OPTIMIZED HONEYCOMB CORE AND ACOUSTIC ATTENUATION PANEL INCLUDING IT FOR AN AIRCRAFT TURBOMACHINE

Abstract

The invention relates to a core (34) for an acoustic attenuation panel extending between a first and a second end face, said core comprising a plurality of tubular polygonal cells (36) having a principal axis (A) of extension between the end faces, each cell comprising an internal cavity (39) delimited by lateral walls (40) extending between the first and second end faces and opening into each of the two end faces, the cells being expandable in a direction parallel to the end faces, each polygonal cell comprising septa (45, 46) extending transversely with respect to the principal axis and being offset along it, characterized in that each septum (45, 46) is folded in the shape of at least one of the lateral walls (40) and another of the lateral walls and has a through orifice,The openings of two consecutive septa are offset radially with respect to the main axis to form a baffle. Figure for the abbreviation: Figure 3,

Description

Title of the invention: OPTIMIZED HONEYCOMB CORE AND ACOUSTIC ATTENUATION PANEL COMPRISING IT FOR AN AIRCRAFT TURBOMACHINE Technical field

[0001] The present invention relates to a honeycomb core for an acoustic attenuation panel and an acoustic panel comprising this honeycomb core for an aircraft turbomachine. Prior art

[0002] Acoustic attenuation panels are widely used in turbomachines for covering areas subject to significant gas passages, such as for example a fan casing or a secondary flow vein located downstream of the fan in which air circulates, or an exhaust nozzle or an exhaust cone of the turbomachine, better known by the English acronym "plug", subject to a passage of exhaust gas.

[0003] These acoustic attenuation panels are generally made in the form of sandwiches comprising at least a first external skin wall, a second internal skin wall, and a core trapped between these first and second skin walls. The core comprises a plurality of tubular polygonal cells which each have an orientation inclined relative to the first and second skin walls or perpendicular to the first and second skin walls. These cores are generally made in the form of a plate of honeycomb material with hexagonal cells. These cells are hollow over their entire height.

[0004] The height of the cells is an essential parameter which determines the capacity of the panel to absorb sound waves. It has in fact been demonstrated that the absorption of sound waves is optimal for a conventional panel when the cells have a height corresponding substantially to a quarter of the wavelength X of the sound wave to be treated.

[0005] Currently, for reasons of integration and compactness, it is desirable to produce sound attenuation panels that are thinner than conventional panels. However, it is not possible to reduce the height of the cells without this resulting in a loss of the panel's ability to absorb sound waves.

[0006] However, it has also been found that the capacity of a cell to absorb an acoustic wave of wavelength X is directly dependent on the length of the path traveled by the acoustic wave in the cell, that is to say, according to the above, a length corresponding to a quarter of the wavelength, or X / 4. However, we also have found that the length of this path does not necessarily have to be along a straight path. In other words, acoustic background can be effectively absorbed as long as the path traveled has a total length close to X / 4, but this length can be distributed along a non-straight path.

[0007] Patents FR-3 070 529-B1 and FR-3 070 530-B1 propose an acoustic panel which has a limited thickness while being suitable for attenuating low frequencies. Said acoustic panel comprises in particular cells housing at least one partial obstacle which extends transversely to the main axis of the cell concerned to increase the length of the path traveled by the sound waves through the cell. Each obstacle has a free end edge which delimits a passage with a part of the wall of the cell opposite.

[0008] Such an acoustic panel makes it possible, at equal height and without loss of acoustically treated surface, to attenuate lower noise frequencies and consequently, to limit the size of the acoustic panel as well as its mass.

[0009] Patents FR-3 070 529-B1 and FR-3 070 530-B1 also propose methods particularly suited to the manufacture of such acoustic panels. In particular, an obstacle in a cell is formed from a strip fixed between two plates. The strip is shaped in particular by folding along fold lines between the two plates intended to form peripheral walls of the cells of a core. The obstacle is formed by a tensile force on the plates fixed to each other tending to separate the plates from each other. This tensile force, taking into account the gluing of the plates by nodal parts and the fold zones provided on the plates, makes it possible to shape the plates to constitute the cells of the cellular core.

[0010] However, the number of baffles created by the folds between the obstacles in a cavity mentioned in these patents is too low and limits the capacity to reduce the height of the honeycomb core.

[0011] There is therefore a real need for an acoustic attenuation panel that is thinner than a conventional panel and yet retains the same level of acoustic attenuation, or even improves it.

[0012] The objective of the present invention is thus to propose a honeycomb core and an acoustic attenuation panel comprising it making it possible to overcome at least some of these drawbacks. Summary of the invention

[0013] To this end, the invention relates to a cellular core for an acoustic attenuation panel extending between a first and a second end face, said core comprising a plurality of tubular polygonal cells having a main axis of extension between the end faces, each cell comprising an internal cavity delimited by side walls extending between the first and second end faces and opening into each of the two end faces, the cells being expandable in a direction parallel to the end faces, each polygonal cell comprising septa extending transversely relative to the main axis of the associated cell and being offset along the main axis.

[0014] According to the invention, each septum is folded in a W shape at least between one of the side walls and another of the side walls and has a through orifice, the orifices of two consecutive septums being offset radially relative to the main axis to form a baffle.

[0015] Thus, the invention makes it possible to integrate more partial transverse obstacles in the same height of cellular core and consequently to increase the length of the path traveled by the sound waves through the cell.

[0016] Indeed, the invention makes it possible to reduce the distance between two successive transverse partitions thanks to the W-shaped folding of each septum or partial transverse partitions provided with a through orifice which has a reduced height compared to a V-shaped folding.

[0017] The invention thus makes it possible, at equal height, to create an air passage path longer than the simple height of the core, and by associating the core with two skins to attenuate lower noise frequencies or to reduce the height of the cells while attenuating the same target frequencies effectively. Consequently, the invention makes it possible to limit the size of the acoustic panel as well as its mass.

[0018] The cellular core according to the invention may comprise one or more of the following characteristics, taken in isolation from one another or in combination with one another in any technically possible combination: - each polygonal cell is formed from a first plate and a second plate fixed together by nodal portions, and the septa each extend through the nodal portions of the first and second plates; - two consecutive septa are independent of each other; - each septum comprises a first fixing tab fixed to a non-nodal portion of the first plate and a second fixing tab fixed to a non-nodal portion of the second plate, the first and second fixing tabs each extending towards one end of the associated cell; - each septum extends transversely between its first fixing leg and its second fixing leg; - each septum is offset along the main axis relative to its first and second fixing legs; - the core includes an additional ribbon arranged between the first and the second plates, attached to the first and second plates and shaped to form at least two consecutive septa; - two consecutive septa are formed from an additional ribbon, said additional ribbon comprising: a first part glued to the first plate and forming a first tab for fixing a first of the septa to a non-nodal part of the first plate, a first adjacent part folded along three fold lines, a second part glued to the second plate and continuously forming a second tab for fixing the first septa and a first tab for fixing the second of the septa to a non-nodal part of the second plate, a second adjacent part folded along three fold lines, and a third part glued to the first plate and forming a second tab for fixing the second septa to a non-nodal part of the first plate; - the additional ribbon is shaped to continuously form consecutive septa, the additional ribbon comprising a succession of parts glued alternately to the first plate or to the second plate and parts folded along three fold lines; - at least one of the septa comprises at least one portion made of acoustically permeable material.

[0019] The invention also relates to an acoustic panel for a turbomachine comprising a first skin wall, a second skin wall and a core according to the invention and as described previously, the core being arranged between the first and second skin walls and secured to at least one of the skin walls, the first skin wall being acoustically permeable, and the second skin wall being acoustically opaque.

[0020] According to one embodiment, the core is integral with the first external skin wall and the second internal skin wall.

[0021] Alternatively, the core is fixed only to one of the skin walls: the first external skin wall or the second internal skin wall, the other of the walls not being integral with the core.

[0022] The invention also relates to an aircraft propulsion unit turbomachine, comprising at least one acoustic panel according to the invention and as described previously. Brief description of the drawings

[0023] The present invention will be better understood and other details, characteristics and advantages of the present invention will appear more clearly on reading the description of a non-limiting example which follows, with reference to the appended drawings in which: - [Fig.l] is a schematic view of the locations of acoustic attenuation panels in a turbomachine; - [Fig.2] is a perspective view of an acoustic attenuation panel according to the invention; - [Fig.3] is a schematic cross-sectional view illustrating cells of one of the acoustic panels of [Fig.2] equipped with several locally perforated septa forming a baffle; - [Fig.4] schematically illustrates a top view of a cell during a forming step; - [Fig.5] is a top view of a cell of the alveolar core in its expanded position as well as expanded cross-sectional views thereof; - [Fig.6] is a perspective view of a W-shaped septum for a cell of the core of an acoustic attenuation panel according to the invention; - [Fig.7] is a perspective view of an example of a cell equipped with two septa according to the invention; - [Fig.8] is a sectional view of another example of a cell equipped with two septa according to the invention; - [Fig.9] is a sectional view of another example of a cell equipped with two septa according to the invention; - [Fig. 10] is a sectional view of another example of a cell equipped with two septa according to the invention; - [Fig.l 1] schematically illustrates a top view of a cell as well as various positions of the septum orifices; - [Fig. 12] schematically represents an example of fixing the tape on the plates; - [Fig. 13] schematically illustrates another example of fixing the tape to the plates; - [Fig.14] schematically represents an example of a cell of an acoustic panel according to the invention; - [Fig. 15] schematically represents another example of a cell of an acoustic panel according to the invention; and - [Fig. 16] schematically represents another example of a cell of an acoustic panel according to the invention. Elements having the same functions in different implementations have the same references in the figures.

[0024] In the description and the claims, the terminology longitudinal, vertical and transverse will be adopted without limitation with reference to the trihedron L, V, T indicated in the figures. Description of the embodiments

[0025] The invention applies to any turbomachine intended to be mounted on an aircraft.

[0026] [Fig. 1] schematically shows such a turbomachine 10 and in particular a nacelle of the turbomachine. In a known manner, the turbomachine 10 comprises, from upstream to downstream, a fan casing 12, an intermediate casing 14, a secondary flow channel 16 delimited by an inner casing 18 and by an outer casing 20, and a nozzle 22 which is delimited by an exhaust casing 24 and by an exhaust cone 26, also known by the English acronym “plug”.

[0027] The fan casing 12 is crossed by an incoming air flow F, the secondary flow channel 16 is crossed by a secondary air flow S and the nozzle 22 allows the ejection of a primary gas flow P. These different flows F, S, P, can propagate acoustic waves coming from the rotating parts of the turbomachine, such as the fan, the compressors and the turbines.

[0028] To reduce these noises, the fan casing 12, the secondary flow channel 16 and the nozzle 22 may have their walls covered with acoustic attenuation panels. Thus the fan casing 12, the inner casing 18 and the outer casing 20, the exhaust casing 24 and the exhaust cone 26 may be covered by acoustic attenuation panels 30, illustrated schematically in bold lines in [Fig.l]. The acoustic attenuation panels 30 are designed to attenuate the noise emitted by the components of the turbomachine 10 which are housed in the nacelle, such as an engine or a fan (not shown).

[0029] Some or all of these, or other acoustic panels may be fully or partially equipped with honeycomb cores according to the invention.

[0030] As illustrated in [Fig. 2], such a panel 30 according to the invention is generally produced in the form of a sandwich comprising in its simplest form, from front to back along the vertical axis V, at least a first external skin wall 31, a second internal skin wall 32, and at least one cellular core 34. Of course, the sandwich may comprise other intermediate layers and in particular other cores superimposed on the core 34.

[0031] In a known manner, the core is fixed to the first outer skin wall and to the second inner skin wall. Alternatively, the core may be fixed only to one of the skin walls: the first outer skin wall or the second inner skin wall, the other of the walls not being integral with the core. The core is secured to one or both skin walls by screwing, riveting, gluing or welding.

[0032] The core 34 extends between a first end face 41 and a second end face 42.

[0033] The core 34 comprises a plurality of tubular polygonal cells 36 which have each an orientation along a main axis A of extension between the first and second end faces. In the examples illustrated and in the remainder of the description, the main axis A is perpendicular to the first and second end faces, and consequently to the first and second skin walls 31, 32. However, the invention also applies to a core whose main axis A of the polygonal cells is inclined relative to the first and second end faces without being perpendicular to them, for example at an angle of between 0° and 45° with the normal to one of the end faces.

[0034] In other words, each cell 36 extends along a vertical main axis A, corresponding overall to an axis of propagation of the sound waves, from a front end 41 of the cell 36 resting on the first external skin wall 31, to a rear end 42 resting on the second external skin wall 42.

[0035] The cells 36 are joined transversely and longitudinally to each other to form a hollow structure. Such a core 34 is generally produced in the form of a plate of material with polygonal cells 36, which are hollow over their entire height H. In the illustrated and non-limiting example, the core 34 is produced in the form of a plate of honeycomb material with hexagonal cells 36, which are hollow over their entire height H.

[0036] The first outer skin wall 31 comprises a plurality of holes 38 which open into the interior of the cells 36. The sound waves penetrate into the cells 36 through these holes 38 where they are partly trapped and thus attenuated. Alternatively, the first outer skin wall 31 has a permeability formed by a grid adapted to allow the sound waves to penetrate into the cellular core 34. In other words, the first outer skin wall 31 is acoustically opaque.

[0037] The second inner skin wall 32 is solid. In other words, the second inner skin wall 32 is acoustically permeable.

[0038] The first outer skin wall 31 and the second inner skin wall 32 extend substantially parallel to each other and transversely. By substantially parallel, it is meant that the angle between the first outer skin wall 31 and the second inner skin wall 32 is less than 10°, preferably less than 5°. The vertical direction V is substantially perpendicular to the local surface of the first outer skin wall 31.

[0039] Each cell 36 comprises an internal cavity 39 delimited by side walls extending between the first and second skin walls 31, 32 and perpendicularly (in the illustrated examples) to the first and second skin walls 31, 32. In other words, the longitudinal walls 40 extend substantially parallel to the vertical direction V from the first skin wall 31 to the second skin wall 32. The internal cavity 39 communicates with at least one of the holes 38 formed in the first skin wall 31.

[0040] However, in the case where the main axis A of the polygonal cells is inclined relative to the first and second end faces, the longitudinal walls 40 extend substantially parallel to the main axis A from the first skin wall 31 to the second skin wall 32.

[0041] With reference to [Fig. 3] which is a schematic cross-sectional view illustrating cells of one of the acoustic panels of [Fig. 2], according to the invention, each polygonal cell comprises at least two septa 45, 46 each extending generally transversely relative to the main axis A of the associated cell from a side wall 40 of the cell. The septa 45, 46 each extend generally perpendicular to the main axis A of the associated cell. More precisely, the mean plane of each septum is perpendicular to the main axis A of the associated cell.

[0042] The two septa are offset in depth along the main longitudinal axis A of the associated cell 36.

[0043] For the purposes of the invention and in the remainder of the description, a septum is by definition an obstacle arranged in a cell of the cellular core, integral with several walls delimiting the cell and extending in a transverse direction relative to the main axis A of the cell. The septums can be in different forms: an acoustically opaque shutter having a local free passage, a partially acoustically opaque obstacle and comprising a portion of acoustically permeable surface (for example in the form of a micro-grid or a zone finely perforated with holes of diameter generally between 0.2 and 0.4 mm for an opening rate generally between 0.5% and 5%), an acoustically permeable obstacle over the entire surface delimited between the walls of the cell (for example in the form of a micro-grid or a finely perforated film).The septum is thus at least locally, that is to say in portions, transparent to certain frequency ranges and impermeable to other frequency ranges.

[0044] In addition, each septum 45, 46 that is not entirely acoustically permeable has a through-orifice 49 or a local acoustically permeable zone, the orifices of two successive septums being offset radially relative to the main axis A to form a baffle for the propagation of the sound waves. The relative position of the through-orifices of two consecutive septums is chosen so as to obtain path lengths of the sound waves to be attenuated that are compatible with their frequency. Thus, the sound waves follow a sinuous trajectory (arrow F) between the septums 45, 46 from the front end 41 to the rear end 42 of the associated cell 36. This sinuous trajectory is therefore longer than the straight-line distance between the two front 41 and rear 42 end faces.

[0045] Furthermore, each septum 45, 46 is folded in a W shape at least between one of the side walls and another of the side walls, preferably non-adjacent. Preferably, each septum 45, 46 is folded in a W shape between at least two opposite side walls, i.e. arranged opposite each other. In other words, each septum 45, 46 has at least three folds P1, P2, P3 along fold lines parallel to each other, two consecutive fold lines being arranged on either side of the mean plane P of the associated septum. Such a septum is illustrated in [Fig. 6]. Thus, the fold lines P1 and P3 are on one side of the plane P and the fold line P2 is on the other side of the plane P.

[0046] Such a honeycomb core 34 is advantageously obtained by forming each cell 36 from two plates 50, 55 and at least one ribbon 60 to form the septa. Of course, several consecutive cells can be formed from these two plates 50 and one or more ribbons 60.

[0047] More precisely, each cell 36 is formed from a first plate 50 and a second plate 55 fixed together by nodal parts. Each plate further comprises so-called non-nodal parts intended to constitute walls delimiting the cells of the alveolar core.

[0048] The ribbons 60 or septa 45, 46 also comprise several functional zones, in particular at least one fixing zone 62 (hatched zone of the ribbon) and a so-called free zone 64, with reference to [Fig. 6]. The free zone 64 has at least three folds P1, P2 and P3.

[0049] First, each fixing zone 62 associated with the first plate 50 of the ribbon 60 is fixed to the first plate 50, for example by gluing, soldering or welding during a step of fixing the ribbon to the first plate 50.

[0050] Similarly, each attachment zone 62 associated with the second plate 55 of the ribbon 60 is attached to the second plate 55, for example by gluing, soldering or welding during a step of attaching the ribbon to the second plate 55.

[0051] Then, during a step of fixing the second plate 55 to the first plate 50, the nodal parts 56 of the first plate 50 are fixed, for example by gluing, brazing or welding, to nodal parts 56 of the second plate 55 located opposite the nodal parts 56 of the first plate 50.

[0052] Finally, during a forming step, the non-nodal parts of the first and second plates 50, 55 are shaped by expansion so that each non-nodal part constitutes, with a non-nodal part opposite the second plate, the side walls delimiting a corresponding cell of the cellular core, and so that at least one part of the ribbon(s) forms the at least two septa 45, 46 in this cell.

[0053] In a first embodiment illustrated in [Fig.12], a ribbon 60 extends between several successive cells delimited by the plates 50 and 55 crossing the nodal zones 56 connecting the plates 50 and 55 and separating said cells. Preferably in this case, at the right of the nodal zone 56 between 50 and 55, all the faces of the ribbon are fixed to each other and to the plates 50 and 55 in order to maintain the uniqueness of the nodal zone 56. Alternatively, the plates 50 and 55 are linked together, only in zones where the ribbon 60 does not obstruct the connection between the plates 50 and 55. In other words, in this alternative case, the plates are secured to each other directly.

[0054] In a second embodiment illustrated in [Fig. 13], the ends of the ribbon are limited between two successive nodal zones connecting the first plate 50 to the second plate 55. In other words, no ribbon extends through the nodal parts of the first plate 50 or the second plate 55. The ribbons are then fixed in each cell of the cellular material after deployment of the cellular material.

[0055] These two embodiments can be present in the same core according to the invention.

[0056] The forming is carried out by applying a tensile force (illustrated by the arrows F1 and F2 in [Fig.4]) on the plates fixed to each other (along the direction T), this tensile force tending to separate the plates from each other or from each other. This tensile force, taking into account the gluing of the plates by nodal parts and the deformability of the plates, makes it possible to shape the plates to constitute the cells of the cellular core. The separation of the plates causes the deployment of the free parts 64 of the ribbon(s) 60 or septums. This can be accentuated after shaping of the cells, by applying a tensile force on these free parts (along the direction T) so as to deploy the septums 45, 46 in the respective cells.

[0057] [Fig.4] schematically illustrates the forming step during the deployment of the free parts 64 forming a septum by applying tensile forces according to the arrows F1 and F2 on the first and second plates 50, 55.

[0058] [Fig. 5] represents a top view of a cell 36 of the expanded cellular core, i.e. after the forming step, as well as transverse sectional views of the assembly and in particular of the septum deployed in: - a plane AA passing through nodal parts of the first and second plates 50, 55. In this zone of attachment of the nodal parts of the plates 50, 55, all of the folds PI, P2 and P3 of the septum are glued with the nodal parts of the two plates 50, 55 so that they remain linked during expansion of the cell. - in a plane BB passing through an area in which the lateral walls of the cell flare out and in which the septum 45 shows the beginning of deployment, and - in a CC plane passing through a central area of ​​the cell containing the main axis of the cell 36, illustrating a maximum deployment of the septum 45.

[0059] The lengths of the W-shaped folds must be such that the total free length 64 is equal to or greater than the maximum distance between two plates obtained during expansion.

[0060] According to one embodiment, the septa of the same cell are independent of each other. In particular, for each pair of consecutive septa (in the height direction, i.e. along the vertical direction V), the two consecutive septa are independent of each other. For the purposes of the invention, two consecutive septa in the same cell are said to be independent if they are formed from two separate ribbons as illustrated in Figures 7 and 8 and unlike Figures 9 and 10.

[0061] With reference to the examples illustrated in Figures 7 and 8, each septum 45, 46 comprises a first fixing tab fixed to a non-nodal portion of the first plate and a second fixing tab fixed to a non-nodal portion of the second plate.

[0062] More precisely, the first septum 45 comprises a first fixing lug 62A fixed to a non-nodal part of the first plate 50 and a second fixing lug 62B fixed to a non-nodal part of the second plate 55. The second septum 46, consecutive to the first septum 45 in the cell considered, comprises a first fixing lug 62C fixed to a non-nodal part of the first plate 50 and a second fixing lug 62D fixed to a non-nodal part of the second plate 55.

[0063] The first fixing tabs 62A, 62C and the second fixing tabs 62B, 62D of the septa 45, 46 all extend towards one end of the associated cell, i.e. either towards the first skin wall 31 or towards the second skin wall 32.

[0064] According to the example illustrated in [Fig.7], each septum 45, 46 is offset along the main axis A, which is parallel to the vertical direction V according to the example illustrated, relative to its first fixing lugs 62A, 62C and its second fixing lugs 62B, 62D. In other words, the average plane P of the septa does not pass through the fixing tabs 62A, 62C, 62B, 62D of the septa 45, 46. As a reminder, the average plane P is defined in such a way that the folds PI, P2 and P3 are alternated on either side of this plane and located at equal distances from it.

[0065] Alternatively, and with reference to the example illustrated in [Fig.8], each septum 45, 46 extends transversely between its first fixing lug 62A, 62C and its second fixing lug 62B, 62D. This configuration is called a hat configuration. In other words, the average plane P of the septa passes through the fixing legs 62A, 62C, 62B, 62D of the septa 45, 46.

[0066] According to other embodiments, the septa of the same cell may be dependent on each other, at least taken two by two consecutively. For the purposes of the invention, two consecutive septa in the same cell are said to be dependent if they are formed from the same ribbon or the same additional plate as illustrated in FIGS. 9 and 10. In this case, the septa are coupled by the same fixing lug to one of the first or second plates 50, 55.

[0067] According to the example illustrated in [Fig.9], the cell 36 comprises an additional ribbon 70 arranged between the first and second plates 50, 55 and fixed to the first and second plates. The additional ribbon 70 is shaped to form at least two consecutive septa 45, 46.

[0068] The additional ribbon 70 successively comprises: - a first part 71 glued to the first plate 50 and forming a first fixing tab 62A of the first septum 45 to a non-nodal part of the first plate 50, - a first adjacent part 72 folded along three fold lines PI, P2 and P3 to form the free W-shaped part of the first septum 45, - a second part 73 glued to the second plate 55 and continuously forming a second fixing tab 62B of the first septum 45 and a first fixing tab 62D of the second septum 46 to a non-nodal part of the second plate 55, - a second adjacent part 74 folded along three fold lines P4, P5, P6 to form the free W-shaped part of the second septum 46, and - a third part 75 glued to the first plate 50 and forming a second fixing tab 62C of the second septum 46 to a non-nodal part of the first plate 50.

[0069] The septums 45 and 46 are thus arranged “head to tail” or “in opposition”, that is to say that the fold lines P2 and P5 are very far from each other while the fold lines P3 and P4 on the one hand and P1 and P6 on the other hand are very close to each other.

[0070] In a variant not illustrated, the additional strip 70 is of closed tubular section. In other words, this variant differs from the case of [Fig.9] in that the first glued part 71 and the third glued part 75 are formed in continuity of material and therefore glued together to form a single tab for fixing the first septum 45 and the second septum 46 to a non-nodal part of the first plate 50. Advantageously, this solution protects the lines PI, P3, P4 and P6 from possible smears of glue connecting the ribbon 70 to the plates 50 and 55, thus not hindering the deployment of the cells and the W to the expansion of the core following the forces F of the [Fig.4].

[0071] The example illustrated in [Fig. 10], called the “zigzag” configuration, differs from that of [Fig. 9] in that the additional strip 80 comprises a succession of parts glued alternately to the first plate 50 or to the second plate 55 and parts folded along three fold lines P1, P2, P3 intended to each form the free W-shaped part of a septum.

[0072] Indeed, in the example illustrated, the additional ribbon 80 makes it possible to form five consecutive septums.

[0073] The additional ribbon 80 successively comprises: - a first part 81 glued to the first plate 50 and forming a first fixing tab of a first septum 45A to a non-nodal part of the first plate 50, - a second part 82 adjacent and folded along three fold lines PI, P2 and P3 to form the free W-shaped part of the first septum 45A, - a third part 83 adjacent and glued to the second plate 55 and forming a common fixing lug of the first septum 45A and of a second septum 45B (consecutive of the first) to a non-nodal part of the second plate 55, - a fourth part 84 adjacent and folded along three fold lines to form the free W-shaped part of the second septum 45B, - a fifth part 85 adjacent and glued to the first plate 50 and forming a common fixing lug of the second septum 45B and of a third septum 45C (consecutive to the second) to a non-nodal part of the first plate 50, and so on until the formation and fixing of the fifth septum 45E.

[0074] [Fig. 11] illustrates different positions of through-holes 39A, 39B, 39C possible in the septum of [Fig.4] for the passage of sound waves. The orifice 39A is made in the zone of greatest deployment of the septum. The orifices 39B and 39C are made in the widening part of the cell.

[0075] The arrangement of the positions of the orifices in the successive septums of the same cell is preferentially organized to maximize the free path to be traveled by the sound wave. Thus it is preferable to combine a succession of orifices according to a position 39A by alternating the proximity plate at each septum, the first plate 50 then the second plate 55.

[0076] According to another embodiment, it is possible to combine orifices of a first position 39C with orifices of a second position 39A, or a single type of position while alternating from one septum to the other the nodal zone near the orifice.

[0077] Preferably, the orifices have a section of at least 2 mm2, more preferably at least 4 mm2 and a smaller width of at least 1 mm and preferably at least 2 mm, the width being the dimension of the orifice in the transverse direction. In other words, the width is the small diameter of an orifice elliptical, the width of a rectangular orifice, or the diameter of the circle inscribed in a triangular orifice.

[0078] Preferably, the orifices are arranged as close as possible to the fixing lugs, that is to say as close as possible to the plates 50, 55 delimiting the cell, so as to maximize the distance between two free edges of orifices of two successive septa, thus maximizing the length of the path to be covered by the sound source.

[0079] Some of the septa of the same cell or of the same acoustic attenuation panel may comprise a permeable material over the entire transverse surface of the cavity or over a part of the transverse surface of the cavity so that they constitute acoustic septa creating a 2DOF functionality according to the English acronym 2-Degrees Of Freedom or 3DOF according to the English acronym 3-Degrees Of Freedom.

[0080] [Fig. 14] illustrates an exemplary embodiment of a cell of a core according to the invention comprising three septa offset radially relative to the main axis A (here vertical) to form a baffle for the propagation of sound waves and thus create a 2DOF functionality. In the illustrated example: - the upper septum 46A is acoustically transparent at least partially and over its entire surface, - the middle 46B and bottom 46C septa are acoustically opaque and each have an orifice 39. The orifices 39 are offset from each other in a direction perpendicular to the vertical axis V (here transverse T) in order to form a baffle for the propagation of sound waves.

[0081] [Fig. 15] illustrates another example of embodiment of a cell of a core according to the invention comprising three septa offset radially with respect to the main axis A (here vertical) to form a baffle for the propagation of sound waves and thus create a 2DOF functionality. In the illustrated example: - the upper septum 46A is acoustically opaque except for a zone Z which is at least partially acoustically transparent, - the middle 46B and lower 46C septa are acoustically opaque and each have an orifice 39. The orifices 39 and the zone Z are offset from each other in a direction perpendicular to the vertical axis V (here transverse T) in order to form baffles for the propagation of sound waves. There are thus three acoustically opaque zones (one per septum) arranged in baffles to maximize the path of the acoustic waves.

[0082] [Fig. 16] illustrates another example of embodiment of a cell of a core according to the invention comprising three septa offset radially with respect to the main axis A (here vertical) to form a baffle for the propagation of sound waves and thus create a 3DOF functionality. In the illustrated example: - the upper septum 46A is acoustically transparent at least partially and over its entire surface, - the middle septa 46B and 46C are acoustically opaque and each have an orifice 39, - the septum of the lower 46D is acoustically opaque except for a zone Z which is at least partially acoustically transparent. The orifices 39 and the zone Z are offset from each other in a direction perpendicular to the vertical axis V (here transverse T) in order to form baffles for the propagation of sound waves. There are thus three acoustically opaque zones arranged in baffles to maximize the path of the acoustic waves.

Claims

Claims

1. Core (34) for an acoustic attenuation panel extending between a first and a second end face, said core comprising a plurality of tubular polygonal cells (36) having a main axis (A) of extension between the end faces, each cell comprising an internal cavity (39) delimited by side walls (40) extending between the first and second end faces and opening into each of the two end faces, the cells being expandable in a direction parallel to the end faces, each polygonal cell comprising septums (45, 46) extending transversely with respect to the main axis of the associated cell and being offset along the main axis (A), characterized in that each septum (45, 46) is folded in a W shape at least between one of the side walls (40) and another of the side walls and has a through orifice,the orifices of two consecutive septa being offset radially relative to the main axis to form a chicane.,

2. The core of claim 1, wherein each polygonal cell (36) is formed from a first plate (50) and a second plate (55) secured together by nodal portions, and the septa (45, 46) each extend through the nodal portions of the first and second plates.

3. A core according to claim 2, wherein two consecutive septa (45, 46) are independent of each other.

4. A core according to any preceding claim, wherein each septum (45, 46) includes a first attachment tab (62A, 62C) attached to a non-nodal portion of the first plate (50) and a second attachment tab (62B, 62D) attached to a non-nodal portion of the second plate (55), the first and second attachment tabs each extending toward an end of the associated cell.

5. A core according to claim 4, wherein each septum extends transversely between its first attachment leg and its second attachment leg.

6. A core according to claim 4, wherein each septum is offset along the main axis relative to its first and second attachment tabs.

7. A core according to claim 2, comprising an additional ribbon (70; 80) arranged between the first and second plates (50, 55), fixed to the first and second plates and shaped to form at least two consecutive septa.

8. Core according to the preceding claim 7, in which two consecutive septums are formed from an additional ribbon (70), said additional ribbon comprising: - a first part glued to the first plate and forming a first tab for fixing a first of the septums to a non-nodal part of the first plate, - a first adjacent part folded along three fold lines, - a second part glued to the second plate and continuously forming a second tab for fixing the first septum and a first tab for fixing the second of the septums to a non-nodal part of the second plate, - a second adjacent part folded along three fold lines, and - a third part glued to the first plate and forming a second tab for fixing the second septum to a non-nodal part of the first plate.

9. A core according to claim 7, wherein the additional ribbon (80) is shaped to continuously form consecutive septa, the additional ribbon comprising a succession of parts glued alternately to the first plate or to the second plate and parts folded along three fold lines.

10. A core according to any preceding claim, wherein at least one of the septa comprises at least one portion of acoustically permeable material.

11. Acoustic attenuation panel (30) for a turbomachine comprising a first skin wall (31), a second skin wall (32) and a core according to one of the preceding claims, the core being arranged between the first and second skin walls and integral with at least one of the skin walls (31, 32), the first skin wall (31) being acoustically permeable, and the second skin wall (32) being acoustically opaque.

12. Acoustic attenuation panel according to claim 11, in which the core is integral with the first skin wall (31) and the second skin wall (32).

13. Aircraft propulsion unit turbomachine, comprising at least one acoustic panel according to any one of the preceding claims 11 or 12.

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