Frank Simon

Liner impedance eduction technique based on velocity fields

This paper describes a new liner impedance eduction technique. The two-dimensional Linearized Euler Equations are solved in the frequency domain with a Discontinuous Galerkin formulation. The objective function which is minimized for impedance eduction is based on two-dimensional acoustic velocity elds measured by Laser Doppler Anemometry. The optimization scheme is based on direct and adjoint equations, which allows easily the

ONERA-NASA Cooperative Effort on Liner Impedance Eduction

As part of a cooperation between ONERA and NASA, the liner impedance eduction methods developed by the two research centers are compared. The NASA technique relies on an objective function built on acoustic pressure measurements located on the wall opposite the test liner, and the propagation code solves the convected Helmholtz equation in uniform ow using a finite element method that implements a continuous Galerkin discretization. The ONERA method uses an objective function based either on wall acoustic pressure or on acoustic velocity acquired above the liner by Laser Doppler Anemometry, and the propagation code solves the linearized Euler equations by a discontinuous Galerkin discretization. Two acoustic liners are tested in both ONERA and NASA ow ducts and the measured data are treated with the corresponding impedance eduction method. The first liner is a wire mesh facesheet mounted onto a honeycomb core, designed to be linear with respect to incident sound pressure level and to grazing ow velocity. The second one is a conventional, nonlinear, perforate-over-honeycomb single layer liner. Configurations without and with ow are considered. For the nonlinear liner, the comparison of liner impedance educed by NASA and ONERA shows a sensitivity to the experimental conditions, namely to the nature of the source and to the sample width.

Non-intrusive planar velocity-based nearfield acoustic holography in moving fluid mediuma)

Nearfield Acoustic Holography (NAH) is a powerful acoustic imaging method, but its application in aeronautics can be limited by intrusive measurements of acoustic field. In this paper, a moving fluid medium NAH procedure using non-intrusive velocity measurements is proposed. This method is based on convective Kirchhoff-Helmholtz integral formula. Convective equations and convective Greens function are used to derive convective real-space propagators including airflow effects. Discrete Fourier transforms of these propagators allow the assessment of acoustic fields from acoustic pressure or normal acoustic velocity measurements. As the aim is to derive an in-flow velocity-based NAH method, this study is especially focused on real convective velocity-to-pressure propagator. In order to validate this procedure, simulations in the case of monopole sources radiating in various uniform subsonic flows have been performed. NAH provides very favorable results when compared to the simulated fields. A comparison of results obtained by the real propagator and those obtained by the wave number-frequency-domain one developed by Kwon et al. [J. Acoust. Soc. Am. 128(4), 1823-1832 (2010)] shows the interest of using the real-form in the case of pressure backward propagation from velocity measurements. The efficiency of the developed procedure is confirmed by a wind tunnel campaign with a flush-mounted loudspeaker and non-intrusive Laser Doppler Velocimetry velocity measurements.

Design, manufacturing and demonstration of acoustic liners for air conditioning systems

Until now, acoustic liners for air conditioning systems are made of porous materials, very efficient for sound absorption in the high-frequency range. However, Liebherr Aerospace is developing an electrically driven air system composed of an air pump which generates low-and mid-frequency noise. New kinds of acoustic liners are therefore required to mitigate this noise source. The ALIAS project (Acoustic Liners for Air conditioning Systems) has been launched in the framework of Clean Sky Systems for Green Operations platform for addressing this issue. The project aimed at designing optimized locally reacting treatments for mid-frequency acoustic attenuation in air conditioning systems. SDOF and DDOF liners , which are standard solutions for acoustic treatments in turbofan engines, were adapted to the air conditioning system environment. The French Aerospace Lab (ONERA) and the SME ATECA, specialized in Advanced Materials & Systems, combined their research and technological capabilities to propose solutions that meet the industrial requirements. This was achieved by implementing a simulation-based design process, performing dedicated laboratory experiments, manufacturing full scale prototypes and demonstrating their efficiency in the anechoic facility of Liebherr Aerospace.

Vibroacoustics behavior of self-similarly loaded sandwich structures

The reduction of structural vibrations and acoustic radiation is increasingly challenging due to lightweight material use driven by mass reduction, particularly in the aerospace industry. Trim panels are well-known materials with high acoustic transmission and radiation. This study combines semi-analytical modeling, numerical simulations and experiments on the vibroacoustic behavior of sandwich constructions locally overloaded by a pre-fractal mass distribution. The research originality lies in the overload distribution (a self-similar pattern inspired by the Cantor set) which is directly slotted within the honeycomb core, so that the structure load-bearing capability is not altered. As the mass effect spatially localizes the vibrations, the pre-fractal pattern focuses the effect of localization on some specific frequency band gaps, which reduces the total vibrational energy and thus the acoustic radiation. Before the extension to composite plates, simulations of a homogenized beam model have been compute...

Vibrational behavior of sandwich structures overloaded by fractal distribution of masses

This paper presents the modeling and numerical simulations of the vibrational behavior of sandwich panels locally overloaded by a fractal distribution of masses. The structural model hypotheses consist in a homogenized material, which is overloaded by small masses distributed following a fractal pattern. The flat panel is then uniformly discretized and the spatial derivatives operators are approximated using finite differences. The time-harmonic equation is recast into an eigenvalue problem and solved to find natural frequencies and mode shapes. The panel is simply-supported on all of its edges. Simulations of a non-overloaded panel are compared to analytic results in term of wavenumbers and mode shapes. Simulations of fractally overloaded panels exhibit localization phenomena when the inter-mass distance is comparable to half the structural wavelength. The influence of the fractal distribution is investigated through the fractal order, the modal frequencies, and the evolution of the modal density. The ma...

Impedance eduction of perforated plates at low Stouhal numbers and high bias flow Mach number

The knowledge of the acoustic behavior of a multiperforated plate is important to design a combustion chamber. It is characterized by the acoustic impedance or the Rayleigh conductivity. Experiments have been performed in the ONERA flow duct to study the acoustic response of a multiperforated plate, without and with bias flow. The geometry of the perforated plate is representative of industrial applications, as well as the bias flowMach number (around 0.3) and the Strouhal number (around5.10−3). An inverse method isused to educe the impedance of the plate from Laser Doppler Anemometry measurements of the acoustic velocity above the plate. The issue of the accuracy of the educed value is specifically adressed. It is shown that the perforated plate is highly reflective with bias flow,as predicted by the models available in the literature. This is due to the combination of the very low Strouhal number and high bias flow Mach number. Consequently, on contrary to the case without flow, in the configuration with bias flow the eduction process shows one’s limits and cannot give a very precise impedance value.