Acoustic Liner Research Articles

Broadband and Low-Frequency Acoustic Liner Investigations at NASA and ONERA

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.

Designing a Facility for Measuring Higher-Order Acoustic Modes in Uniform Heated Flows

An experimental facility was recently developed at the Georgia Tech Research Institute to measure the acoustic scattering coefficients of a test article under mean flow, high-temperature, high-amplitude, and higher-order mode propagation conditions. The experimental facility acts like a two-sided impedance tube, in which the measurement section is a circular duct with 16 acoustic drivers and 16 microphones mounted on each side of the test article. Such a facility can be configured for grazing and bias flow measurements of acoustic liners with sound traveling in and against the direction of the flow. The new facility is designed to maximize the accuracy of acoustic measurements of higher-order modes in heated flows. The influence of experimental uncertainties on the acoustic measurements is analyzed using a Monte Carlo method. Measurements of half-wave resonators in quiescent air and heated flows demonstrate that the experimental uncertainty decreases as the absorption coefficient increases. High-sensitivity microphones produce extremely low uncertainty, and even lower-sensitivity pressure transducers are sufficiently accurate for many applications. This paper is intended to serve as a reference for designing and analyzing an experimental facility to measure the acoustic scattering coefficients in heated-flow and higher-order mode propagation conditions.

Sound Absorption Properties of Porous Metals Under Grazing Flow Conditions

Porous metals could potentially be used for acoustic liners that can operate in adverse conditions such as high temperature and high pressure. Recent developments in synthetic techniques for porous metals have made it possible to produce them for specific applications, which are intended to provide increased sound absorption. However, if a porous metal is employed in the presence of grazing flow, it is difficult to predict its sound absorption properties. In this study, the acoustic properties of two well-known types of stainless-steel porous materials with different porosities, that is, sintered fiber and cellular foam, were investigated using impedance tube and flow duct experiments. The impedance in flow duct experiments in the absence of flow is almost the same as that in impedance tube experiments, except for the cellular foam sample with high porosity, which may not be a locally reacting material. Furthermore, the effects of grazing flow on the impedance of the porous sample can be separated into those due to the average flow velocity and those due to the flow velocity gradient in the wall-normal direction. Both effects depend on the pore structure and porosity of the porous samples.

Influence of Hole-to-Hole Interaction on the Acoustic Behavior of Multi-Orifice Perforated Plates

Abstract A key factor for developing low-emission combustion systems in modern gas turbines and aero-engines is the acoustic liner's optimized design. Several models are available in the literature for the acoustic impedance of perforated acoustic liners. Most of these impedance models neglect the interaction effect between the orifices. In practice, the orifices are generally closely distributed such that the interactions between acoustic radiation from neighboring orifices can affect their acoustical behavior. The hole-to-hole interaction effect may change the resonance frequency of the resonator due to the nonplanar wave propagation in the cavity, the orifices in the perforated plate, and the near-wall region in the combustor. Considering this effect may help to predict the resonance frequency of the resonator accurately. In this work, a three-dimensional (3D) analytical approach is developed to account for the nonplanar wave propagation in the cavity and orifices on the perforated plate. The present study employs the proposed 3D analytical method to determine the hole-to-hole interaction end correction of multi-orifice perforated plates. Additionally, the hole-to-hole interaction end correction from a series of perforated plates with different orifice radii and spacings is obtained via the finite element method. Perforated plate specimens with different center-to-center hole spacing are tested using an impedance tube. Experimental results show that the resonance frequency is shifted toward a lower frequency with decreasing holes' spacing. The resulting model is compared with the experiments and the end-correction models available in the literature. The comparison shows that the available end-correction models cannot capture the hole-to-hole interaction effect, which is observed in experiments. In contrast, the proposed model can reproduce measurements with high quality. The resulting model demonstrates that the acoustic end-correction length for orifices is closely related to the perforated plate's porosity ratio and orifice radius. The proposed model is readily applicable in the design of multi-orifice perforated plates.

Further assessment of a time domain impedance boundary condition for the numerical simulation of noise-absorbing materials

In regard to the mitigation of environmental noise across major industry sectors, the present study focuses on the numerical prediction of passive noise reduction devices. Here, it is further explored how the noise attenuation induced by locally reacting noise absorbing materials (also called acoustic liners) can be simulated using a time domain highly accurate Computational AeroAcoustics (CAA) method. To this end, it is assessed how a classical Time Domain Impedance Boundary Condition (TDIBC) can effectively model acoustic liners of practical interest, including when the latter are exposed to realistic conditions (grazing flow and noise excitation). The investigation consists in numerically reproducing two experimental campaigns initially performed at NASA Langley Research Center. Two different materials are considered (honeycomb superimposed with perforate or wiremesh resistive face-sheet), each being characterized by a specific noise attenuation behaviour ( e.g. dependency on the flow conditions and/or noise excitation). Each material is tested under various flow conditions ( e.g. grazing flow of Mach up to 0.5) and/or noise source excitation ( e.g. multiple tones of level up to 140 dB each). The results demonstrate the ability of the underlying CAA/TDIBC approach to simulate realistic acoustic liners in non-trivial configurations, with enough physical accuracy ( e.g. correct capture of the noise attenuation characteristics) and numerical robustness ( e.g. absence of instabilities). The study also reveals that, independent from the CAA/TDIBC approach itself, some specific pre-processing tasks (e.g. impedance eduction and subsequent TDIBC calibration) may play a bigger role than expected, in practice.

Modelling of ducted noise sources in the proximity of acoustic liners

This paper focuses on modelling and predicting the acoustic field generated by ducted point sources in close proximity to an acoustic liner. Two analytical models are presented. The first model comprises a point monopole or dipole source in an infinite lined duct based on an existing Green’s function. The predictions are compared with classic solutions for a source over an infinite lined plane and with a high frequency asymptotic duct approximation. The second model extends the Green’s function to include a liner section of finite length connected to hard-wall extensions by using mode-matching techniques. The new model features the inclusion of point sources in the vicinity of an impedance discontinuity. The accuracy of this model is demonstrated by comparison with reference FE solutions. Both models indicate a significant impact of the source proximity to the liner surface in the source power output. These models offer insight on the source modification effects beyond the conventional approach in the design of liners solely based on acoustic absorption.

Innovative Acoustic Treatments of Nacelle Intakes Based on Optimised Metamaterials

Modern turbofans with high bypass ratios, low blade passage frequencies and short nacelles require continuous development of acoustic linings to achieve the noise reductions expected by the international aviation authorities. Metamaterials and metafluids have been recently proposed as promising technologies for designing innovative acoustic treatments dedicated to reducing aeronautic turbofan noise emissions. In this work, a phase-gradient metasurface treatment is investigated as a way to tackle the noise radiation from an axially symmetric nacelle. This paper aims to study the potential benefits of the mentioned technology, and is not an attempt to design a complete new liner or nacelle. The metasurface is modelled through an equivalent metafluid, and a simulation-based optimisation is used in defining the design parameters. The tonal contribution of the blade passage frequency is considered, and the numerical results with the metafluid optimised on one azimuthal mode at a time show a significant effect in terms of acoustic levels and directivity over an arc of virtual receivers.

Experimental and Numerical Study on Wall Impedance Effects on Tollmien–Schlichting Waves

This paper investigates the effects of a porous wall on the development of Tollmien–Schlichting (TS) waves propagating inside a boundary layer. An experimental study performed in a low-speed wind tunnel showed that microperforated panels over hollow chambers could promote laminar–turbulent transition. First, measurements of the streamwise evolution of the TS instabilities reveal that greater amplification occurs over perforated panels compared to a solid wall. This effect was attributed to a wall impedance condition imposed by the perforated panels over the chambers. This setup is somewhat analogous to an acoustic liner for which the wall response is represented by a complex impedance. The impedance of the experimental setup was thus measured in situ. These experimental impedance values were then used as wall boundary conditions of a linear local stability calculation to quantify the influence of the wall impedance condition on TS wave growth. The approach was applied to predict the corresponding transition positions. Comparison between experimental and numerical results shows that the use of impedance boundary conditions is a promising approach to predict transition location, particularly in the case of the panel with the lowest resistance.

Editorial for special issue on acoustic liners

Editorial for special issue on acoustic liners

Effects of acoustic liner on thermoacoustic instabilities in a premixed swirl combustor

Acoustic liner is generally adopted to damp noise in aero-engines and gas turbines. This paper presents a systematic study about the effects of an acoustic liner on thermoacoustic instabilities in a laboratory scale burner. A premixed swirl combustion system is constructed and a specially designed single layer acoustic liner is located at downstream of the flame zone. A low order network model is built to recognize the thermoacoustic modes of the combustion system. The Transfer Element Method (TEM) method is applied to analyze the absorption of the acoustic liner with bias flow. Experiments under the boundaries of the rigid wall and the acoustic liner without and with tunable bias flow are carried out, respectively. Furthermore, the temperature of the bias flow is adjusted to evaluate its effects on combustion oscillations. The result of low order modeling shows that the low-frequency mode around 116 Hz is the Helmholtz mode of the upstream plenum, while the higher frequency mode near 300 Hz is the quarter wave mode of the combustion chamber. The experimental result shows that the instability of the Helmholtz mode can be triggered and further enhanced with the increase of the bias flow Mach number. Meanwhile, the instability of the 1/4 wave mode is completely suppressed. The Helmholtz mode that is triggered by the bias flow can be attenuated by raising the temperature of the bias flow, while no substantial changes are observed in the quarter wave mode.

High fidelity modeling tools for engine liner design and screening of advanced concepts

With aircraft engines trending toward ultra-high bypass ratios, resulting in lower fan pressure ratios, lower fan RPM, and therefore lower blade pass frequency, the aircraft engine liner design space has been dramatically altered. This result is also due to the associated reduction in both the available acoustic treatment area (axial extent) as well as thickness (liner depth). As a consequence, there is current need for novel acoustic liner technologies that are able to meet multiple physical constraints and simultaneously provide enhanced noise attenuation capabilities. In addition, recent advances in additive manufacturing have enabled the consideration of complex liner backing structures that would traditionally be limited to honeycomb cores. This paper provides an overview of engine liner modeling and a description of the key physical mechanisms, with some emphasis on the use of low to high-fidelity tools such as empirical models and commercially available software such as COMSOL, Actran, and PowerFLOW. It is shown that the higher fidelity tools are a critical enabler for the evaluation and construction of future complex liner structures. A systematic study is conducted to predict the acoustic performance of traditional single degree of freedom liners and comparisons are made to experimental data. The effects of grazing flow and bias flow are briefly addressed. Finally, a more advanced structure, a metamaterial, is modeled and the acoustic performance is discussed.

Proposal of Acoustic Liners Combined with Fine-Perforated-Film

This paper deals with a resonant type liner panel with a special surface structure. A typical resonant type liner panel generally consists of a perforated face plate, cells, and a back rigid plate. One of the technical challenges of the acoustic liners applied to the future ultra-high bypass ratio engines is to increase the sound absorption efficiency under grazing conditions because the nacelle, covering of the engine, tends to reduce its length and the lined area. It is known that the sound absorption of the conventional liners tends to deteriorate as grazing flow increases. The authors introduced a special thin acoustically transparent film over the face plate of the acoustic liner. The film, a fine perforated film (FPF), is expected to prevent the interaction of the grazing flow with the opening of the liner face plate. An experimental result with a flow duct rig in JAXA confirmed that the proposed combination of the acoustic liner and the FPF improved the absorption in acoustic energy under grazing conditions, compared with the sole acoustic liner and simple treatment of the FPF.

Design and optimization of acoustic liners with a shear grazing flow: OPAL software platform description

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).

Design and optimization of acoustic liners with a shear grazing flow: OPAL software platform applications

In the context of noise reduction in diverse applications where a shear grazing flow is present (i.e., engine nacelle, jet pump, landing gear), improved acoustic liner solutions are being sought. This is particularly true in the low-frequency regime, where space constraints currently limit the efficiency of classic liner technology. To perform the required multi-objective optimization of complex meta-surface liner candidates, a software platform called OPAL was developed. Its first goal is to allow the user to assemble a large panel of parallel/serial assembly of unit acoustic elements, including the recent concept of LEONAR materials. Then, the physical properties of this liner can be optimized, relatively to given weighted objectives (noise reduction, total size of the sample, weight), for a given configuration. Alternatively, properties such as the different impedances of liner unit surfaces can be optimized. To accelerate the process, different nested levels of optimization are considered, from 0D analytical coarse designs in order to reduce the parameter space, up to 2D plan or axisymmetric high-order Discontinuous Galerkin resolution of the Linearized Euler Equations. The presentation will focus on the different aspects of liner design considered in OPAL, and present an application on different samples made for a small scale aeroacoustic bench.

Acoustic space filling curve metamaterials for grazing flow in Jet engine inlets

Acoustic metamaterials research has grown exponentially in the past 10 years driven by the advances in manufacturing and an increased understanding of damaging environment noise. 2020 was the first noise reduction target as set by Advisory Council for Aircraft Research and Innovation in Europe with a relative 50% decrease. This was missed by current Jet engine noise control technology; however, metamaterials offer an encouraging alternative. Space Filling Curves (SFC) have the potential to provide a lightweight, thin, high performance acoustic liner. SFC have a history in mathematical geometry dating back to the 1890's but are a comparatively new addition to acoustics. They are designed with a sub-wavelength curled cross-section creating a maze-like pattern which slows acoustic wave propagation through the liner enabling characteristics such as negative refraction and low frequency attenuation. This paper contains a comparison of some of the most promising SFC metamaterial acoustic liner designs, in terms of the fundamental theory of the design category and a discussion of the reflection, absorption and transmission characteristics in terms of a grazing flow conditions. Computer simulation and impedance tube based experimental testing compares the designs. The paper concludes with future application for aeroacoustics with particular focus on the engine inlet.

A review of variable-impedance acoustic liner concepts developed at NASA

This paper presents results attained in the NASA Langley Research Center test rigs using concepts for which the impedance varies over the surface of the liner. These liners are typically designed for significant sound absorption over a wide frequency range, but it is also possible to tune the design to achieve increased absorption at selected frequencies. A brief review is provided regarding a number of variable-impedance concepts. The first is a modified version of a conventional two-layer liner, in which the embedded septum location and acoustic properties are different for adjacent core chambers. Two concepts employ core chambers with different lengths, one with bent chambers to allow packaging within a limited volume, and the other with shared inlet ports to reduce the surface porosity. The last employs a perforated facesheet in which the hole diameter and porosity are varied over the surface of the liner. Data acquired in the NASA normal incidence and grazing flow impedance tubes are used to demonstrate the capabilities of these concepts. Impedance prediction models are also presented for comparison with these measured data.

A flow and acoustic facility for characterization of liner and meta-acoustic surfaces under grazing flow condition

The Hong Kong University of Science and Technology (HKUST) has designed and assembled a new facility, a grazing flow tube, for aeroacoustic characteristics measurement of acoustic liners, e.g. transmission loss, impedance, etc., under a high-speed grazing flow. The cross-section of the test section of the tube has a dimension of 50 mm × 50 mm, and the grazing flow speed can be up to 0.3 Ma. A settling chamber, a long-enough flow development section and a multi-stage anechoic termination are adopted to ensure the high-quality flow field and acoustic field. This paper presents the detailed designs of the key components of the facility, as well as the calibrations of the velocity profile in a series of cross-section surfaces of the duct along the streamwise direction and sound pressure distributions in the axial and circumferential directions. Pitot tube, Hotwire and PIV are used to obtain the flow field measurement results. The overall performance of the diagnostic facility is verified by comparing the impedance results of acoustic liners acquired from an impedance tube under the static condition and the theoretical variation of axial wavenumber with Ma number under the grazing flow.

A generalized compliance model for study of acoustic damping behavior of mixed porosity segmented perforated liner

Acoustic liners play a vital role in the reduction of aero-engine noise and combustion instabilities. In the present work, acoustic damping behavior of perforated cylindrical liners of mixed porosities is investigated both analytically and experimentally. The aim of the paper is to propose a generalized analytical model for the compliance of mixed porosity liners. The analytical results are obtained by considering the compliance of each segment of distinct porosities. Experimental results are compared with the analytical predictions to validate the proposed generalized compliance expression. Predicted results are in reasonable good agreement with the experimental results. A parametric study is carried out for different flow with variably segmented liner configurations. The effect of number of liner segments is studied. Liners with higher number of segments are shown to perform better than the liners having lesser number of segments.

An introduction to NASA’s broadband acoustic absorbers that resemble natural reeds

Thin, lightweight, and durable broadband acoustic absorbers capable of absorbing sounds over a wide frequency range, especially below 1000 Hz, while also surviving harsh operational conditions such as exposure to sprays of liquid and solid debris and high temperatures are desired for many noise control applications. While today’s commercially available broadband acoustic liners are impressive, such as melamine foam and perforate-over-honeycomb structures, each style has its limitations. Motivated by the need to reduce aircraft engine noise pollution NASA has recently patented a broadband acoustic absorber that claims some benefit over existing acoustic liners. Inspired by nature, these structures resemble the geometry and acoustic absorption of bundles of natural reeds, slender grasses that grow in wetlands across the world. Proof-of-concept experiments have begun at NASA. This report summarizes the design, fabrication, and normal incidence impedance tube tests performed for assemblies of natural reeds and additively-manufactured plastic prototypes that resemble the irregular geometry of bundles of natural reeds. Some synthetic prototypes were tested with and without perforated face sheets. Results indicate that there are a number of synthetic designs that exhibit substantial acoustic absorption in the frequency range of 500 Hz to 3000 Hz, and especially below 1000 Hz, as compared to baseline acoustic absorbers of similar thicknesses and weights. Many of these prototypes have an average acoustic absorption coefficient greater than 0.6. Additionally, an annular prototype was designed and printed but not yet subjected to tests. This annular prototype of a multifunctional structure designed to transfer heat and absorb sound was developed to fit inside the NASA Glenn Research Center’s DGEN Aeropropulsion Research Turbofan engine testbed. This invention can be considered and developed for a variety of aerospace, automotive, industrial, and architectural noise control applications.

Investigations of three over-the-rotor liner concepts at various technology readiness levels

The emphasis on increased turbofan fuel efficiency requires advanced turbofan designs that will integrate higher engine bypass ratios and shorter nacelles. The resulting acoustic signature of these designs will have a more broadband character as well as a smaller available area for liner installation. This two-fold impact compels a need for an improvement in the state of the art in liner technology. Increasing the acoustic absorption efficacy over a broader frequency range is a means to address this need. NASA investigated over-the-rotor acoustic liners for turbofan applications as a potential solution. This type of liner represents a significant advance over traditional liners due to placement in close proximity to the rotor. An advantage of placing treatment in this region is a modification of the acoustic near field, thereby inhibiting noise generation mechanisms. This can result in higher attenuation levels than could be achieved by the conventional sound absorption means. In addition, there is potential to integrate the liner with fan rub-strip and containment components, reducing engine components and thus weight, enabling a systematic enhancement in noise reduction and engine performance. This article reviews the development and evaluation process of three unique over-the-rotor concepts focusing on the discrete tests conducted across the Technology Readiness Level span.