1,720,977 research outputs found
On the integration of acoustic phase-gradient metasurfaces in aeronautics
No full textMetamaterials might be one of the breakthrough technologies needed from the aeronautic industry to achieve the more and more challenging targets set by the international authorities, especially about noise emissions. In this article, a theoretical link between Transformation Acoustics and Generalized Snell’s Law, two widely used metamaterial models, is demonstrated analytically and applied to case studies. The relevance of the connection in the aeroacoustic field is discussed along with the consequent computational advantages for numerical simulations. This is exploited to perform a simulation-based design optimization of a phase-graded metasurface acoustic lining of a 2 D duct in presence of flow. Results show promising abilities of the optimized device to modify and control the directivity of the noise emitted from the duct by means of unconventional reflections. The noise reduction in the desired direction is obtained through constructive and destructive interference, with no absorption from the boundaries
Erratum to ‘An integral equation approach to acoustic cloaking’ [Journal of Sound and Vibration (2012)4629–4643]
No full textRadial basis functions for stochastic metamodels tailored to aeroacoustic applications
No full textThe paper presents a preliminary investigation on the applicability of stochastic Radial Basis Functions (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
Noise shielding metamodels based on stochastic radial basis functions
No full textThis paper deals with the development of suitable surrogate models for the acoustic assessment of the shielding effects due to engine installation on top of unconventional aircraft configurations. The interest of the aeronautic community towards finding solutions to reduce the environmental impact has increased tremendously in the last decade, in particular to reduce acoustic emissions. The Advisory Council for Aviation Research and Innovation in Europe (ACARE) has in fact set ambitious noise reduction targets for 2050, which can only be reached through important technological advances such as the development of highly innovative configurations. Among solutions proposed in literature, one of the most promising and most investigated is the Blended Wing Body (BWB) aircraft configurations. Because of the peculiar center-body shape of this concept, the engines can be installed on top of the aircraft, making its noise shielding properties particular appealing. Therefore, the choice of the optimal position of the engines to maximize the masking effect represents a crucial aspect that must be taken into account from the early design stages. However, as experimental and historical data are not available in this context, there are no alternative to perform direct simulations for the acoustic assessment needed in conceptual design phase, which would lead to an unacceptable increase of computational resources for optimization purposes. A possible solution is to limit the number of performed simulation by using them to develop metamodels based on stochastic radial basic functions for the evaluation of the acoustic shielding factor, instead of calculating the actual value at each call of the objective functions. Specifically, starting from few high-fidelity simulations by means of a bidimensional integral solver based on the convective Helmholtz equation, an adaptive metamodel is developed for the evaluation of the shielding factor of airfoils, which takes into account the variation of crucial design variable
Noise shielding models for the conceptual design of unconventional aircraft
No full textAir traffic growth and the expansion of urban areas around airports make the aviation community noise a key aspect of aircraft design. The analysis of innovative configurations is becoming activity of primary interest within the scientific community. The current effort to attain the strict noise–abatement targets imposed by the authorities risks to be profitless without a significant technological breakthrough. A promising innovative configuration in terms of noise reduction is the Blended–Wing–Body aircraft. Indeed, the installation of the propulsion system above the centre–body surface ensures a significant masking effect of the engine noise components. The lack of experimental data needed to validate the advantages introduced by these technologies, and therefore carry out parametric analysis, can only be overcome by formulating reliable and cost–effective models to predict the noise shielding induced by engines installation on unconventional geometries. The present work introduces the development of tailored models for the correction of the existing semi-empirical prediction techniques. Such models are embedded within the modules of the existing conceptual design framework FRIDA. Without excluding other disciplines, the introduction of a noise prediction tool allows to cast a wide net in the research of optimal configuration in the future aviation context
Optimized metamaterials for enhanced noise shielding of innovative aircraft configurations
No full textMetasurfaces that exploit the so-called generalized Snell's law are suitable for the manipulation of acoustic waves in the subwavelength regime, exhibiting extraordinary refracting and reflecting behaviours. This attracted the interest of the aeroacoustic community for their potential as a disruptive technology for noise abatement and acoustic field control. New acoustic treatments have, hence, been imagined for highly innovative aircraft configurations, to be placed also in unconventional positions over the airframe, fully exploiting their characteristics. However, the direct simulation of metamaterials and metasurfaces in the aeroacoustic context can be computationally very expensive, even when involved geometries are not particularly complex, limiting the possibility to include metamaterial effects into multidisciplinary design process for applications of aeronautical interest. In this paper a generalized Snell law-based lining installed on the upper side of an airfoil is explored as a way to enhance its shielding of the noise emitted by a point source. This shall be seen as a 2 dimensional schematization of the center body of a Blended Wing Body configuration with the propulsion system mounted on top of it. A model of metafluid is used to reduce the computational burden, allowing a simulation-based optimization process to be conducted, maximizing the shielding factor of the airfoil
Numerical acoustic characterisation of a Kelvin cell structure under normal and grazing incidence
No full textOver the last decade, metamaterials based on periodic structures have been largely studied to deeply control and manipulate an acoustic field, exploiting the exotic acoustic behaviours that can be achieved, such as band gaps, hyperfocusing and noise trapping capabilities. In this class of materials, the acoustic response depends on the geometric characteristics, i.e. shape, size, orientation and arrangements of meta-atoms, rather than on the mechanical properties of the base material. Metabehaviours are thus related to constructive and disruptive interactions between incident and reflected waves within the periodic lattice. The present paper deals with the numerical acoustic characterisation of a Kelvin-cell-based layer under normal and grazing incidence. Due to its intricate periodic structure, the Kelvin cell can exhibit interesting acoustic meta-properties, while its high-porosity lattice makes it a good candidate for aeronautical applications, where weight limitation is urgent. Despite an extensive literature on acoustic metamaterials and periodic structures, limited applications on the aeroacoustic responses of these type of structures have been presented so far. The influence of angle of incidence is key to disclose the enormous potential of acoustic metamaterials in aeroacoustics, and possibly open the path to the analysis of the response in more complex situations such as those of interest to the aeronautics community. Uiltimately this could make possible the exploitation of unconventional metabehaviours to mitigate the aviation community noise. In this paper, a numerical parametric analysis of the acoustic response of the Kelvin-cell-based specimen is presented, focusing on the search for transmission band-gaps, directivity pattern modifications and energy absorption
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