Sound-absorbing metamaterial and process for its production

Abstract

Sound-absorbing meta-material and method for producing same The present disclosure relates to a sound-absorbing meta-material (100) comprising a plurality of stacked layers (101x, 101y) and a method for producing same. Each layer (101x, 101y) is formed by a plurality of parallel filaments (102), and the plurality of layers (101x, 101y) comprises layers whose filaments are oriented in orthogonal directions, arranged so as to form an orthotropic lattice with at least two adjacent parallel filaments (102) in mutual contact or a filament (102) with a substantially wider cross-section orthogonally to a stacking direction (Z) of the layers (101x, 101y) than in the stacking direction (Z) of the layers (101x, 101y). Figure for abstract: Fig. 2.

Description

Title of the invention: Acoustic absorbing meta-material and method for its production Technical field

[0001] The present invention relates to the field of acoustic absorbing metamaterials, as well as to their production. Prior art

[0002] Climate change is a major concern for many legislative and regulatory bodies around the world. Indeed, various restrictions on carbon emissions have been, are being or will be adopted by various states. In particular, an ambitious standard applies both to new types of aircraft but also to those in circulation requiring the implementation of technological solutions in order to make them compliant with current regulations. Civil aviation has been mobilizing for several years now to make a contribution to the fight against climate change.

[0003] Technological research efforts have already made it possible to significantly improve the environmental performance of aircraft. The Applicant takes into consideration the impact factors in all phases of design and development to obtain less energy-intensive, more environmentally friendly aeronautical components and products whose integration and use in civil aviation have moderate environmental consequences with the aim of improving the energy efficiency of aircraft.

[0004] Consequently, the Applicant is constantly working to reduce its negative climate impact by using methods and operating virtuous development and manufacturing processes and minimizing greenhouse gas emissions to the minimum possible in order to reduce the environmental footprint of its activity.

[0005] This sustained research and development work covers new generations of aircraft engines, the lightening of aircraft, in particular through the materials used and lighter on-board equipment, the development of the use of electrical technologies to ensure propulsion, and, as essential complements to technological progress, aeronautical biofuels.

[0006] Among the most common aircraft engines are turbofans. A turbofan comprises a fan and a gas generator incorporating at least a compressor, a combustion chamber, a turbine and a nozzle. The total noise produced by such a turbofan turbofan engines can therefore include jet, combustion, fan, compressor and turbine noise. However, the most dominant noise is usually that emitted by the fan, which can extend over a wide frequency band, as illustrated in [Fig. 10], with tonal components corresponding to the fan blade passing frequencies. In order to increase the energy efficiency of turbofans, the general trend is to increase their bypass ratio, i.e., the proportion of airflow driven by the fan compared to that used for combustion in the gas generator, and therefore the fan diameter. As a result, the fans of the latest generations of turbofans tend to rotate more slowly, and therefore to emit noise at lower frequencies.

[0007] In order to reduce the noise emitted by aircraft engines, it is therefore common to cover certain areas, such as the nacelles containing these engines, with acoustic absorbers such as honeycomb sandwich panels. In this type of acoustic absorber, each cell of the honeycomb can function as a Helmholtz resonator to attenuate the noise. However, the frequency range of acoustic attenuation of such absorbers is limited and, to be effective at low frequencies, they must be particularly large, which is all the more penalizing since the surface area to be covered can be very large for turbojet engines with a double flow and very high bypass ratio.

[0008] As an alternative to honeycomb sandwich panels, it has therefore been proposed to use porous materials. However, most of the available porous materials have too low a mechanical strength, while the strongest ones, such as for example the metallic material disclosed in US 7,963,364 B2, are excessively heavy. Beyond these disadvantages, the minimum frequency of perfect absorption of porous materials is usually reached when their thickness is approximately equal to a quarter of the acoustic wavelength. Consequently, to obtain high noise absorption at 1000 or 500Hz, for example, this thickness must be approximately 86 or 171mm, respectively, resulting in elements that are much too bulky for an increasingly restricted space in the new generations of engines with high or ultra-high bypass ratios.

[0009] The use of additive manufacturing has been proposed by Z. Liu, J. Zhan, M. Fard, and J.L. Davy in “Acoustic properties of a porous polycarbonate material produced by additive manufacturing,” Materials Letters, vol. 181, pp. 296-299, (Oct. 2016) to produce sound absorbers with microchannels. However, these sound absorbers also have only a fairly narrow absorption frequency range. Other ordered porous materials produced by additive manufacturing have been proposed by JP Singh and PM Pandey in “Fabrication and assessment of mechanical properties of open cell porous regular interconnected metallic structure through rapid manufacturing route”, Rapid Prototyping Journal, vol. 24, no. 1, p. 138-149, Jan. 2018, and by T. Zielinski et al. in “Reproducibility of sound-absorbing periodic porous materials using additive manufacturing technologies: Round robin study”, Additive Manufacturing, vol. 36, pp. 101564, 2020.

[0010] It has also been proposed, for example by Qian, YJ, Kong, DY, Liu, SM, Sun, SM, & Zhao, Z., in "Investigation on micro-perforated panel absorber with ultra-micro perforations.", Applied Acoustics, 74(7), pp. 931-935 (2013), to use micro-perforated panels as sound absorbers. In order to broaden the sound absorption frequency range, Liu, Z., Zhan, J., Fard, M., & Davy, J., in "Acoustic properties of multilayer sound absorbers with a 3D printed micro-perforated panel." Applied Acoustics, 121, pp. 25-32 (2017), and Yang, W., Bai, X., Zhu, W., Kiran, R., An, J., Chua, CK, & Zhou, K. in “3D Printing of Polymeric Multi-Layer Micro-Perforated Panels for Tunable Wideband Sound Absorption”. Polymers, 12(2), p. 360 (2020) also proposed to superimpose several of these panels and produce them by additive manufacturing.However, these relatively fragile sound absorbers seem difficult to apply in environments in which they would be subject to abrasion or other mechanical constraints, such as aircraft engine nacelles.

[0011] Acoustic metamaterials produced by additive manufacturing have been proposed by T. Cavalieri et al. in “Rapid additive manufacturing of optimized anisotropic metaporous surfaces for broadband absorption”, J. Appl. Phys. 129, 115102 (2021), as well as by J. Boulvert et al. in “Folded metaporous material for sub-wavelength and broadband perfect sound absorption”, Appl. Phys. Lett. 117, 251902 (2020) and in “Optimally graded porous material for broadband perfect absorption of sound”, Journal of Applied Physics 126, 175101 (2019). An acoustic metamaterial can be understood as a periodically structured medium whose periodically repeated constituent units collectively affect the passage of acoustic waves.

[0012] Other acoustic metamaterials produced by additive manufacturing, with several layers superimposed in the thickness direction, each of which may have a lattice with a different periodicity, so as to broaden their attenuation frequency ranges, have been proposed in the French patent application publication FR 1 761 722, as well as by Guild, MD, Rohde, C., Rothko, MC, & Sieck, CF in “3D printed acoustic metamaterial sound absorbers using functionally-graded sonie crystals”, Proceedings of Euronoise (2018). However, their acoustic absorption is more reduced at the lowest frequencies, which are particularly suitable for absorption in the context of motors very high bypass ratio blower.

[0013] Other acoustically applied microlattices produced by additive manufacturing have been proposed by E. R. Fotsing, A. Dubourg, A. Ross, and J. Mardjono in “Acoustic properties of periodic micro-structures obtained by additive manufacturing,” Applied Acoustics, vol. 148, pp. 322-331, May 2019, by X. Cai, J. Yang, and G. Hu in “Optimization on microlattice materials for sound absorption by an integrated transfer matrix method,” The Journal of the Acoustical Society of America, vol. 137, no. 4, pp. EL334-EL339, Apr. 2015, by A. Dubourg in “Integration of innovative acoustic absorbing structures within a turbofan,” p. 142, 2015, by S. Kuschmitz, TP Ring, H. Watschke, SC Langer, and T. Vietor “Design and Additive Manufacturing of Porous Sound Absorbers—A Machine-Learning Approach”, Materials 2021, 14, 1747, by TP Ring and SCLanger in “Design, Experimental and Numerical Characterization of 3D-Printed Porous Absorbers”, Materials 2019, 12, 3397, J. Boulvert et al. in “Folded metaporous material for sub-wavelength and broadband perfect sound absorption”, Appl. Phys. Lett. 117, 251902 (2020) and in “Optimally graded porous material for broadband perfect absorption of sound”, Journal of Applied Physics 126, 175101 (2019). .

[0014] However, even when they exhibit property gradients, these metamaterials obtained by additive manufacturing continue to exhibit limitations, particularly with regard to the maximum impedance that can be achieved at low frequencies with reduced bulk. Statement of the invention

[0015] The present disclosure aims to overcome these drawbacks, and in particular to achieve optimal acoustic impedances even at low frequencies with limited thicknesses. To this end, the disclosure is the result of technological research aimed at very significantly improving the performance of aircraft and, in this sense, contributes to reducing the environmental impact of aircraft.For this purpose, a first aspect of the present disclosure relates to an acoustic absorbing metamaterial comprising a plurality of stacked layers, each layer of which is formed by a plurality of parallel filaments, the plurality of layers comprising layers whose filaments are oriented in orthogonal directions, arranged so as to form an orthotropic lattice with at least two adjacent parallel filaments in mutual contact or a filament with a substantially wider cross-section orthogonally to a layer stacking direction than in the layer stacking direction. By "substantially wider" is meant that the difference is greater than that due to natural settling of the molten filament during its deposition and before its curing. Thus, the filament may be at . less than 20%, or at least 50% wider orthogonally to the stacking direction than in the stacking direction,

[0016] Thanks to the orthotropic character of the lattice, easily adaptable by grouping the parallel filaments differently according to the layers, additional degrees of freedom are obtained compared to lattices with transverse isotropy to achieve optimal acoustic impedances even at low frequencies, as well as possibly mechanical properties allowing the acoustic meta-material to be better adapted to its application, in particular in gas turbine engines.

[0017] The filaments of at least one layer of the plurality of layers may be arranged periodically.

[0018] The sound-absorbing metamaterial may comprise filaments with different cross-sections, even within the same layer of the plurality of layers.

[0019] However, the acoustic absorbing metamaterial may also comprise at least one filament with a cross-section evolving along a longitudinal direction.

[0020] A second aspect of the present disclosure relates to a structure comprising the acoustic absorbing meta-material according to the first aspect and at least one additional superimposed alveolar layer.

[0021] A third aspect of the present disclosure relates to a gas turbine engine incorporating the sound absorbing metamaterial according to the first aspect.

[0022] A fourth aspect of the present disclosure relates to an aircraft incorporating the sound-absorbing metamaterial according to the first aspect.

[0023] A fifth aspect of the present disclosure relates to a method for producing the acoustic absorbing metamaterial according to the first aspect, comprising an additive manufacturing step. This additive manufacturing step can in particular be carried out by depositing the filaments, in particular by an extruder head. Brief description of the drawings

[0024] The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of embodiments shown as non-limiting examples. The description refers to the appended drawings in which:

[0025] [Fig. 1] [Fig. 1] schematically illustrates a gas turbine engine for the propulsion of an aircraft.

[0026] [Fig.2] [Fig.2] represents an acoustic absorbing meta-material according to a first embodiment.

[0027] [Fig.3] [Fig.3] represents an acoustic absorbing meta-material according to a second embodiment.

[0028] [Fig.4] [Fig.4] represents an acoustic absorbing meta-material following a third embodiment.

[0029] [Fig.5] [Fig.5] represents an acoustic absorbing meta-material according to a fourth embodiment.

[0030] [Fig.6] [Fig.6] represents an acoustic absorbing meta-material according to a fifth embodiment.

[0031] [Fig.7] [Fig.7] represents an acoustic absorbing meta-material according to a sixth embodiment.

[0032] [Fig.8] [Fig.8] shows several alternative cross-sections for the filaments of the acoustic absorbing meta-materials according to the previous embodiments.

[0033] [Fig.9] [Fig.9] a structure incorporating an acoustic absorbing meta-material according to any of the preceding embodiments, and at least one additional alveolar layer.

[0034] [Fig. 10] [Fig. 10] illustrates a method of additively manufacturing an acoustic metamaterial according to any of the preceding embodiments.

[0035] [Fig. 11] [Fig. 11] is a graph illustrating the sound pressure level emitted by a turbofan engine as a function of frequency. Description of the embodiments

[0036] [Fig.l] schematically illustrates a gas turbine engine 1. In the direction of fluid flow, this gas turbine engine 1 may comprise a fan 2, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6, a low-pressure turbine 7 and a nozzle 8. The assembly may be surrounded by a nacelle 9. The compressors 3, 4, the combustion chamber 5 and the turbines 6, 7 together form the gas generator 10, which may itself be surrounded by a fairing 11 ending in the nozzle 8. Thus, an air stream 12 of the fan 2 may be defined between the fairing 11 of the gas generator 10 and an internal wall 13 of the nacelle 9.The high-pressure turbine 6 may be connected to the high-pressure compressor 4 by a first rotary shaft 14 for driving the latter, while the low-pressure turbine 7 may be connected to the fan 2 and to the low-pressure compressor 3 by a second rotary shaft 15 coaxial with the first rotary shaft 14, in a similar manner. In the context of high and very high bypass ratio engines, a reducer 16 may be mechanically interposed between the second rotary shaft 15 and the fan 2, in order to reduce the rotational speed of the fan 2 and prevent the blade tips of the fan 2 from reaching excessive speeds.

[0037] Each of these elements of the gas turbine engine 1 can generate noise, but the noise generated by the fan 2 is generally dominant. Furthermore, in engines with high and very high bypass ratios, and in particular in those equipped with a reduction gear 16, a large part of the noise of the fan 2 can be concentrated in low frequencies, as illustrated in [Fig. 11], showing the sound pressure level (SPL) as a function of the frequency f. In order to absorb at least part of the noise of the fan 2, noise absorbers 17 can be integrated into the internal wall 13 of the nacelle 9, in particular upstream and downstream of the blades of the fan 2. As illustrated, it is however also possible to integrate sound absorbers 17 into the fairing 11 of the gas generator 10, or even into the casing of the latter.

[0038] Typically, the sound absorbers 17 are formed by honeycomb sandwich panels. However, in engines with high, or even very high, bypass ratios, these panels can represent a significant penalty in terms of mass and size. In addition, it can be difficult to arrange them directly opposite the tips of the fan blades, where the noise emission can nevertheless be the most intense, since the internal wall 13 of the nacelle 9 typically comprises an abradable material 18 at this location, in order to absorb the occasional friction of the tips of the fan blades 2 due to their transient deformations.

[0039] An acoustic absorbing meta-material 100 according to a first embodiment, illustrated in [Fig.2], and which may be intended for the acoustic absorbers of [Fig.l], may comprise a plurality of layers 101 x, 10ly stacked in a Z direction. Each layer 101x is formed by a plurality of parallel filaments 102 oriented in a direction X orthogonal to the direction Z, while each layer 10ly is formed by a plurality of parallel filaments 102 oriented in a direction Y orthogonal to the directions X and Z. As illustrated, all the filaments 102 may have the same continuous cross-section, for example round, with a diameter which may be for example between 1 μm and 1 mm.As illustrated, the layers 101x and 101y may be alternated, while in each layer 101x, 101y, the filaments 102 may be grouped by bundles 103 of several adjacent filaments 102, for example three, where each filament 102 of each bundle 103 is in contact with at least one adjacent filament 102 of the same bundle 103, while the bundles 103 of the same layer 101x, 101y may be laterally spaced periodically with a distance d between them. Thus, the stacked layers 101x, 101y may form an orthotropic lattice, with different properties in the Z direction to those in the X and Y directions.

[0040] Alternatively, however, rather than grouping the filaments into bundles of several adjacent filaments, it is also conceivable to use filaments 102 with substantially wider cross-sections, for example at least 20%, or even at least 50%, in a Y, X direction substantially orthogonal to the Z direction of stacking of the layers 101 x, 10ly than in this Z direction, as in a second embodiment as illustrated in [Fig. 3]. Thus, the layers 101x, 101 stacked y can form an orthotropic lattice, with different properties in the Z direction from those in the X and Y directions, similarly to the first embodiment.

[0041] It is nevertheless also possible to group the filaments in bundles of different numbers of filaments and / or with different spacings according to the layers. Thus, as in a third embodiment as illustrated in [Fig.4], the acoustic meta-material 100 may also comprise a plurality of layers 10lx, 10ly stacked alternately along a Z direction, where each layer 10lx is formed by a plurality of parallel filaments 102 oriented along an X direction orthogonal to the Z direction, while each layer 101 y is formed by a plurality of parallel filaments 102 oriented along a Y direction orthogonal to the Y and Z directions, but where the filaments 102 of the layers 10lx and those of the layers 10ly may be respectively grouped in bundles 103x, 103y of different numbers of filaments 102, for example bundles 103x of five filaments 102 in the layers 10lx, spaced apart by a distance dx, and bundles 103y of two filaments 102 in the layers 10ly, spaced apart by a distance dy. Thus, the stacked lOlx, lOly layers can form an orthotropic lattice, with different properties in each of the X, Y and Z directions.

[0042] Although in the first three embodiments the layers of filaments oriented in a first direction are alternated with those of filaments oriented in a second direction orthogonal to the first, it is also possible to stack several adjacent layers of filaments oriented in the same direction. Thus, as in a fourth embodiment as illustrated in [Fig. 5], the acoustic absorbing meta-material may comprise groups of several adjacent layers 101x, for example two, formed by filaments 102 oriented in the same direction X, each group of several adjacent layers 101x being able to be interposed between layers 101y formed by filaments 102 oriented in an orthogonal direction Y.As in the first and third embodiments, the filaments 102 may be grouped in each layer 101x, 101y by bundles 103 of several adjacent filaments 102, for example three, where each filament 102 of each bundle 103 is in contact with at least one adjacent filament 102 of the same bundle 103, while the bundles 103 of the same layer 101x, 101y may be laterally spaced periodically with a distance d between them. Alternatively, however, it is also conceivable to use filaments with a flattened cross-section, as in the second embodiment.

[0043] Although in the fourth embodiment the filaments are grouped in bundles having the same number of filaments and spacing in each layer, it is also conceivable, as in the second embodiment, to group the filaments in bundles of different numbers of filaments and / or with different spacings depending on the layers, including among the layers of filaments oriented in the same direction. Thus, in a fifth embodiment as illustrated in [Fig.6], the acoustic absorbing meta-material 100 may comprise groups of several adjacent layers 101x, 101x', formed by filaments 102 oriented in a direction X, but grouped in bundles 103, 103' each having a different spacing and / or number of filaments 102, or even at least one layer 101x” comprising only filaments 102 laterally separated from each other and not incorporated in adjacent bundles of filaments.

[0044] Furthermore, although in the preceding embodiments the number of filaments per bundle and the spacing between adjacent bundles are constant in the individual layers, it is also possible to vary them within the same layer. Thus, in a sixth embodiment as illustrated in [Fig.7], the acoustic absorbing meta-material 100, otherwise similar to that of any of the preceding embodiments, may comprise at least one layer 101x, formed by filaments 102 oriented in a direction X, grouped in bundles 103, 103' each having a different spacing and / or number of filaments 102, or even individual filaments 102 laterally separated from the others and from each other and not incorporated into bundles of adjacent filaments.

[0045] Furthermore, although for each of the preceding embodiments the filaments have been presented as having a substantially round or oval cross-section, other shapes are also conceivable, such as for example each of the polygonal geometries illustrated in [Fig.8]. Furthermore, the same acoustic meta-material 100 according to the present disclosure could comprise filaments of different cross-sections, even in the same layer, or even one or more filaments whose cross-section evolves along the transverse direction.

[0046] Acoustic absorbers 17 such as those of the gas turbine engine 1 of [Fig.l], particularly in the aeronautical field, may incorporate structures comprising the acoustic absorbing meta-material 100 according to any of the preceding embodiments. For this purpose, this acoustic absorbing meta-material 100 may be used alone or, as illustrated in [Fig.9], be combined in the structure of the acoustic absorber 17 with one or more additional layers 160, superimposed on or between the layers of the acoustic absorbing meta-material 100. These additional layers 160 may, for example, be cellular such as conventional honeycomb structures. The acoustic absorbing meta-material 100 may thus, for example, form resistive veils within this structure to reinforce the acoustic absorption by the cells.

[0047] The sound-absorbing metamaterial 100 may be produced by a method of fa additive manufacturing based on material extrusion, such as the fused deposition process used for thermoplastic materials. These processes comprise several consecutive material deposition steps. In each of these steps, an extruder head 200 can move along a path in a transverse XY plane depositing each filament 102, which then solidifies, to form a layer 101x or 101y. By moving this transverse XY plane in an orthogonal Z direction after the deposition of the filaments 102 of each layer 101x or 101y, it is possible to stack these layers 101x, 101y to form the sound-absorbing metamaterial 100, as illustrated in [Fig. 10]. In order to vary the cross-section of the filaments 102, the shape of the extruder head 200 can change during the execution of this process.The filaments 102 may in particular be made of thermoplastic polymer such as polyetheretherketone (known by its English acronym PEEK) or polyethyleneimine (PEI), alone or reinforced for example with fibers.

[0048] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that various modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments discussed may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

Claims

1. An acoustic absorbing meta-material (100) comprising a plurality of stacked layers (101x, 101y), each layer (101x, 101y) being formed by a plurality of parallel filaments (102), the plurality of layers (101x, 101y) comprising layers whose filaments are oriented in orthogonal directions, arranged so as to form an orthotropic lattice with at least two adjacent parallel filaments (102) in mutual contact or a filament (102) with a substantially wider cross-section orthogonally to a stacking direction (Z) of the layers (101x, 101y) than in the stacking direction (Z) of the layers (101x, 101y).

2. The sound-absorbing meta-material (100) of claim 1, wherein the filaments (102) of at least one of the plurality of layers (101x, 101y) are arranged periodically.

3. An acoustic absorbing meta-material (100) according to any preceding claim, comprising filaments (102) with different cross-sections.

4. An acoustic absorbing meta-material (100) according to claim 3, comprising filaments (102) with different cross-sections in a same layer of the plurality of layers (101x, 101y).

5. Acoustic absorbing meta-material (100) according to any one of the preceding claims, comprising at least one filament (102) with a cross-section evolving along a longitudinal direction.

6. Structure comprising an acoustic absorbing meta-material (100) according to any one of the preceding claims, and at least one additional superimposed alveolar layer (110).

7. A gas turbine engine (1) incorporating the sound absorbing metamaterial (100) of any one of claims 1 to 6.

8. An aircraft incorporating the sound absorbing metamaterial (100) of any one of claims 1 to 6.

9. A method of producing the sound-absorbing metamaterial (100) according to any one of claims 1 to 6, comprising an additive manufacturing step.

10. A production method according to claim 9, wherein the additive manufacturing step is carried out by depositing the filaments (102).

11. A production method according to claim 10, wherein the filaments (102) are deposited by an extruder head (200).

Images