1,720,997 research outputs found
An integrated approach to the direct simulation of brasses in the performance environment
No full textThe 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
Optimization of metasurfaces for the design of noise trapping metadevices
No full textThe article deals with the design of a metadevice able to trap acoustic waves in a duct. The acoustic perturbation is produced by a source placed inside the duct. The aim is to limit the outgoing acoustic power and confine the perturbation inside the duct exploiting the unconventional reflection of the optimized metasurface. The metabehaviour is modeled by means of the generalized Snell's law for reflection from acoustically rigid surfaces. The realization of the device relies on a modular concept, which building set is made of eight elementary cells, able to induce a reflected field suitably phase-delayed with respect to the incident wave. The set spans the whole 0-2π phase delay range, and the anomalous reflection is obtained by the tailored design of the phase delay gradient profile on the metasurface. The cells are designed in order to extend the effective frequency range of the device, keeping the overall thickness of the metadevice smaller than a quarter of the design wavelength. The duct and the source are considered co-moving within the fluid at rest. The numerical analysis is performed in the frequency domain in a frame of reference rigidly connected to the duct, and considering several values for the Mach numbe
On aeroacoustic space-time curvature for certain aerodynamic flows
No full textThe research on acoustic metamaterials is probably the most lively research field in classical mechanics since the early achievemnts dating back to 2006. One of the most intriguing and challenging aspects emerged during the last decade deals with the theoretical modeling of the acoustic meta-behavior in presence of a background aerodynamic flow. Indeed, the structure of the equations governing the propagation of an acoustic perturbation in a moving medium is substantially different, due to the effect of the convecive terms, mixing space and time derivatives through the components of the aerodynamic velocity vector field. An effective approach to cope with this aspect relies in the spacetime reinterpretation of the (aero)acoustic equations adopting the analytical tools developed within the framework of the special and general relativity. Indeed, the Minkowskian structure of the equations governing the propagtion of waves in quiescent media turns out to be Lorentzian in presence of a non-uniform background aerodynamic, with the component of the veloicity field acting as space-time-bending elements. The present paper analyses the actual structure of the aeroacoustic spacetime for certain types of flows of interest in many applications. The analysis will take advantage of the existence of analytical solutions to derive the structure of the metric tensor in closed form and make some consideration about the related Ricci's tensor an the corresponding scalar curvarture. The paper will consider potential flows around simple geometries, includig the effect of contact discontinuities (wakes), and potential vortices. The final goal of the research is a deeper understanding of the underlying structure of the acoustic spacetime for those classes of applications where the aerodynamic convections plays a key role, in order to contribute to the disclosure of breakthrough modeling approaches
Design of metacontinua in the aeroacoustic spacetime
Full text linkThe 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∞≤ 0.35
Spacetime design of aeroacoustic metacontinua through optimal response matching
No full textThe paper deals with an integrated approach to the design of a metacontinuum capable to attain a specific target response when operating in a flow. The context of the research is related to the development of a general framework for identification of the mechanical properties required by a metamaterial-based, noise-abatement device for aeronautical application. The bulk metamaterial is here represented as an unconventional continuum with peculiar constitutive equations. The propagation of an acoustic perturbation in such a continuum is governed by a generalized wave equation written here in the aeroacoustic spacetime. The spacetime metrics of the generalized D'Alembertian reveals that the acoustic perturbations propagate through a curved spacetime, whose curvature depends jointly by the metacontinuum properties and the background aerodynamic flow. The identification of the mechanical properties of the metacontinuum is achieved by matching its acoustic response with a pre-defined target using numerical optimization. A suitable measure of the distance between the acoustic responses is used as the objective function to be minimized. The problem is completed by the relevant constraints and solved numerically using a meta-heuristic algorithm. Preliminary numerical results show how the developed method can effectively identify the elasticity tensor that induces an acoustic response compatible with the target
SpaceTime-bending transformations in aeroacoustics
No full textThe paper deals with the spacetime reinterpretation of the phenomenon of aeroacoustic convection within the framework of the Lorentzian differential geometry. Specifically, the effect of the background aerodynamic velocity field on the propagation of the acoustic disturbances is revisited in terms of spacetime curvature. The analysis explores the local properties of the curved aeroacoustic spacetime to correlate the background aerodynamics with the local value of the Riemann's scalar, and the latter with the convective effect. The metric associated to the aeroacoustic spacetime is compare with that induced by the inverse Taylor transformation, which is based on the O(M) approximation of the convective wave equation. Although limited by the low-speed assumption, the inverse Taylor transformation bends the Minkowskian (i.e., flat) acoustic spacetime taking into account the spatial variation of the background velocity potential. Numerical simulations are conducted to verify and visualise the spacetime propertie
Radial 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
An integrated toolchain for the design of aeroacoustic metamaterials: The H2020 project AERIALIST
No full textThe 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
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
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