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This 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 variables.

Presentations of the AERIALIST cycle of seminars given on January 8-9, 2020 at Peking University, College of Engineering.

The present document contains the report of the Annual Network Event and Promotion of Research Effort and the 2nd Scientific Workshop sponsored by ANIMA WP6. The 2019 edition of the events has been hosted by the Department of Engineering of the Roma Tre University, on September 25-27 2019. The next section contains a short report of the scientific workshop, as well as the link to the repository of the technical presentations.

The 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 properties.

The 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 phasedelayed with respect to the incident wave. The set spans the whole 0-2pi 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 number.

The objective of the paper is a critical review within the context of aeronautical applications of the Ana- logue Transformation Acoustics (ATA) approach for the modelling of acoustic metamaterials. The ATA approach has been introduced to overcome the limitation of the metamaterials design methods based on the Standard Transformation Acoustics (STA) imposed by the requirement of a strict formal invariance of the governing equations. Indeed, in case of acoustic perturbations propagating within moving media, the convective terms are responsible of the failure of formal invariance under the action of conformal mappings as a consequence of the combination of space and time derivatives. The ATA is based on the concept of analogue spacetimes and fully relies on the analytical tools of Lorentzian differential geome- try. The great advantages of the method is the possibility to handle a background flow, conversely to the STA which fails in presence of convection. Despite the undoubted appeal of this feature in the eyes of an aeroacoustician, its potential has not been completely disclosed yet. The class of transformations that yield a physically meaningful aerodynamics in the virtual analogue spacetime has not been explored in details, preventing the application of the method in aeronautics. The present paper analyses the relation- ship between the the analogue velocity field with a realistic potential flow. The analytical comparison is validated through numerical simulations of two classic benchmarks: the irrotational flow of an inviscid, incompressible fluid around a circular cylinder and the flow through a bumped wall. The target acoustic behaviour is the cancellation of the scattering (cloaking) induced by the cylinder and the bump.

The present paper examines different methods for the design of acoustic metamaterial devices required to operate in presence of an aerodynamic flow. The ultimate goal of the research is the exploitation of the acoustic properties of acoustic metafluids in the aeronautical context. Particular attention is paid to the development of innovative devices capable to modify the propagation pattern of aviation noise to mitigate its effect on the population. Possible applications are a new generation of liners to achieve the virtual scarfing of nacelle intakes or special surface treatments to enhance the shielding of engine noise by wings and fuselage. The paper is focused on aerodynamic flows at Mach number not higher than 0.3, which is compatible with the take-off and landing conditions of commercial aircraft. Different approaches are used to define appropriate corrections of static metamaterial designs to recover, partially or completely, the efficiency of the meta-response lost as a consequence of the aerodynamic convection. All the approaches are based on classic aeroacoustic coordinate transformations, suitably revisited and adapted to specific application at hand. All the corrections presented are independent on the technique used in the static design of the device. Preliminary numerical results are obtained in the scattering abatement (cloaking) of obstacle with simple geometries. The numerical simulations are obtained using an original, general integral formulation of the problem, solved using an extended boundary element method.

The 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 chal- lenging 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 equa- tions 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 ana- lytical 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 com- ponent 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.

Air trac growth and the expansion of urban areas around airports make the aviation community noise a key aspect of aircraft design. The analysis of innovative con gu- rations is becoming activity of primary interest within the scienti c community. The current e ort to attain the strict noise{abatement targets imposed by the authorities risks to be pro tless without a signi cant technological breakthrough. A promising innovative con guration in terms of noise reduction is the Blended{Wing{Body air- craft. Indeed, the installation of the propulsion system above the centre{body surface ensures a signi cant masking e ect of the engine noise components. The lack of ex- perimental 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{e ective models to predict the noise shielding induced by engines installation on unconventional geometries. The present work introduces the develop- ment 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 con guration in the future aviation context.

Air trac growth and the expansion of urban areas around airports make the aviation community noise a key aspect of aircraft design. The analysis of innovative con gu- rations is becoming activity of primary interest within the scienti c community. The current e ort to attain the strict noise{abatement targets imposed by the authorities risks to be pro tless without a signi cant technological breakthrough. A promising innovative con guration in terms of noise reduction is the Blended{Wing{Body air- craft. Indeed, the installation of the propulsion system above the centre{body surface ensures a signi cant masking e ect of the engine noise components. The lack of ex- perimental 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{e ective models to predict the noise shielding induced by engines installation on unconventional geometries. The present work introduces the develop- ment 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 con guration in the future aviation context.