Broadband sound absorption structure suitable for undercarriage cabin door noise reduction and design method

Abstract

The invention provides a broadband sound absorption structure suitable for undercarriage cabin door noise reduction and a design method, and belongs to the field of aircraft noise reduction. The sound absorption structure comprises a plurality of perforated plate type Helmholtz cavity unit cells and a plurality of embedded pipe type Helmholtz cavity unit cells. In the perforated plate type unit cells, one or more first through holes are formed in the perforated plate. And in the embedded pipe type unit cells, the embedded pipes penetrate through the perforated holes in the perforated plates and extend into the cavities. Cavities of the two single cells face the same direction, and the perforated plate forms a curved surface conformal with an undercarriage cabin door. The effective length of the neck of the perforated plate is increased by using the embedded pipe, the low-frequency sound absorption performance is improved, the sound absorption performance and the sound absorption bandwidth of the middle-frequency band are improved by using the perforated plate type Helmholtz cavity, meanwhile, the curved surface conformal design is highly matched with the special-shaped cabin door structure, and the sound absorption performance is improved. The low-frequency and medium-frequency sound absorption performance can be guaranteed, and meanwhile the light and thin requirements of an aircraft can be met; and a pure passive mechanical structure is adopted, so that the control complexity and stability risks of an active noise reduction system can be avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of aircraft noise reduction, and particularly relates to a broadband sound absorbing structure suitable for noise reduction of a landing gear door, a landing gear door using the broadband sound absorbing structure, and a design method for the landing gear door. Background Art

[0002] With the rapid development of the aviation industry, aircraft noise control has become a critical issue. During takeoff and landing, the complex cavity formed by the landing gear and door structure generates significant broadband aerodynamic noise, typically ranging from 200 to 5000 Hz. This noise not only exacerbates metal structural fatigue but also transmits through the fuselage, resulting in a low-frequency rumble in the cabin, seriously affecting passenger comfort.

[0003] Currently, there are two main technical solutions for landing gear door noise reduction:

[0004] Composite sound-absorbing material solution: Most landing gear doors utilize a "honeycomb sandwich + porous fiber" composite layer. This structure exhibits excellent sound absorption performance for mid- and high-frequency noise (>1500Hz) (sound absorption coefficient exceeding 0.8), but its efficiency drops sharply for low-frequency noise (<800Hz), typically below 0.3. Improving mid- and low-frequency performance would require significantly increasing the material thickness, which fundamentally conflicts with the design requirements for lightweight and thin aircraft doors.

[0005] Helmholtz resonator solutions: While cavity resonance can achieve efficient sound absorption at specific frequencies, its inherent narrowband nature results in a narrow absorption bandwidth and relies on a strict match between the cavity volume and the neck dimensions. To achieve low-frequency targets (e.g., 150 Hz), the required cavity depth often exceeds 25 cm. Existing solutions for extending the frequency band require juxtaposing multiple resonator arrays of varying sizes, significantly increasing the overall structural thickness and failing to meet the space constraints of the hatch structure.

[0006] Active noise reduction technologies, such as those using acoustic speakers and feedback control systems installed inside cabin doors, have emerged in recent years. While they can theoretically cover a wide range of noise, they suffer from three inherent drawbacks: they rely heavily on sensor layout accuracy and real-time control algorithms; they are susceptible to airflow disturbances, resulting in instability; and their systems are complex and expensive to maintain. This makes them difficult to implement in high-reliability aircraft scenarios. Summary of the Invention

[0007] The present invention aims to address the problems in existing landing gear door noise reduction, such as the difficulty of using composite sound-absorbing materials to achieve both low-frequency performance and a lightweight design, the inability of Helmholtz resonators to overcome the mutually exclusive limitations of narrow-band sound absorption and space occupancy, and the insufficient reliability and excessive cost of active control solutions. The present invention provides a broadband sound-absorbing structure suitable for landing gear door noise reduction, a landing gear door using the broadband sound-absorbing structure, and a design method for the landing gear door.

[0008] To achieve the above objectives, the technical solutions provided by the present invention are:

[0009] On the one hand, a broadband sound absorption structure suitable for noise reduction of a landing gear door is provided, comprising a plurality of perforated plate-type Helmholtz cavity cells and a plurality of embedded tube-type Helmholtz cavity cells;

[0010] Each perforated plate type Helmholtz cavity unit cell includes a first perforated plate and a first cavity, wherein the first perforated plate covers the open end of the first cavity, and one or more first perforations are formed on the first perforated plate;

[0011] Each embedded tube-type Helmholtz cavity unit cell includes a second perforated plate, an embedded tube, and a second cavity. The second perforated plate covers the open end of the second cavity. A second perforated hole is formed on the second perforated plate. The embedded tube is coaxial with the second perforated hole. One end of the embedded tube extends into the second cavity, and the other end is connected to a surface of the second perforated plate facing the second cavity.

[0012] The perforated plate-type Helmholtz cavity unit cell and the embedded tube-type Helmholtz cavity unit cell are arranged so that: the first cavity and the second cavity are oriented in the same direction, the first perforated plate and the second perforated plate form a curved surface conformal to the landing gear door, the axis of the first perforated hole on each first perforated plate and the axis of the second perforated hole on each second perforated plate are parallel to each other, and the surfaces of the first cavity and the second cavity opposite to their respective perforated plates are located in the same horizontal plane.

[0013] Furthermore, the number of first holes provided on the first perforated plate of each perforated plate type Helmholtz cavity unit cell is different and ranges from 1 to 5.

[0014] Furthermore, the aperture of the first perforation is 0.5-2 mm, and the aperture of the first perforation on each first perforated plate is the same. The aperture of the second perforation is the same as that of the embedded tube, which is 1-3 mm. The wall thickness of the embedded tube is 0.5-1 mm.

[0015] Furthermore, the thickness of the first perforated plate and the second perforated plate is 1 to 3 mm, and the porosity is 0.1 to 2.5%.

[0016] Furthermore, the length of the embedded tube is 1 to 23 mm.

[0017] Furthermore, the cross-sections of the perforated plate-type Helmholtz cavity unit cell and the embedded tube-type Helmholtz cavity unit cell are both square, and the cross-sectional dimensions of each unit cell are the same.

[0018] Furthermore, the first perforated plate and the first cavity, the second perforated plate and the second cavity are obtained by 3D printing and bonded with epoxy resin.

[0019] On the other hand, a landing gear door using the above-mentioned broadband sound absorption structure is provided, comprising a plurality of Helmholtz cavity cell units composed of a plurality of perforated plate-type Helmholtz cavity cells and a plurality of embedded tube-type Helmholtz cavity cells. In the Helmholtz cavity cell unit, the plurality of perforated plate-type Helmholtz cavity cells and the plurality of embedded tube-type Helmholtz cavity cells are adjacently arranged in a square or rectangle.

[0020] Another aspect provides a method for designing the landing gear door, comprising the following steps:

[0021] Step 1: Calculate the sound absorption coefficient of the Helmholtz cavity unit cell:

[0022]

[0023] Where Z is the surface acoustic impedance of the Helmholtz cavity unit cell, Z0 is the characteristic impedance of air, Z0 = ρ0c0, ρ0 and c0 are the density and sound speed of air respectively;

[0024] Each Helmholtz cavity unit cell includes p perforated plate-type Helmholtz cavity cells and q embedded tube-type Helmholtz cavity cells, and the surface acoustic impedance is:

[0025]

[0026] Where Z p represents the surface acoustic impedance of the pth perforated plate Helmholtz cavity unit cell, m represents the number of perforated plate Helmholtz cavity unit cells, Z q represents the surface acoustic impedance of the qth embedded tubular Helmholtz cavity unit cell, and n represents the number of embedded tubular Helmholtz cavity unit cells:

[0027]

[0028] Where σ p is the porosity of the first perforated plate in the pth perforated plate Helmholtz cavity unit cell, is the intermediate variable, d p is the diameter of the first hole in the pth perforated plate Helmholtz cavity unit cell, t p is the thickness of the first perforated plate in the pth perforated plate Helmholtz cavity unit cell, h pis the inner cavity depth of the first cavity in the pth perforated plate Helmholtz cavity unit cell, η is the air dynamic viscosity, ω is the angular frequency, k0 is the wave number of the sound wave, j is the imaginary unit; γ is the specific heat ratio of air, A is the total cross-sectional area of the embedded tube Helmholtz cavity unit cell, S q is the inner cross-sectional area of the embedded tube in the qth embedded tube-type Helmholtz cavity unit cell, l q is the length of the embedded tube in the qth embedded tube-type Helmholtz cavity unit cell, V q is the volume of the second cavity excluding the neck of the qth embedded tubular Helmholtz cavity unit cell, ρ c,q and c c are the complex density and complex sound speed of air, ρ c,q =ρ0 / Ψ v,q , δ q is the terminal correction coefficient of the sound wave, k c,q ,Ψ v,q ,Ψ h,q are functions of the complex wave number of air and the viscous and thermal fields respectively;

[0029] Step 2: Determine the noise reduction frequency band of the landing gear door and the number of cells in each Helmholtz cavity unit: Determine the noise reduction frequency band f1~f2 according to the actual noise spectrum, f1 is the starting frequency, f2 is the ending frequency, and then determine the number of cells to be M×N, where M represents the number of cell rows and N represents the number of cell columns. The frequency interval is divided into multiple single-frequency noises;

[0030] Step 3: Determine the thickness of the sound absorbing structure based on the structure of the landing gear door;

[0031] Step 4, designing and determining the Helmholtz cavity unit cell, includes the following sub-steps:

[0032] Step 4.1, select the unit cell type according to the determined thickness of the sound absorbing structure and the size of the divided single frequency;

[0033] Step 4.2: By adjusting the diameter of the first perforation, the number of the first perforations, the thickness of the first perforated plate, the depth of the first cavity, the diameter of the second perforation and the embedded tube, the length of the embedded tube, and the depth of the second cavity 23, a Helmholtz cavity unit cell including a perforated plate-type Helmholtz cavity cell and an embedded tube-type Helmholtz cavity cell of multiple single frequencies is designed;

[0034] Step 5: According to the structure of the landing gear door, the obtained Helmholtz cavity unit cells are combined to form the landing gear door.

[0035] The advantages of the present invention are:

[0036] 1. The broadband sound absorption structure suitable for landing gear door noise reduction of the present invention improves low-frequency sound absorption performance by increasing the effective length of the perforated plate neck using an embedded tube. Furthermore, it enhances mid-frequency sound absorption performance and absorption bandwidth by using a perforated plate-type Helmholtz cavity. Broadband sound absorption can be achieved by combining different sound absorption units. The overall thickness of the sound absorption structure is compressed to within the spatial constraints of the door. The curved surface conformal design allows for a high degree of compatibility with special-shaped door structures. Therefore, the structure is suitable for landing gear door noise reduction requiring both broadband sound absorption performance and a lightweight structure.

[0037] 2. The present invention uses multiple Helmholtz cavity unit cells to form a landing gear door, wherein each Helmholtz cavity unit cell is a square or rectangular structure composed of multiple perforated plate-type Helmholtz cavity units and multiple embedded tube-type Helmholtz cavity units. The resulting landing gear door can meet the aircraft's requirements for lightweight and thinness while ensuring low-frequency and mid-frequency sound absorption performance.

[0038] 3. The present invention calculates the sound absorption coefficient of a Helmholtz cavity unit cell and designs the structural dimensions and quantity of the two units in each unit cell based on the structure and noise reduction frequency band of the landing gear door. This can produce a landing gear door that meets the low-frequency and mid-frequency sound absorption performance while also meeting the requirements for lightweight aircraft.

[0039] 4. The landing gear door noise reduction system of the present invention adopts a purely passive mechanical structure, which can avoid the control complexity and stability risks of the active noise reduction system, significantly reduce maintenance costs and improve environmental adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The above and / or other features and advantages of the present invention will become more readily understood through the following description with reference to the accompanying drawings, which are not drawn to scale and in which some features are exaggerated or minimized to show details of particular components.

[0041] Figure 1 This is a schematic diagram of the broadband sound absorbing structure for noise reduction of a landing gear door according to the present invention applied to the landing gear door, wherein the enlarged view is a Helmholtz cavity unit cell;

[0042] Figure 2 Schematic diagram of the structure of the perforated plate type Helmholtz cavity unit cell in the present invention;

[0043] Figure 3 Schematic diagram of the structure of the embedded tubular Helmholtz cavity unit cell in the present invention;

[0044] Figure 4 Schematic diagram of the structure of the rigid curved perforated plate in the present invention;

[0045] Figure 5 Schematic diagram of a finite element simulation model of the broadband sound absorbing structure of the present invention;

[0046] Figure 6 This is a finite element simulation sound absorption coefficient diagram of the broadband sound absorption structure of the present invention;

[0047] Figure 7 3. It is a schematic diagram of a finite element simulation model of the noise reduction effect of a landing gear door equipped with the broadband sound absorbing structure of the present invention;

[0048] Figure 8 This is a comparison chart of the sound pressure levels of the landing gear door before and after the broadband sound absorbing structure of the present invention is installed.

[0049] In the figure: 1-perforated plate type Helmholtz cavity unit cell, 11-first perforated plate, 12-first cavity, 13-first perforation; 2-embedded tube type Helmholtz cavity unit cell, 21-second perforated plate, 22-embedded tube, 23-second cavity, 24-second perforation. DETAILED DESCRIPTION

[0050] The present invention will be described in detail below with reference to the accompanying drawings by means of exemplary embodiments of the present invention. It should be noted that the following detailed description of the present invention is only for the purpose of illustration and is not intended to limit the present invention.

[0051] It should be pointed out that, in the context of the present invention, the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "inside", "outside", "clockwise" and "counterclockwise" etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operate in a specific direction, and therefore should not be understood as limiting the present invention.

[0052] Furthermore, terms such as "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or to implicitly indicate the quantity of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of the features.

[0053] The present invention provides a broadband sound-absorbing structure suitable for landing gear door noise reduction, a landing gear door using the broadband sound-absorbing structure, and a design method for the landing gear door. The structure improves low- and mid-frequency sound absorption performance by extending the acoustic path through an embedded tube (low-frequency noise reduction characteristics) and by coupling resonance with multiple Helmholtz cavities (mid- and high-frequency noise reduction characteristics). The structure also achieves balanced broadband sound absorption in a thin structure by integrating the mid-frequency bandwidth expansion of perforated plates with the synergistic effect of multiple unit cells. Furthermore, a conformal curved surface design is implemented for aircraft landing gear doors, ensuring acoustic performance while meeting the stringent requirements for lightweight and thinness of aircraft.

[0054] First, the overall reference Figure 1 The broadband sound absorption structure for landing gear door noise reduction provided by the present invention comprises multiple perforated plate Helmholtz cavity cells 1 and multiple embedded tube Helmholtz cavity cells 2. The perforated plate Helmholtz cavity 1 improves mid-frequency sound absorption performance and bandwidth, while the embedded tubes of the embedded tube Helmholtz cavity cells 2 increase the effective length of the perforated plate neck, improving low-frequency sound absorption. The perforated plate Helmholtz cavity cells 1 and the embedded tube Helmholtz cavity cells 2 are arranged adjacent to each other in multiple rows and columns.

[0055] Combine Figure 2 Each perforated plate-type Helmholtz cavity unit cell 1 includes a first perforated plate 11 and a first cavity 12. The first cavity 12 is composed of a bottom plate and a side plate surrounding the bottom plate. The first perforated plate 11 covers the open end of the first cavity 12. The first perforated plate 11 is provided with one or more first perforations 13. According to the present invention, the number of first perforations 13 provided on the first perforated plate 11 of each perforated plate-type Helmholtz cavity unit cell 1 is different and ranges from 1 to 5. Specifically, as shown in the figure, taking the unit cell cross-section as a square as an example, some perforated plate type Helmholtz cavity unit cells 1 are provided with one first perforation 13, which can be set at the center of the first perforated plate 11, some are provided with two first perforations 13, which can be arranged at equal intervals on the central axis of the first perforated plate 11, some are provided with three, which can be arranged in the center to form an equilateral triangle, some are provided with four first perforations 13, which are arranged in a manner to form a square, and some are provided with five, four of which can form a square, and the side length of the square is parallel to the side length of the first perforated plate, and the other perforation can be located at the center of the square. It should be pointed out that this arrangement structure is only an example and is not intended to limit the present invention. Those skilled in the art can choose other different arrangements according to their needs. For example, in the case of five first perforations, the side length of the square formed by four of the perforations may not be parallel to the side length of the first perforated plate.

[0056] In some embodiments of the present invention, all perforated plate-type Helmholtz cavity cells 1 have the same shape, being rectangular parallelepipeds with square cross-sections. That is, the cross-sections of the first cavities 12 are all square. Specifically, the thickness of the first perforated plate 11 is 1 to 3 mm, and the porosity is 0.1 to 2.5%. Furthermore, when the first perforated plate 11 has two or more first perforations 13, the apertures of each first perforation 13 are identical, and the sizes of the first perforations 13 between different first perforated plates 11 can be the same or different. The aperture of the first perforations 13 can be 0.5 to 2 mm.

[0057] In addition, the total thickness of the perforated plate type Helmholtz cavity unit cell 1 can be 20mm-30mm, and the total thickness of the structure needs to be less than the thickness of the landing gear door. The depth of the first cavity 12 can be 16mm-28mm, and the cavity wall thickness is 0.5-2mm.

[0058] The first perforated plate 11 and the first cavity 12 can be obtained by 3D printing respectively, wherein the first cavity 12 is integrally formed, and the first perforated plate 11 and the first cavity 12 can be bonded together by using epoxy resin.

[0059] Combine Figure 3 Each embedded tube-type Helmholtz cavity unit cell 2 includes a second perforated plate 21, an embedded tube 22, and a second cavity 23. The second cavity 23 is composed of a bottom plate and a side plate surrounding the bottom plate. The second perforated plate 21 covers the open end of the second cavity 23. Each second perforated plate 21 is provided with a second perforation 24, preferably located at the center of the second perforated plate 21. The embedded tube 22 is coaxial with the second perforation 24, with one end of the embedded tube 22 extending into the second cavity 23 and the other end connected to the surface of the second perforated plate 21 facing the second cavity 23. With this arrangement, the embedded tube 22 is in communication with the outside world.

[0060] In a specific embodiment, all embedded tube-type Helmholtz cavity unit cells 2 have the same shape, are rectangular parallelepipeds, and have square cross-sections. That is, the cross-sections of the second cavity 23 are all square. In addition, the cross-sectional dimensions of the perforated plate-type Helmholtz cavity unit cell 1 and the embedded tube-type Helmholtz cavity unit cell 2 can also be the same.

[0061] The diameter of the second perforation 24 is the same as that of the embedded tube 22, which is 1 to 3 mm. The wall thickness of the embedded tube 22 can be 0.5 to 1 mm. Like the first perforated plate 11, the second perforated plate 21 also has a thickness of 1 to 3 mm and a porosity of 0.1 to 2.5%. Preferably, the first perforated plate 11 and the second perforated plate 21 have the same thickness but different porosities. In addition, the diameter of the second perforation 24 and the inner diameter of the embedded tube 22 in all embedded tube-type Helmholtz cavity cells 2 are the same.

[0062] In some embodiments of the present invention, the total thickness of the embedded tube-type Helmholtz cavity cell 2 can be 20mm-30mm, and the total thickness of the structure needs to be less than the thickness of the landing gear door. The depth of the second cavity 23 can be 16mm-28mm, and the cavity wall thickness can be 0.5-2mm. These parameters can be the same as those of the perforated plate-type Helmholtz cavity cell 1; the lengths of the embedded tubes 22 in different embedded tube-type Helmholtz cavity cells 2 can be the same or different, specifically 1-28mm, which needs to be less than the inner cavity depth and greater than the thickness of the second perforated plate.

[0063] In addition, the second perforated plate 21, the embedded tube 22 and the second cavity 23 can be obtained by 3D printing respectively, wherein the second cavity 23 is integrally formed, and the second perforated plate 21, the embedded tube 22 and the second cavity 23 can be bonded together using epoxy resin.

[0064] In the present invention, the perforated plate type Helmholtz cavity unit cell 1 and the embedded tube type Helmholtz cavity unit cell 2 are arranged so that: the first cavity 12 and the second cavity 23 are oriented in the same direction, and the first perforated plate 11 and the second perforated plate 21 form a curved surface conforming to the landing gear door, as shown in FIG. Figure 4 As shown; the axes of the first perforations 13 on each first perforated plate 11 and the axes of the second perforations 24 on each second perforated plate 21 are parallel to each other, and the surfaces of the first cavity 12 and the second cavity 23 opposite to their respective perforated plates are located in the same horizontal plane.

[0065] With this structure, the present invention increases the effective length of the perforated plate neck by using an embedded tube, thereby improving low-frequency sound absorption performance. It also uses a perforated plate-type Helmholtz cavity to improve mid-frequency sound absorption performance and sound absorption bandwidth. A broadband sound absorption effect can be achieved by combining different sound-absorbing units. At the same time, the overall thickness of the sound-absorbing structure is compressed to within the space constraint limit of the door. The curved surface conformal design allows for a high degree of compatibility with special-shaped door structures. Therefore, the present invention is suitable for noise reduction of landing gear doors that require both broadband sound absorption performance and a lightweight structure.

[0066] Return to reference Figure 1 As shown in the enlarged view, the landing gear door provided by the present invention using the above-mentioned broadband sound absorbing structure may include multiple Helmholtz cavity cell units, which are composed of multiple perforated plate type Helmholtz cavity cells 1 and multiple embedded tube type Helmholtz cavity cells 2. The multiple perforated plate type Helmholtz cavity cells 1 and the multiple embedded tube type Helmholtz cavity cells 2 can be adjacently arranged in a square or rectangle.

[0067] In an optional embodiment, the total number of the two types of cells in each unit cell is 4-16, and the number of rows and columns is the same. Preferably, each unit cell is composed of 8 square-cross-section perforated plate-type Helmholtz cavity cells 1 and 8 square-cross-section embedded tube-type Helmholtz cavity cells 2, a total of 16 cells arranged in a square structure of four rows and four columns. However, this is not intended to limit the present invention. Those skilled in the art will understand that two types of cells with other structures can also be used to form unit cells of other structures. For example, two rectangular cells can be used to form a square or rectangular unit cell structure, as long as the formed unit cells can be arranged to form the structure of a landing gear door.

[0068] Therefore, the present invention adopts multiple Helmholtz cavity unit cells to form the landing gear door, wherein each Helmholtz cavity unit cell is a square or rectangular structure composed of multiple perforated plate type Helmholtz cavity unit cells and multiple embedded tube type Helmholtz cavity unit cells. The obtained landing gear door can meet the aircraft's requirements for lightness and thinness while ensuring low-frequency and mid-frequency sound absorption performance.

[0069] The design method for the landing gear door provided by the present invention may include the following steps:

[0070] Step S1, calculate the sound absorption coefficient of the Helmholtz cavity unit cell:

[0071]

[0072] Where Z is the surface acoustic impedance of the Helmholtz cavity unit cell, Z0 is the characteristic impedance of air, Z0 = ρ0c0, ρ0 and c0 are the density and sound speed of air respectively;

[0073] Each Helmholtz cavity unit cell includes p perforated plate-type Helmholtz cavity cells and q embedded tube-type Helmholtz cavity cells, and the surface acoustic impedance is:

[0074]

[0075] Where Z p represents the surface acoustic impedance of the pth perforated plate Helmholtz cavity unit cell, m represents the number of perforated plate Helmholtz cavity unit cells, Z q represents the surface acoustic impedance of the qth embedded tubular Helmholtz cavity unit cell, and n represents the number of embedded tubular Helmholtz cavity unit cells:

[0076]

[0077] Where σ p is the porosity of the first perforated plate in the pth perforated plate Helmholtz cavity unit cell, is the intermediate variable, d p is the diameter of the first hole in the pth perforated plate Helmholtz cavity unit cell, t p is the thickness of the first perforated plate in the pth perforated plate Helmholtz cavity unit cell, h p is the inner cavity depth of the first cavity in the pth perforated plate Helmholtz cavity unit cell, η is the air dynamic viscosity, ω is the angular frequency, k0 is the wave number of the sound wave, j is the imaginary unit; γ is the specific heat ratio of air, A is the total cross-sectional area of the embedded tube-type Helmholtz cavity unit cell, when the unit cell cross-section is square, A=l c 2 , l c is the unit cell side length, is the inner cross-sectional area of the embedded tube in the qth embedded tube-type Helmholtz cavity unit cell, d q is the inner diameter of the second perforation and embedded tube in the qth embedded tube-type Helmholtz cavity unit cell, l q is the length of the embedded tube in the qth embedded tube-type Helmholtz cavity unit cell, V q is the volume of the second cavity excluding the neck of the qth embedded tubular Helmholtz cavity unit cell, ρ c,q is the complex density of the air in the qth embedded tubular Helmholtz cavity unit cell, c c is the complex sound speed of air, ρ c,q =ρ0 / Ψ v,q , δ q =(4 / 3π)d q [1+(1-1.25d q / l c )] is the end correction coefficient of the acoustic wave of the qth embedded tubular Helmholtz cavity unit cell, k c,q ,Ψ v,q ,Ψ h,q are functions of the complex wave number, viscosity field, and thermal field of the air in the qth embedded tubular Helmholtz cavity unit cell, respectively. The expressions are as follows:

[0078]

[0079] Where k v = -jωρ0 / μ and k h =-jωρ0C / κ are viscous wave number and thermal wave number respectively, μ is the kinematic viscosity of air, C p is the constant pressure specific heat capacity of air, κ is the thermal conductivity of air, J n (n=0,2) is the nth-order Bessel function;

[0080] Step S2, determine the noise reduction frequency band of the landing gear door and the number of cells in each Helmholtz cavity unit cell: determine the noise reduction frequency band f1~f2 according to the actual noise spectrum, f1 is the starting frequency, f2 is the ending frequency, and then determine the number of cells to be M×N, where M represents the number of cell rows and N represents the number of cell columns. The frequency interval is divided into multiple single-frequency noises. It should be pointed out here that the positions of the two types of unit cells in the unit cell unit can be arranged arbitrarily in terms of the sound absorption coefficient;

[0081] Step S3, determining the thickness of the sound absorbing structure according to the structure of the landing gear door;

[0082] Step S4, designing and determining the Helmholtz cavity unit cell, includes the following sub-steps:

[0083] Step S4.1: Select the unit cell type based on the determined sound absorption structure thickness and the single-frequency range. A perforated plate Helmholtz cavity unit cell is preferred. If the sound absorption coefficient cannot reach 0.5 or above at this frequency by adjusting the geometric parameters within the structural thickness limit, an embedded tube Helmholtz cavity unit cell is used instead.

[0084] Step S4.2: By adjusting the parameters of the diameter of the first perforation 13, the number of holes in the first perforation 13, the thickness of the first perforated plate 11, the depth of the first cavity 12, the diameter of the second perforation 24 and the embedded tube 22, the length of the embedded tube 22, and the depth of the second cavity 23, a Helmholtz cavity unit cell including a perforated plate-type Helmholtz cavity cell 1 and an embedded tube-type Helmholtz cavity cell 2 with multiple single frequencies is designed.

[0085] Step S5: According to the structure of the landing gear door, the obtained Helmholtz cavity unit cells are combined to form the landing gear door.

[0086] As described, the present invention calculates the sound absorption coefficient of the Helmholtz cavity unit cell and designs the structural dimensions and quantity of the two units in each unit cell according to the structure and noise reduction frequency band of the landing gear door. This can obtain a landing gear door that meets the low-frequency and mid-frequency sound absorption performance and at the same time meets the requirements of lightweight aircraft.

[0087] In addition, the landing gear door noise reduction of the present invention adopts a purely passive mechanical structure, which can avoid the control complexity and stability risks of the active noise reduction system, significantly reduce maintenance costs and improve environmental adaptability.

[0088] Next, the broadband sound absorbing structure applicable to noise reduction of a landing gear door provided by the present invention will be further described with reference to examples.

[0089] Reference Figure 5 In this example, the broadband sound absorption structure consists of a total of 16 sound absorption cells, arranged in an array of four rows and four columns. Among them, 8 cells are perforated plate-type Helmholtz cavity cells with a square cross-section, which are mainly responsible for absorbing sound waves in the frequency band of 1000-2000Hz. The other 8 cells are embedded tube-type Helmholtz cavity cells with a square cross-section, which are mainly responsible for absorbing sound waves in the frequency band of 500-1000Hz.

[0090] The perforated plate composed of the first perforated plate and the second perforated plate is a square curved plate of 48 mm × 48 mm with a thickness of 1-2 mm. The inner cavity depth of the Helmholtz cavity of the two unit cells is uniformly 22 mm, the inner cavity side length of the two unit cells is 10 mm, the diameter of the first perforation is 1.2-1.7 mm, the diameter of the second perforation and the embedded tube is 2.2-3 mm, the length of the embedded tube is 7.5-17 mm, the wall thickness of the first cavity and the second cavity is 1 mm, and the total thickness of the structure is 25 mm. The geometric parameters of the two unit cells are shown in Tables 1 and 2.

[0091] Table 1 Unit cell parameters of perforated plate Helmholtz cavity

[0092]

[0093] Table 2 Parameters of the unit cell of the embedded tubular Helmholtz cavity

[0094]

[0095]

[0096] The sound absorption performance of the structure was evaluated by theoretical and finite element simulation. The finite element simulation model is as follows: Figure 5 As shown, a background pressure field is set above the sound-absorbing structure. The background pressure field applies a negative sound pressure field in the Z-axis with a magnitude of 1 Pa. As the sound source, the sound wave enters the first cavity through the first perforation and enters the second cavity through the second perforation and the embedded tube. A perfect matching layer is applied above the background pressure field to absorb reflected sound waves and simulate a non-reflective boundary.

[0097] The calculated sound absorption coefficient is Figure 6 As shown in the figure, the structure can effectively absorb sound in the frequency range of 500-2000Hz, and the theoretical and simulated average sound absorption coefficients are 0.70 and 0.68 respectively, which has a good broadband sound absorption effect.

[0098] At the same time, the broadband sound absorption structure is used as a Helmholtz cavity unit cell to evaluate the noise reduction effect of the landing gear door. A line source 0.5m above the door is used to simulate the noise inside the landing gear compartment, and an observation point is set 0.5m below the door. Based on this, a relevant finite element simulation model is established, such as Figure 7 As shown in the figure, a sphere is set outside the hatch to simulate the propagation of sound waves in the air. The sphere is set as a perfect matching boundary to simulate a non-reflective boundary. The broadband sound absorption structure is simulated by setting the surface impedance on the hatch surface. Finally, the sound pressure level curve before and after the broadband sound absorption structure is loaded is calculated, as shown in the figure. Figure 8As shown in the figures, after the sound-absorbing structure is installed, the sound pressure levels in the 500-2000 Hz frequency band decrease to varying degrees, indicating that the landing gear door equipped with the sound-absorbing structure of the present invention has a good noise reduction effect. Therefore, the present invention has a good market prospect in application environments with space constraints such as aircraft landing gear doors.

[0099] Finally, it should be noted that the features mentioned and / or illustrated in the above description of the exemplary embodiments of the present invention may be incorporated into one or more other embodiments in the same or similar manner, combined with features in other embodiments, or substituted for corresponding features in other implementations. The technical solutions obtained by such combination or substitution shall also be deemed to be included in the scope of protection of the present invention.

Claims

1. A broadband sound absorbing structure suitable for reducing noise in landing gear doors, characterized by: It includes a plurality of perforated plate type Helmholtz cavity unit cells and a plurality of embedded tube type Helmholtz cavity unit cells; Each perforated plate type Helmholtz cavity unit cell includes a first perforated plate and a first cavity, wherein the first perforated plate covers the open end of the first cavity, and one or more first perforations are formed on the first perforated plate; Each embedded tube-type Helmholtz cavity unit cell includes a second perforated plate, an embedded tube, and a second cavity. The second perforated plate covers the open end of the second cavity. A second perforated hole is formed on the second perforated plate. The embedded tube is coaxial with the second perforated hole. One end of the embedded tube extends into the second cavity, and the other end is connected to the surface of the second perforated plate facing the second cavity. The perforated plate-type Helmholtz cavity unit cell and the embedded tube-type Helmholtz cavity unit cell are arranged so that: the first cavity and the second cavity are oriented in the same direction, the first perforated plate and the second perforated plate form a curved surface conformal to the landing gear door, the axis of the first perforated hole on each first perforated plate and the axis of the second perforated hole on each second perforated plate are parallel to each other, and the surfaces of the first cavity and the second cavity opposite to their respective perforated plates are located in the same horizontal plane.

2. The broadband sound absorbing structure suitable for landing gear door noise reduction according to claim 1, characterized in that: The number of first holes provided on the first perforated plate of each perforated plate type Helmholtz cavity unit cell is different and ranges from 1 to 5.

3. The broadband sound absorbing structure for landing gear door noise reduction according to claim 1 or 2, characterized in that: The aperture of the first perforation is 0.5-2 mm, and the aperture of the first perforation on each first perforated plate is the same. The aperture of the second perforation is the same as that of the embedded tube, which is 1-3 mm. The wall thickness of the embedded tube is 0.5-1 mm.

4. The broadband sound absorbing structure suitable for landing gear door noise reduction according to claim 3, characterized in that: The thickness of the first perforated plate and the second perforated plate is 1-3 mm, and the porosity is 0.1-2.5%.

5. The broadband sound absorbing structure suitable for landing gear door noise reduction according to claim 3, characterized in that: The length of the embedded tube is 1 to 23 mm.

6. The broadband sound absorbing structure for landing gear door noise reduction according to claim 1 or 2, characterized in that: The cross sections of the perforated plate type Helmholtz cavity unit cell and the embedded tube type Helmholtz cavity unit cell are both square, and the cross section size of each unit cell is the same.

7. The broadband sound absorbing structure for landing gear door noise reduction according to claim 1 or 2, characterized in that: The first perforated plate and the first cavity, the second perforated plate and the second cavity are obtained by 3D printing and bonded with epoxy resin.

8. A landing gear door using the broadband sound absorbing structure according to any one of claims 1 to 7, characterized in that: It comprises a plurality of Helmholtz cavity cell units consisting of a plurality of perforated plate-type Helmholtz cavity cells and a plurality of embedded tube-type Helmholtz cavity cells. In the Helmholtz cavity cell unit, the plurality of perforated plate-type Helmholtz cavity cells and the plurality of embedded tube-type Helmholtz cavity cells are adjacently arranged in a square or a rectangle.

9. A method for designing a landing gear door according to claim 8, characterized in that: The following steps are involved: Step 1: Calculate the sound absorption coefficient of the Helmholtz cavity unit cell: Where Z is the surface acoustic impedance of the Helmholtz cavity unit cell, Z0 is the characteristic impedance of air, Z0 = ρ0c0, ρ0 and c0 are the density and sound speed of air respectively; Each Helmholtz cavity unit cell includes p perforated plate-type Helmholtz cavity cells and q embedded tube-type Helmholtz cavity cells, and the surface acoustic impedance is: Where Z p represents the surface acoustic impedance of the pth perforated plate Helmholtz cavity unit cell, m represents the number of perforated plate Helmholtz cavity unit cells, Z q represents the surface acoustic impedance of the qth embedded tubular Helmholtz cavity unit cell, and n represents the number of embedded tubular Helmholtz cavity unit cells: Where σ p is the porosity of the first perforated plate in the pth perforated plate Helmholtz cavity unit cell, is the intermediate variable, d p is the diameter of the first hole in the pth perforated plate Helmholtz cavity unit cell, t p is the thickness of the first perforated plate in the pth perforated plate Helmholtz cavity unit cell, h p is the inner cavity depth of the first cavity in the pth perforated plate Helmholtz cavity unit cell, η is the air dynamic viscosity, ω is the angular frequency, k0 is the wave number of the sound wave, j is the imaginary unit; γ is the specific heat ratio of air, A is the total cross-sectional area of the embedded tube Helmholtz cavity unit cell, S q is the inner cross-sectional area of the embedded tube in the qth embedded tube-type Helmholtz cavity unit cell, l q is the length of the embedded tube in the qth embedded tube-type Helmholtz cavity unit cell, V q is the volume of the second cavity excluding the neck of the qth embedded tubular Helmholtz cavity unit cell, ρ c,q and c c are the complex density and complex sound speed of air, ρ c,q =ρ0 / Ψ v,q , δ q is the terminal correction coefficient of the sound wave, k c,q ,Ψ v,q ,Ψ h,q are functions of the complex wave number of air and the viscous and thermal fields respectively; Step 2: Determine the noise reduction frequency band of the landing gear door and the number of cells in each Helmholtz cavity unit: Determine the noise reduction frequency band f1~f2 according to the actual noise spectrum, f1 is the starting frequency, f2 is the ending frequency, and then determine the number of cells to be M×N, where M represents the number of cell rows and N represents the number of cell columns. The frequency interval is divided into multiple single-frequency noises; Step 3: Determine the thickness of the sound absorbing structure based on the structure of the landing gear door; Step 4, designing and determining the Helmholtz cavity unit cell, includes the following sub-steps: Step 4.1, select the unit cell type according to the determined thickness of the sound absorbing structure and the size of the divided single frequency; Step 4.2, by adjusting the aperture of the first perforation, the number of the first perforations, the thickness of the first perforated plate, the depth of the first cavity, and the aperture of the second perforation and the embedded tube, the length of the embedded tube, and the depth of the second cavity, a Helmholtz cavity unit cell including a perforated plate-type Helmholtz cavity cell and an embedded tube-type Helmholtz cavity cell of multiple single frequencies is designed; Step 5: According to the structure of the landing gear door, the obtained Helmholtz cavity unit cells are combined to form the landing gear door.

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