solution of the unsteady perturbation potential flow about steady-state reference wing configurations. The resulting transfer functions between structural Lagrangean variables and generalized aerodynamic forces are approximated by means of rational expressions, and the aeroelastic ROM is identified by coupling them with the structural operator. With the aeroelastic operator recast in a reduced-order form, transonic flutter boundaries are detected through a classical eigenvalue analysis and the time-domain state–space aeroelastic model is also obtained. Applications of the methodology presented to a widely known aeroelastic test case reveal a remarkable agreement with the measured speed and frequency of flutter.

A reliability-based robust design optimization (RBRDO) for ship hulls is presented. A real ocean environment is considered, including stochastic sea state and speed. The optimization problem has two objectives: (a) the reduction of the expected value of the total resistance in waves and (b) the increase of the ship operability (reliability). Analysis tools include a URANS solver, uncertainty quantification methods and metamodels, developed and validated in earlier research. The design space is defined by an orthogonal four- dimensional representation of shape modifications, based on the Karhunen-Lo` eve expansion of free-form defor- mations of the original hull. The objective of the present paper is the assessment of deterministic derivative-free multi-objective optimization algorithms for the solution of the RBRDO problem, with focus on multi-objective extensions of the deterministic particle swarm optimization (DPSO) algorithm. Three evaluation metrics provide the assessment of the proximity of the solutions to a reference Pareto front and their wideness.

A high-order boundary element method (BEM) for the analysis of the steady two-dimensional full-potential transonic equation is presented. The use of a high-order (piecewise cubic on the boundary and piecewise bi-cubic in the field) numerical formulation, the main novelty of the present work, is important in two respects: first, the convergence of the solution as h vanishes is faster than the zeroth-order (the piecewise constant) one, yielding more accurate results with coarser grid resolutions. In addition, in supercritical flows, the derivation of the velocity field from the high-order representation for potential gives, in the vicinity of the shock, a sharper discontinuity, and allows for an in-depth analysis of the shock properties. Both aspects are analyzed in the present paper through applications to steady, two-dimensional, full-potential flows. All the results obtained using the present method are validated through comparison to other computational fluid dynamics (CFD) solutions of the full-potential flows and, when applicable, Euler equations. A comparison to a zeroth-order BEM, based on the same integral formulation is also included.

Simulation-based design (SBD) optimization assists the designer in the design process of complex engineering systems. In this context, real-world problems are affected by different sources of uncertainties (operational, environmental, geometrical or numerical) and require reliability-based robust design optimization (RBRDO) formulations to identify the optimal solution. RBRDO is usually computationally very costly (especially if high-fidelity simulations are used) and may be achieved by means of metamodels, with efficient optimization algorithms. Herein, a RBRDO for ship design is solved, for real ocean environment including stochastic sea state and speed.

The paper presents a nature-inspired derivative-free global optimization method, namely the fish shoal algorithm (FSA), for solving engineering optimization problems with costly objective functions. The method is intended for unconstrained single-objective maximization and is based on a simplified social model of a fish shoal in search for food. Derivative-free global optimization approaches are usually preferred to derivative-based local approaches, when objectives are noisy, derivatives are unknown and the existence of multiple local optima cannot be excluded, as often encountered in simulation-based design (SBD) optimization. When global techniques are used with CPU- time expensive solvers, the optimization process is computationally expensive and its effectiveness and efficiency remain an algorithmic and technological challenge.

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.

to be addressed. The objective of the paper is to present a formulation for the evaluation of the power spectral density of the acoustic pressure at any given point in the field in terms of the power spectral density of the transpiration velocity: this is a quantity defined in terms of the vorticity and is closely related to the equivalent source concept introduced by Lighthill 7 (the relationship between the two is addressed in Appendix B). Specifically, the formulation used allows one to obtain, in the frequency domain (Fourier transform), a matrix relationship between the transpiration velocity at a number of points on the surface of the object (those arising from the boundary‐element discretization) and the pressure at given points in the irrotational region. 1 From this, the relationship between the corresponding power spectral densities is easily obtained using the Wiener-Khintchine theorem. The approach used here is based upon a formulation introduced for aerodynamics in Ref. 11, and refined in Refs. 13 and 14. The commonality between aerodynamics and aeroacoustics is addressed in Ref. 12 (which provides a synthesis of all the preceding work), and is exploited here. Although applications to aeroacoustics implicitly imply compressibility, for the sake of clarity in the main body of the paper the formulation is presented for an incompressible flow (the formulation for compressible flows is presented in Appendix C).

This paper deals with the acoustic field generated by rigid bodies in arbitrary motion. The Ffowcs Williams and Hawkings equation is the starting point for the derivation of a boundary integral formulation for the acoustic disturbance. It yields a unified approach for the prediction of both sound generated aerodynamically and sound scattered by arbitrarily moving, (eventually) vibrating, surfaces impinged by an acoustic signal. The advantages of the acoustic formulation presented here with respect to other approaches that are commonly used are discussed. A boundary element method is applied for its numerical solution and the resulting prediction tool is validated by analysing the problem of low-frequency and high-frequency plane waves impinging on a rigid sphere. Then, the formulation is applied to an aeronautical configuration for the determination of the scattered pressure field generated by a fuselage impinged by the incident pressure wave emitted by a propeller.

In the present work we address the problem of the dynamic response of an acoustoaeroelastic system, i.e., a fluid-filled elastic structure in motion within a compressible fluid in presence of a sound source. The analysis is accomplished by a fully integrated approach, taking into account the feedbacks between the structure, the cavity, and the exterior flow, leading to a single acoustoaeroelastic (matrix) equation. This is formulated in terms of the amplitudes of the structural models, considered in vacuo. Such an approach is developed by expressing the pressure in the cavity using its acoustic natural modes of vibration, while for the pressure in the exterior field we rely on a direct boundary integral formulation, numerically solved using BEM. Dealing with the BEM applied to the exterior problem, we address the issue of the so-called FED (Fictitious Eigenvalues Difficulty). Such difficulty involves the arising of fictitious (not physical) resonances for frequency values coincident with the eigenvalues of the associated interior problem. We present a reduction to the finite-state aerodynamics for the matrix resulting from the BEM, which seems convenient to bypass the FED. (Author)

An extension of the Karhunen-Loeve decomposition (KLD) specifically aimed at the evaluation of the natural modes of n-dimensional structures (n=1,2,3) having nonhomogeneous density is presented. The KLD (also known as proper orthogonal decomposition) is a numerical method to obtain an "optimal" basis, capable of extracting from a data ensemble the maximum energy content. The extension under consideration consists of modifying the Hilbert space that embeds the formulation so as to have an inner product with a weight equal to the density. This yields a modified Karhunen-Loeve integral operator, whose kernel is represented by the time-averaged autocorrelation tensor of the ensemble of data multiplied by the density function. The basis functions are obtained as the eigenfunctions of this operator; the corresponding eigenvalues represent the Hilbert-space-norm energy associated with each eigenfunction in the phenomenon analyzed. It is shown under what conditions the eigenfunctions, obtained using the proposed extension of the KLD, coincide with the natural modes of vibration of the structure (linear normal modes). An efficient numerical procedure for the implementation of the method is also presented.

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.

The present paper deals with an innovative decision making approach for the selection of aircraft design concepts and operational procedures aimed at the reduction of the community noise annoyance. Specifically, the least annoying solutions pertaining to the Pareto front resulting from a multidisciplinary, multi-objective design and procedure optimization process, are selected on the basis of psychoacoustic requirements. The need of such an approach stems from the fact that the environmental requirements imposed to the commercial aviation are becoming stricter and stricter, as the trend of the traffic increase is rapidly growing following the market demand. The fulfillment of these requirements often leads to design decisions that can be conflicting, thus forcing the designer to look for trade-off solutions since the early conceptual phase of the design process. The multi-objective optimization is the methodological approach most suited to cope with this kind of problems. Using this approach, the Pareto optimality is achieved as the set of non-dominated solutions (the Pareto front), i.e., the solutions such that none of the objectives can be improved without deteriorate at least one of the others. The criterion used to select one of the non-dominated solutions along the Pareto front represents a key point. Within the EC-funded project COSMA, the outcomes of an extensive campaign of psychometric tests aimed at the identification of the most annoying characteristics of the aircraft noise are used to rank the points on the Pareto fronts with respect to the level of annoyance produced on the residential community. The approach is highly innovative, being the first time that psychoacoustic considerations are directly used in the design and procedural optimization process. The results obtained for different classes of aircraft in several operating conditions will be presented.

Aircraft noise is nowadays one of the most urgent concerns in the development of the air transportation system. The tremendous grow rate of the number of passengers during the last two decades has generated a significant increase of the number of airports, with a commercial rebirth of secondary air terminal. As a consequence, the number of people affected every day by the noise produced by aircraft operations has sensibly increased. Within this framework, the effort of the research community during the last 20 years has concentrated, according to the regulation, on the abatement of the noise level. Lately, the possibility to include in this process considerations related to the annoyance produced by the exposition to aviation noise has received a major improvement. Such an approach would allow the designer to explore unconventional possibilities, widening the horizon of the future technological scenarios and multiplying the options available to reduce the impact of the community noise on the population. One of the major difficulties in the integration of perception-based considerations resides in the not easy translation of psychoacoustic indicators in terms of design variables. The present papers describes the approaches developed during the EU-funded research projects SEFA (Sound Engineering for Aircraft, FP6/2004-2007) and its follow-up COSMA (Community Oriented Solutions to Minimize aircraft noise Annoyance, FP7/2009 on). The problem is here approached from the point of view of a research engineer in aeronautical sciences. Specifically, the mechanism identified to drive the optimization process to weakly annoying configuration is described. Special attention is payed to the description of the relevance of annoyance related objectives in the resulting workflow, and how these objectives are capable to drive the designers' choices.

The paper deals with the modeling of woodwind musical instruments using a Boundary Integral Equation (BIE) formulation. Specifically, the BIE is used to model the acoustic response of the instrument bore, and is numerically solved by means of a Boundary Element Method (BEM). The latter takes advantage of an analytical solution for the calculation of the BEM coefficients, thus allowing for the representation of the problem as an open‐domain problem. This peculiarity avoids the use of approximated boundary conditions at the open end of the pipe. The formulation is used to: evaluate the input impedance of the resonating air column; identify the frequency dependent transfer function relating the inflow of the instrument with the signal measured at a specified location (the Reed‐To‐Microphone transfer function); extend the analysis to a realistic performing environment to obtain the "Reed‐To‐Listener" transfer function. Standard techniques are used to take into account the interaction of the bore with the nonl...

The environment challenge and the expected growth of the market demand impose a technology breakthrough within the aeronautical industry. Although several innovative configurations of airplanes have been analyzed, none of them has been yet chosen as the most promising candidate. One of the reasons is related to the not yet known economic impacts of these architectures, since the life-cycle costs and infrastructural investments analysis are still unclear and therefore the current modules cannot be reliably directly applied. Such cost categories, in the last decades, have been mainly related to design and operational factors, as aircraft fuel-weight and empty-weight, with only an indirect link to the aircraft performance, particularly from the environmental viewpoint. This work presents a novel approach to include up-to-date models of life-cycle costs for low-noise aircraft from an airliner perspective. The situation is here addressed as a multidisciplinary conceptual design problem including the above up-to-date models. The possibility to estimate at the conceptual design stage the economic and financial implications of a radically new concept would substantially increase the possibility for the manufacturers to rank and select the most promising ideas.

The approach exploited within the the EC funded project COSMA (Community Oriented Solu- tions to Minimize Aircraft Annoyance, FP7) for multi-objective optimization aimed at optimal take-off and landing trajectories (minimizing noise emissions and fuel burn) is presented. The methodology has been recently extended to include aircraft design variables, performing a si- multaneous design/procedure optimization. As noise indicator, the area bounded by the SEL 60dBA contour is chosen, whereas the fuel burn refers to the specified procedure. A binary coded multi-objective genetic algorithm (MOGA) is applied to find the front of non-dominated solutions (Pareto front) and an optimal compromise solution is selected. The trajectory is par- titioned into segments, and the 3D spatial coordinates of each segment extreme, along with the aircraft speed and high-lift devices settings are taken as optimization variables. Aircraft performance and noise emissions are evaluated through the multidisciplinary simulation, de- sign and optimization tool FRIDA (Framework for Innovative Design in Aeronautics). The approach is applied to three classes of aircraft, from mid to long range and the relevant results for mid range aircraft are presented herein.