Acoustic absorption structure comprising at least one rotationally-indexed acoustic element and aircraft propulsion assembly comprising said acoustic absorption structure

An acoustic absorption structure includes: at least one acoustic element which has at least one cavity delimited by at least one enclosure comprising at least one first drainage orifice passing through the enclosure, and a rotational indexing system making it possible to position the acoustic element so that at least one first drainage orifice is positioned in proximity to or at a lowest point of the cavity. An aircraft propulsion assembly including such an acoustic absorption structure is also described.

FIELD OF THE INVENTION

The present application relates to an acoustic absorption structure comprising at least one rotationally-indexed acoustic element and a propulsion assembly comprising such an acoustic absorption structure.

BACKGROUND OF THE INVENTION

According to an embodiment of the prior art, a propulsion assembly comprises a nacelle and a turbofan engine, positioned inside the nacelle. Some surfaces of the nacelle and of the turbofan engine comprise acoustic absorption structures for attenuating the sound nuisances. According to one embodiment, an acoustic absorption structure comprises a porous layer, a honeycomb cellular layer and a reflective layer.

An ultrahigh bypass ratio (UHBR) turbofan engine has a fan revolving at lower frequencies than some turbofan engines currently on the market so that the acoustic absorption structures have to be configured to attenuate low frequency soundwaves. To attenuate such soundwaves, the honeycomb cellular layer needs to have a significant height which is detrimental in terms of weight, of bulk and of production.

An acoustic absorption structure that makes it possible to absorb low frequency soundwaves is described in the document “aero-acoustic liner applications of the broadband special acoustic absorber concept, American Institute of Aeronautics and Astronautics, AIAA 2013-2176, 19th AIAA/CEAS Aeroacoustics Conference May 27-29, 2013, Berlin, Germany” It comprises a plurality of capsules sealed by a skin in contact with the medium in which the soundwaves are propagated so as to each delimit a cavity in which is positioned a hollow cone, remote from the capsule, which has a base emerging at the skin. Each cone comprises at least one acoustic orifice, making it possible to connect the interior of the cone with the space between the cone and the capsule, positioned and dimensioned as a function of the acoustic characteristics sought. In addition, the skin is porous at least in line with each cone.

A duly formed acoustic absorption structure, based on the same principle as a Helmholtz resonator and a quarter wave resonator, makes it possible to efficiently attenuate the low frequency sounds emitted by a turbofan engine of UHBR type.

Since the skin is porous in line with the cones and the latter are perforated, water or any other liquid can accumulate and stagnate, in operation, inside each cone and in the space delimited by each cone and its capsule. This accumulation of water or of liquid effects the correct operation of the acoustic absorption structure. Furthermore, in case of ice, the water transformed into ice can damage it.

BRIEF SUMMARY OF THE INVENTION

Aspects of the present invention may remedy all or some of the drawbacks of the prior art.

An embodiment of the invention is an acoustic absorption structure comprising:an at least partially porous surface layer which has an outer surface and an inner layer opposite the outer surface,a support layer linked to the surface layer,at least one acoustic element, positioned in the support layer, which has at least one cavity delimited by at least one enclosure.

According to an embodiment of the invention, the acoustic absorption structure comprises a drainage system which comprises at least one first drainage orifice passing through the enclosure and a rotational indexing system making it possible to position the acoustic element in the support layer so that at least one first drainage orifice is positioned in proximity to or at a lowest point of the cavity.

By virtue of the rotational indexing system, at least one of the drainage orifices of the drainage system is positioned at a low point of the cavity greatly limiting the stagnation of liquid in said cavity.

According to another feature, the drainage system comprises at most two first diametrically opposite drainage orifices.

According to one embodiment, the rotational indexing system comprises, at the acoustic element, an outer lateral wall which is not of revolution and, at the support layer, an indentation for said acoustic element which has forms complementary or identical to those of the outer lateral wall of the acoustic element.

According to another feature, the outer lateral wall has two planes of symmetry and the drainage system comprises two first drainage orifices positioned with respect to the planes of symmetry so that one of the two first drainage orifices is positioned at or in proximity to the low point of the cavity when the acoustic element is in its indentation.

According to another feature, a straight line passing through the two first drainage orifices is contained in one of the planes of symmetry.

According to another feature, the outer lateral wall is tubular and delimited by a first edge, oriented towards the surface layer and describing a circle, and by a second edge, opposite the first edge, describing an ellipse.

According to another feature, the outer lateral wall is tubular and delimited by a first edge, oriented towards the surface layer and describing a circle, and by a second edge opposite the first edge describing an oblong form.

According to another embodiment, the rotational indexing system comprises a polarizing means, integral to the acoustic element, protruding with respect to an outer lateral wall, and a recess, hollowed out in the support layer, the polarizing means and the recess having complementary or identical forms so that the acoustic element can be positioned in its indentation only in a single position. In addition, the drainage system comprises a single drainage orifice positioned at or in proximity to a low point of the cavity when the polarizing means cooperates with the recess.

According to another feature, the acoustic element comprises:a first enclosure, which has a first aperture pressed against the inner surface of the surface layer so that the first enclosure and the surface layer delimit a first cavity,a second enclosure, in which is positioned the first enclosure, at least partially spaced apart from the first enclosure, which has a second aperture pressed against the first enclosure and/or possibly the inner surface so that the second enclosure and the first enclosure and possibly the surface layer delimit a second cavity,at least one acoustic orifice passing through the first enclosure to connect the first and second cavities,the first drainage orifice or orifices passing through the second enclosure.

According to another feature, the acoustic absorption structure comprises at least one second drainage orifice passing through the first enclosure.

According to another feature, each drainage orifice is positioned in the same radial plane as a first drainage orifice passing through the second enclosure.

Another embodiment of the invention is an aircraft propulsion assembly comprising at least one acoustic absorption structure according to one of the preceding features.

DETAILED DESCRIPTION

FIG. 1shows an aircraft10which has a fuselage12, two wings14, arranged on either side of the fuselage12, and propulsion assemblies16fixed under the wings14. Each propulsion assembly16comprises a nacelle18and a turbofan engine20positioned inside the nacelle18.

According to an embodiment visible inFIG. 2, the turbofan engine20comprises, at the rear, a primary ejection duct22, through which gases burned in the turbofan engine20escape, which is delimited on the outside by a primary nozzle24and on the inside by an inner structure26extended by a nozzle cone28.

According to one configuration, the inner structure26comprises an acoustic absorption structure30, positioned at a skin32, which delimits the primary ejection duct22and which has an outer surface SE, in contact with the burnt gases, and an inner surface SI, opposite the outer surface SE.

Although described as applied to a primary ejection duct22, the invention is not limited to that application. Thus, the acoustic absorption structure30can be positioned at any skin32which has an outer skin SE in contact with a medium in which soundwaves are propagated, such as, for example, a lip and a duct of an air inlet of an aircraft nacelle, a fan casing of an aircraft nacelle or any other surface of the propulsion assembly16. Thus, whatever the configuration, the propulsion assembly16comprises at least one acoustic absorption structure30.

According to an embodiment visible inFIG. 3, the acoustic absorption structure30comprises a porous layer34of which one face forms the outer surface SE, at least one cellular layer36, a reflective layer38and a plurality of acoustic elements40, positioned in the cellular layer36, which each have at least one cavity blocked off by the porous layer34. According to one configuration, the cellular layer36is a honeycomb structure which has indentations62(visible inFIG. 12) for housing the acoustic elements40.

According to the configurations, the porous layer34can be porous over all of its surface or comprise porous zones only in line with the acoustic elements40.

Whatever the embodiment, the acoustic absorption structure30comprises:an at least partially porous surface layer34which has an outer surface SE in contact with a medium in which soundwaves are propagated and an inner layer SI, opposite the outer surface SE,a support layer36, linked to the surface layer34, which can be, in a non-exhaustive manner, a honeycomb cellular layer, a foam layer, or the like,at least one acoustic element40, positioned in the support layer36, which has at least one cavity connected with the medium in which the soundwaves are propagated.

According to an embodiment visible inFIG. 3, an acoustic element40comprises:a first indentation42, also called cone, which has a first aperture44delimited by an edge pressed against the inner surface SI of the surface layer34so that the first enclosure42and the surface layer34delimit a first cavity46connected, via the porous surface layer34, with the medium in which the soundwaves are propagated,a second enclosure48, also called capsule, in which is positioned the first enclosure42, at least partially spaced apart from the first enclosure42, which has a second aperture50delimited by an edge pressed against the first enclosure42and/or possibly the inner surface SI of the surface layer34so that the second enclosure48and the first enclosure42(and possibly the surface layer34) delimit a second cavity52,at least one acoustic orifice54passing through the first enclosure42to connect the first and second cavities46,52.

According to an arrangement visible inFIG. 3, the acoustic elements40are arranged in several rows and several columns. Other arrangements could be envisaged.

According to an embodiment visible inFIGS. 3 and 4, the first enclosure42is tapered and comprises an inner lateral wall56, delimited by a first edge56.1, oriented towards the surface layer34and forming the first aperture44, and by a second edge56.2, opposite the first edge56.1, which delimits the edge of the acoustic orifice54.

According to one configuration, the first and second edges56.1,56.2of the inner lateral wall56are approximately circular and the inner lateral wall56has an axis of revolution Ax.

Obviously, the invention is not limited to this geometry for the first enclosure42. Thus, it can be tapered or cylindrical. It can comprise, in addition to the inner lateral wall56, an inner bottom wall, to close the first cavity42, positioned at the second edge56.2of the inner lateral wall56. Depending on the configurations, the acoustic orifice or orifices54is (or are) positioned on the inner lateral wall56and/or onthe inner bottom wall.

According to an embodiment visible inFIGS. 3 to 6, the second enclosure48comprises an outer lateral wall58, delimited by a first edge58.1, oriented towards the surface layer34and forming the second aperture50, and by a second edge58.2opposite the first edge58.1, and an outer bottom wall60positioned at the second edge58.2of the outer lateral wall58so as to close the second cavity52.

According to one configuration, the first edge58.1of the outer lateral wall58is approximately circular.

Whatever the geometry of the acoustic element40, the support layer36comprises, for each acoustic element40, an indentation62which has a form that makes it possible to house the acoustic element40so that the first aperture44of the first enclosure42is pressed against the surface layer34. According to one configuration, the indentation62has a form complementary or identical to the outer lateral wall58of the second enclosure48of the acoustic element.

According to a feature of the invention, the acoustic element40comprises a drainage system to avoid the stagnation of a liquid inside at least one of the two cavities46,52.

The drainage system comprises at least one drainage orifice64passing through the second enclosure48, positioned at or in proximity to the second edge58.2of the outer lateral wall58. Depending on the embodiments, the drainage orifice64is positioned:on the outer bottom wall60and in proximity to the second edge58.2of the outer lateral wall58,on the outer lateral wall58, in proximity to the outer bottom wall60and the second edge58.2, orstraddling the outer lateral wall58and the outer bottom wall60, at the second edge58.2.

According to a first embodiment visible inFIGS. 6, 8 to 11, the second enclosure48comprises two diametrically opposite drainage orifices64,64′. In this case, the acoustic element40must be correctly oriented for one of the two drainage orifices64,64′ to be positioned at or in proximity to a low point of the second cavity52in order for the volume of liquid stagnating inside the second cavity52to be as small as possible. To this end, the acoustic absorption structure30comprises a rotational indexing system66configured to position the acoustic element40according to two given positions with respect to the support layer36.

According to the first embodiment, the rotational indexing system66comprises, at the acoustic element40, the outer lateral wall58of the second enclosure48which has two planes of symmetry P1and P2and, at the support layer36, an indentation62which has forms complementary or identical to those of the outer lateral wall58of the second enclosure48.

According to the first embodiment visible in particular inFIG. 8, the second edge58.2of the outer lateral wall58describes an ellipse and has two planes of symmetry P1and P2.

According to a second embodiment visible inFIG. 9, the second edge58.2of the outer lateral wall58describes an oblong form which has two planes of symmetry P1and P2and comprises two rectilinear and parallel sections68.1,68.2linked by two semi-circular sections68.3,68.4.

According to these two embodiments, the outer lateral wall58has a set tubular surface which bears at a first end on the first circular edge58.1and at a second end on the second edge58.2. The outer lateral wall58is not of revolution.

According to these two embodiments, the two drainage orifices,64,64′ are positioned with respect to the planes of symmetry of the outer lateral wall58of the second enclosure48so that one of the two drainage orifices64,64′ is positioned at or in proximity to the low point of the second cavity52when the acoustic element is in its indentation62.

According to one configuration, the straight line passing through the two drainage orifices64,64′ is contained in one of the planes of symmetry of the outer lateral wall58of the second enclosure48.

As illustrated inFIGS. 10 and 11, when the acoustic element40is in its indentation62, one of the orifices64,64′ is necessarily positioned at or in proximity to the lowest point of the second cavity52which makes it possible to prevent the accumulation of liquid in said second cavity52.

According to a third embodiment visible inFIG. 13, the second enclosure48comprises a single drainage orifice64. In this case, the acoustic element40must be correctly oriented for the single drainage orifice64to be positioned at or in proximity to a low point of the second cavity52in order for the volume of liquid stagnating inside the second cavity52to be as small as possible. To this end, the acoustic absorption structure30comprises a rotational indexing system66configured to position the acoustic element40according to a single given position with respect to the support layer36.

According to the third embodiment visible inFIG. 13, the rotational indexing system66comprises a first form, called polarizing means70, integral to the acoustic element40, protruding with respect to the outer lateral wall58, and a second form, called recess72, hollowed out in the support layer36, the polarizing means70and the recess72having complementary or identical forms so that the acoustic element40can be positioned in its indentation62only in a single position.

The polarizing means70and the single drainage orifice64are positioned so that, when the polarizing means70cooperates with its recess72, the single drainage orifice64is positioned at or in proximity to a low point of the second cavity52.

This third embodiment of the rotational indexing system can be used even if the outer lateral wall58of the second enclosure48has a form of revolution.

Whatever the embodiment, the second enclosure comprises a smaller number of drainage orifices64,64′, at most two. This limited number of drainage orifices64,64′ makes it possible to limit the impact of the drainage orifices on the acoustic efficiencies of the acoustic element40.

Generally, the liquids present in the first cavity46can flow through the porous surface layer34in line with the first cavity46and/or via the acoustic orifice54.

In certain circumstances, the acoustic element40comprises at least one drainage orifice74passing through the first enclosure42. The drainage orifice or orifices74is (or are) positioned at or in proximity to the first edge56.1and/or the second edge56.1of the inner lateral wall56of the first enclosure42. According to a first configuration visible inFIG. 4, a plurality of drainage orifices74are positioned on the periphery of the inner lateral wall56. According to another configuration visible inFIGS. 10 and 11, each drainage orifice74is positioned in the same radial plane (plane passing through the axis of revolution Ax) as a drainage orifice64,64′ passing through the second enclosure48. Thus, when the acoustic element40is in its indentation62, at least one of the drainage orifices74passing through the first enclosure42is positioned at or in proximity to the lowest point of the first cavity46.

Whatever the embodiment, the drainage system comprises at least one drainage orifice64,64′,74passing through the first and/or the second enclosure42,48and a rotational indexing system66that makes it possible to rotationally immobilize the acoustic element40in the enclosure62of the support layer36so that at least one drainage orifice64,64′,74is positioned in proximity to or at a lowest point of the first and/or second cavity46,52. The low point of a cavity extends from a point of the cavity to which a fluid flows by virtue of gravity.

The drainage system comprises, at one and the same enclosure, at most two drainage orifices64,64′,74so as not to excessively impact the operation of the acoustic element.

When the drainage system comprises at least one pair of first and second drainage orifices64,74each passing through a first and second enclosure42,48, the first and second drainage orifices64,74of one and the same pair are arranged in one and the same radial plane.

Whatever the embodiment, by virtue of the rotational indexing system, at least one of the drainage orifices64,74of the drainage system is positioned at a low point of the first and/or second cavity46,52greatly limiting the stagnation of liquid in said first and/or second cavity46,52.