The use of passive noise control techniques for low Reynolds rotors, such as tripping of the boundary layer, has provided evidence of a possible improvement of aerodynamic performance and reduction of radiated noise if a trade-off of performance/noise is found. Recently, in another study, the integration of an acoustic liner inside the blades of a small academic rotor was investigated. The experimental results showed noise reduction of the first harmonics of the blade-passing frequency (BPF), but discrepancies were found with the predictions, raising questions about the nature of the noise reduction, whether it is caused by the impedance effect of the liner or by the tripping effect of the roughness added to the blades by the liner. In the present study, an experimental campaign on a set of rotor prototypes supplied with surface defects was conducted. The defects are defined as negative defects (such as holes or gaps) and positive defects (like bosses or bumps), and each rotor configuration provided with defects on its blades will be tested in an anechoic room under hovering conditions. This study aims to measure the aerodynamic and acoustic performance of each rotor configuration to gain insight into the nature of the noise reduction originally found with the liner integrated inside the blades.

Unmanned aerial vehicles (UAVs) are currently being used for reconnaissance missions, tactical surveillance, and infrastructure inspection. These missions and operations could be performed near inhabited areas, possibly leading to noise complaints. For most UAVs, the rotor blades are the dominant source of noise. While some UAV concepts are equipped with rotor ducts for aerodynamic and safety reasons, few studies focus on the acoustic benefit of rotor ducts with integrated acoustic liners, similar to implementations on turbofan aircraft. The liner concept used in this study, called a Long Elastic Open Neck Acoustic Resonator (LEONAR), achieves good low frequency performance in a limited available space. The LEONAR concept extends the holes of the facesheet perforations into the resonator cavities with hollow tubes. As a result, the columns of air in the facesheet holes are prolongated without increasing facesheet thickness, thereby shifting the resonant frequencies of the liner lower. The objectives of this paper are to describe the simulation of noise radiated from a rotor in a duct with a LEONAR liner, the design of a liner integrated along the duct inner surface to mitigate radiated noise between 1000-5000 Hz, and impedance tube tests of liner samples to validate the predicted impedances.

The trim panels of aircraft cabin are frequently made up of multilayer materials including a honeycomb, typically in the form of sandwich panels, which have the advantage of being lightweight and space-saving, but lack good acoustic insulation. It has been shown that, for 10% more mass, weights arranged in a fractal pattern in the honeycomb affect the vibratory pattern of a sandwich panel by creating multiple reflections of the bending waves on the inclusions. They thus generate a phenomenon of focusing vibratory waves providing localized modes while attenuating the structure's acoustic radiation. The objectives of this paper are to describe: first, the vibro-acoustic model of an overloaded homogenized bending panel (to mimic a trim panel with a fractal pattern); second, the physical phenomena associated to this concept; third, the impact on the modal behavior and the vibratory and acoustic responses.

A helicopter's anti-torque system is a significant contributor to radiated noise. The Fenestron, used on Airbus Helicopters, allows to mitigate the anti-torque noise by masking effect and modulation of the blade distribution. However, unlike aircraft engine nacelles, it does not contain acoustic liners inside the duct to absorb the noise radiated by the rotor blades. This paper describes: first, the FEM modelization of the noise radiation for a 1/3-scale fenestron mock-up at the 1st BPFs, in static conditions; second, the assessment of the effect of an absorbing treatment introduced inside the collector or the diffuser and defined by its acoustic specific impedance; third, the design optimization applied to different acoustic liner types, adapted to the diffuser of the mock-up. Because of the limited duct length, forward and afterward radiations interact, generating interference pressure patterns. Several types of liners (conventional and unconventional), designed to deliver a high absorption, appear adapted to dimensional constraints: SDOF or DDOF type, with a perforated plate above cavities or based on the concept LEONAR ("Long Elastic Open Neck Acoustic Resonator"). Another paper completes the study with (aero-)acoustic tests, applied to elementary samples to validate the impedance target, and performed in an anechoic room with the mock-up.

No AccessEngineering NotesAcoustic Liner Demonstrator for a Turning Vane of S1MA Wind TunnelFabien Méry, Rémi Roncen, Frank Simon, Marlon Botte and Loïc OstoreroFabien Méry https://orcid.org/0000-0003-1408-2003ONERA—University of Toulouse, F-31055 Toulouse, France, Rémi RoncenONERA—University of Toulouse, F-31055 Toulouse, France, Frank SimonONERA—University of Toulouse, F-31055 Toulouse, France, Marlon BotteONERA, F-73500 Modane, France and Loïc OstoreroONERA, F-73500 Modane, FrancePublished Online:6 Feb 2023https://doi.org/10.2514/1.C037048SectionsRead Now ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail About References [1] Chung J. Y., "Rejection of Flow Noise Using a Coherence Function Method," Journal of the Acoustical Society of America, Vol. 62, No. 2, 1977, pp. 388–395. https://doi.org/10.1121/1.381537 CrossrefGoogle Scholar[2] Blacodon D., "Spectral Estimation Method for Noisy Data Using a Noise Reference," Applied Acoustics, Vol. 72, No. 1, 2011, pp. 11–21. https://doi.org/10.1016/j.apacoust.2010.09.004 CrossrefGoogle Scholar[3] Blacodon D. and Mohammad-Djafari A., "Separation of Acoustical Source Power Spectral Densities with Bayesian Sparsity Enforcing," Journal of Sound and Vibration, Vol. 480, 2020, Paper 115334. https://doi.org/10.1016/j.jsv.2020.115334 CrossrefGoogle Scholar[4] Fleury V., Coste L., Davy R., Mignosi A., Cariou C. and Prosper J.-M., "Optimization of Microphone Array Wall Mountings in Closed-Section Wind Tunnels," AIAA Journal, Vol. 50, No. 11, 2012, pp. 2325–2335. https://doi.org/10.2514/1.J051336 LinkGoogle Scholar[5] Remillieux M., Crede E., Camargo H., Burdisso R., Devenport W., Rasnick M., Seeters P. 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R. and Singh R., "Measurement of Acoustic Intensity in the Presence of One-Dimensional Fluid Flow," Journal of the Acoustical Society of America, Vol. 72, No. 1, 1982, pp. 7–12. https://doi.org/10.1121/1.388027 CrossrefGoogle Scholar[18] Simon F., "Adaptive Active Control of Acoustic Intensity in Flow," Internoise 2012, edited by Burroughs C., New York, Aug. 2012. Google Scholar Previous article Next article FiguresReferencesRelatedDetails What's Popular Volume 60, Number 4July 2023 CrossmarkInformationCopyright © 2023 by ONERA. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. All requests for copying and permission to reprint should be submitted to CCC at www.copyright.com; employ the eISSN 1533-3868 to initiate your request. See also AIAA Rights and Permissions www.aiaa.org/randp. TopicsAcousticsAeroacousticsAerodynamicsAeronauticsAerospace SciencesComputing, Information, and CommunicationContinuum MechanicsFluid DynamicsMaterials and Structural MechanicsSignal ProcessingSolid MechanicsStructural MechanicsWind Tunnels KeywordsHybrid Laminar Flow ControlOverall Sound Pressure LevelTransonic Wind TunnelSignal ProcessingStagnation TemperatureAcoustic AttenuationAcknowledgmentsThis study was performed in the framework of Revêtement Acoustique pour les MesureS d'Essais en Soufflerie (RAMSES), which is a project funded by the Direction Générale de l'Aviation Civile. The authors wish to thank Timothée Chabert and the S1MA team for running the tests. The second author wishes to thank Christophe Peyret for his help in setting up the discontinuous-Galerkin simulations.PDF Received18 May 2022Accepted17 January 2023Published online6 February 2023

 The last sentence of the second paragraph of III.B.2.Geometrical parameters in page 14 must read:For that, a maximum number of 5 tubes can be inserted and considering that the surface to define the porosity is delimited by the width of the cavity (5.4 mm) and 20% of the blade span (25 mm), the porosity cannot be greater than 2%.

The present paper presents the design and the experimental assessment of an acoustic liner placed on the pressure side of a turning vane of the large S1MA windtunnel in order to reduce the noise in the test section due to drive fans noise. This paper shows the full methodology applied and the assessment of a demonstrator in real conditions

The evolution of turbofan engines has led to designs with larger fan diameters, shorter inlets, and thinner nacelle walls. This has forced the acoustic liners used therein to be placed closer to engine components that undergo significant thermal heating. This experimental study investigates the combined effects of important thermal gradients, grazing flow, and acoustic level on liners applied to the Long Elastic Open Neck Acoustic Resonator (LEONAR) liner concept. Previous studies have shown that a coupling between these three effects can exist. The objective is to compare such a liner concept to a classical equivalent single-degree-of-freedom liner and to highlight differences in this coupling behavior. Experiments are conducted in the grazing flow duct at ONERA (B2A), where the flow temperature can be accurately regulated and several types of acoustic excitation can be provided. A test section with a heating or cooling device is used to obtain a thermal gradient between the backplate and the perforated plate of the liner sample, and infrared thermography is used to measure the temperature distribution on the perforated plate. The measurement is conducted on several configurations to assess the behaviors of the LEONAR liner concept in the context of combined influence of grazing flow, thermal gradients, and high sound levels.

NASA and ONERA have explored a number of acoustic liner concepts over the last few decades. This paper begins with a brief review regarding conventional liners as well as the recent implementation of multi-degree-of-freedom liners enabled by embedded mesh caps. Six novel liner concepts are presented, along with the accompanying impedance prediction models used in their design. Each of the NASA concepts is designed to vary the impedance over the surface of the liner in a controlled manner, whereas the ONERA concepts make use of long neck acoustic resonators. Selected results are presented for each of these liners evaluated in the NASA and ONERA test rigs. Finally, a set of aeroacoustic metrics is defined for comparison standardization between conventional and innovative acoustic liners and all the concepts are compared on this basis.

In the context of aircraft noise reduction in varied applications where a cold or hot shear grazing flow is present (i.e., engine nacelle, combustion chamber, jet pump, landing gear), improved acoustic liner solutions are being sought. This is particularly true in the low-frequency regime, where space constraints limit the efficiency of conventional liner technology. Therefore, liner design must take into account the dimensional and phenomenological characteristics of constituent materials, assembly specifications and industrial requirements involving multiphysical phenomena. To perform the single/multi-objective optimization of complex meta-surface liner candidates, a software platform coined OPAL (OPtimisation of Acoustic Liners) was developed. Its first goal is to allow the user to assemble a large panel of parallel/serial elementary acoustic layers along a given duct. Then, the physical properties of this liner can be optimized, relatively to weighted objectives, for a given flow and frequency range: impedance target, maximum absorption coefficient or transmission loss with a total sample size and weight... The presentation will focus on the different elementary bricks and assembly of a problem (from 0D analytical coarse designs in order to reduce the parameter space, up to 2D plan or axisymmetric high-order Discontinuous Galerkin simulations of the Linearized Euler Equations).

Passive acoustic liners, used in aeronautic engine nacelles to reduce radiated fan noise, have a quarter-wavelength behavior. The simplest systems are SDOF-type (single degree of freedom), consisting of a perforated sheet backed with a honeycomb, whose absorption ability is limited to frequencies near the Helmholtz frequency. Thus, to widen the absorption frequency range, manufacturers use a 2DOF (double degree of freedom) system, with an internal layer over another honeycomb (stack of two resonators). However, one constraint is the limited thickness of the overall system, which reduces the space allotted to each honeycomb. A possible approach, based on a previous concept called LEONAR (long elastic open-neck acoustic resonator), could be to link each perforated layer to hollow tubes inserted in each honeycomb layer, in order to shift resonance frequencies to lower frequencies by extending the air column lengths. The presence of an empty chamber on both sides of the internal perforated layer also allows the tube length to be increased through tubes crossing both cavities, preserving the liner thickness. The main aim of this article is to mathematically describe the principle of a 2DOF LEONAR and to show the relevance of the mathematical model through FEM simulations and experiments performed in an impedance tube. Moreover, its behavior is analyzed through a parametric study, in order to explore its potential for an aeronautic application. A remarkable feature of 2DOF LEONAR-type materials with insertion of bottom tubes in the higher cavity is the possibility of maintaining the low frequency band provided by the original LEONAR concept, while adding a second absorption peak at a higher frequency, by the second layer and the accompanying tubes. There is a fundamental difference from classical SDOF/2DOF resonators, for which the thicknesses are obviously different.

The development of turbofans with larger fan diameters, shorter inlets, and thinner walls forces the acoustic liners to be placed closer to the hot parts of engines. This experimental study investigates the combined effects of important thermal gradients, grazing flow and acoustic level on the acoustic behavior of liners applied to the LEONAR (Long Elastic Open Neck Acoustic Resonator) liner concept. Previous studies have shown that a coupling between these three effects can exist. The objective is thus to understand the underlying coupled phenomena, in order to emphasize the interest of such a liner concept regarding a classical equivalent SDOF liner. In the ONERA B2A grazing flow acoustic liner facility, the flow temperature can be accurately regulated and several types of acoustic excitation can be provided. A test section with a heating or cooling device is used to obtain a thermal gradient between the backplate and the perforated plate of the liner sample, and infrared (IR) thermography is used to measure the temperature distribution on the perforated plate. The measurement is conducted on several configurations and with different types of liners, to assess the behaviors of the LEONAR liner concept in the context of the UHBR.

The development of turbofans with larger fan diameters, shorter inlets, and thinner walls forces the acoustic liners to be placed closer to the hot parts of engines. This experimental study investigates the combined effects of large thermal gradients, grazing flow and acoustic level on the impedance of liners. Previous studies have shown that a coupling between these three effects can exist. The objective is thus to understand the underlying coupled phenomena, in order to extract the driving parameters for a more accurate impedance modeling. In the ONERA B2A grazing flow acoustic liner facility, the flow temperature can be accurately regulated and several types of acoustic excitation can be provided. For the purposes of this study, a test section with a heating device is used to obtain a thermal gradient between the backplate and the perforated plate of the liner sample. Infrared (IR) thermography is used to measure the temperature distribution on the perforated plate. The measurement is conducted for several configurations, to determine in which conditions the coupling between thermal and acoustic dissipation effects exists. In particular, a possible control of surface temperature by high sound pressure level is highlighted.

Acoustic liners for aeronautics have mostly a Helmholtz resonator behavior provided by perforated sheets backed by honeycombs. They may be used, for example, at the surface of a landing gear door to reduce the noise generated by a landing gear at landing. However, their acoustic absorption ability is naturally limited to medium and high frequencies due to thickness constraints. The LEONAR concept is proposed to overcome the problem of available space in case of “low frequency” range. The design consists in a meta-surface in which a perforated plate is coupled with tubes of variable lengths. This meta-surface covers a back cavity with limited volume and generate a significant shift in the frequency range of absorption, towards lower frequencies. First, an optimization problem is carried out to obtain the LEONAR meta-surface design that allows a maximal absorption coefficient between 400 and 1000 Hz, for a low thickness and a grazing flow up to Mach 0.2. Then the effect of impedance surface is checked by simulation of the radiated pressure field produced by a monopole source in presence of a landing gear door and compared with wind tunnel tests. The measurements showed significant attenuation within the prescribed range (D(OASPL) up to 1.5dBA).

An experimental campaign dedicated to the characterization of the wall-normal velocity correlations in a zero pressure gradient turbulent boundary layer was performed. A double set of laser Doppler velocimetry (LDV) benches were used to access two-point two-time correlations of the wall-normal velocity. The measurements analysis confirms several important hypotheses classically made to model wall pressure spectra from the velocity correlations. In particular, the ratio of the wall-normal Reynolds stress to the turbulent shear stress is confirmed to exhibit a large plateau in the logarithmic region. In addition, Taylor's hypothesis of frozen turbulence is well recovered for the wall-normal velocity fluctuations. The convection velocity for the wall-normal velocity fluctuations is also shown to evolve across the boundary layer, according to the mean velocity profile. Furthermore, the decorrelation time scale of velocity correlations appears to be increasing throughout the boundary layer thickness in accordance with the increase of the convection velocity. The results obtained with this original campaign will help improving models for wall pressure spectra, especially those based on the resolution of the Poisson equation for the pressure for which the wall pressure correlations are related to the wall-normal velocity correlations.

This paper investigates the combined effects of high sound pressure level and grazing flow on impedance eduction for acoustical liners. Experiments are conducted in the grazing flow duct at ONERA (B2A). The impedance is then educed with an inverse method adapted to a shear flow. To take into account the effects of incident sound pressure level, a strategy for impedance eduction using a space-dependent impedance is considered. This strategy is applied to different experimental cases, and the results are compared with those obtained with a reference method in which the impedance is assumed constant.