EP2071561B1 - Absorbent structure for reducing the noise generated in particular by a rotor and fairing comprising such a structure - Google Patents

Description

The present invention relates to the general technical field of acoustic treatment to reduce noise pollution emitted by rotors, motors or others. Such an acoustic treatment often proves to be essential in the aeronautical field and in particular on helicopters.

The present invention relates more particularly to an acoustic treatment of an anti-torque ducted rotor vein also called “fenestron”.

In general, we find in the noise spectrum generated by the shrouded tail rotor and by the resulting air circulation, lines corresponding to pure tones whose frequency is related to the speed of rotation of the rotor, to the number of rotor blades, the geometric configuration of the rotor and a stator, the shape and structure of the fairing.

Any rotor rotating in a vein, fed by more or less turbulent air, will generate acoustic waves which can be organized or random.

Organized waves constitute what is commonly called rotational noise, which is characterized in the noise spectrum by discrete frequencies (lines) corresponding to the frequencies of rotation of the blades, of the transmission shaft, of their sub-harmonics and harmonics or at frequencies modulated by an angular phase shift of the blades or of the rotational speed.

Random waves are characterized in the noise spectrum by a high spectral density over a very wide band of frequencies. These random waves generate so-called “broadband” noise.

It is known to use absorbent structures to reduce the propagation of acoustic waves emitted by noisy devices of the rotor or motor type, comprising a rigid partition, a porous wall and separation means for placing the porous wall at a determined distance. of the rigid partition, by delimiting cavities between said porous wall and said rigid partition, the height of which is determined to obtain maximum absorption of a given frequency of the acoustic waves emitted.

So-called “quarter wave” materials are known which have cavities of a height corresponding to a quarter of the wavelength of the base frequency which should be attenuated as a priority. These materials, however, suffer from a number of drawbacks.

In fact, in a certain number of applications and in particular in applications relating to shrouded anticouple rotors of helicopters, the audible acoustic waves emitted are most often composed of random and organized waves, distributed in a wide band of frequencies, making the known materials insufficiently efficient to effectively attenuate, in any flight envelope, the acoustic waves thus composed. It is necessary for example to treat pure tones and their harmonics but also noise sources operating over a wide range of speed variation as is the case for aircraft and operating over a temperature range of - 40 ° VS at + 40 ° C. The sources of parasitic noise that must be treated are therefore numerous and very diverse.

The document US 6,114,652 describes, for example, a method for producing acoustic attenuation chambers using a honeycomb structure. The cells comprise at least two absorbent and porous layers in which perforations are formed by means of a laser. The material constituting the layers is based on polymers and is chosen for its energy absorption properties according to a given radiation frequency of the laser. The layers thus have perforations of different diameter, distributed differently, to optimize the sound absorption properties.

This document describes an absorbent structure for reducing the propagation of acoustic waves comprising a rigid partition, at least one porous wall and separation means for placing the porous wall at a determined distance from the rigid partition, by delimiting cavities of a height given between said porous wall and said rigid partition.

The document EP 1 111 584 also describes a method of making acoustic attenuation chambers for aircraft jet engine nacelles. Such chambers are thus produced using a honeycomb structure forming a separation between a wave reflective bottom and an acoustically resistive layer. This acoustically resistive layer is formed by a woven metal fabric on which are placed in superposition son impregnated with resin oriented in a predetermined direction.

However, such an embodiment of the acoustically resistive layer requires assembly and fabrication operations that are complex and costly to carry out.

In fact, such an acoustically resistive layer requires, on the one hand, a complex weaving operation carried out with metallic weft and warp threads which are therefore substantially rigid.

The weaving of these yarns thus generates a significant trapping specific to the fabrics and therefore consumes raw material. In addition, such a weaving of the threads confers flexibility on the layer thus produced which goes against the function allowing the mechanical retention of another layer.

On the other hand, the acoustically resistive layer also requires an operation consisting in orienting the impregnated threads relatively with respect to the threads constituting the fabric of metallic threads oriented in at least two distinct directions.

We also know, as described in the document GB 2 059 341 , absorbent structures comprising a rigid partition, separation means, a porous wall and complementary absorption means. However, such a porous wall comprises a first layer formed by a perforated sheet and a second layer of fiber felt.

Further in this document, the first layer of the porous wall is arranged towards the interior of the structure and the second layer of the porous wall is arranged the exterior.

The objects of the present invention are therefore aimed at providing a novel absorbing structure making it possible to attenuate pure tones as well as to exhibit a high efficiency of absorption of acoustic waves in a wide frequency band. The absorbent structure in accordance with the invention thus makes it possible to process groups of pure tones and / or so-called “broadband” noises. There is thus obtained a substantial and audible reduction of the parasitic noises generated.

Another object of the present invention aims to provide an absorbent structure providing an acoustic covering on the one hand and constituting a rigid structural element on the other hand. Thus, in the application relating to faired anti-torque rotors of helicopters, the absorbent structure constitutes the air flow duct of said anti-torque rotors.

Another object of the present invention aims to provide an absorbent structure which does not increase significantly. significant the weight and / or size of the elements on which or in which it is used as a replacement for metal elements in single sheet or single walls in composite materials.

The objects assigned to the present invention are achieved using an absorbent structure to reduce the propagation of acoustic waves emitted by noisy devices such as rotors or motors, comprising a rigid partition, at least one porous wall and means separation to dispose the porous wall at a determined distance from the rigid partition, by delimiting cavities of a height h1 between said porous wall and said rigid partition, said height h1 being determined to obtain maximum absorption of a base frequency F1 given of the acoustic waves emitted, said structure comprising complementary absorption means to obtain a maximum absorption of the acoustic waves emitted at at least one additional base frequency Fi, of the spectrum of the acoustic waves emitted, i being a whole number greater than or equal at 2, the porous wall comprises at least a first layer and at least a second layer of fiber felt, characterized in that said first layer is formed of fine mesh mesh and in that said first layer is positioned on said second layer of fiber felt to constitute an assembly of two layers, said second layer of fiber felt being positioned on the means of seperation.

The combination of these two layers makes it possible to obtain, on the one hand, optimum porosity and, on the other hand, sufficient mechanical retention of the felt, thanks to the mesh.

The complementary absorption means, in combination with the porous wall and the cavities therefore make it possible to obtain a maximum absorption coefficient of 100% for at least one base frequency F1 and Fi and an absorption coefficient of approximately 80% around these base frequencies F1 and Fi, and this over a wide frequency band ranging for example from 0.77Fi to 1.3.Fi.

The absorbent structure according to the invention also has the advantage of having, in addition to a maximum attenuation for each base frequency F1 or Fi, a maximum attenuation for multiples of the base frequencies corresponding to (2n + 1) .Fi, where n is an integer greater than or equal to 1.

As an example, a noise attenuation of 100% can be obtained for the center frequencies F1 of 1000 Hz and F2 = 2.F1 of 2000 Hz as well as a noise attenuation of 80% in frequency ranges from preferably and respectively from a value of two thirds of each of the base frequencies to a value of four thirds of each of said base frequencies. The total attenuation of a line at 1000 Hz is therefore accompanied by an attenuation of about 80% of the other lines of the noise spectrum, representative of noise at frequencies between 667 Hz and 1333 Hz and preferably between 700 Hz and 1300 Hz and those between 1400 Hz and 2600 Hz.

According to an exemplary embodiment in accordance with the invention, the additional absorption means comprise a complementary porous wall, arranged in the cavities, at an intermediate height h2. The heights h1 and h2 consequently correspond respectively to the attenuation of the respective frequencies F1 and F2. The cavities of height h1 and h2 are thus arranged in parallel, in this way reducing the overall thickness of the absorbent structure with respect to an arrangement in series of two successive cavities of height h1 and h2.

According to another exemplary embodiment according to the invention, the additional absorption means are materialized by an inclination of the rigid partition with respect to to the porous wall so as to continuously modify, in at least one direction, the height h1 from one cavity to another. Such a design makes it possible to promote the processing of noise over a wide band of frequencies. It is therefore advantageous, according to another exemplary embodiment in accordance with the invention, to combine these complementary absorption means with complementary absorption means favoring the processing of noise at one or more base frequencies Fi.

According to another exemplary embodiment in accordance with the invention, the complementary absorption means comprise, alternately with the cavities of height h1, additional cavities of height h3, said height h3 being less than height h1. These additional cavities of height h3 are for example produced with a deposit of an absorbent material on the rigid partition in certain cavities of height h1, for example in every other cavity.

Without departing from the scope of the present invention, it is possible in certain cases to combine different embodiments described above in order to improve the performance of the absorbent structure.

According to an exemplary embodiment of the absorbent structure according to the invention, the cavities are delimited with rising partitions, extending substantially orthogonally from the rigid partition to a porous wall.

According to one embodiment of the absorbent structure according to the invention, the mesh and / or the felt are preferably made of metallic or composite materials.

According to an exemplary embodiment of the absorbent structure according to the invention, the first layer and the second layer are assembled by gluing or welding. These operations, as well as the assembly of a porous wall and of the rigid partition delimiting the cavities, can easily be automated during the manufacture of the absorbent structure.

According to an exemplary embodiment of the absorbent structure according to the invention, the rigid partition is preferably made of glass fibers. It is the same, preferably, for the rising partitions. This gives the rigidity, strength and lightness required in particular in the field of helicopters.

The objects assigned to the present invention are also achieved with the aid of an anti-torque rotor vein for helicopters constituted at least in part of an absorbent structure as presented.

The objects assigned to the present invention are also achieved with the aid of a faired anti-torque rotor for helicopters comprising a fairing constituted at least in part of an absorbent structure as presented.

The objects assigned to the present invention are also achieved by means of a fairing for parts of helicopters, said fairing comprising an absorbent structure as shown.

• la figure 1 illustre un exemple de réalisation d'une structure absorbante connue et conforme à l'art antérieur;
• la figure 2 illustre un exemple de réalisation d'une structure absorbante conforme à l'invention;
• la figure 3 illustre un autre exemple de réalisation d'une structure absorbante conforme à la invention;
• la figure 4 illustre un autre exemple de réalisation d'une structure absorbante conforme à la mention;
• la figure 5 illustre selon une vue schématique et en coupe transversale, un rotor caréné d'hélicoptère agencé dans une veine comportant une structure absorbante conforme à la mention;
• la figure 6 illustre une vue de dessous de la vue schématique de la figure 5 ;
• la figure 7 illustre une section transversale d'un rotor caréné d'hélicoptère comportant une veine pourvue d'une structure absorbante conforme à l'invention ainsi qu'un moyeu de rotor comportant également une structure absorbante conforme à la mention ;
• La figure 8 est un diagramme représentant le coefficient d'absorption du bruit en fonction de la fréquence, correspondant à une structure absorbante conçue pour traiter les fréquences F1 et F2=2.F1.

Other features and advantages of the invention will emerge in more detail on reading the description which follows, as well as with the aid of the appended drawings given purely by way of illustration and not by way of limitation, among which:

• the figure 1 illustrates an exemplary embodiment of a known absorbent structure in accordance with the prior art;
• the figure 2 illustrates an exemplary embodiment of an absorbent structure according to the invention;
• the figure 3 illustrates another exemplary embodiment of an absorbent structure according to the invention;
• the figure 4 illustrates another embodiment of an absorbent structure according to the mention;
• the figure 5 illustrates, in a schematic view and in cross section, a ducted rotor of a helicopter arranged in a duct comprising an absorbent structure in accordance with the mention;
• the figure 6 illustrates a bottom view of the schematic view of the figure 5 ;
• the figure 7 illustrates a cross section of a faired rotor of a helicopter comprising a duct provided with an absorbent structure according to the invention as well as a rotor hub also comprising an absorbent structure according to the mention;
• The figure 8 is a diagram representing the noise absorption coefficient as a function of frequency, corresponding to an absorbing structure designed to handle the frequencies F1 and F2 = 2.F1.

The absorbent structure according to the invention, part of which is illustrated on figure 1 , comprises a rigid partition 1, for example of glass fibers, as well as rising partitions 2 extending substantially orthogonally from the rigid partition 1 in order to define cavities 3. The rising partitions 2, for example of glass fibers, extend to a porous wall 4 and constitute means of separation between the rigid partition 1 and the porous wall 4.

où c est une constante, F étant la fréquence à absorber, est connue en tant que telle.The cavities 3 have a height h1 whose value, with a good approximation, is proportional to the inverse of the base frequency F that should be absorbed, and this at a given temperature T. This relation: $$\mathrm{h}=\mathrm{vs}.{\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="imgf0001.png"\; state="\{\{state\}\}">T</figure-callout>}}^{1/2}.1/\mathrm{F}$$

where c is a constant, F being the frequency to be absorbed, is known as such.

The value h corresponds approximately to a quarter or to a multiple of a quarter of the wavelength of the frequency F which should be absorbed.

The porous wall 4 comprises a first layer 4a of fine or very fine mesh metal mesh and a second layer 4b of felt of metal fibers. The mesh and the felt can also be made of composite materials. The layers 4a and 4b are for example assembled by gluing or by welding.

The figure 2 illustrates an exemplary embodiment of the absorbent structure according to the invention. The latter comprises a second porous wall 5 arranged between the rigid partition 1 and the porous wall 4. Each of the cavities 3 is thus divided into two by means of the second porous wall 5.

The porous wall 5 is moved away from the rigid partition 1 by extending to a height h2 less than h1. The height h2 is determined by the same relation as that determining h1 and specified above.

The porous wall 5 is preferably identical or similar to the porous wall 4 and comprises a first layer 5a in wire mesh with fine mesh and a second 5b in felt of metallic fibers.

This absorbing structure makes it possible to absorb two base frequencies F1 and F2, corresponding to two distinct lines of the noise spectrum which should be attenuated.

The figure 3 illustrates another exemplary embodiment of the absorbent structure according to the invention. In this embodiment in accordance with the invention, the complementary absorption means comprise additional cavities 7 having a height h3, alternating with cavities of height h1. The height h3 is also determined by the relation specified above.

The additional cavities 7 are obtained by depositing an absorbent material 7a on the rigid partition 1, in certain cavities 3. By way of example, every other cavity 3 can thus be transformed into an additional cavity 7 having a height h3. . As a variant, it is also possible to envisage transforming one in three or out of four cavities into additional cavity 7, for example.

The cavities 3 and the additional cavities 7 thus make it possible to absorb respectively acoustic waves of frequencies F1 and F3 distinct from the spectrum of the emitted noise.

The figure 4 shows another embodiment of the absorbent structure according to the invention, in which the additional absorption means are obtained by an inclination of the rigid partition 1 relative to the porous wall 4. This results in rising partitions 2 having a different height h1 _(n) when passing from one riser 2 to the next.

Particular cavities 8 are thus obtained having a rising partition 2 of height h1 _(n) and a neighboring rising partition 2 of height h1 _{(n + 1)} . The variation in height from one rigid partition to the next is of course determined by the inclination of the rigid partition 1. Such an absorbing structure consequently attenuates a certain number of lines in the spectrum of the noise emitted, and more preferably a wide band. frequencies corresponding to so-called “broadband” noises.

The figure 5 schematizes in section an embodiment of a ducted anti-torque rotor of a helicopter. The anti-torque rotor comprises a hub 10 driving the blades 11.

Retaining plates 12 are provided for, on the one hand, maintaining the hub 10 in position in an air circulation duct 13 and, on the other hand, ensuring a straightening of the air expelled by said rotor. This recovery is obtained by a particular orientation of the retaining plates 12, for example a radial orientation 12a for one 12a and a quasi-radial orientation for the other 12b of the retaining plates 12, shown for example on figure 6 .

The air sucked in by the anti-torque rotor is shown by arrows A. The air sucked in enters the air circulation stream 13 through an inlet 13a of the stream 13, and is expelled via an outlet 13b of the stream 13.

The inlet 13a and the outlet 13b of the duct 13 are delimited by a fairing 15 of the rotor. This fairing 15 is produced by means of absorbent structure elements according to the invention or by elements coated with an absorbent structure according to the invention.

The air circulation duct 13 also comprises a constriction 16 positioned around the path of the ends of the blades 11.

The retaining plates 12a, 12b are for example provided on each of their faces with an absorbent structure according to the invention. Preferably, all the parts of the fairing 15 delimiting the air circulation duct 13 comprises a coating of an absorbent structure in accordance with the invention.

As a variant, these parts can also be produced directly with elements of absorbent structure. The latter thus constitute rigid structural elements of the anti-torque rotor.

The figure 7 illustrates a cross-sectional view of a ducted anti-torque rotor of a helicopter, in which the hub 10 transmits to the blades 11 a rotational movement via a transmission shaft 17. The hub 10 comprises a housing 10a and a cover element 10b coated or made of an absorbent structure according to the invention.

The air circulation stream 13 is delimited in particular by air inlet lips 18 and by a diffusion cone 19 coated with or formed with an absorbent structure in accordance with the invention. The whole of the air circulation vein 13 is preferably treated, namely coated or made up, with the absorbent structure according to the invention.

The anti-torque rotor as shown in figure 7 , can also operate in reverse mode, in which the air circulation through the duct 13 takes place in the opposite direction shown by the arrows R. The air circulation duct 13 retains its noise attenuation properties also in reverse mode.

The figure 8 represents for an exemplary embodiment of an absorbent structure according to the invention, the absorption coefficient CA as a function of the frequency F. In this particular case the base frequencies F1 and F2 = 2.F1, as well as the frequencies 3.F1, 5.F1 and 3.F2 are attenuated to 100%. Other harmonics, also attenuated to 100%, are not shown for reasons of clarity. A wide frequency band of about +/- 30% of the above frequencies is also attenuated to at least 80%. At least 80% noise attenuation is thus obtained for frequencies between 2.1.F2 and 3.9.F2.

Claims (12)

1. Absorbent structure for reducing the propagation of sound waves transmitted by noisy devices of the rotor or motor type, comprising a rigid partition (1), at least one porous wall (4) and separation means (2) for arranging the porous wall (4) at a predetermined distance from the rigid partition (1), delimiting cavities (3) having a height h1 between the porous wall (4) and the rigid partition (1), the height h1 being determined in order to obtain maximum absorption of the sound waves transmitted at a given basic frequency F1, the structure comprising complementary absorption means in order to obtain maximum absorption of the sound waves transmitted at least at one additional basic frequency Fi, i being a whole number greater than or equal to 2,
   the porous wall (4, 5) comprising at least a first layer (4a, 5a) and at least a second layer (4b, 5b) of fibre felt,
   characterised in that the first layer (4a, 5a) is formed as a grid with fine mesh and in that the first layer (4a, 5a) is positioned on the second layer (4b, 5b) of fibre felt in order to constitute an assembly of two layers,
   the second layer (4b, 5b) of fibre felt being positioned on the separation means (2).
2. Absorbent structure according to claim 1,
   characterised in that the grid and/or the felt are produced from metal or composite materials.
3. Absorbent structure according to claim 1 or 2,
   characterised in that the first layer (4a, 5a) and the second layer (4b, 5b) are assembled by means of adhesive bonding or by means of welding.
4. Absorbent structure according to any one of claims 1 to 3,
   characterised in that the complementary absorption means comprise at least one complementary porous wall (5) which is arranged in the cavities (3) at an intermediate height h2 in order to obtain maximum absorption for a basic frequency F2.
5. Absorbent structure according to either claim 1 or 4,
   characterised in that the complementary absorption means are produced by an inclination of the rigid partition (1) relative to the porous wall (4) in order to modify in at least one direction the height h1 from one specific cavity (8) to the following one.
6. Absorbent structure according to any one of claims 1 to 5,
   characterised in that the complementary absorption means comprise, in a manner alternating with cavities (3) having a height h1, additional cavities (7) having a height h3, the height h3 being less than the height h1.
7. Absorbent structure according to claim 6,
   characterised in that the additional cavities (7) are produced with a deposit of an absorbent material (7a) on the rigid partition (1) in specific cavities (3) having a height h1.
8. Absorbent structure according to any one of claims 1 to 7,
   characterised in that the cavities (3, 7) are delimited with rising partitions (2) which extend substantially orthogonally from the rigid partition (1) as far as a porous wall (4, 5).
9. Absorbent structure according to any one of claims 1 to 8,
   characterised in that the rigid partition (1) is at least partially made of glass fibres.
10. Anti-torque rotor duct (13) for a helicopter,
    characterised in that it is constituted, at least partially, by an absorbent structure according to any one of claims 1 to 9.
11. Ducted anti-torque rotor for a helicopter,
    characterised in that it comprises a fairing (15) which is constituted at least partially by an absorbent structure according to any one of claims 1 to 9.
12. Fairing (15) for helicopter components,
    characterised in that it comprises an absorbent structure according to any one of claims 1 to 9.

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