CN117916800A

CN117916800A - Acoustic metamaterial and additive manufacturing method thereof

Info

Publication number: CN117916800A
Application number: CN202280039618.1A
Authority: CN
Inventors: 约书亚·科斯塔·巴普蒂斯塔; 伊迪丝-罗兰·福钦; 安妮·罗斯; 杰克·诺维·马尔卓诺; 吉恩·波威尔特; 丹尼尔·泰里奥
Original assignee: Centre National de la Recherche Scientifique CNRS; Le Mans Universite; SNECMA SAS
Current assignee: Safran Aircraft Engines SAS; Centre National de la Recherche Scientifique CNRS; Le Mans Universite
Priority date: 2021-05-04
Filing date: 2022-05-03
Publication date: 2024-04-19
Also published as: CA3117015A1

Abstract

The invention relates to an acoustic metamaterial (100, 100') and a method for manufacturing the same. The acoustic metamaterial comprises a plurality of channels (101) or pillars (101'), each channel or pillar having the same cross section with a hydraulic radius of between 5 μm and 300 μm, arranged with periodic spacing (t, s) of between 2 μm and 600 μm. This results in a high density network structure capable of providing optimized sound absorption and/or acoustic impedance over a wide frequency band. The method of manufacture comprises additive manufacturing by a plurality of successive material deposition steps to form in each step a layer (203) comprising a plurality of periodically repeating cells (204) separated by walls (104). The layers (203) deposited in successive material deposition steps are stacked with their respective cells (204) aligned to form channels (101, 206').

Description

Acoustic metamaterial and additive manufacturing method thereof

Technical Field

The present invention relates to the field of acoustic metamaterials and their manufacture.

Background

Sound absorbers have a wide range of applications. In particular, these applications include aeronautics, in which such elements are used to at least partially absorb the noise generated by an aeroengine and thus reduce its transmission to the external environment. Turbofan engines are one of the most common aeroengines. A turbofan engine includes a fan and a gas generator including at least a compressor, a combustion chamber, a turbine, and a nozzle. Thus, the total noise generated by such turbofan engines may include injection noise, combustion noise, fan noise, compressor noise, and turbine noise. However, the most dominant noise is typically that emitted by fans, which can extend over a wide frequency band, as shown in FIG. 16, where the tonal component corresponds to the frequency of passage of the fan blades. In order to increase the energy efficiency of turbofan engines, the general trend is to increase their bypass ratio (i.e. the ratio of the air flow driven by the fan compared to the air flow for combustion in the gasifier) and thus to increase the diameter of the fan. Therefore, the fans of the latest generation turbofan engines tend to rotate slower and thus emit noise at a lower frequency.

Therefore, in order to reduce the noise emitted by aircraft engines, it is common to cover certain areas (such as the cabins housing these engines) with sound absorbers (such as honeycomb sandwich panels). In this type of acoustic absorber, each cell in the honeycomb can act as a helmholtz resonator (Helmholtz resonator) to attenuate noise. However, the frequency range of acoustic attenuation of such absorbers is limited and in order to be effective at low frequencies they must be particularly heavy, as this makes the surface to be covered very large for extremely high bypass ratio turbofan engines, even more disadvantageous.

Thus, as an alternative to honeycomb sandwich panels, it has been proposed to use porous materials, the individual pores of which act as helmholtz resonators. However, most available porous materials have too low mechanical strength, while the most robust materials (such as the metallic materials disclosed in e.g. US 7,963,364 B2) are too heavy. Furthermore, they provide significant attenuation only at resonance frequencies, and are not able to absorb noise over a wide frequency range.

The use of additive manufacturing for the production of sound absorbers including microchannels is proposed in "acoustic properties (Acoustic properties of a porous polycarbonate material produced by additive manufacturing) of porous polycarbonate materials produced by additive manufacturing" by z.liu, j.zhan, m.bird and j.l.davy, volume 181, pages 296-299 (2016, 10 month) of materials rapid report (MATERIAL LETTERS). However, these sound absorbers also have only a fairly narrow absorption frequency range.

For example qian, y.j., kong, d.y., liu, s.m., sun, s.m., and zhao, z. The use of microperforated panels as sound absorbers is proposed in "study of microperforated panel absorbers with ultrafine perforations (Investigation on micro-perforated panel absorber with ultra-micro perforations)" by application of acoustics (Applied Acoustics) volume 74 (7) pages 931-935 (2013). In order to widen the frequency range of sound absorption, liu, z., zhan, j., fard, m, and davy, j., in "acoustic properties of a multilayer sound absorber with 3D printed microperforated panel (Acoustic properties of multilayer sound absorbers with a D printed micro-perforated panel)" and yang, w., bai, x., zhu, w., kiran, r., an, j, chua, c.k., and zhou, j., in "3D printing (3D Printing of Polymeric Multi-Layer Micro-Perforated Panels for Tunable Wideband Sound Absorption)" of a polymer multilayer microperforated panel for tunable wideband sound absorption" on pages 25-32 (2017) of volume 121 of applied acoustics (Applied Acoustics), and in "polymer multilayer microperforated panel for tunable wideband sound absorption" of volume 12 (2), it is also proposed to stack several such panels and produce them by additive manufacturing. However, these relatively fragile sound absorbers appear to be difficult to apply in environments where they would be subject to wear or other mechanical stresses, such as in particular the cabin of an aircraft engine.

Metamaterials produced by additive manufacturing with multiple layers superimposed in the thickness direction are proposed in french patent application publications FR 1 761 722 and Guild, m.d., rohde, c., rothko, m.c., and Sieck, c.f., in the "european noise seminar (Proceedings of Euronoise)" (2018) "3D printed acoustic metamaterial sound absorber (3D printed acoustic metamaterial sound absorbers using functionally-graded sonic crystals) using functionally graded acoustic crystals. An acoustic metamaterial is understood to be a periodically structured medium whose periodically repeating constituent units together influence the passage of sound waves. In the case of the above metamaterials, each superimposed layer may have a periodically different lattice in order to widen its attenuation frequency range.

Disclosure of Invention

In a first aspect, the present disclosure is intended to propose an acoustic metamaterial that combines a high level of sound absorption with good mechanical strength (including abrasiveness). The acoustic metamaterial may comprise a plurality of channels, each channel having the same cross section with a hydraulic radius of between 5 μm and 300 μm, the channels being arranged at periodic intervals between adjacent channels of between 2 μm and 600 μm. Thus, a high density array of acoustic micro-channels may be obtained that may provide optimal acoustic absorption and/or acoustic impedance over a wide frequency band, and that may have maximum absorption at least at certain low frequencies, such as those frequencies that predominate in the emission spectrum of the fan of a turbofan engine at high and very high bypass ratios.

The channels may have a generally polygonal cross-section, such as triangular, square, rectangular or hexagonal cross-section. By "substantially polygonal" is meant that the corners of the cross-section may be rounded due to manufacturing limitations. However, the cross-section may also be generally circular or elliptical. By "generally circular or oval" is meant that the profile of the cross-section may also have flat spots due to manufacturing limitations.

To widen its acoustic absorption band, an acoustic metamaterial may comprise several sets of multiple channels, each set of multiple channels having a different cross-section and/or periodic spacing of channels. In particular, these different pluralities of channels may be arranged in directly adjacent layers in the thickness direction of the metamaterial, such that the acoustic metamaterial comprises several layers stacked in the thickness direction, each layer comprising a plurality of channels having different cross sections and/or periodic channel spacing. However, the cross section and/or the channel spacing may also be varied in a plane perpendicular to the thickness direction of the metamaterial.

In order to increase the length of the channels without increasing the thickness of the acoustic metamaterial, one or more of the channels may be inclined with respect to the thickness direction of the metamaterial, and in particular be spiral-shaped. Alternatively or additionally, they may be curved to increase their length.

A second aspect of the invention relates to a method of additive manufacturing of the acoustic metamaterial of the first aspect. The additive manufacturing method may comprise several successive material deposition steps to form a layer comprising a plurality of periodically repeating cells separated by walls in each step. The layers deposited in successive material deposition steps may be stacked with their respective cells aligned to form the channels.

The material used in the method according to this second aspect may comprise a thermoplastic polymer and may therefore be deposited by fuse deposition in order to allow the fabrication of sufficiently fine structures. Alternatively, however, the material used in the method may comprise a thermosetting resin, and the deposition of the material may be performed in a manner similar to that of fuse deposition by extrusion of the thermosetting resin. In order to mechanically reinforce acoustic metamaterials, the materials used in the method may include suspended solid particles, such as in particular fibers, more in particular carbon fibers, in addition to thermoplastic polymers or thermosetting resins. Other types of solid particles are also possible, such as in particular nanoparticles or microbeads, in particular made of silica. Due to these solid particles, acoustic metamaterials will be able to have significant mechanical strength and heat resistance as well as abrasion resistance.

A third aspect of the present disclosure relates to another method for manufacturing acoustic metamaterials that also combines a high level of sound absorption with good mechanical strength (including abrasiveness). In a first additive manufacturing step of the method for manufacturing acoustic metamaterials, a mold may be manufactured by depositing a plurality of stacked layers, each of which may include a plurality of periodically repeating cells separated by walls, which may be aligned to form channels. In a second step of the method, the channels may be filled with a fluid material, which may then be cured before the mould is removed.

By following the method of this third aspect, a metamaterial comprising highly dense periodically arranged pillars may be produced which may also provide optimal acoustic absorption and/or acoustic impedance over a wide frequency band, and which has maximum absorption at least at certain low frequencies, such as those frequencies dominant in the emission spectrum of the fans of high and very high bypass ratio turbofan engines.

The hollow cells may in particular have a hydraulic radius of between 5 μm and 300 μm in order to obtain pillars of corresponding width in the acoustic metamaterial, while the walls may have a minimum width of between 2 μm and 600 μm in order to thereby obtain corresponding lateral distances between the pillars. With these dimensions, an acoustic crystal with optimal absorption and acoustic impedance over a wide frequency range including dominant frequencies in the emission spectrum of fans of high and very high bypass ratio turbofan engines can be obtained.

The mould channels may have a length of between 1mm and 150mm in order to obtain a corresponding height of the column. Thus, the acoustic metamaterial obtained by the method may have a thickness that is hardly larger than this length, facilitating its integration, in particular in and around aeroengines.

The cells may be generally polygonal, circular or oval in shape in order to obtain pillars of the same cross-section in the resulting acoustic metamaterial. It is also possible to combine differently shaped cells in the same mould, or even in the same layer of the mould.

Furthermore, in the stacked layers, the shape and/or size of the cells of the different layers may be different in order to change the cross section of the channel and thus the cross section of the post over the length of the channel, in particular in order to optimize the acoustic response of the acoustic metamaterial at several frequency bands.

The mold may also include one or more lateral conduits between the channels to form spacing structures and other lateral reinforcements between the pillars of acoustic metamaterial during their filling with fluid material and curing of the fluid material.

To facilitate the step of removing the mold, it may be made of a water-soluble material including, for example, polyvinyl alcohol (PVA), a copolymer of butylene glycol and vinyl alcohol (BVOH), or polylactic acid (PLA). The step of removing the mould may then be performed by leaching, in particular by leaching in an ultrasonic bath.

To allow the manufacture of sufficiently fine structures, additive manufacturing of the die may be performed by depositing a wire of extruded material, in particular by a fuse deposition method. Thus, the material used to make the mold may comprise a thermoplastic polymer, but may also be a thermosetting resin.

The fluid material used in the mold filling step may comprise a resin (such as, for example, an epoxy resin), and the step of curing the mold material comprises polymerization of the resin. Such polymerization may be thermally activated and/or thermally accelerated, but other means of activation (e.g., by ultraviolet light) are also possible. In addition, it is also possible to use a molten thermoplastic polymer as fluid material in the filling step.

To mechanically enhance the resulting acoustic metamaterial, the fluidic material may comprise suspended solid particles, such as in particular silica microbeads or nanoparticles, or fibers (in particular carbon fibers).

A fourth aspect of the present disclosure is directed to an acoustic metamaterial manufactured by the manufacturing method of the third aspect and comprising a plurality of pillars extending from a common substrate.

Finally, a fifth aspect of the present disclosure relates to a turbine, in particular a gas turbine engine such as a turbofan engine, comprising the acoustic metamaterial of the first or fourth aspect as an acoustic absorber. In particular, in turbofan engines, acoustic metamaterials may be integrated into the walls defining the fan airflow path and/or into the gasifier shell.

Drawings

The invention will be better understood and its advantages will emerge better upon reading the following detailed description of several embodiments presented by way of non-limiting example. This description relates to the accompanying drawings, in which:

FIG. 1 is a longitudinal cross-sectional view of a turbofan engine;

FIG. 2 is a cut-away thickness view of a first acoustic metamaterial suitable for use as an acoustic absorber in the turbofan engine of FIG. 1;

FIGS. 3A-3G are cross-sectional views along plane III-III of different possible alternative shapes of the metamaterial channel of FIG. 2;

FIG. 4 is a graph showing the change in acoustic absorption coefficient with frequency for several acoustic metamaterials having channels of different shapes and widths;

FIG. 5 is a thickness cross-sectional view of an alternative embodiment of an acoustic metamaterial, wherein channels with different widths are on different layers of the acoustic metamaterial;

FIG. 6 is a graph showing the sound absorption coefficient as a function of frequency for several examples of multi-layer acoustic metamaterials;

FIGS. 7A, 7B, and 7C are thickness cross-sectional views of several other alternative embodiments of acoustic metamaterials;

FIG. 8 illustrates an apparatus for implementing an additive manufacturing method;

FIGS. 9A and 9B show two alternative material deposition patterns for the fabrication of layers;

FIG. 10 is a perspective view of a second acoustic metamaterial suitable for use as a sound absorber in the turbofan engine of FIG. 1;

FIGS. 11A-11G are cross-sectional views along plane XI-XI of different possible alternative shapes of the post of the metamaterial of FIG. 10;

FIG. 12 is a graph showing the change in acoustic absorption coefficient with frequency for several acoustic metamaterials having channels of different shapes and widths;

FIG. 13 is a thickness cross-sectional view of an alternative embodiment of a second acoustic metamaterial, wherein pillars having different widths on different layers of the acoustic metamaterial;

FIG. 14 is a thickness cross-sectional view of another alternative embodiment of a second acoustic metamaterial, wherein spacer structures laterally connect pillars of the acoustic metamaterial;

FIG. 15 shows a step of filling a mold with a second acoustic metamaterial; and

Fig. 16 is a graph of intensity of noise emitted from a turbofan engine as a function of frequency.

Detailed Description

Fig. 1 schematically shows a turbine 1, more specifically a turbofan engine. In the direction of fluid flow, the turbofan engine may include a fan 2, a low pressure compressor 3, a high pressure compressor 4, a combustor 5, a high pressure turbine 6, a low pressure turbine 7, and a nozzle 8. The assembly may be surrounded by a compartment 9. The compressors 3 and 4, the combustion chamber 5, and the turbines 6 and 7 together form a gas generator 10, which gas generator 10 may in turn be surrounded by a shroud 11 leading to the nozzle 8. Thus, the airflow path 12 of the fan 2 may be defined between the shroud 11 of the gasifier 10 and the inner wall 13 of the nacelle 9. The high-pressure turbine 6 may be connected to the high-pressure compressor 4 by a first rotation shaft 14 for driving the high-pressure compressor 4, and the low-pressure turbine 7 may be similarly connected to the fan 2 and the low-pressure compressor 3 by a second rotation shaft 15 coaxial with the first rotation shaft 14. In the case of high and extremely high bypass ratio engines, the reduction gear device 16 may be mechanically interposed between the second rotation shaft 15 and the fan 2 in order to reduce the rotation speed of the fan 2 and prevent the blade tips of the fan 2 from reaching excessive speeds.

Each of these elements of the turbine 1 produces noise, but the noise produced by the fan 2 is typically dominant. Further, in the high and extremely high bypass ratio engine, particularly in the engine equipped with the reduction gear device 16, most of the noise from the fan 2 may concentrate in the low frequency, as shown in fig. 16, showing the variation of the Sound Pressure Level (SPL) with the frequency f. In order to absorb at least a part of the noise from the fan 2, a noise absorber 17 may be integrated into the inner wall 13 of the nacelle 9, in particular upstream and downstream of the blades of the fan 2. However, as shown, the noise absorber 17 may also be integrated into the shroud 11 of the gasifier 10, or even into the housing of the gasifier 10.

Typically, the sound absorber 17 is formed by a honeycomb sandwich panel. However, in engines with high or even very high bypass ratios, these plates may present significant disadvantages in terms of quality and size. Furthermore, since the inner wall 13 of the nacelle 9 typically comprises abradable material 18 at this location in order to absorb the occasional friction of the tips of the blades of the fan 2 due to their transient deformations, it is difficult to arrange the honeycomb sandwich panel directly facing the tips of the fan blades, where the noise emission is still the most intense.

Fig. 2-3G illustrate several embodiments of a noise absorber 17 formed from an acoustic metamaterial 100 that can effectively replace a honeycomb sandwich panel noise absorber, with less weight and space requirements, and even be disposed directly opposite the blades of the fan 2 as an abradable material 18.

As shown in fig. 2, such an acoustic metamaterial 100 may comprise a plurality of channels 101 having a high density and periodically arranged and extending from an exposed surface 102 of the metamaterial 100 to a substrate 103 thereof. The channels 101 may be separated from each other by walls 104.

As shown in fig. 2 and 3A, each channel 101 may have a generally square profile in cross-section. However, other generally polygonal shapes, such as generally rectangular, diamond, triangular, or hexagonal shapes, may also be used, as shown in fig. 3B, 3C, 3D, and 3E, respectively. But may also be a non-polygonal shape such as, for example, a generally circular or oval shape, as shown in fig. 3F and 3G, respectively.

The hydraulic radius r _h of the cross-section of each channel 101 may be defined according to the formula r _h =2a/P, where a and P represent the area and perimeter, respectively, of the cross-section of the channel 101. Independent of the shape of its cross-section, each channel 101 may have a hydraulic radius r _h, for example, between 5 μm and 300 μm, which corresponds to a width W between 10 μm and 600 μm for channels 101 having square or circular cross-sections, but the shape factor may also be applied to take into account the edge effect of channels having differently shaped cross-sections. The periodic spacing t between adjacent channels 101 may be, for example, between 2 μm and 600 μm.

The sound absorption of different frequencies may vary significantly with the variation of the hydraulic radius r _h and thus with the variation of the width W and with the variation of the periodic interval t of the channel 101. Thus, FIG. 4 shows the variation of the absorption coefficient ALPHA with the acoustic frequency f of an acoustic metamaterial 100 having, for example, different values of width W and periodic intervals t of channels 101. Thus, curves 401, 402, 403, and 404 correspond to an ultrasonic material 100 having substantially square channels 101 with widths W and periodic intervals t of 133 μm and 2 μm, 175 μm and 50 μm, 215 μm and 100 μm, and 265 μm and 155 μm, respectively. It will be appreciated that although the maximum absorption coefficient is close to 1 and corresponds to approximately the same frequency f between 2000Hz and 3000Hz for different values of W and t, the absorption band widens as W and t decrease.

To widen the sound absorption range of the metamaterial 100, a plurality of channels 101 having cross sections of different periodic intervals and/or different shapes and sizes may be combined in the same metamaterial 100. Thus, the metamaterial 100 can be considered to comprise several layers superimposed in one direction of thickness, the channels 101 of each layer having different cross sections and/or different spacing. It is even possible to include layers with functions other than sound absorption and thus not include regularly spaced channels or have the claimed dimensions. To avoid that the channels 101 of one layer are blocked by adjacent layers, the channels of different layers may be aligned and the grid spacing (i.e., the sum of the width W and the spacing t) corresponding to each layer is an integer multiple of the smallest grid spacing in the different layers. In particular, the grid spacing of each layer may be 2 ⁿ times the minimum grid spacing in the different layers, where n is an integer. With a constant spacing t and a minimum width W _min, the width W will thus follow the equation: w= (W _min+t)ⁿ -t).

Fig. 5 shows a first example of an acoustic metamaterial 100 having five superimposed layers 100 ₁、100₂、100₃、100₄ and 100 ₅, each of the respective thicknesses h ₁、h₂、h₃、h₄ and h ₅ being 6mm, and the channel 101 being square in cross section, and wherein the width W ₁ of the channel of the first layer 100 ₁ is 496 μm, the width W ₂ of the channel of the second layer 100 ₂ is 148 μm, the width W ₃ of the channel of the third layer 100 ₃ is 496 μm, the width W ₄ of the channel of the fourth layer 100 ₄ is 1192 μm, and the width W ₅ of the channel of the fifth layer 1005 is 496 μm, wherein the constant spacing t between the channels 101 in each layer is 200 μm, so as to obtain an absorption coefficient α of approximately 1 over a wide frequency range f of 2500Hz to 6500Hz, as shown by curve 601 of fig. 6.

Other multi-layer constructions are also possible. Thus, according to a second example, the acoustic metamaterial may comprise only two superimposed layers with a thickness of 1mm and 29mm, respectively, and wherein the width of the channels of the first layer is 100 μm and the width of the channels of the second layer is 9mm, the constant spacing t between the channels 101 in each layer being 200 μm, so as to obtain a high absorption coefficient α in a frequency range f ranging from 1000Hz to 3000Hz, as illustrated by curve 602 of fig. 6. According to a third example, the acoustic metamaterial may comprise thirty stacked layers, each layer having a thickness of 1mm and a constant spacing t between channels of 200 μm, the width of the channels being 4.11mm for layers 1, 6, 12, 15 to 17, 20 and 22 to 24; for layers 2, 8, 11, 18, 27 and 29, the width of the channels was 8.42mm; for layers 3, 19, 21, 25 and 26, the width of the channels was 69.4 μm; for layers 4,5, 7, 13, 14 and 30, the width of the channels is 1.95mm; for layers 9, 10 and 28, the width of the channels was 338.8 μm; so as to obtain a high absorption coefficient alpha over a wide frequency range f of 1000Hz to 4500Hz, as indicated by the curve 603 in fig. 6.

The channels 101 may also be inclined with respect to the direction of the thickness T of the metamaterial 100, as shown in fig. 7A, or even turned over, as shown in fig. 7B, in order to maximize the length of the channels 101 for a limited thickness T of the metamaterial 100 between its base 103 and its exposed surface 102. Furthermore, at least some of the channels 101 may be helically wound around the central axis in the same metamaterial, as shown in fig. 7C.

The base 103 and walls 104 of the acoustic metamaterial 100 may be made of a thermoplastic polymer, such as Polyetherimide (PEI) or Polyetheretherketone (PEEK); or from thermosetting resins, e.g. epoxy resins, similar to those formed fromBy name/>Epoxy resins for abradable materials sold by EC-3524B/A. In order to strengthen this material, in particular when the acoustic metamaterial 100 is to be arranged opposite to a rotating component, in particular a rotating blade of the fan 2, it may be reinforced by solid particles, such as fibers, in particular carbon fibers, embedded in the material; microspheres, such as glass microspheres; or nanoparticles such as silica powder. Depending on the materials and reinforcing materials used to make the acoustic metamaterial, the reinforcing materials may have significant mechanical strength and heat resistance as well as wear resistance properties.

The acoustic properties (e.g., impedance and absorption) of the acoustic metamaterial 100 can be modeled using a transfer matrix method or "TMM". In this method, the equivalent fluid wavenumber and equivalent characteristic impedance can be calculated based on six parameters of porosity, tortuosity, viscosity and thermal length, and viscosity and thermal permeability using a semi-unique johnson-Champoux-Allard-Laffarge (JCAL) model describing the viscous inertial dissipative effect inside porous media, which can be modeled with a multiscale asymptotic method or "MAM". When acoustic metamaterial 100 has several different layers, the equivalent fluid wavenumbers and equivalent characteristic impedances can be calculated for each layer separately.

The shape, size, and arrangement of the channels 101 of the acoustic metamaterial 100 may be defined by applying an optimization algorithm (such as, for example, a Nelder-Mead iterative optimization method) according to a frequency range that desirably achieves optimal acoustic impedance and/or acoustic absorption, according to a model that allows calculation of acoustic properties of the metamaterial 100. In each iteration of the optimization algorithm, these dimensional parameters of the acoustic metamaterial 100 may be adjusted to satisfy other constraints, such as, for example, avoiding the channels 101 of each layer from being obstructed by adjacent layers.

The acoustic metamaterial 100 may be produced by additive manufacturing methods based on extrusion of materials, such as, for example, fused deposition methods for thermoplastic materials. These methods are particularly suitable for manufacturing complex shapes with thin walls, which methods comprise several successive material deposition steps. In each of these steps, extruder head 200 may be moved along path 201 in transverse plane X-Y by depositing material 202, and then material 202 is cured to form layer 203. By moving the transverse plane X-Y in the orthogonal direction Z after deposition of each layer 203, the layers 203 may be stacked to form an acoustic metamaterial 100, as shown in fig. 8. To form the channel 101, each layer 203 may include a plurality of cells 204 that are periodically repeated, separated by walls 104 formed by deposition of material 202, and the layers 203 deposited in successive material deposition steps may be stacked with their respective cells 204 aligned.

To at least partially avoid crossing of the extruded material 202 during deposition of the layer 203 (which may result in the formation of voids between the channels 101), the path 201 may be zigzagged, as shown in fig. 9A. To avoid accumulation of material and formation of voids at the intersections between walls 104, a distance O may be maintained between corners 205 of path 201 at these intersections.

However, for the same shape of cells 204, it is also possible to have paths 201 with long intersecting sections, as shown in fig. 9B.

As shown in fig. 10, an acoustic metamaterial 100 'according to another embodiment may include a plurality of posts 101' that are periodically arranged and extend from a common base 103 'of the metamaterial 100 to an exposed face 102'. The posts 101 'may be separated from one another by gaps 104'. The total height H of each column 101' may be, for example, between 1mm and 150 mm.

As shown in fig. 10 and 11A, in cross-section, each post 101' may have a generally square profile. However, other generally polygonal shapes such as, for example, generally rectangular, diamond, triangular, or hexagonal shapes may also be used, as shown in fig. 11B, 11C, 11D, and 11E, respectively. But may also be a non-polygonal shape such as, for example, a generally circular or oval shape, as shown in fig. 11F and 11G, respectively.

The hydraulic radius r _h of the cross section of each column 101 'can be defined according to the formula r _h =2a/P, where a and P represent the area and perimeter, respectively, of the cross section of the column 101'. Independent of its shape, the cross section of each column 101 'may have a hydraulic radius r _h, for example, between 5 μm and 300 μm, which corresponds to a width W between 10 μm and 600 μm for columns 101' with square or circular cross sections, but the shape factor may also be applied to take into account the edge effect of columns with cross sections of different shapes. The pillars 101 'may have a periodic spacing s between adjacent pillars 101', for example, between 2 μm and 600 μm. As shown in the graph in fig. 12, the dimensions in these spacings allow for particularly high absorption coefficients ALPHA for frequencies f between 200Hz and 10000Hz, i.e. frequencies that are typically dominant in the noise of high or very high bypass ratios turbofan engines.

In FIG. 12, curve 1201 illustrates the variation of absorption coefficient ALPHA with frequency for a metamaterial 100 'comprising pillars 101' having a square cross section with a width W of 130 μm and a periodic spacing s of 100 μm, having a height of 30mm, while curve 1202 illustrates the variation of absorption coefficient ALPHA with frequency for a metamaterial 100 'comprising pillars 101' having a square cross section and the same height but a width W of 1.15mm and a periodic spacing s of 200 μm. It will be appreciated that although in both cases the maximum absorption coefficient is close to 1 and corresponds to a frequency f between 2500Hz and 3000Hz, in curve 1201 the absorption coefficient ALPHA remains high over a much wider frequency range than curve 1202.

The pillars 101' having cross-sections of different shapes and sizes may be combined in the same metamaterial 100', or may even have different shapes and sizes (e.g., different maximum widths) at different heights from the substrate, in order to adapt the acoustic metamaterial 100' to attenuation of several different acoustic frequencies, as shown in fig. 13. It is even possible to include layers with functions other than sound absorption and thus not include regularly spaced pillars or have the aforementioned dimensions. In addition, in order to laterally strengthen the post 101', adjacent posts 101' may be partially connected by spacer structures 105' integrally formed with the posts, as shown in fig. 14. The substrate 103' and the pillars 101' of the acoustic metamaterial 100' may be made of a polymer (e.g., polyepoxide).

The acoustic metamaterial 100' may be produced by molding. In a first step, the die 210 may be produced by an additive manufacturing method based on extrusion of a material, such as for example a fuse deposition method for thermoplastic materials. In each of the successive material deposition steps of the method, the extruder head 200 may be moved along the path 201 in the transverse plane X-Y by depositing the material 202, and then the material 202 is cured to form the layer 203. By moving the transverse plane X-Y in the orthogonal direction Z after deposition of each layer 203, the layers 203 may be stacked to form a mold 210, as shown in fig. 8. Each layer 203 may include a plurality of periodically repeating cells 204 separated by walls 205 formed by deposition of material 202, and the layers 203 deposited in successive material deposition steps may be stacked with their respective cells 204 aligned to form channels 206 having a size, shape, and spacing corresponding to the size, shape, and spacing of the pillars 101'. Thus, the maximum width of the channels 206 may be substantially equal to the maximum width W of the posts 101', the minimum thickness of the walls 205 may be substantially equal to the minimum spacing t ' between the posts 101', and the length of the channels 206 may be substantially equal to the height H ' of the posts 101 '. As with the cross-section of the post 101', the cross-section of each channel 206' in the transverse plane X-Y may vary depending on the height in the orthogonal direction. The mold 210 may also include lateral conduits between the channels 206 to form the spacer structures 105'.

When the metamaterial 100 'must include several layers and have posts 101' with widths W and/or spacings s that vary from layer to layer, to avoid the channels 206 'corresponding to one layer being obstructed by the walls 104 of an adjacent layer, the channels 206' of different layers may be aligned and the grid spacing corresponding to each layer (i.e., the sum of the widths W and spacings s) may be an integer multiple of the minimum grid spacing in the different layers. In particular, the grid spacing of each layer may be 2 ⁿ times the minimum grid spacing in the different layers, where n is an integer. With a constant spacing s and a minimum width W _min, the width W will thus follow the equation w= (W _min+s)ⁿ -s.

To at least partially avoid crossing of the extruded material 202 during deposition of the layer 203 (which may otherwise result in the formation of voids between the channels 104), the path 201 may be zigzagged, as shown in fig. 9A.

After the mold 210 is so fabricated, in a subsequent step, a fluid material 220 may be introduced into the mold 210 to fill the channels 206 and other cavities of the mold 210, as shown in fig. 15. The fluid material 220 may be a thermosetting resin, particularly an epoxy resin mixed with a cross-linking agent, such as to formBy name/>Epoxy resins for abradable materials sold by EC-3524B/A. However, it may also be, for example, a molten thermoplastic polymer such as Polyetherimide (PEI) or Polyetheretherketone (PEEK). To enhance the acoustic metamaterial 100', the fluid material 220 may also comprise suspended solid particles, such as silica beads or nanoparticles or fibers (e.g. carbon fibers), which will remain embedded in the material after the fluid material 220 is cured, especially when it is to be disposed opposite to a rotating component, in particular a rotating blade of the fan 2. Filling the cavity of the mold 210 with the fluid material 220 may be done simply by gravity, or at least aided by a pressure gradient.

Once the fluid material 220 fills the cavities of the mold 210, it may harden within those cavities. Such curing may be thermally induced or at least accelerated during the firing step, particularly when the fluid material 220 is a thermosetting resin. After such curing has formed the acoustic metamaterial 100 'in the cavity of the mold 210, the mold 210 may be removed to release the acoustic metamaterial 100'. For this purpose, the material of the mould 210 may be a water-soluble material, and in particular a water-soluble thermoplastic polymer, such as for example polyvinyl alcohol (PVA), a copolymer of butanediol and vinyl alcohol (BVOH) or polylactic acid (PLA), and the removal of the mould 210 may be performed by leaching the water-soluble material, for example in an ultrasonic bath, optionally heated to a temperature of for example 60 to 80 ℃ for 3 to 5 hours. After the acoustic metamaterial 100' is thus released from its mould 210, it may be dried, for example, in an oven at 70 ℃ for one hour.

Although the invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these examples without departing from the broader scope of the invention as set forth in the claims. Furthermore, various features of the different embodiments discussed may be combined in additional embodiments. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Claims

1. A metamaterial (100) comprising a plurality of channels (101), each channel having the same cross section, a hydraulic radius of between 5 μm and 300 μm, and a periodic spacing (t) between adjacent channels (101) of between 2 μm and 600 μm.

2. The metamaterial (100) according to any preceding claim, wherein the channel (101) has a substantially polygonal cross section.

3. The metamaterial (100) according to any preceding claim, wherein the channel (101) has a substantially circular or elliptical cross section.

4. The metamaterial (100) according to any preceding claim, comprising a plurality of sets of a plurality of channels (101), each set of a plurality of channels (101) having a different cross section and/or periodic spacing of channels (101).

5. The metamaterial (100) according to claim 4, comprising several layers stacked in a thickness direction, each layer comprising a plurality of channels (101) having different cross sections and/or periodic spacing of the channels (101).

6. The metamaterial (100) according to any preceding claim, wherein one or more of the channels (101) are inclined with respect to a thickness direction of the acoustic metamaterial (100).

7. The metamaterial (100) according to claim 6, wherein one or more of the channels (101) are spiral-shaped.

8. A method for additive manufacturing of an acoustic metamaterial (100) according to any of the preceding claims, comprising several successive material deposition steps to form a layer (203) in each step, the layer comprising a plurality of periodically repeated cells (204) separated by walls (104), the layers (203) deposited in the successive material deposition steps being stacked with their respective cells (204) aligned to form a channel (101).

9. The additive manufacturing method of claim 8, wherein the material comprises a thermoplastic polymer.

10. An additive manufacturing method according to claim 9, wherein the deposition of material is performed by fuse deposition.

11. The additive manufacturing method of claim 8, wherein the material comprises a thermosetting resin.

12. An additive manufacturing method according to any one of claims 9 to 11, wherein the material comprises suspended solid particles.

13. The additive manufacturing method of claim 12, wherein the solid particles are fibers.

14. A method for manufacturing an acoustic metamaterial (100'), comprising the steps of:

Manufacturing a mold (210) by depositing a plurality of stacked layers (203), each layer comprising a plurality of periodically repeating cells (204) separated by walls (205), the cells (204) of the plurality of stacked layers (203) being aligned to form channels (206);

Filling the channel (206) with a fluid material (220);

-solidifying the fluid material (220); and

The mold (210) is removed.

15. The manufacturing method according to claim 14, wherein the hydraulic radius of the cells (204) is between 5 and 600 μm.

16. The manufacturing method according to any one of claims 14 or 15, wherein the wall (205) has a minimum width (S) between 2 μιη and 600 μιη.

17. The manufacturing method according to any one of claims 14 to 16, wherein the length (H') of the channel (206) is between 1mm and 150 mm.

18. The manufacturing method according to any one of claims 14 to 17, wherein the cells (204) are substantially polygonal.

19. The manufacturing method according to any one of claims 14 to 18, wherein the cells (204) are substantially circular or elliptical.

20. The manufacturing method according to any one of claims 14 to 19, wherein in the stacked layers (203) the shape and/or size of the cells (204) of different layers (203) are different.

21. The manufacturing method according to any one of claims 14 to 20, wherein the mould (210) further comprises one or more lateral ducts between the channels (206).

22. The manufacturing method according to any one of claims 14 to 21, wherein the mould (210) is made of a water-soluble material and the step of removing the mould is performed by leaching.

23. The manufacturing method according to any one of claims 14 to 22, wherein the additive manufacturing of the mould (210) is performed by fuse deposition.

24. The manufacturing method according to any one of claims 14 to 23, wherein the fluid material (220) comprises a resin, and the step of curing the fluid material comprises polymerizing the resin.

25. The manufacturing method according to any one of claims 14 to 24, wherein the fluid material (220) comprises suspended solid particles.

26. A metamaterial (100 ') manufactured by the manufacturing method according to any one of claims 14 to 25, comprising a plurality of pillars (101 ') extending from a common substrate (103 ').

27. A turbine (1) comprising an acoustic metamaterial (100, 100') according to any one of claims 1 to 7 and 26 as an acoustic absorber.