Acoustic Liner Research Articles

Feasibility of an acoustic liner applied to a Fenestron: experimentation

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 (aero-)acoustic tests applied to different acoustic liners optimised to be integrated into a 1/3-scale fenestron mock-up, in order to provide a specific impedance (measurement in impedance tube and aeroacoustic bench); second, the configuration of a mock-up installed in an anechoic room, with an acoustic antenna; third, the acoustic pressure mitigation obtained for several diffusers equiped with liners (variation of RPM and pitch for the purpose of the experiment). It appears that the tested liners are likely to provide a high absorption coefficient in a large frequency range and a significant attenuation when installed in the Fenestron mock-up, in accordance with preliminary simulations. Another paper completes the study with a preliminary numerical assessment of the effect of an absorbing treatment introduced inside the collector or the diffuser, and a design optimization applied to different acoustic liner types, adapted to the mock-up.

Feasibility of an acoustic liner applied to a Fenestron: simulation

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.

Direct aeroacoutic simulation of a ducted axial fan with acoustic liners using a volume penalization lattice Boltzmann solver on heterogeneous massively parallel platforms

Aeroacoustic noise of an axial fan is directly solved to understand its mechanism, using our in-house Lattice Boltzmann Method (LBM) solver. To make the LBM solver industrially successful, we must reduce calculation time while maintaining its accuracy. We adopt Volume Penalization method for moving boundaries to achieve independent parallelism, and to utilize GPU (Graphical Processing Units) for parallel computation. In this study, we present a comparison between numerical and experimental results of an axial fan in a lined duct. The acoustic liner is designed to reduce 2nd BPF noise, and the noise reduction of 9.8dB is observed in the experiment. With our LBM code, the noise reduction was reproduced within an accuracy of 2.5dB, and profound insights are provided through the detailed investigation of the numerical results.

Design and evaluation of an acoustic liner applied to a UAV ducted rotor in hover

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.

Electroacoustic Liner for tonal and broadband noise attenuation of turbofans

The new generation of Ultra-High-By-Pass-Ratio (UHBR) turbofan engine while considerably reducing fuel consumption, threatens higher noise levels at low frequencies because of its larger diameter, lower number of blades and rotational speed. This is accompanied by a shorter nacelle, leaving less available space for acoustic treatments. Therefore, alternative solutions to classic liners are required. The SALUTE H2020 project has taken up this challenge, proposing electro-active acoustic liners, made up of loudspeakers (actuators) and microphones (sensors). The electro-active means allow to program the surface impedance on the electroacoustic liner, but also to conceive alternative boundary laws. Test-rigs of gradually increasing complexities have allowed to raise the Technology Readiness Level (TRL) up to 3-4. In this paper, we illustrate the control strategies and the experimental results achieved on the Phare2 test-bench. Phare2 is a reproduction of a real turbofan (scale 1:3), available in the Laboratory of Fluid Mechanics and Acoustics in the Ecole Centrale of Lyon. The noise attenuation accomplished by the electroacoustic liner is assessed by an antenna of microphones surrounding the turbofan inlet, and two rings of microphones placed upstream and downstream the liner in the nacelle. Both broadband and tonal noise are targeted with very promising results.

Inverse and direct impedance eduction of liners in the MAINE Flow facility

The assessment of acoustic liners typically involves conducting experiments with airflow in a duct under various incident sound pressure levels and flow velocities. Impedance, as the most intrinsic parameter characterizing an acoustic wall (due to its independence from the duct section), is often the primary focus. Experimental techniques for its determination can be categorized into two classes: direct and inverse eduction methods. This paper contrasts these two methodological classes within the context of the MAINE Flow facility-a 150mm x 280mm rectangular duct capable of generating flow Mach numbers up to 0.63 and multimodal sound fields of Sound Pressure Levels up to 150 dB. Direct impedance determination necessitates a substantial degree of attenuation along the liner. However, in the current implementation, inverse impedance determination is constrained by the uniform flow hypothesis, which is invalid for large Mach numbers in such facilities. The findings illustrate that both methods can yield valuable impedance data, enabling the examination of the effects of flow and incident modal content on the measured impedance.

CONCEPTUAL ANALYSIS ON THE IMPACT OF ACOUSTIC LINERS IN FAN FLUTTER

Abstract This work investigates the impact of acoustically treated intakes on fan flutter. The propagation of the fan acoustic perturbations is studied using a simplified model, where the frequency of the waves in the intake is the natural frequency of the fan blade in the stationary frame of reference. The model consists of a constant cross-section annular duct, where the acoustic modes are computed for hard wall and lined wall regions, assuming a uniform axial flow. The interfaces between the lined and hard wall sections are solved by doing mode matching. The acoustic reflections in the zero-thickness intake are computed using the Wiener-Hopf technique following Rienstra's model. Analytical results show that the waves propagating from the fan are reflected and scattered radially at the interfaces between the hard wall and the liner. Furthermore, the liner damps and changes the phase velocity of acoustic modes. Due to the low frequency of the flutter waves, the radial scattering can activate different cut-off modes, which need to be retained in order to make accurate predictions. The likelihood of fan flutter is estimated using the phase of the reflected cut-on waves reaching the fan. This strategy is used to assess the impact of the acoustic liners on the stability of a realistic fan. It is concluded that the impact of the phasing and reflections induced by the acoustic liners is comparable to that of the intake and can alter the flutter characteristics of the fan significantly.

Study on multi-degree-of-freedom septum liners for broadband noise reduction

Silent aeroengine nacelle highlights the demand for advanced acoustic liners providing broadband absorption. This paper studies the multi-degree-of-freedom (MDOF) septum liner that is a promising candidate. The steady-flow resistance of the septum as the necessary input parameter is measured on a steady-flow resistance tube herein, and an impedance prediction model is introduced and validated through the experimental measurement and the numerical simulation. Then, three liners with embedded septa of varied types, numbers and depths are designed, and the acoustic characteristics are analyzed in a wide frequency range under different incident sound pressure levels (SPL). It indicates that such liners can perform well in an ultra-broadband range up to 10000 Hz, and the impedance is relatively insensitive to the incident SPL. It is worth noting that, achieving broadband absorption for such liners is influenced by the coupling effect between zeros of the reflection coefficient, which correspond to the resonance frequencies of the liner. Moreover, the simulated results illustrate that the significant absorption for MDOF septum liners stems mainly from the sound energy dissipation at specific spatial positions depending on the frequency.

Thermo-elastic vibration analysis and optimization of a nanoresonator composed of a coupled nanotube system for acoustic liner application using the hybrid deep neural network–based white shark algorithm

This study explores the potential of nanoresonator systems composed of coupled nanotubes with graphene nanoparticles for enhancing the performance of acoustic liners in aircraft engines. The objective of this study is to develop an analytical model and predict its behavior under various conditions. The acoustic liners consist of perforated metal sheets and honeycomb cavities, which are essential for noise reduction in aircraft engines. The model is tested for variations in natural frequency, mode number (m), size effects (e0a), viscous constant (C), temperature (T), localness factor (L), and stiffness constant (K). A hybrid deep neural network–based white shark algorithm (DNN-WSA) is used to predict and optimize the performance of nanoresonator-coupled nanotube systems. Four theories were compared, such as wave propagation theory, nonlocal elasticity theory, polynomial eigenvalue approach, and governing equations with respect to natural frequencies in nanoresonator-coupled nanotube systems. The wave propagation theory yielded the lowest natural frequency, which was selected for detailed analysis. The optimized values of size effect of 2 nm, temperature of 5 K, and frequency of 1.971975 THz were obtained. When C = 0.3, K = 10, T = 300, and L = 10×e−9, the root mean square error (RMSE) value is 0.8421, which indicates improved predictive performance as it continues to decrease. The study’s findings showed that changes in viscous constants impact natural frequencies, while size effects have a minor influence. Temperature variations also affect natural frequencies, with higher temperatures leading to higher frequencies. The optimized model demonstrates enhanced predictive performance, which contributes to a better understanding of nanoresonator systems and their application in noise reduction for aircraft engines.

Effect of Over-the-Rotor Liner on Fan Flutter Stability

Previously published literature has proved excellent performance of the over-the-rotor acoustic liner (OTRL) in noise reduction. This work investigates the impact of the OTRL on fan flutter stability in a subsonic duct using a three-dimensional coupled singularity method. The effects of the axial position and the acoustic impedance of the liner on the fan flutter stability are examined. Results show significant near-field coupling between the acoustic liner and the rotor, affecting the unsteady blade loading distributions and intensifying as the liner and the rotor overlap axially. The fan flutter stability shows clear sensitivity to the liner’s acoustic resistance, while its dependency on acoustic reactance appears more complex. An energy-based analysis shows that secondary unsteady loading induced by rotor–OTRL interaction may inverse the phase of the total unsteady loading, thus affecting the fan flutter stability.

A simple method improving acoustic mode identification capability based on genetic algorithms

This letter develops a simple approach of duct mode identification and reconstruction based on genetic algorithms, which can extend the azimuthal mode order range compared to the conventional method based on the (spatial) discrete Fourier transform. The underlying principle is reconstructing the dominant mode from the modal identification forward model through optimization by exploiting the sparsity of the mode amplitude vector. The performance is experimentally demonstrated for detections of one and two azimuthal modes under noisy conditions with nondominant modes. Overall, the proposed genetic-algorithm-based framework for solving acoustic inverse problems is beneficial to duct acoustic testing, particularly design evaluations of fan blades and acoustic liners for aeroengines.

The Advection Boundary Law in absence of mean flow: Passivity, nonreciprocity and enhanced noise transmission attenuation

Sound attenuation along a waveguide is intensively studied for applications ranging from heating and air-conditioning ventilation systems, to aircraft turbofan engines. In particular, the new generation of Ultra-High-By-Pass-Ratio turbofan requires higher attenuation at low frequencies, in less space for liner treatment. This demands to go beyond the classical acoustic liner concepts and overcome their limitations. In this paper, we discuss an unconventional boundary operator, called Advection Boundary Law, which can be artificially synthesized by electroactive means, such as Electroacoustic Resonators. This boundary condition entails nonreciprocal propagation, meanwhile enhancing noise transmission attenuation with respect to purely locally-reacting boundaries, along one sense of propagation. Because of its artificial nature though, its acoustical passivity limits are yet to be defined. A thorough numerical study is provided to assess the performances of the Advection Boundary Law, in absence of mean flow. An experimental test-bench validates the numerical outcomes in terms of passivity limits, non-reciprocal propagation and enhanced isolation with respect to local impedance operators. Guidelines are outlined to properly implement the Advection Boundary Law for optimal noise transmission attenuation. Moreover, the tools and criteria provided here can also be employed for the design and characterization of other innovative liners.

Composite acoustic hole segmentation by semi-supervised learning for robotic multi-spindle drilling of aero-engine nacelle acoustic liners

The large-scale tiny acoustic holes densely distributed on acoustic liners are essential in aero-engine noise reduction. Accurate segmentation of those holes is fundamental for a robotic multi-spindle drilling system. This paper introduces a novel semi-supervised segmentation method for acoustic holes on composite acoustic liners. This method uses perturbation consistency loss to ensure output consistency while solving the problem of data volume imbalance. Afterward, a multi-reliability enhancement method and a pseudo-label reliability enhancement module are utilized to enhance the model’s robustness and accuracy. The segmentation experiments show that our method is superior to UniMatch and FixMatch. When using only 30% of labeled data, our method achieves an IoU of 96.39%, and the porosity differs only 0.038% from the ground truth, which is better than the fully-supervised segmentation method using all data. Our method can meet the accuracy and efficiency requirements of aero-engine nacelle production in China with only limited labeled data.

Low-frequency sound attenuation by coiled-up meta-liner with nonuniform cross sections under grazing flow

We report a kind of coiled-up meta-liners with nonuniform cross sections (CMNC), which can efficiently attenuate low-frequency sound waves under grazing flow with a deep subwavelength thickness (e.g., ∼λ/17 at 500 Hz). At a grazing flow Mach number of 0.26, the average transmission loss of the meta-liner is 12.6 dB at 500–1000 Hz, which is twice as much as that of a double-degree-of-freedom acoustic liner of the same size. Physically, the nonuniform cross-sectional distribution and significant cross-sectional area ratio enhances vortex shedding, thus resulting in severe acoustic energy dissipation. The excellent low-frequency acoustic attenuation performance of CMNC is investigated thoroughly with experimental, theoretical, and numerical methods. This work provides an avenue for low-frequency noise reduction in grazing flow scenarios (e.g., in a high bypass ratio turbofan engine).

Domain decomposition method based on One-Way approaches for sound propagation in a partially lined duct

A numerical factorization method of the unidirectional propagation operators induced by the linearized Euler and Navier–Stokes equations is performed to construct a new One-Way approach to compute the sound radiation in a partially lined duct. The complex phenomena of reflection and transmission of incident waves originating from discontinuities in the duct wall are accurately taken into account by an iterative domain decomposition method. Furthermore, the proposed factorization approach allows both the derivation of the lined duct scattering matrix and an in-depth understanding of the impact of the acoustic liner on left- and right-going waves coming from a transmission or a reflection. Finally, the efficiency of the numerical method is shown based on a classical benchmark with two types of baseflows (laminar and turbulent Poiseuille flows) and by comparison with other numerical and experimental results. An unstable surface mode is observed at 1000 Hz, and we find good agreement with experimental data for the turbulent mean flow associated with the One-Way Navier–Stokes equations.

Physics-Informed Acoustic Liner Optimization: Balancing Drag and Noise

Pore-resolved direct numerical simulations (DNS) of turbulent flows grazing over acoustic liners with aerodynamically and/or acoustically optimized orifice configurations are presented. The DNS explore a large parameter space, studying various families of orifice geometries, including the influence of orifice shape, orientation, and the number of orifices. All flow cases show an increase in drag compared to the smooth wall. However, the added drag can be reduced by as much as approximately 55% as compared to conventional acoustic liners by simply altering the shape of the orifice or its orientation, in the case of a noncircular orifice. Complementary acoustic simulations demonstrate that this reduced drag may be achieved while maintaining the same noise reduction properties over a wide range of frequencies.

On the application of acoustic liners for the reduction of jet-flap installation noise

In this work, an experimental and numerical analysis of the feasibility of using acoustic liners to reduce the acoustic far-field radiation of a subsonic jet near a flat plate is presented. The Boundary Element Method (BEM) with a wavepacket model of the jet noise and considering an acoustic impedance boundary condition was used to calculate the far-field noise for different conditions. A parametric optimization was performed in order to find an optimum impedance in terms of the Overall Sound Pressure Level reduction. Semi-empirical models of acoustic liners were then used to design a liner with an impedance as close as possible to the optimum impedance. The liners were built using additive manufacturing and tested in terms of their acoustic impedance and the resulting noise reduction when applied to a region of the flat plate. The BEM simulations, considering the impedance, showed good agreement with experimental data. Overall, by means of the BEM model, optimization and liner semi-empirical models, it was possible to design an acoustic liner which significantly reduced the installation noise at the far-field, with the overall noise reduction between 2 dB to 4 dB depending on observation angle.

Acoustic Liners Combined with Fine Perforated Film

The presence of grazing flow can alter the noise reduction performance of resonant liner panels. One possible cause is the flow entering the liner opening. In this study, a fine perforated film was applied to the surface of acoustic liners to prevent such flow effects. Three types of liner samples were fabricated and tested in a flow duct rig at the Japan Aerospace Exploration Agency. The first sample was a conventional resonant liner (“baseline”), the second sample had a fine perforated film directly attached to the baseline liner surface (“direct fine perforated film”), and the third sample had a gap fixed between the film and the liner surface (“fine perforated film with a gap”). The impedance eduction results show that the fine perforated film with a gap was effective in separating grazing flow from the liner openings and in reducing the effect of the flow on the impedance. The transmission loss in the duct improved. However, the effectiveness of the direct fine perforated film was limited. The fine perforated film with a gap structure showed potential to facilitate the design of high-efficiency liners under grazing flow conditions.

Passive acoustic liners for aero-engine noise control: A review

Aero-engine noise reduction has always been a critical challenge in the field of science and engineering. Passive acoustic liners are considered one of the most effective structures for reducing aero-engine noise, significantly improving comfort aboard aircraft, protecting personnel health and reducing the noise footprint of aircrafts. This review summarizes the progress in passive acoustic liners, including locally reacting liners, non-locally reacting liners and metamaterial liners. We provide an overview of the fundamental structures, performances, and absorption mechanisms associated with these liners. Firstly, we briefly introduce the absorption mechanism of locally reacting liners and non-locally reacting liners respectively, and summarize the different types of each. Secondly, the unique ways in which acoustic metamaterials control sound waves due to their negative physical parameters are illustrated. Furthermore, we review the applications of acoustic metamaterials in passive acoustic liners. Finally, the challenges and opportunities of passive acoustic liners are analyzed and prospected.

Numerical simulation of sound attenuation in an acoustically lined duct in high-temperature air flows

Noise reduction structures are important for the vibration and noise reduction design of aerospace engines. The design of noise-reducing structures often needs to be quickly evaluated via numerical simulations. Hence, the simulation results of the corresponding system are very important for guiding the design of noise-reducing structures. High temperature is one of the key environmental factors that need to be considered when evaluating the sound attenuation process via numerical methods. In this study, numerical simulations of acoustic wave propagation on an acoustic liner structure considering air temperature variations are carried out by using compressible Navier–Stokes equations and the ideal gas equation of state. The results showed that the effect of temperature on sound attenuation under grazing flow conditions is complex. Moreover, an increase in temperature will reduce the transmission loss of the acoustic liner in the grazing flow at high air speed.