The analysis of porous materials for aeroacoustic noise mitigation requires high-fidelity numerical tools to simulate the sound propagation and interaction in porous media. The manuscript presents the calibration and validation of a direct-hybrid LES/CAA method used to model the aerodynamically generated sound. The porous micro-structures are modeled with a volume-averaging approach. Additional terms, emerging in the governing equations, owing to the force exerted by the porous matrix on the fluid, are closed by means of Darcy's law and the quadratic Forchheimer term. The sound generation and propagation is predicted by a solution of the acoustic perturbation equations via a discontinuous Galerkin method. The model for the porous material is separately calibrated for the CFD and CAA with the parameters retrieved by the characterization of a melamine foam in experimental tests. The sound predicted by an absorbing layer of the material is compared with measurements performed in an impedance tube. Secondly, the transmission coefficient of the melamine foam placed in an acoustic liner configuration inside a duct is studied numerically. The results are then compared with experiments conducted in absence of a flow and also with a flow velocity of 30 m/s in order to assess the accuracy of the model calibration.

The present work aims at evaluating the effectiveness of the use of porous materials for reducing airfoil turbulence-impingement noise. To pursue this objective, a porous NACA-0024 profile filled with melamine foam has been designed for comparison with a solid model. The porous media constituting the airfoil has been fully characterized in order to determine the parameters that describe the material. The two profiles have been tested in a rod airfoil configuration in the anechoic chamber of von Karman Institute for Fluid Dynamics. A free-stream velocity of 30 m/s (corresponding to a chord-based Reynolds number of 3.2·10⁵) and an angle of attack of 0^◦ have been considered for the tests. Hot-wire anemometry measurements have been performed with the aim of characterizing the boundary layers around the two airfoils and investigating the eventual nose mitigation mechanisms that occur due to the porous treatment. Moreover, the static pressure distributions along the two surfaces have been studied to find correlations with the velocity profiles. The results show that the use of a porous treatment leads to a decrease of turbulence-impingement noise in the low-frequency range and to an increase in the high-frequency one, mostly due to surface roughness noise. The application of an inverse beamforming technique has lead to a qualitative comparison of noise distribution maps at different one-third octave band frequencies for the two cases. Furthermore, the pressure values of each map point within a rectangular region surrounding the leading edge have been summed in order to retrieve the integrated one-third octave band spectra.

Thanks to their ability to deal with distributed and coherent acoustic sources, inverse beamforming methods have grown in popularity amongst the aeroacoustic community in the last few decades. This paper outlines the implementation of an improved version of the Generalized Inverse Beamforming (GIBF) with the purpose of ensuring an accurate source localization and a robust source strength reconstruction. In this respect, the development of a technique for the determination of the optimal regularization parameters at each iteration of the beamforming algorithm is of key importance. The regularization strategy is based on the Quasi-optimality criterion and on the L-curve method and, for its validation, GIBF code is applied to an experimental benchmark dataset labeled as NASA2. This test case concerns the analysis of a small-scale open-jet facility, the NASA Langley Quiet Flow Facility (QFF), for the characterization of airfoil self-noise. In order to evaluate the performances of the algorithm and to illustrate the potentialities and the benefits of its use for complex applications, a comparison with a Conventional Beamforming (CB) technique and with other array analysis methods applied to the same test case is performed. The analysis comprehends the qualitative evaluation of the source maps for several one-third octave frequency bands and the quantitative estimation of the integrated one-third octave band spectra of the leading edge and trailing edge regions of the model. The results show that, with a proper handling of the regularization strategy, GIBF can accurately resolve distributed acoustic sources. The source maps present improvements in terms of readability and reconstruction of the distributed nature of the source, whereas the integrated levels are in close agreement with the ones predicted by the other advanced methods.