Acoustic absorbing metamaterial and process for its production

Abstract

Acoustic Absorbing Metamaterial and Process for its Production This disclosure relates to an acoustic absorbing metamaterial (100) comprising a plurality of stacked layers (101x, 101y), and a process for its production. 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 along orthogonal directions, arranged to form an orthotropic lattice with at least two adjacent parallel filaments (102) in mutual contact or a filament (102) with a substantially larger 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 the abstract: Fig. 2.

Description

Title of the invention: Acoustic absorbing meta-material and process for its production technical field

[0001] The present invention relates to the field of acoustic absorbing meta-materials, as well as to their production. Previous technique

[0002] Climate change is a major concern for many legislative and regulatory bodies worldwide. Indeed, various restrictions on carbon emissions have been, are being, or will be adopted by various states. In particular, an ambitious standard applies to both new types of aircraft and those already in operation, requiring the implementation of technological solutions to bring them into compliance with current regulations. Civil aviation has been actively working for several years now to contribute to the fight against climate change.

[0003] Technological research efforts have already led to very significant improvements in the environmental performance of aircraft. The Applicant takes into account the factors impacting all phases of design and development in order to obtain aeronautical components and products that are less energy-intensive, more environmentally friendly, and 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 that minimize greenhouse gas emissions to the minimum possible in order to reduce the environmental footprint of its activity.

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

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

[0007] In order to reduce the noise emitted by aircraft engines, it is therefore common practice 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 act 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 problematic as the surface area to be covered can be very large for turbofan engines with very high bypass ratios.

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

[0009] The use of additive manufacturing was 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) for producing acoustic absorbers with microchannels. However, these acoustic absorbers also have a rather narrow absorption frequency range. Other ordered porous materials produced by additive manufacturing were proposed by J.P. Singh and P.M. 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, p. 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 acoustic absorbers. In order to broaden the frequency range of acoustic absorption, 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 layering several of these panels and producing them by additive manufacturing.However, these relatively fragile acoustic absorbers seem difficult to apply in environments where they would be subject to abrasion or other mechanical stresses, 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), and 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 superimposed layers 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 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 limited at lower frequencies, which it is particularly important to absorb in the context of engines with very high bypass ratio blowers.

[0013] Other micro-lattices for acoustic applications produced by additive manufacturing have been proposed by ER Fotsing, A. Dubourg, A. Ross, and J. Mardjono in "Acoustic properties of periodic micro-structures obtained by additive manufacturing", Applied Acoustics, vol. 148, p. 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, p. EL334-EL339, Apr. 2015, by A. Dubourg in "Integration of Innovative Acoustic Absorbing Structures within a Turboblower", 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 gradients of properties, these meta-materials obtained by additive manufacturing continue to have limitations, particularly with regard to the maximum impedance that can be achieved at low frequencies with a reduced footprint. Description of the invention

[0015] This disclosure aims to address 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 significantly improving aircraft performance and, in this respect, contributes to reducing the environmental impact of aircraft.To this end, a first aspect of this disclosure relates to a sound-absorbing metamaterial comprising a plurality of stacked layers, each layer being formed by a plurality of parallel filaments. The plurality of layers includes layers whose filaments are oriented along orthogonal directions, arranged to form an orthotropic lattice with at least two adjacent parallel filaments in mutual contact, or a filament with a cross-section substantially larger orthogonally to a layer stacking direction than in the layer stacking direction. By "substantially larger," we mean that the difference is greater than that due to natural settling. of the molten filament during its deposition and before it hardens. Thus, the filament can be at least 20%, or even at least 50%, wider orthogonally to the stacking direction than in the stacking direction.

[0016] Thanks to the orthotropic nature of the lattice, easily adaptable by grouping the parallel filaments differently depending on 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, particularly in gas turbine engines.

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

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

[0019] However, the acoustic absorbing meta-material may also include 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 this disclosure relates to a gas turbine engine incorporating the acoustic absorbing meta-material according to the first aspect.

[0022] A fourth aspect of this disclosure relates to an aircraft incorporating the acoustic-absorbing meta-material according to the first aspect.

[0023] A fifth aspect of this 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 notably be carried out by depositing filaments, in particular by an extruder head. Brief description of the drawings

[0024] The invention will be better understood and its advantages will become more apparent upon reading the following detailed description of embodiments shown by way of non-limiting examples. The description refers to the accompanying drawings in which:

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

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

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

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

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

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

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

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

[0033] [Fig.9] The [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] The [Fig. 10] illustrates an additive manufacturing process of an acoustic meta-material according to any one of the preceding embodiments.

[0035] [Fig. 11] The [Fig. 11] is a graph illustrating the level of acoustic pressure emitted by a turbofan engine as a function of frequency. Description of the implementation methods

[0036] Figure 1 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 shroud 11 leading into the nozzle 8. Thus, an air stream 12 from the fan 2 may be defined between the shroud 11 of the gas generator 10 and an internal wall 13 of the nacelle 9.The high-pressure turbine 6 can be connected to the high-pressure compressor 4 by a first rotating shaft 14 for driving the latter, while the low-pressure turbine 7 can be connected to the blower 2 and the low-pressure compressor 3 by a second rotating shaft 15 coaxial with the first rotating shaft 14, in a similar manner. In the context of high and very high bypass ratio engines, a reduction gear 16 can be mechanically interposed between the second rotating shaft 15 and the blower 2, in order to reduce the rotational speed of the blower 2 and prevent the blade tips of the blower 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 the engines At high and very high dilution rates, and particularly in those equipped with a reducer 16, a large part of the noise from the fan 2 can be concentrated in low frequencies, as illustrated in [Fig. 11], showing the sound pressure level (SPL) as a function of frequency f. In order to absorb at least some of the noise from the fan 2, noise absorbers 17 can be integrated into the inner wall 13 of the nacelle 9, particularly upstream and downstream of the fan 2 blades. As illustrated, however, it is also possible to integrate acoustic absorbers 17 into the fairing 11 of the gas generator 10, or even into the casing of the latter.

[0038] Typically, the acoustic 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. Furthermore, it can be difficult to position them directly opposite the fan blade tips, where noise emission may be most intense, since the inner wall 13 of the nacelle 9 typically includes an abradable material 18 at this location, in order to absorb the occasional friction of the fan blade tips 2 due to their transient deformations.

[0039] A sound-absorbing metamaterial 100 according to a first embodiment, illustrated in [Fig. 2], and which can be used for the sound absorbers of [Fig. 1], can comprise a plurality of layers 101x, lOly stacked along a Z direction. Each layer lOlx is formed by a plurality of parallel filaments 102 oriented along an X direction orthogonal to the Z direction, while each layer lOly is formed by a plurality of parallel filaments 102 oriented along a Y direction orthogonal to the X and Z directions. As illustrated, all the filaments 102 can have the same continuous cross-section, for example round, with a diameter that can be, for example, between 1 µm and 1 mm.As illustrated, the lOlx and lOly layers can alternate, while in each lOlx, lOly layer, the filaments 102 can 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 lOlx, lOly layer can be laterally spaced periodically with a distance d between them. Thus, the stacked lOlx, lOly layers can form an orthotropic lattice, with different properties in the Z direction than 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 larger 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 101 layers x, lOly than in this Z direction, as in a second embodiment as illustrated in [Fig.3]. Thus, the stacked lOlx, 101 y layers can form an orthotropic lattice, with different properties in the Z direction than in the X and Y directions, in a manner analogous to the first embodiment.

[0041] It is nevertheless also possible to group the filaments into bundles with different numbers of filaments and / or with different spacings depending on the layers. Thus, as in a third embodiment as illustrated in [Fig.4], the acoustic meta-material 100 can also comprise a plurality of layers lOlx, lOly stacked alternately along a Z direction, where each layer lOlx is formed by a plurality of parallel filaments 102 oriented along a direction X orthogonal to the direction Z, while each layer 101 y is formed by a plurality of parallel filaments 102 oriented along a direction Y orthogonal to the directions Y and Z, but where the filaments 102 of the layers lOlx and those of the layers lOly can respectively be grouped into bundles 103x, 103y of different numbers of filaments 102, for example bundles 103x of five filaments 102 in the layers lOlx, spaced apart by a distance dx, and bundles 103y of two filaments 102 in the layers lOly, 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 conceivable to stack several adjacent layers of filaments oriented along the same direction. Thus, as in a fourth embodiment as illustrated in [Fig. 5], the acoustic absorbing metamaterial can comprise groups of several adjacent lOlx layers, for example two, formed by filaments 102 oriented along the same X direction, each group of several adjacent lOlx layers being able to be intercalated between lOly layers formed by filaments 102 oriented along an orthogonal Y direction.As in the first and third embodiments, the filaments 102 can be grouped in each layer 101x, lOly 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, lOly can 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 into bundles having the same number of filaments and spacing in each layer, it It is also conceivable, as in the second embodiment, to group the filaments into bundles with different numbers of filaments and / or different spacings between layers, including among layers of filaments oriented in the same direction. Thus, in a fifth embodiment as illustrated in [Fig. 6], the acoustic absorbing meta-material 100 can comprise groups of several adjacent layers lOlx, 101x', formed by filaments 102 oriented along a direction X, but grouped into bundles 103, 103' each having a different spacing and / or number of filaments 102, or even at least one layer lOlx” comprising only filaments 102 separated laterally from each other and not incorporated into bundles of adjacent 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 analogous to that of any of the preceding embodiments, can comprise at least one layer 101x, formed by filaments 102 oriented along a direction X, grouped in bundles 103, 103' each having a different spacing and / or number of filaments 102, or even individual filaments 102 separated laterally from the others and from each other and not incorporated into bundles of adjacent filaments.

[0045] Furthermore, although in 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 each of the polygonal geometries illustrated in [Fig. 8]. Moreover, the same acoustic metamaterial 100 according to the present disclosure could comprise filaments with different cross-sections, even in the same layer, or even one or more filaments whose cross-section varies along the transverse direction.

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

[0047] The acoustic absorbing metamaterial 100 can be produced by an additive manufacturing process based on material extrusion, such as the fused deposition modeling (FDM) process used for thermoplastics. 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 lOlx or lOly. By moving this transverse XY plane along an orthogonal Z direction after the filaments 102 have been deposited in each layer lOlx or lOly, it is possible to stack these layers lOlx, lOly to form the acoustic 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 be changed during the execution of this process.102 filaments can notably 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 embodiments, it is evident that various modifications and changes can 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 mentioned can be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

Demands

1. 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 along orthogonal directions, arranged so as to form an orthotropic lattice with at least two adjacent parallel filaments (102) in mutual contact orthogonally to a layer stacking direction.

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

3. Acoustic absorbing meta-material (100) according to any one of the preceding claims, comprising filaments (102) with different cross-sections.

4. Acoustic absorbing meta-material (100) according to claim 3, comprising filaments (102) with different cross-sections in the 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. Gas turbine engine (1) incorporating the sound-absorbing meta-material (100) according to any one of claims 1 to 6.

8. Aircraft incorporating the acoustic absorbing meta-material (100) according to any one of claims 1 to 6.

9. A method for producing the acoustic absorbing meta-material (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 filaments (102).

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