Patent Publication Number: US-2023154447-A1

Title: Porous skin comprising a plurality of strips and an acoustic attenuation panel comprising such a porous skin

Description:
RELATED APPLICATION 
     This application incorporates by reference and claims priority to French patent application 2112149, filed Nov. 17, 2021. 
     TECHNICAL FIELD 
     The present invention concerns a porous skin for an acoustic attenuation panel and an acoustic attenuation panel comprising such a porous skin. 
     BACKGROUND 
     In many fields loud disturbing sounds are generated, for example by noisy equipment or machines, and may be undesirable. In particular, it is known that on an aircraft, for example a transport aircraft, loud noises are often generated, in particular by the propulsion systems (or jet engines) of the aircraft. For reasons of comfort and also because of standards, these noises constitute a nuisance that it is necessary to reduce. 
     To this end it is known to use, for example at the level of the nacelles of the jet engines, panels offering good acoustic absorption in order to reduce the disturbing sounds. In particular, there is known from the documents EP 3552951A1 (corresponding to US 2019/309682A1) an acoustic attenuation panel for an aircraft. 
     An acoustic attenuation panel usually and generally comprises an acoustic structure provided with a plate having good acoustic absorption properties, in particular for the frequencies of the most disturbing noises. This acoustic structure used for acoustic absorption may be a cellular material, but also a material based on foam or a fibrous material. An acoustic attenuation panel of this kind is generally also provided on one of its two faces with a porous (that is to say perforated) skin termed a “resistive” skin and on the other face with an unperforated, so-called “reflective” rear skin. The resistive skin allows the sound waves (or acoustic waves) to penetrate into the acoustic structure, where they are attenuated. 
     However, as the resistive skin is perforated, it can also allow a flow, for example a flow of a fluid such as air, to penetrate into the acoustic structure. A flow of this kind that enters the acoustic structure is liable to disturb the acoustic attenuation, in particular by creating undesirable phenomena that reduce the effective portion of the openings in the resistive skin. This solution is open to improvement. 
     SUMMARY 
     An inventive porous skin for an acoustic attenuation panel is disclosed herein, wherein the porous skin being intended to be subjected to a fluid flow. 
     In accordance with an embodiment invention, the porous skin includes a plurality of strips that are arranged transversely to the fluid flow and which, successively, overlap each other laterally, each strip upstream of the fluid flow overlapping the strip or strips downstream of said fluid flow, each of said strips includes a so-called upstream first lateral edge and a so-called downstream second lateral edge, at least one of said lateral edges having a contour forming a line with deviations, and the arrangement of the strips and the contour or contours of their lateral edges are configured to form between the strips openings connecting an external face of the porous skin with an internal face of said porous skin and to prevent the fluid flow from penetrating into said openings. 
     In the context of the present invention a “line with deviations” refers to a non-straight line, as specified hereinafter. 
     Accordingly, thanks to the invention, a porous skin is obtained that is configured so that sound waves can pass through it while being adapted, if it is subjected to a fluid flow, to prevent that fluid flow from passing through it. Also, when it is used as a resistive skin on an acoustic attenuation panel it is able, by preventing any fluid flow from passing through it, to prevent the appearance of the aforementioned undesirable phenomena and thus to remedy the disadvantage mentioned hereinabove. 
     Moreover, the overlapping of the strips advantageously creates contact surfaces between two successive strips and said strips are fixed together at the level of at least some of said contact surfaces. 
     In an embodiment, at least some of said openings are created by shapes adapted to match on the one hand the contour of the downstream lateral edge of a first strip and on the other hand the contour of the upstream lateral edge of a second strip directly downstream of said first strip, each of the openings created in this way being covered by a third strip that is directly upstream of said first strip and that overlies this first strip in the downstream direction. 
     In a first embodiment at least part of the contour of at least one of the lateral edges comprises a curved line. 
     Moreover, alternatively or additionally, in a second embodiment at least part of the contour of at least one of the lateral edges comprises a broken line. 
     Moreover, in aa embodiment said strips are of identical shape to one another. In another embodiment at least some of said strips are different shapes to one another. 
     Moreover, strips advantageously have on at least one of their lateral edges on one of their faces a discontinuity forming at least one plane surface. 
     The present invention also concerns an acoustic attenuation panel comprising at least one acoustic structure provided with a porous skin as described hereinabove. 
     Moreover, in an embodiment the acoustic structure comprises at least one cellular structure provided on one of its faces with a so-called reflective rear skin and on the other face with said porous skin. 
    
    
     
       SUMMARY OF FIGURES 
       The appended figures will clearly explain how the invention may be reduced to practice. In these figures identical references designate similar elements. 
         FIG.  1    is a perspective view of an embodiment of a porous skin including strips having lateral edges having a contour of sinusoidal shape. 
         FIG.  2    is a perspective view of one particular embodiment of a strip of the porous skin from  FIG.  1   . 
         FIG.  3    is a perspective view of one particular embodiment of three strips conforming to the  FIG.  2    embodiment that overlap and form openings between them. 
         FIG.  4    is a view in section taken along a section line C-C in  FIG.  3   . 
         FIG.  5    is a perspective view of one particular embodiment of a porous skin including strips having lateral edges having a contour formed of trapezoidal crenellations. 
         FIG.  6    is a perspective view of one particular embodiment of a strip of the porous skin from  FIG.  5   . 
         FIG.  7    is a perspective view of a part of an acoustic attenuation panel including a porous skin conforming to the  FIG.  1    embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The porous skin  1  in accordance with the invention and one embodiment of which is represented schematically in  FIG.  1    is a skin intended to be disposed in an acoustic attenuation panel ( FIG.  7   ). 
     By “acoustic attenuation panel” is meant an element, such as a plate, intended to be fixed to a surface in order to reduce the surrounding noises, for example by trapping and/or damping sound waves (or acoustic waves). In the present description it is considered that an acoustic attenuation panel may correspond to any structure having acoustic absorption properties. There is also considered as “exterior” or “external” to such a panel the environment from which the acoustic waves to be attenuated come and as “interior” or “internal” the space contained within said panel in which said acoustic waves are trapped and/or damped. 
     An external face of the porous skin  1  is liable to be subjected to a fluid flow E, for example a flow of air, as illustrated by an arrow in  FIGS.  1  to  7   . As described hereinafter the porous skin  1  is configured to be permeable to sound waves while preventing the fluid flow E from passing through it. 
     To this end, as represented in  FIG.  1   , the porous skin  1  includes a plurality of strips  2  that successively overlap laterally. In the context of the present invention by two strips “overlapping” is meant the fact that one of said strips is overlaid on the other strip in part with surface contact. Moreover, “lateral” overlapping of two strips denotes the arrangement in the direction of the width of said strips. That arrangement will be described in more detail in the remainder of the description. 
     The strips  2  may be made of aluminum and have for example a thickness of the order of one hundredth of a millimeter. However, they may equally be made of other materials, and in particular of a metal other than aluminum, a plastic material or a composite material. 
     As represented in  FIGS.  1  to  3    in particular, each of the strips  2  of the porous skin  1  has an elongate shape extending in a so-called longitudinal direction X that is substantially orthogonal to a so-called lateral direction Y. 
     In the examples from  FIGS.  1  to  7    the strips  2  and the porous skin  1  are arranged so that the direction Y (the lateral direction of the strips) corresponds to the direction of the fluid flow E. In other embodiments the strips  2  and the porous skin  1  may be arranged so that the lateral direction Y of the strips and the direction of the fluid flow E have between them a non-zero angle, although always an angle less than 90 degrees. 
     Moreover, as represented in  FIG.  1    the porous skin  1  has a so-called external face  3  intended to be subjected to the fluid flow E and a so-called internal face  4  opposite the face  3 . The direction X and the direction Y define a so-called horizontal plane P that is orthogonal to a so-called vertical direction Z. In particular, the direction Z is oriented upward. The face  3  is oriented in the sense of the direction Z and the face  4  is oriented in the sense opposite the direction Z. 
     In a similar way, as represented in  FIG.  2    each strip  2  has an external face  5  oriented in the sense of the direction Z and an internal face  6  oriented in the sense opposite the direction Z. The faces  5  and  6  may be parallel to one another. However, in particular embodiments they may be non-parallel. 
     Moreover, as illustrated in  FIG.  4    the strips  2  are inclined so that their internal and external faces are at an angle α relative to the horizontal plane P (and therefore substantially so relative to the fluid flow E). 
     The strips  2  also have two lateral edges, namely the edges oriented in the direction Y. As represented in  FIG.  2    each strip  2  in particular has a so-called “upstream” lateral edge  7  (situated upstream in the direction Y, and therefore upstream in the direction of the fluid flow E) and a so-called “downstream” lateral edge  8  (situated downstream in the direction Y, and therefore downstream in the direction of the fluid flow E). The strips  2  overlapping each other laterally, the lateral edge  7  of each of them is never in direct contact with the fluid flow E while the lateral edge  8  is at least partially in contact with said fluid flow E. The lateral edge  7  has a contour  9  and the lateral edge  8  has a contour  10 . Each of the contours  9  and  10  is a line with deviations. 
     In the context of the present invention a “line with deviations” denotes a non-straight line, that is to say a line that may include a broken line (or polygonal line), namely a succession of straight-line segments with different orientations, one or more curved lines, a serpentine shape, sinusoidal, repeating triangular shapes and/or a zig-zag shape. A contour forming a line with deviations is therefore a contour that is not completely straight (or not completely rectilinear). 
     Moreover, the strips  2  are arranged transversely to the fluid flow E so that each strip upstream of said flow overlaps (that is to say partly covers) the downstream strip. In particular, the arrangement of the strips  2  and the shape of the contours  9  and  10  of the lateral edges  7  and  8  are configured to form openings  11  between two successive strips  2 . Moreover, the strips  2  being inclined relative to the horizontal plane P, said openings  11  formed between two strips  2  are covered at least in part by a third strip  2  situated upstream of the first two. The openings  11  are therefore accessible only from downstream of that third strip. 
     Moreover, the openings  11  created in this way are through-openings, and this allows sound waves to pass through said openings  11  in order to pass through the porous skin  1  following a path represented schematically by an arrow S in  FIG.  4   . 
     Nevertheless, the openings  11  make it possible to prevent the fluid flow E from penetrating into the openings  11  and therefore passing through the porous skin  1 . In fact, the inclination of the strips  2  and the covering of the openings  11  by said strips  2  as described hereinabove have the effect of directing the fluid flow E along the external face  3  of the porous skin  1  over the strips  2 . The fluid flow E being a movement of material oriented substantially in a given direction, it runs along the external faces  5  of the strips  2  with no possibility of accessing the openings  11 . On the other hand, the sound waves that correspond to mechanical vibrations, that is to say compression and expansion of the surrounding medium, are therefore able to propagate (progressively) in all directions and in particular through the openings  11  accessible from downstream. 
     Moreover, the dimensions (length and section) of the openings  11  can be modified by modifying at least one of the following parameters: the shape of the contours of the lateral edges  7  and  8 , the arrangement of the strips  2  relative to one another and the thickness of the strips  2 . 
     Moreover, the strips  2  overlapping creates contact surfaces  19  ( FIG.  4   ) between two successive strips. The contact surfaces  19  between two successive strips correspond to the interfaces (with contact) formed by longitudinal portions of the internal face of the upstream strip overlapping longitudinal portions of the external face of the downstream strip. 
     Moreover, the strips  2  may be fixed together at the level of at least some of these contact surfaces  19 . In particular, they are fixed by gluing or by welding or any other means of known type. 
     In the embodiment represented in  FIGS.  1  and  5   , all the strips  2  of the porous skin  1  are identical. An embodiment of this kind, using a single type of strip, enables simplification of the production of the porous skin  1  and reduction of its production cost. 
     However, in a variant of this embodiment at least some of the strips  2  are different from one another and for example have different lengths, different widths and/or different lateral edges. In one particular embodiment of this variant the porous skin  1  includes a succession of at least two strips  2  different from one another, said succession being repeated laterally (in the direction Y) in order to form the plurality of strips  2  of the porous skin  1 . 
     In the embodiment represented in  FIGS.  1  to  3    in particular each of the two contours  9  and  10  of the lateral edges  7  and  8  is a line with deviations. As represented in  FIG.  2   , said contours  9  and  10  may be identical, that is to say have the same shape, and one of said contours  9  and  10  may be obtained by movement of the other contour in translation in the direction Y. 
     In a variant (not represented) of the preceding embodiment the shapes of the contours  9  and  10  are offset longitudinally (in the direction X) relative to one another so that one of the contours can be obtained by movement of the other contour in translation in a direction other than the direction Y. 
     In another variant (not represented) of the preceding embodiment the contours of the lateral edges  7  and  8  have different shapes. 
     Moreover, in a first particular embodiment represented in  FIGS.  1  to  4    the contours  9  and  10  of the lateral edges  7  and  8  have a curved shape (that is to say a line with curves). In the example represented the curved shape is a sinusoidal shape. 
     Moreover, in a second particular embodiment represented in  FIGS.  5  and  6    the lateral edges  7  and  8  respectively include a contour  27  and a contour  28 . Both contours  27  and  28  have a shape comprising a broken line (or polygonal line). In the example represented the shape of the broken line forming the contours  26  and  27  corresponds to a series of trapezoidal crenellations. In variants of this embodiment square, rectangular or triangular crenellations may be provided, for example. 
     In a third embodiment (not represented) the contours of the lateral edges comprise at least one curved line and at least one broken line. 
     By modifying the size and the shape of the strips  2  and in particular by modifying the shapes of the contours of their lateral edges  7  and  8  it is possible to vary the acoustic properties and/or the aerodynamic properties of the porous skin  1  and to adapt them to suit the envisaged application. 
     Moreover, in one particular embodiment the strips  2  have a discontinuity  12  ( FIG.  2   ), specified hereinafter, on their external face  5  at the level of their downstream lateral edge  8 . The discontinuity  12  forms a plane surface  13  at the free edge of the lateral edge  8  ( FIGS.  2  and  6   ). The plane surface  13  is slightly inclined relative to the external face  5 . The plane surface  13  may in particular be at an angle to the face  5  that is of the order of a few degrees. The discontinuity  12  extends over all the length of the strip  2  parallel to the direction X. As the contour  10 ,  28  of the lateral edge  8  is not straight and the discontinuity  12  is situated at the free edge of the lateral edge  8 , said discontinuity  12  forms a plurality of plane surfaces  13 . Each plane surface  13  is situated at a summit or a trough of the contour  10 ,  28 , as illustrated in  FIGS.  2  and  6   . The plane surfaces  13  of the strip  2  are formed in such a manner as to be coplanar. 
     The plane surfaces  13  may be formed in such a manner as to lie in the same plane parallel to the horizontal plane P. This plane may in particular be the plane defined by the external face  3  of the porous skin  1 . Thus, the plane surfaces  13  make it possible to minimize irregularities present on the surface of the external face  3  of the porous skin  1  and consequently to prevent the aerodynamic properties of the porous skin  1  from being degraded, in particular by limiting drag. 
     In this particular embodiment the discontinuity  12  is a chamfer. Nevertheless, in a variant of this embodiment the discontinuity  12  may equally be a bevel. 
     Moreover, in one particular embodiment represented in  FIG.  4    the strips  2  also include on their internal face  6  at the level of the free edge of their upstream lateral edge  7  a discontinuity  14  forming plane surfaces  15 . The plane surfaces  15  are slightly inclined relative to the internal face  6 . The plane surfaces  15  may in particular be at an angle to the face  6  that is of the order of a few degrees. The plane surfaces  13  and  15  may be parallel to one another. Consequently, the plane surfaces  15  lie in substantially the same plane parallel to the horizontal plane P. That plane in particular may be the plane defined by the internal face  4  of the porous skin  1 . Thus, the plane surfaces  15  make it possible to obtain a substantially plane face  4  that can serve as a contact surface for arranging the porous skin  1  on a wall or a face of an acoustic attenuation panel for example. 
     In one particular embodiment the porous skin  1  also includes a structural support  16  configured to support the plurality of strips  2 . 
     In one particular embodiment represented in  FIGS.  1  and  5    the support  16  comprises a first plate  17  arranged upstream of the porous skin  1 . The plate  17  overlaps the strip or strips  2  situated most upstream of the porous skin  1 . The support  16  also comprises a second plate  18  arranged downstream of the plurality of strips  2  and overlaps the strip or strips  2  situated most downstream of the porous skin  1 . The strips  2  connected to the plates  17  and  18  may be fixed to the latter, for example by gluing, by welding or otherwise. 
     In a variant of this particular embodiment the support  17  may be any structure able to support at least some or all of the strips  2 , such as a frame or a grid structure for example. 
     By way of example, and to explain the configuration of the porous skin  1  better, one particular embodiment of a part of the porous skin  1  comprising three successive strips is illustrated in  FIGS.  3  and  4   . A fourth strip is represented in thinner line in  FIG.  4    in order clearly to specify the arrangement of the three successive strips described hereinafter in a porous skin  1  comprising more than three strips. 
     In this particular embodiment the porous skin  1  includes a first so-called upstream strip  2 A, a second so-called intermediate strip  2 B and a third so-called downstream strip  2 C. In the remainder of the description the letter A, B or C has been added to the elements respectively forming part of the strips  2 A,  2 B or  2 C to make clearer the arrangement of the various elements. 
     In this particular embodiment the strips  2 A,  2 B and  2 C have the same shape. They are arranged parallel to one another and substantially perpendicular to the fluid flow E. Moreover, as represented in  FIG.  4   , which is a view in section of the porous skin  1  in  FIG.  3    on a section plane C-C, the strips  2 A,  2 B and  2 C are inclined relative to the horizontal plane P. 
     To be more precise, the external faces  5 A,  5 B and  5 C are at an angle α relative to the plane P. Thus, the external faces  5 A,  5 B and  5 C are arranged in such a manner as to be partly in contact with the fluid flow E. The angle α may be of the order of a few degrees, e.g., 2 to 5 degrees, to obtain an internal face  3  of the porous skin  1  that is aerodynamic. 
     Moreover, the strip  2 A is arranged in such a manner to overlap the strip  2 B. In particular, part of the internal face  6 A of the strip  2 A is in (surface) contact with part of the external face  5 B of the strip  2 B. 
     In a similar way, the strip  2 B is arranged in such a manner as to overlap the strip  2 C. In particular, part of the internal face  6 B of the strip  2 B is in (surface) contact with part of the external face  5 C of the strip  2 C. 
     Similarly, the strip  2 C is arranged in such a manner as to overlap another adjacent strip situated downstream (represented in thinner line in  FIG.  4   ) 
     The arrangement of the strips  2 A,  2 B and  2 C as described hereinabove and the shape of the contours of their lateral edges enable the openings  11  to be formed between said strips  2 A,  2 B and  2 C. In particular, the contours of the lateral edges of the strips  2 B and  2 C enable a through-passage to be formed between the external face  3  and the internal face  4  of the porous skin  1 . Moreover, as illustrated in  FIG.  4    on the section plane C-C, the downstream lateral edge of the strip  2 A enables this through-passage to be covered by overlying the strip  2 B. The through-passage covered in this way by the strip  2 A corresponds to the opening  11  and is no longer accessible from downstream of the porous skin  1  (that is to say it is not possible for the fluid flow E moving from upstream to downstream to penetrate into the opening  11 ). However, the strip  2 A is not in contact with the strip  2 C. The opening  11  therefore remains accessible from downstream, that is to say it is possible to penetrate into said opening  11  and to pass through the porous skin  1  by moving from downstream to upstream, as illustrated for sound waves by the arrow S. 
     In an application the porous skin  1  as described hereinabove may be arranged in an acoustic attenuation panel  20  as represented in  FIG.  7   . 
     The acoustic attenuation panel  20  includes an acoustic structure  21  having the porous skin  1  on one of its faces. In the context of the present invention “acoustic structure” denotes any element having acoustic absorption properties, namely able to trap, damp and/or cancel out sound waves. In particular, this may be a structure based on foam or on fibrous material. 
     In the embodiment represented in  FIG.  7    the acoustic structure  21  of the acoustic attenuation panel  20  includes a cellular structure  22  having an upper face  23  and a lower face  24  opposite the upper face  23 . The cellular structure  22  comprises cells  25  between the upper face  23  and the lower face  24 . 
     In an implementation of this embodiment the cellular structure  22  is a honeycomb structure. However, in particular embodiments it may be other cellular structures having varied shapes and arrangements of cells. 
     The acoustic structure  21  also includes a so-called reflective rear skin  26  arranged on the lower face  24  of the cellular structure  22 . The acoustic attenuation panel  20  may be arranged on a wall, for example by fixing the rear skin  26  to said wall. 
     The acoustic structure  21  also includes the porous skin  1  that is arranged on the upper face  23  of the cellular structure  22 . The internal face  4  of the porous skin  1  is fixed to the upper face  23  of the cellular structure  22 , for example by gluing or by welding. The porous skin  1  can in particular be fixed to the acoustic attenuation panel  20  by means of the support  16  ( FIGS.  1  and  5   ). Moreover, the porous skin  1  is arranged so that the openings  11  face the cells  25 . Thus, the space inside the cells  25  is accessible from the outside of the acoustic attenuation panel  20 . Sound waves are therefore able to penetrate into the cells  25 , as represented by the arrow S in  FIG.  7   , while the fluid flow E is not able to pass through the porous skin  1 . 
     It is possible to adapt the position of the openings  11  of the porous skin  1  to varied architectures of the cellular structure  22  by modifying the size and the shape of the strips  2  and in particular by modifying the shapes of the contours of the lateral edges  7  and  8 . Thus it is possible in particular to optimize the compatibility of the porous skin  1  with the cellular structure  22 . 
     An acoustic attenuation panel  20  of this kind is particularly suitable for attenuating noises generated by an aircraft, for example a transport aircraft, and in particular by a propulsion system (or jet engine) of the aircraft. 
     The acoustic attenuation panel  20  is intended to be fixed to a wall, for example an air intake wall of a nacelle of a propulsion system. To this end the acoustic attenuation panel  20  may correspond to a plane plate, as in the example illustrated in  FIG.  7   . However, in particular embodiments the acoustic attenuation panel  20  may have varied shapes in order to espouse the shape of the wall on which it is intended to be arranged. 
     The porous skin  1  as described hereinabove that is applied to an acoustic attenuation panel  20  has numerous advantages. In particular: 
     it enables an acoustic attenuation panel  20  to be obtained the acoustic attenuation properties of which are very little impacted by a fluid flow E at the surface of said acoustic attenuation panel  20 , the porous skin  1  in fact allowing the sound waves to penetrate to the interior of the acoustic attenuation panel  20  while preventing the fluid flow E from passing through it; 
     it enables limitation of irregularities on the external surface of the acoustic attenuation panel  20  and therefore good aerodynamic properties to be obtained by limiting drag, thanks to the particular arrangement of the strips  2  and to the plane surfaces  13  forming the external face  3  of the porous skin  1 ; 
     it enables modification of the acoustic and/or aerodynamic properties of the external surface of the acoustic attenuation panel  20  by adapting the size and the shape of the strips  2 ; and 
     it enables an external surface of the acoustic attenuation panel  20  to be obtained on which the positions of the openings  11  can be adapted to suit varied architectures of the acoustic structure  21  and in particular of the cellular structure  22 , in particular by adapting the size and the shape of the strips  2 . 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both, unless the application states otherwise. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.