Acoustic Liner Research Articles

Hybrid space filling curve metamaterials for transmissive 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. 2050 was the first noise reduction target as set by Advisory Council for Aircraft Research and Innovation in Europe with a relative 65% decrease. This ambitions target will not be met by current 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 unique hybrid designs combining 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. Experimental impedance tube testing compares 3D printed designs to the traditional Helmholtz resonator. The paper concludes with future application for aeroacoustics with particular focus on the engine inlet.

Programmable time-serial resonances for broadband spectrum matching

Spatially parallel and cascading acoustic liners are commonly used to treat duct noise. Their performance depends on the spectral match with source characteristics. Such liners rely on subtle geometrical and mechanical designs which are not difficult to adapt to different noise sources in a passive device. Here, we propose a temporal analogy, which is a shunted electromechanical diaphragm (SEMD) having different resonant frequencies in different time segments, namely, a time-serial resonator. Its sound absorption spectrum can be easily adapted by a program and yet the device functions passively. The acoustic impedance of the SEMD is determined by the shunt circuit, hence its resonant frequency and absorption peak. A multiple set of branch circuits are used to shunt the diaphragm. A MOSFET, which is a voltage-controlled ultrafast electronic switch, is introduced to cascade and switch each circuit branch. By activating circuit branches in a pre-defined time sequence, we obtain a series of resonance and the absorption which are effective for specific frequency bands at different time segments. When viewed over a long period, the averaged effective sound absorption spectrum is broadened and can be easily shaped by the working duty cycle of each circuit branch. Note that the switching voltage changes the circuit states without supplying power to the SEMD. Both numerical results and experimental demonstrations are presented. This study would open the new era of temporal design of acoustic liners.

Broadband noise attenuation in the flow duct using metamaterial-based acoustic liners

An acoustic liner design based on metamaterial is proposed for broadband duct noise reduction. The material is constructed by four periodically arranged partitions-embedded units covered by a perforated plate, forming a linear reflected phase-shifting within 0 to 2π in a wide target frequency range. The sound absorption performance under normal incident waves is firstly examined by both numerical simulation and impedance tube measurement, leading to close agreement to the target design. Then, the meta-liner is installed in a flow tube to assess its capability of reducing broadband noise in a duct with aerodynamic flows. The experiments investigations are performed in an advanced grazing flow tube developed at the Hong Kong University of Science and Technology. The effect of sound levels (ranging from 120dB to 140dB), flow speeds with the Mach number up to 0.3, and the position of sound source (at upstream and downstream sides of meta-liner) are investigated. Results show that a nearly flat transmission loss is achieved in the target frequency range by the metamaterial-based acoustic liner in various conditions, showing its potential for practical industrial applications.

Effect of the microstructure on the acoustic performance of porous material liner in the duct

Acoustic liners can efficiently reduce the sound pressure level in the duct. The porous material liner has recently received wide attention for the excellent attenuation performance at mid- to high- frequencies. However, more attention was paid to the sound attenuation of the given porous material liner in the duct rather than the design of the porous material liner for a duct noise problem. In this work, from a micro perspective, the influences of the average fiber radius and gap of the porous material liner on the acoustic field in a duct are systematically analyzed, based on the bottom-up method and Johnson-Champoux-Allard model. The multimodal method is utilized to evaluate the duct acoustic field. The work may guide the selection and design of the porous material during the application of the porous material in the duct noise problem.

Noise reduction by perforated cascades in annular ducts

One of the latest trends in noise control relating to aeroacoustics is to mimic the silent flight capability of owls. Particularly, porosity is most often applied on cascades in ducts with axial flows, such as stator structures in an aero-engine. However, current acoustic scattering models of perforated cascades are based on two-dimensional methods without including the three-dimensional effects. In this paper, we present a fully three-dimensional acoustic scattering model for perforated cascades based on the lifting surface theory in which the dominant sound source reduces to dipoles alone under the thin airfoil assumption. Accordingly, the acoustic scattering of perforated cascades with single-mode incident wave was studied and obvious noise reduction was observed. The optimum Rayleigh conductivity and the maximum noise-reducing capability of the porosity varied substantially with different incident duct modes, whilst a larger cascade chord-length could achieve more noise reduction at optimum porosity. With a background flow, the Kutta condition can greatly influence the overall distribution of the unsteady loading on vanes. Additionally, the unsteady vortex shedding at the trailing edge offers extra sound energy dissipation mechanism. Therefore, the implementation of porosity on cascades is much different to the design of a traditional acoustic liner.

A high-order stabilized finite-element model for the Linearized Navier-Stokes equations

A numerical approach based on high-order stabilized finite elements is proposed to solve thermo-acoustic and visco-acoustic problems accounting for non-uniform mean flow effects. The approach is based on the Linearized Navier-Stokes equations written in conservative form in the frequency domain. An adaptive polynomial Finite-Element Method (FEM) using hierarchic shape functions is applied for accuracy and ease-of-use. A new enrichment strategy, inspired by the extended Finite Element Method (X-FEM), is developed to resolve the finer scales near the walls at a reasonable computational cost. It relies on a re-orthogonalization procedure proposed to preserve both the continuity of the solution and the conditioning properties of the discrete model. The performance of the method is first evaluated by performing two-dimensional simulations of acoustic waves affected by visco-thermal wall losses and mean flow effects while propagating in a duct. Numerical results are in good agreement with the analytical solution. The applicability of this new methodology is then demonstrated by computing the sound absorption of an acoustic liner installed in an impedance tube in the presence of a grazing flow in 2D. Numerical predictions of the sound pressure levels in the tube are compared to experimental data from the literature.

Fan-noise reduction of data centre telecommunications’ server racks, with an acoustic metamaterial broadband, low-frequency sound-absorbing liner

An acoustic metamaterial is evaluated successfully as a means to reduce axial fan noise in an IT server rack application. The continuing growth in demand for cloud-based memory storage is outpacing sustainable energy management which still requires the axial fan as a fundamental component in temperature control technologies. As chip temperatures increase, additional numbers of more powerful fans are required to run faster with the result that data centres have become unsafe working environments due to the acoustic output exceeding prescribed occupational noise level limits. Noise isolation using soundproof cabinets is expensive and impractical in data centres which seek to reduce floor space and communication losses. Instead, in this work, a 15 mm deep acoustic liner solution is proposed being the approach taken in the aeronautics sector to reduce aeroengine noise which, similarly, is a flow duct problem. The acoustic metamaterial adopted here is the recently developed Segmented Membrane Sound Absorber (SeMSA) which can be tailored for low frequency, broadband and tonal noise in a thin form-factor and which can be used in a grazing flow configuration. An equivalent circuit model developed for the SeMSA allows its design to be optimised to attenuate a typical server fan’s frequency spectrum. An array of manufactured rectangular SeMSA cells was embedded in a “deflector plate”, which is an existing component in a server rack. The deflector plate was implemented in a custom-built fan sound power rig, tested in an hemi-anechoic chamber and compared to a hardwall case and to PU foam. A relative overall decrease of 2.5 dBA was achieved for the SeMSA deflector plate liner which corresponds to a 43.9% decrease in overall sound emissions, which using a material thickness of only 15 mm, signifies an almost twofold increase in the time required for hearing damage to occur. A 3 dBA drop was achieved at the blade passing frequency of 620 Hz, while relative drops of 1–2.5 dBA were achieved from 2000–8000 Hz.

Fan Noise Reduction by Acoustic Liners Combined with Fine-Perforated-Film: Noise Tests by a Small Turbofan Engine DGEN 380

Resonant-type liner panels are one the primary countermeasures for the fan noise of aircraft engines, though the sound absorption performances of the liners are known to be tolerated by the grazing flow streaming on their surface. Therefore, suppressing the effect of the flow on the liners is greatly important for higher sound absorption efficiency. In this study, liner panels with special surface structures were manufactured and applied to an outdoor test using a small turbofan engine, DGEN 380. A special thin film, Fine-Perforated-Film (FPF), was applied to two liner samples. In the first sample, FPF was applied directly to the surface of a typical liner. In the second sample, a gap was fixed between the FPF and liner surface. In addition, a rigid wall and a typical liner were used for comparison, to investigate the effect of the structures. During the test, the samples were installed to the exhaust bypass duct of the DGEN engine. The acoustic pressure inside the duct and in far field was measured. Analysis and comparison of the results showed that the new structures suppressed the effect of the grazing flow and caused a larger amount of noise reduction, compared to the typical liner sample.

Acoustic Experimental Technology for Aircraft Nacelle Liner

An aircraft nacelle acoustic liner is a key mean of aircraft noise reduction. The success of its design depends strongly on the development of experimental technology, which is generally divided into two stages: impedance eduction and the modal verification of acoustic performance. The comparative study summarizes the impedance eduction technology based on the in-situ method and the straight forward method, and the acoustic modal measurement and control technology, as well as their applications in the design of the acoustic liner of an engine intake and exhaust ducts. The results show that the in-situ method has higher accuracy at low frequencies, and the accuracies of both methods are decreased in the high frequency range. Both methods show an acceptable accuracy and good applicability in the mid-frequency range. A modal generator was designed and used to emit separate and pure acoustic modes in sequence, and a comparative test was carried out on the two types of acoustic liner. Compared with the seamed acoustic liner, the seamless acoustic liner significantly improved its noise reduction effect at the multi-acoustic modes and target frequencies, which further increases the overall reduction up to 5.2 dB. Through research, reliable and validated technologies of acoustic performance tests for a nacelle acoustic liner were established.

Metasurface acoustic liner of engine based on asymmetric absorber

In order to solve the problem of low frequency noise of engine, based on the principle of dual port asymmetric sound absorber, a kind of gradually changing size sound absorbing metasurface is designed to reduce the noise of engine acoustic liner. Firstly, the theoretical analysis model and simulation analysis model of the asymmetric resonance sound absorber are established, the noise reduction mechanism is revealed, and the influencing factors of the noise reduction effect are analyzed. Then an acoustic metasurface acoustic liner is designed based on the asymmetric resonance sound absorber. The noise reduction effect of the acoustic liner is analyzed in depth by using three methods: full model theoretical calculation, equivalent impedance theoretical calculation and COMSOL finite element simulation. Then, the parameters of this structure are optimized, and the influence of flow velocity on the noise reduction effect is considered by using the full model theoretical calculation and equivalent impedance theoretical calculation. The research results show that the acoustic metasurface acoustic liner designed based on asymmetric sound absorber can achieve noise reduction effect of more than 3 dB in a frequency band range from 252 to 692 Hz when the thickness is only 2.5 cm (only 1/54 of the corresponding wavelength of 252 Hz), which provides a new idea for designing engine noise reduction.

An investigation on the modelling of a finite porous liner for the sound propagation in a rectangular duct

Acoustic liners take an essential role in the noise attenuation in ducts. In this work, the multimodal method based on the finite-difference method is extended to predict the acoustic field in a rectangular duct lined with a finite porous material and validated by the experiment. A modified immersed interface method is developed for the air–porous interface problem in the extended multimodal method without flow. When the flow exists, the treatments of the air–porous interface problem and the continuity of pressure between the wall and liner are also given. Three ways of describing the porous liner using the surface impedance at normal incidence named normal impedance, the surface impedance at grazing incidence named shear impedance and a cavity filled with the equivalent fluid of porous material are introduced. The comparison among the three ways reveals that the extended method of treating liners using a cavity filled with the equivalent fluid is more accurate for the acoustic evaluation of porous liners. The analysis finds that the shear impedance can reasonably present the influence of porous material during the comparisons of transmission loss curves of the liners with different lengths and depths at different Mach numbers. In most cases, the prediction by the shear impedance is closer to that simulated by the cavity filled with porous material than the normal impedance. Normal impedance is hardly utilized to describe the porous material and is only reliable when the cavity is very short and deep.

Experimental and Numerical Investigation of Novel Acoustic Liners and Their Design for Aero-Engine Applications

This paper presents a combined experimental and numerical investigation on a novel liner concept for enhanced low-frequency and broadband acoustic attenuation. In particular, two different realizations, derived from conventional Helmholtz resonators (HR) and plate resonators (PR) are investigated, which both deploy flexible materials with material inherent damping. In this context, a comprehensive experimental investigation was carried out focusing the identification and evaluation of various geometric parameters and material properties on the acoustics dissipation and related properties of various materials in a simplified setup of a single Helmholtz resonator with flexible walls (FHR concept). Furthermore, a parameter study based on analytical models was performed for both liner concepts, taking into account material as well as geometric parameters and their effects on transmission loss. In addition, design concepts that enable cylindrical or otherwise curved liner structures and the corresponding manufacturing technologies are presented, while considering essential structural features such as drainage. With respect to the potential application in jet engines, a structural–mechanical analysis considering the relevant load cases to compare and discuss the mechanical performance of a classical HR and the FHR concept liner is presented. Finally, both concepts are evaluated and possible challenges and potentials for further implementation are described.

Topological Surface States in a Gyroid Acoustic Crystal

The acoustic properties of an acoustic crystal consisting of acoustic channels designed according to the gyroid minimal surface embedded in a 3D rigid material are investigated. The resulting gyroid acoustic crystal is characterized by a spin-1 Weyl and a charge-2 Dirac degenerate points that are enforced by its nonsymmorphic symmetry. The gyroid geometry and its symmetries produce multi-fold topological degeneracies that occur naturally without the need for ad hoc geometry designs. The non-trivial topology of the acoustic dispersion produces chiral surface states with open arcs, which manifest themselves as waves whose propagation is highly directional and remains confined to the surfaces of a 3D material. Experiments on an additively manufactured sample validate the predictions of surface arc states and produce negative refraction of waves at the interface between adjoining surfaces. The topological surface states in a gyroid acoustic crystal shed light on nontrivial bulk and edge physics in symmetry-based compact continuum materials, whose capabilities augment those observed in ad hoc designs. The continuous shape design of the considered acoustic channels and the ensuing anomalous acoustic performance suggest this class of phononic materials with semimetal-like topology as effective building blocks for acoustic liners and load-carrying structural components with sound proofing functionality.

Neck-embedded acoustic meta-liner for the broadband sound-absorbing under the grazing flow with a wide speed range

Acoustic liners are the most commonly used dissipative muffler for aero-engines but remain a challenge to design a broadband acoustic liner under the grazing flow. In this work, a novel neck-embedded meta-liner is proposed for the broadband sound-absorbing under the grazing flow with a wide speed range. The meta-liner is composed of many neck-embedded Helmholtz Resonators, and the idea of bending the subsequent cavity is introduced to improve low-frequency sound absorption performance. The acoustic impedance model of the meta-liner with neck-embedded slit is first detailly established by analyzing the acoustic characteristics of the equivalent elliptical pipe and using the matrix transfer method. Then by designing the local coupling between channels, efficient broadband impedance modulation and broadband matching with air impedance under grazing flow conditions are realized. The proposed meta-liner exhibits the excellent sound attenuation capacity in the frequency domain of 500–3000 Hz under the grazing flow speeds from 0 m s−1 to 98 m s−1 and the sound absorption performance is greatly boosted compared with the traditional Double- Degree-Of-Freedom liner. Features mentioned above have been verified numerically and in a series of comparative experiments. Besides, the thickness of the meta-liner is only 50 mm. Our work provides a possible design reference for the new generation of acoustic liners and has potential applications in noise reduction engineering.

Design of hole matrix unit for high-precision batch hole making of acoustic lining assembly of aero-engine

Acoustic lining assemblies with millions of high-density array holes are key structures for high-performance and low-noise aero-engines. However, the material specificity, variable curvature shape structure, high-order number of holes, and small-sized hole spacing of acoustic lining assemblies pose great challenges to the hole-making technology. In this paper, in view of the machining requirements of millions of acoustic lining holes, a hole matrix unit design method for batch hole making is proposed. The optimal hole matrix unit dimension is calculated with the angle deviation of normal vector as the constraint by matching the structural features of the acoustic lining assembly, dividing and interpolating the hole-making surface, and solving the normal vector of the hole position. The experimental results show that the proposed hole matrix unit design method can effectively control the angle deviation of normal vector of the acoustic lining holes and ensure the processing accuracy of the holes.

DMA of TPU Films and the Modelling of Their Viscoelastic Properties for Noise Reduction in Jet Engines.

Due to current developments in jet engine design, the acoustic performance of conventional acoustic liners needs to be improved with respect to lower frequency spectrums and broadband absorption. In this context, the present study aimed to determine the viscoelastic material properties of a thermoplastic polyurethane (TPU) film for targeted application in novel acoustic liners with integrated film material for enhanced noise reduction. Therefore, a dynamic mechanical analysis (DMA) was performed to determine these viscoelastic material properties. Based on the acquired data, the time-temperature shift (TTS) was applied to obtain the material's temperature- and frequency-dependent mechanical properties. In this regard, the William-Landel-Ferry (WLF) method and an alternative polynomial approach determining the shift factors were investigated and compared. Furthermore, a generalized Maxwell model-so-called Prony-series-with and without pre-smoothing utilizing of a fractional rheological model was applied to approximate the measured storage and loss modulus and to provide a material model that can be used in finite element analyses. Finally, the results were discussed concerning the application of the films in acoustic liners under the conditions of a standard flight cycle and the applied loads. The present investigations thus provide a method for characterizing polymer materials, approximating their mechanical behavior for vibration applications at different ambient temperatures and enabling the identification of their operational limits during the application in acoustic liners.

Laser microdrilling of CFRP with a nanosecond-pulsed high-power laser

The global demand for air travel and air transport is expected to further increase in the next couple of years, and so the environmental protection will also increasingly come into focus again. In the aviation sector, this means not only saving fuel and reducing emissions but also reducing the noise pollution caused by aircrafts. For this purpose, sound-absorbing acoustic liners are used, which consist of a sandwich structure with one perforated skin layer. To reduce weight, these outer layers are often made of carbon fiber-reinforced plastic (CFRP). For manufacturing these CFRP skin layers, laser drilling offers specific benefits, but when it comes to industrial applications, further process improvements are necessary. Although laser microdrilling is a well-known process in laser materials’ processing, it has not yet been sufficiently investigated for the processing of carbon fiber-reinforced plastics. This study investigates the quality and efficiency of laser microdrilling with a 1.5 kW nanosecond-pulsed high-power laser for percussion drilling and helical drilling of thin CFRP laminates. The efficiency was evaluated in terms of drilling depth, the amount of energy required to remove a specific volume of material, and the time required to remove a specific volume of the material. The quality evaluation focused on the heat-affected zone and the hole taper. The efficiency and quality results were set in relation to higher-level parameters such as energy fluence in the laser spot, pulse overlap, or total energy applied. This facilitates future transferability to similar laser processes that may use laser machines and system technology with deviating specifications.

The critical layer in quadratic flow boundary layers over acoustic linings

A straight cylindrical duct is considered containing an axial mean flow that is uniform everywhere except within a boundary layer near the wall, which need not be thin. Within this boundary layer the mean flow varies parabolically. The linearized Euler equations are Fourier transformed to give the Pridmore-Brown equation, for which the Green's function is constructed using Frobenius series. The critical layer gives a non-modal contribution from the continuous spectrum branch cut, and dominates the downstream pressure perturbation in certain cases, particularly for thicker boundary layers. The continuous spectrum branch cut is also found to stabilize what are otherwise convectively unstable modes by hiding them behind the branch cut. Overall, the contribution from the critical layer is found to give a neutrally stable non-modal wave when the source is located within the sheared flow region, and to decay algebraically along the duct as $O(x^{-{5}/{2}})$ for a source located with the uniform flow region. The Frobenius expansion, in addition to being numerically accurate close to the critical layer where other numerical methods lose accuracy, is also able to locate modal poles hidden behind the branch cut, which other methods are unable to find; this includes the stabilized hydrodynamic instability. Matlab code is provided to compute the Green's function.

Experimental investigation of factors influencing acoustic liner drag using direct measurement

Nacelle acoustic liners inevitably generate greater aerodynamic drag than smooth walls. It is necessary to study the factors influencing acoustic liner drag to reduce engine fuel consumption and emissions. In this study, experimental measurements were conducted in a grazing flow tube with bulk Mach numbers ranging from 0.1 to 0.6. A drag balance was designed to measure the liner drag directly. This balance can eliminate the interference of the vertical force and moment by arranging the sensors appropriately. A wedge-shaped sidewall was proposed that can eliminate the side effects of the differential pressure force effectively. The test samples involved Helmholtz resonator liners, including conventional acoustic liners and multislot acoustic liners, and a smooth wall set as a reference for comparison. The effects of geometric structures on the acoustic liner drag were studied, including perforation size, plate thickness, porosity, perforation, slot patterns, and fabric mesh. Experimental results show that Mach number (M) 0.4 is the critical point. At M < 0.4, the acoustic liner drag is proportional to the perforation diameter. At M > 0.4, the opposite trend is observed. Moreover, the acoustic liner drag is directly proportional to the porosity and perforated plate thickness at M = 0.1 to 0.6. Experimental results also indicate that some structural characteristics can effectively reduce the acoustic liner drag, including placing a fabric mesh under the perforated panel, adopting a staggered hole layout, and slotting perpendicular to the flow direction. The summarized influencing factors are helpful for avoiding high flow drag structures in the design of acoustic liners and subsequently, reducing the aircraft fuel consumption.

Modeling of Over-Tip-Rotor Liners for the Suppression of Fan Noise

Fan noise is one of the dominant sources of aircraft engine noise, both at approach and at takeoff. Improved attenuation of fan noise with acoustic liners and the reduction of fan noise at the source remain key technology challenges for civil aviation in the foreseeable future. Over-tip-rotor (OTR) acoustic treatments have been investigated experimentally during the last decade, and significant fan noise reductions have been measured, most recently using a single rotor and multiple lined circumferential grooves. This paper describes an analytical OTR prediction model in which the fan noise is modeled with point or distributed, static or rotating monopole and dipole sources based on Green’s functions for infinite hard or lined cylindrical ducts containing uniform mean flow; these are combined with an anechoic or unflanged inlet termination and an embedded finite length lined section in which the rotor-alone source is located with its OTR liner. OTR liner insertion loss predictions are obtained for comparison with data from the W-8 NASA experimental rig. These yield peak broadband in-duct noise reductions of up to 4 dB, in line with the measurements.