Authors:Zhengyu Zheng Abstract: International Journal of Aeroacoustics, Ahead of Print. In this paper, the DBEM/Hybrid LES（Directly Boundary Element Method/Hybrid Large Eddy Simulation）technique is applied to predict the aerodynamic noise generated by tandem circular cylinders immersed in a three-dimensional turbulent flow. Utilizing the Lighthill's Acoustic Analogy, the flow pressure fluctuation near the surface of the cylinder is converted into acoustic dipole sources. Taking the dipole sound sources as the actual sound sources, the aeroacoustic field is simulated and analyzed by DBEM. The research shows that: The strong dipole sources are distributed in the collision zone of the downstream cylindrical surface, where the upstream cylinder's shedding vortex colliding to downstream cylinder surface. Both of the amplitude-frequency response and the phase-frequency response of dipole acoustic source are obtained, which is helpful for further research on aerodynamics noise interference and suppression. Good comparisons are obtained between numerical results and BART (Basic Aerodynamic Research Tunnel) experimental data published by NASA. Citation: International Journal of Aeroacoustics PubDate: 2021-01-11T05:16:12Z DOI: 10.1177/1475472X20984092
Authors:Reda R Mankbadi, Saman Salehian Abstract: International Journal of Aeroacoustics, Ahead of Print. In this work we propose replacing the conventional flat-surface airframe that shields the engine by a wavy surface. The basic principle is to design a wavy pattern to reflect the incoming near-field flow and acoustic perturbations into waves of a particular dominant frequency. The reflected waves will then excite the corresponding frequency of the large-scale structure in the initial region of the jet’s shear layer. By designing the frequency of the reflected waves to be the harmonic of the fundamental frequency that corresponds to the radiated peak noise, the two frequency-modes interact nonlinearly. With the appropriate phase difference, the harmonic dampens the fundamental as it extracts energy from it to amplify. The outcome is a reduction in the peak noise. To evaluate this concept, we conducted Detached Eddy Simulations for a rectangular supersonic jet with and without the wavy shield and verified our numerical results with experimental data for a free jet, as well as, for a jet with an adjacent flat surface. Results show that the proposed wavy surface reduces the jet noise as compared to that of the corresponding flat surface by as much as 4 dB. Citation: International Journal of Aeroacoustics PubDate: 2021-01-08T05:12:43Z DOI: 10.1177/1475472X20978385
Authors:Paruchuri Chaitanya, Pratibha Vellanki Abstract: International Journal of Aeroacoustics, Ahead of Print. This paper presents an optimisation approach for designing low-noise Outlet Guide Vanes (OGVs) for fan broadband noise generated due to the interaction of turbulence and a cascade of 2-dimensional aerofoils. The paper demonstrates the usage of Bayesian optimisation with constraints to reduce the computation cost of optimisation. The prediction is based on Fourier synthesis of the impinging turbulence and the aerofoil response is predicted for each vortical modal component. A linearised unsteady Navier-Stokes solver is used to predict the aerofoil response due to an incoming harmonic vortical gust. This paper shows that to achieve noise reductions of 0.5 dB the penalty on the aerodynamic performance of 33% is observed compared to baseline aerofoil. Hence, the geometry changes such as thickness and nose radius can’t reduce broadband noise without effecting aerodynamic performance. Citation: International Journal of Aeroacoustics PubDate: 2020-12-27T02:43:59Z DOI: 10.1177/1475472X20978395
Authors:CK Sumesh, TJS Jothi Abstract: International Journal of Aeroacoustics, Ahead of Print. This paper investigates the noise emissions from NACA 6412 asymmetric airfoil with different perforated extension plates at the trailing edge. The length of the extension plate is 10 mm, and the pore diameters (D) considered for the study are in the range of 0.689 to 1.665 mm. The experiments are carried out in the flow velocity (U∞) range of 20 to 45 m/s, and geometric angles of attack (αg) values of −10° to +10°. Perforated extensions have an overwhelming response in reducing the low frequency noise (4 kHz) is observed to increase with an increase in the pore diameter. The dominant reduction in the low frequency noise for perforated model airfoils is within the Strouhal number (based on the displacement thickness) of 0.11. The overall sound pressure levels of perforated model airfoils are observed to reduce by a maximum of 2 dB compared to the base airfoil. Finally, by varying the geometric angle of attack from −10° to +10°, the lower frequency noise is seen to increase, while the high frequency noise is observed to decrease. Citation: International Journal of Aeroacoustics PubDate: 2020-12-22T08:05:06Z DOI: 10.1177/1475472X20978388
Authors:Bassem Barhoumi, Jamel Bessrour Abstract: International Journal of Aeroacoustics, Ahead of Print. This paper presents a new numerical analysis approach based on an improved Modal Boundary Element Method (MBEM) formulation for axisymmetric acoustic radiation and propagation problems in a uniform mean flow of arbitrary direction. It is based on the homogeneous Modal Convected Helmholtz Equation (MCHE) and its convected Green’s kernel using a Fourier transform method. In order to simplify the flow terms, a general modal boundary integral solution is formulated explicitly according to two new operators such as the particular and convected kernels. Through the use of modified operators, the improved MBEM approach with flow takes a convective form of the general MBEM approach and has a similar form of the nonflow MBEM formulation. The reference and reduced Helmholtz Integral Equations (HIEs) are implicitly taken into account a new nonreflecting Sommerfeld condition to solve far field axisymmetric regions in a uniform mean flow. For isolating the singular integrations, the modal convected Green’s kernel and its modified normal derivative are performed partly analytically in terms of Laplace coefficients and partly numerically in terms of Fourier coefficients. These coefficients are computed by recursion schemes and Gauss-Legendre quadrature standard formulae. Specifically, standard forms of the free term and its convected angle resulting from the singular integrals can be expressed only in terms of real angles in meridian plane. To demonstrate the application of the improved MBEM formulation, three exterior acoustic case studies are considered. These verification cases are based on new analytic formulations for axisymmetric acoustic sources, such as axisymmetric monopole, axial and radial dipole sources in the presence of an arbitrary uniform mean flow. Directivity plots obtained using the proposed technique are compared with the analytical results. Citation: International Journal of Aeroacoustics PubDate: 2020-12-16T06:31:23Z DOI: 10.1177/1475472X20978384
Authors:Sergi Palleja-Cabre, Brian J Tester, R Jeremy Astley, Hadrien Beriot First page: 277 Abstract: International Journal of Aeroacoustics, Ahead of Print. Experimental investigation of Over-Tip-Rotor circumferential groove liners has shown potential for fan noise suppression in turbofan engines whilst providing minimal penalty in fan aerodynamic performance. The validation of Over-Tip-Rotor liner analytical prediction models against published experimental data requires the modelling of an equivalent impedance for such acoustic treatments. This paper describes the formulation of two analytical groove impedance models as semi-locally reacting liners, that is locally reacting in the axial direction and non-locally reacting in the azimuthal direction. The models are cross-verified by comparison with high-order FEM simulations, and applied to a simplified Over-Tip-Rotor configuration consisting of multiple grooves excited by a monopole point source located close to the grooved surface. Citation: International Journal of Aeroacoustics PubDate: 2020-09-29T10:43:40Z DOI: 10.1177/1475472X20954427
Authors:Giorgio Palma, Lorenzo Burghignoli First page: 294 Abstract: International Journal of Aeroacoustics, Ahead of Print. Metamaterials 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. Citation: International Journal of Aeroacoustics PubDate: 2020-09-10T04:50:19Z DOI: 10.1177/1475472X20954404
Authors:Lara Flanagan, David Heaphy, John Kennedy, Raphaël Leiba, Henry Rice First page: 310 Abstract: International Journal of Aeroacoustics, Ahead of Print. The sound absorptive performance of a proposed “meta-liner” are investigated in this paper. The structure is composed of closely placed plates connected by openings at alternating locations in a stacked format. This system presents multiple band gaps with high absorption and sub-wavelength behaviour (sample thickness equals 0.04λ), achieved through tortuosity within the design. The acoustic response of the single layer is obtained numerically and with experimental verification under normal incidence. The repeating cellular design allows efficiencies in the viscothermal numerical analysis and using a transfer matrix approach, it is demonstrated that the response of the overall system may be efficiently predicted from a detailed model of a unit cell. Both the transfer matrix method and a full viscothermal model are validated against experimental data as a function of system depth. The analysis gives very satisfactory results which could form the basis for future designs. Citation: International Journal of Aeroacoustics PubDate: 2020-09-10T04:50:19Z DOI: 10.1177/1475472X20954894
Authors:Imran Bashir, Michael Carley First page: 324 Abstract: International Journal of Aeroacoustics, Ahead of Print. Low-cost airlines have significantly increased air transport, thus an increase in aviation noise. Therefore, predicting aircraft noise is an important component for designing an aircraft to reduce its impact on environmental noise along with the cost of testing and certification. The aim of this work is to develop a three-dimensional Boundary Element Method (BEM), which can predict the sound propagation and scattering over metamaterials and metasurfaces in mean flow. A methodology for the implementation of metamaterials and metasurfaces in BEM as an impedance patch is presented here. A three-dimensional BEM named as BEM3D has been developed to solve the aero-acoustics problems, which incorporates the Fast Multipole Method to solve large scale acoustics problems, Taylor’s transformation to account for uniform and non-uniform mean flow, impedance and non-local boundary conditions for the implementation of metamaterials. To validate BEM3D, the predictions have been benchmarked against the Finite Element Method (FEM) simulations and experimental data. It has been concluded that for no flow case BEM3D gives identical acoustics potential values against benchmarked FEM (COMSOL) predictions. For Mach number of 0.1, the BEM3D and FEM (COMSOL) predictions show small differences. The difference between BEM3D and FEM (COMSOL) predictions increases further for higher Mach number of 0.2 and 0.3. The increase in difference with Mach number is because Taylor’s Transformation gives an approximate solution for the boundary integral equation. Nevertheless, it has been concluded that Taylor’s transformation gives reasonable predictions for low Mach number of up to 0.3. BEM3D predictions have been validated against experimental data on a flat plate and a duct. Very good agreement has been found between the measured data and BEM3D predictions for sound propagation without and with the mean flow at low Mach number. Citation: International Journal of Aeroacoustics PubDate: 2020-09-04T04:51:23Z DOI: 10.1177/1475472X20954423
Authors:Lennart Rossian, Roland Ewert, Jan W. Delfs First page: 347 Abstract: International Journal of Aeroacoustics, Ahead of Print. In the framework of the German Collaborative Research Center CRC 880: Fundamentals of High Lift for Future Civil Aircraft porous materials as a means towards the reduction of airfoil trailing edge noise are investigated. At DLR, both experimental and numerical approaches are pursued to understand the physics behind the noise reduction. The present paper focuses on the numerical investigations, for which the experimental data serves as an evaluation basis. From the analysis of homogeneous materials, first steps are made towards the design of aeroacoustically tailored materials. It is assumed that materials with locally varying permeability may be suitable to achieve maximum noise reduction, as they provide a smooth transition from the solid airfoil to the free flow in the wake. The simulation results support this understanding, however it is revealed that high local gradients in the material properties themselves may act as acoustic sources. Citation: International Journal of Aeroacoustics PubDate: 2020-09-25T06:51:55Z DOI: 10.1177/1475472X20954410
Authors:K-S Rossignol, A Suryadi, M Herr, J Schmidt, J Tychsen First page: 365 Abstract: International Journal of Aeroacoustics, Ahead of Print. The introduction of quiet short take-off and landing for civil aircraft operations in close proximity to the population poses important technological challenges. One critical aspect is the realization of extreme lift augmentation at low acoustic emissions. The aircraft concept selected to achieve this goal is a high-lift system equipped with an active flow-control non-slotted flap and a droop nose. For this specific configuration, trailing edge noise becomes a dominant noise source. Porous materials as a passive means for trailing-edge noise reduction are selected and characterized. Results of extensive experimental investigations in the acoustic wind tunnel Braunschweig are presented and discussed to point out the potential and limitations of the selected porous devices. Practical issues related to material manufacturing and integration into the wind tunnel model are addressed. The noise reduction potential of passive porous trailing-edge devices is found to strongly depend on both these aspects. Issues related to the characterization of the porous materials properties are described. Although porous materials are found to be successful at reducing trailing-edge noise emissions, the results indicate that there is still a need for more generic investigations to further clarify the parametric dependencies between noise reduction and material properties. Citation: International Journal of Aeroacoustics PubDate: 2020-09-01T07:11:04Z DOI: 10.1177/1475472X20954421
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