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(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

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 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.

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.

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.

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

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.

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 advent, during the first decade of the 21st century, of the concept of acoustic metamaterial has disclosed an incredible potential of development for breakthrough technologies. Unfortunately, the extension of the same concepts to aeroacoustics has turned out to be not a trivial task, because of the different structure of the governing equations, characterized by the presence of the background aerodynamic convection. Some of the approaches recently introduced to circumvent the problem are biased by a fundamental assumption that makes the actual realization of devices extremely unlikely: the metamaterial should guarantee an adapted background aerodynamic convection in order to modify suitably the acoustic field and obtain the desired effect, thus implying the porosity of the cloaking device. In the present paper, we propose an interpretation of the metamaterial design that removes this unlikely assumption, focusing on the identification of an aerodynamically-impermeable metamaterial capable of reproducing the surface impedance profile required to achieve the desired scattering abatement. The attention is focused on a moving obstacle impinged by an acoustic perturbation induced by a co-moving source. The problem is written in a frame of reference rigidly connected to the moving object to couple the convective wave equation in the hosting medium with the inertially-anisotropic wave operator within the cloak. The problem is recast in an integral form and numerically solved through a boundary-field element method. The matching of the local wave vector is used to derive a convective design of the metamaterial applicable to the specific problem analyzed. Preliminary numerical results obtained under the simplifying assumption of a uniform aerodynamic flow reveal a considerable enhancement of the masking capability of the convected design. The numerical method developed shows a remarkable computational efficiency, completing a simulation of the entire field in a few minutes on mid-end workstations. The results are re-interpreted in term of boundary impedance, assuming a locally-reacting behavior of the outer boundary of the cloaking layer. The formulation is currently being extended to the analysis of arbitrarily complex external flows in order to remove the limitation of the background uniform stream in the host.

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.

Simulation-based design optimization methods integrate computer simulations, design modification tools, and optimization algorithms. In hydrodynamic applications, often objective functions are computationally expensive and noisy, their derivatives are not directly provided, and the existence of local minima cannot be excluded a priori, which motivates the use of deterministic derivative-free global optimization algorithms. The enhancement of two algorithms of this type, DIRECT (DIviding RECTangles) and DPSO (Deterministic Particle Swarm Optimization), is presented based on global/local hybridization with derivative-free line search methods. The hull-form optimization of the DTMB 5415 model is solved for the reduction of the calm-water resistance at Fr = 0.25, using potential flow and RANS solvers. Six and eleven design variables are used respectively, modifying both the hull and the sonar dome. Hybrid algorithms show a faster convergence towards the global minimum than the original global methods and are a viable option for ship hydrodynamic optimization. A significant resistance reduction is achieved both by potential flow and RANS-based optimizations, showing the effectiveness of the optimization procedure.

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.

The paper presents a multi-fidelity global metamodel for expensive computer simulations, developed as an essential part of efficient simulation-based design optimization under uncertainty. High- and low-fidelity solvers are managed through a multi-fidelity adaptive sampling procedure. The multi-fidelity approximation is built as the sum of a low-fidelity-trained metamodel and the metamodel of the difference (error) between high- and low-fidelity simulations. The metamodels are based on dynamic stochastic radial basis functions, which provide the prediction along with the associated uncertainty. New training points are placed where the prediction uncertainty is maximum. The prediction uncertainty of both the low-fidelity and the error metamodel is considered for the adaptive refinement of the low- and high-fidelity training set, respectively. The method is demonstrated through three analytical test problems and one simple industrial application in ship hydrodynamics. The fitting error of the multi-fidelity metamodel is used as evaluation metric. The comparison with a high-fidelity-trained metamodel shows the effectiveness of the present method.

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

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