A noise reduction liner for helicopter ducted tail rotors aimed at suppressing low-frequency line spectrum noise.

Abstract

This invention discloses a noise reduction liner for helicopter ducted tail rotors aimed at suppressing low-frequency line spectrum noise, belonging to the field of helicopter ducted tail rotor noise reduction technology. The noise of the ducted tail rotor in this invention belongs to multi-line spectrum harmonic noise, and the noise energy decreases with increasing frequency, meaning the noise energy is concentrated in the first two harmonics. Furthermore, the ducted tail rotor has a low rotational speed and a small number of blades, resulting in a low noise frequency. The micro-perforated plate composite folded back cavity used in this invention serves as a sound-absorbing unit. Through parameter design, the two sound-absorbing resonant frequencies of the sound-absorbing unit coincide with the first two harmonic noises of the tail rotor. This reduces the overall thickness of the sound-absorbing unit while still providing low-frequency multi-line spectrum noise reduction performance, thus reducing weight while effectively suppressing tail rotor noise.

Description

Technical Field

[0001] This invention relates to the field of noise reduction technology for helicopter ducted tail rotors, and in particular to a ducted acoustic liner structure that can suppress multi-line spectrum harmonic noise of the tail rotor. Background Technology

[0002] Helicopters are widely used by militaries worldwide due to their vertical takeoff and landing (VTOL) and low-altitude penetration capabilities. To counter helicopters, various countries have developed a range of anti-helicopter weapons tailored to their flight characteristics, such as Stinger missiles that automatically track helicopters using infrared guidance and anti-helicopter mines that identify characteristic helicopter noise. This demonstrates that noise issues seriously threaten helicopter flight safety. For small and medium-sized helicopters, main rotor noise and tail rotor noise are the primary externally transmitted noise. While main rotor noise reduction technology is now relatively mature, tail rotor noise is gradually becoming an unavoidable factor, thus necessitating a solution to the tail rotor noise problem.

[0003] In 2016, Pongratz et al. from Eurocopter embedded a Helmholtz resonator combined with a quarter-wavelength tube into the inner wall of the EC135 ducted tail rotor to suppress noise generated during high-speed rotation of the helicopter ducted tail rotor. Simulations and experiments showed that this combined sound-absorbing structure could reduce the overall noise of the tail rotor by 3 dBA. However, the noise frequency of the helicopter ducted tail rotor is relatively low, and the energy is concentrated on the first two harmonics. The quarter-wavelength tube is limited by the internal space of the duct, and its sound absorption frequency exceeds 2000 Hz, which cannot suppress the high-energy first two harmonic noise.

[0004] Currently available helicopter duct acoustic liners are mainly designed based on Helmholtz resonators and quarter-wavelength tubes, which can effectively reduce mid-to-high frequency broadband noise. However, helicopter tail rotor noise is mainly concentrated in the low-frequency line spectrum noise generated by the tail rotor. For this type of low-frequency line spectrum noise, the noise suppression performance of current helicopter duct acoustic liners is very poor.

[0005] To address the challenge of low-frequency line spectrum noise reduction in helicopter ducted tail rotors, this invention proposes a novel acoustic liner for helicopter ducted tail rotors based on a composite sound-absorbing structure. A low-frequency line spectrum sound-absorbing unit structure is constructed using a folded back cavity and a micro-perforated plate. Then, a periodic design is performed in conjunction with the geometric characteristics of the duct to obtain a novel ducted tail rotor noise reduction liner, achieving highly efficient suppression of low-frequency line spectrum noise in helicopter tail rotors. Summary of the Invention

[0006] The main objective of this invention is to suppress external noise transmitted by the ducted tail rotor. Ducted tail rotor noise is a multi-line spectrum harmonic noise, with noise energy decreasing as frequency increases, meaning the noise energy is concentrated in the first two harmonics. Furthermore, the ducted tail rotor operates at low speeds and has few blades, resulting in a lower noise frequency. This invention uses a micro-perforated plate composite folded back cavity as a sound-absorbing unit. Through parameter design, the two sound-absorbing resonant frequencies of the unit coincide with the first two harmonic noises of the tail rotor. This reduces the overall thickness of the sound-absorbing unit while still providing low-frequency multi-line spectrum noise reduction performance, thus reducing weight while effectively suppressing tail rotor noise.

[0007] This invention is implemented as follows:

[0008] A noise reduction liner for helicopter ducted tail rotors aimed at suppressing low-frequency line spectrum noise is characterized by comprising a ducted acoustic liner structure. The noise reduction liner targets the first two harmonic noises. The calculation of the first two harmonic noise frequencies of the ducted tail rotor involves first determining the operating speed Ω (rpm) and the number of tail rotor blades N, and then applying the formula f... base =Ω / 60×N to calculate the fundamental frequency f of the ducted tail rotor noise. base The first harmonic noise frequency f1 = f base ×1, the second harmonic noise frequency f2=f base ×2;

[0009] Sound absorption unit structure selection: In order to suppress the first two harmonic noises of the ducted tail rotor, the micro-perforated plate is combined with the folded back cavity. The folded back cavity is a cavity with a partition added behind the micro-perforated plate. By separating the cavity, the sound absorption unit has additional sound absorption peaks, and the partition extends the propagation distance of noise in the cavity. The composite structure ensures that low-frequency multi-line spectrum noise can be suppressed while reducing the overall thickness of the sound absorption unit.

[0010] The sound-absorbing unit structure includes a micro-perforated plate and an outer wall. The micro-perforated plate is located at the top of the sound-absorbing structure, facing the inner side of the duct, and is connected to the air. Its thickness is t, its pore size is d, and its perforation rate is p. The outer wall of the sound-absorbing structure consists of the bottom plate and side plates of the entire structure. The bottom plate is connected to the inner wall of the duct, and the four side plates are connected to the nearby sound-absorbing structure. A first partition and a second partition are installed inside the cavity formed by the micro-perforated plate and the outer wall. The thickness of the first partition and the second partition is t. The length of the first partition is l1, and the length of the second partition is l2. l1 and l2 must be less than the length of the cavity inside the sound-absorbing unit. The width of the first partition and the second partition is consistent with the width of the acoustic cavity inside the sound-absorbing unit. By optimizing the geometric parameters of the cavity, micro-perforated plate, and partitions of the sound-absorbing unit structure, the sound absorption performance of the sound-absorbing unit is changed, thereby achieving the purpose of suppressing the line spectrum noise of the helicopter duct tail rotor.

[0011] Furthermore, the structural parameters of the sound-absorbing unit are optimized as follows: the structural parameters of the sound-absorbing unit are optimized through finite element simulation so that the first two sound absorption resonance peaks of the sound-absorbing unit are consistent with the first two harmonic noise frequencies of the ducted tail rotor.

[0012] First, an impedance tube model is established in COMSOL finite element simulation software (but not limited to COMSOL, such as Virtual Lab, ANSYS and other acoustic finite element simulation software are also acceptable). The direction of the acoustic wave in the impedance tube is perpendicular to the micro-perforated plate. The parameters of the micro-perforated plate and the folded back cavity structure are controlled by user-defined variables, namely, aperture d, plate thickness t, perforation rate p, length of the first partition l1, length of the second partition l2, height of the first channel h1, and height of the second channel h2 (but not limited to these 7 parameters, other parameters such as micro-hole shape, micro-hole layout, and thickness of the first and second partitions can also be used as design variables).

[0013] The Nelder-Mead downhill simplex method (but not limited to the Nelder-Mead downhill simplex method; other optimization algorithms such as gradient descent, Newton's method, and genetic algorithm are also applicable) is used to design the parameters of the sound-absorbing unit. The optimization objective is to maximize the sum of the sound absorption coefficients of the sound-absorbing unit at the first and second harmonic noise frequencies of the tail rotor. The optimization variables are the seven parameters d, t, p, l1, l2, h1, and h2 (other parameters such as micropore shape, micropore layout, and the thickness of the first and second partitions can also be used as optimization variables).

[0014] Furthermore, the acoustic liner of the helicopter ducted tail rotor designed based on the bandgap theory of acoustic metamaterials has broadband line spectrum noise suppression capability. First, the optimized sound-absorbing unit is designed as a single-layer acoustic liner structure. This single-layer acoustic liner structure is used as the unit cell structure of the acoustic metamaterial. By applying the periodic boundary conditions of the acoustic metamaterial, its bandgap characteristics and transmissivity characteristics are obtained and compared with the first two harmonic noise frequencies of the ducted tail rotor. Then, the number of layers of the acoustic liner structure and the geometric installation position and arrangement of each layer of the acoustic liner are determined, thereby achieving the optimal noise reduction effect.

[0015] The ducted tail rotor acoustic liner structure consists of K (K>3) layers of single acoustic liner structure, each layer of which is composed of a designed small-sized low-frequency line spectrum sound-absorbing structure. Finally, the entire acoustic liner structure is embedded inside the duct of the helicopter tail rotor. When the helicopter ducted tail rotor is working, the line spectrum noise it generates will decrease rapidly as the number of acoustic liner layers increases, thereby achieving a good noise suppression effect.

[0016] Furthermore, the configurations of the small-sized low-frequency line spectrum sound-absorbing structures inside the duct acoustic liner are the same, and the unit parameters of the small-sized low-frequency line spectrum sound-absorbing structures are the same or different; the structural parameters of each layer of acoustic liner are the same or different, and are determined based on the bandgap calculation results; if the operating speed fluctuation of the duct tail rotor is within ±5% during operation, the same sound-absorbing unit is used to make the duct acoustic liner; otherwise, it is necessary to optimize the design of the sound-absorbing unit structural parameters with a wide first-order sound absorption frequency band and a wide second-order sound absorption frequency band, and combine different sound-absorbing units to make the duct acoustic liner to achieve a good noise reduction effect.

[0017] The present invention relates to a helicopter ducted tail rotor noise reduction liner for low-frequency line spectrum noise suppression, wherein the fabrication and assembly of the ducted acoustic liner are as follows:

[0018] Fabrication of micro-perforated plates: Micro-perforated plates are made from metal or composite materials. First, a thickness of t and an area of ​​S (S=(π / 36*R)) are machined. 2 The plate surface is then analyzed, and the number of holes M = 4Sp / πd is calculated based on the perforation rate p and the hole diameter d. 2 Next, a corresponding number of evenly distributed microholes are cut on the surface of the plate using a laser. Finally, a wall with a radius of R is processed. The previously processed planar micro-perforated plate is placed on the wall and bent to form a curved micro-perforated plate with the same curvature as the duct wall.

[0019] Fabrication of the curved back cavity: Based on the optimized structural parameters, a three-dimensional model is created in the three-dimensional modeling software and exported in a file format that can be recognized by the 3D printer, such as STL, GCODE, etc. The file is then imported into the 3D printer, and photosensitive resin is selected as the material to complete the fabrication of the curved back cavity.

[0020] Duct acoustic liner preparation: First, the micro-perforated plate is glued to the curved back cavity to form a sound-absorbing unit. To facilitate installation, the sound-absorbing units are glued together in sequence to form a quarter-ring structure. This step is repeated 4K times to obtain 4K identical quarter-ring structures.

[0021] Installation of duct acoustic liner: Cut off part of the inner wall of the duct and leave a space with the same thickness and depth as the acoustic liner; apply glue to the inner wall of the duct and glue the quarter-ring structure from the above steps to the inside of the duct one by one until it is filled.

[0022] The preparation and installation of the noise reduction liner for the ducted tail rotor are now complete.

[0023] The inventive points of this invention include:

[0024] I. Sound-absorbing structure design for low-frequency line spectrum noise suppression of helicopter ducted tail rotors. Since the noise energy of the ducted tail rotor is mainly concentrated in the first two harmonics, this invention focuses on reducing the noise of the first two harmonics. First, the frequencies of the first two harmonic noises of the ducted tail rotor are calculated. The operating speed of the helicopter ducted tail rotor is Ω (rpm), and the number of tail rotor blades is N. Therefore, according to the formula f... base The fundamental frequency f of the ducted tail rotor noise can be calculated by Ω / 60×N. base Therefore, the noise energy of the ducted tail rotor is concentrated at the first harmonic noise frequency f1 = f base ×1 and the second harmonic noise frequency f2=f base ×2. Next, the sound-absorbing unit structure design was carried out. Since the frequencies of the first two harmonic noises are low, and the internal space of the helicopter duct is limited, a small-sized low-frequency sound-absorbing structure needs to be designed. To this end, this invention, on the one hand, increases the sound wave transmission path by folding the back cavity, thus designing a small-sized sound-absorbing unit with good low-frequency noise reduction performance. On the other hand, since the noise reduction target contains two main noise reduction frequency bands, the number of folded back cavity layers was increased, and a micro-perforated plate structure was added. Increasing the number of folded back cavity layers increases the number of noise reduction frequency bands; the micro-perforated plate structure can broaden the noise reduction frequency band. The designed small-sized low-frequency multi-line spectrum sound-absorbing structure is as follows: Figure 1 As shown, the micro-perforated plate is located at the top of the sound-absorbing structure, facing the inner side of the duct and connected to the air. Its thickness is t, pore size is d, and perforation rate is p. The outer wall of the sound-absorbing structure consists of the bottom plate and side plates of the entire structure. The bottom plate is connected to the inner wall of the duct, and the four side plates are connected to the nearby sound-absorbing structures. The first and second partitions are located inside the cavity, with a thickness of t and lengths of l1 and l2 respectively. The length of the partitions must be less than the width of the inner cavity. The sound absorption performance of this sound-absorbing unit depends on the geometric parameters of the cavity, the micro-perforated plate, and the partitions. It needs to be optimized according to the method in point two of the invention to achieve the goal of suppressing the line spectrum noise of the helicopter duct tail rotor.

[0025] II. Parameter Design of Small-Size Low-Frequency Sound Absorbing Structure: To match the sound absorption frequency of the sound absorbing structure with the noise line spectrum frequency of the helicopter ducted tail rotor, optimization design is required. First, finite element models of the perfectly matched layer, background pressure field, micro-perforated plate, and folded back cavity are sequentially established in COMSOL. The pressure amplitude of the background pressure field is defined as 1 Pa, the sound velocity as 343 m / s, and the wave direction is perpendicular to the micro-perforated plate. The micro-perforated plate is directly generated from the internal perforated plate module. The aperture d (0.4mm-1mm), plate thickness t (1mm-3mm), and perforation rate p (1%-5%) of the micro-perforated plate can be defined. For ease of parameter design, the number of folded layers n is set to 3. The remaining parameters are controlled by user-defined variables: the length l1 of the first partition (20mm-30mm), the length l2 of the second partition (20mm-30mm), the height h1 of the first layer channel (1mm-30mm), and the height h2 of the second layer channel (1mm-30mm). The total depth H of the folded back cavity is limited to 30mm. Then, the Nelder-Mead downhill simplex method is used for parameter design. The optimization objective is to maximize the sum of the sound absorption coefficients of the sound-absorbing units at the first and second harmonic frequencies. The optimization variables are the seven parameters: d, t, p, l1, l2, h1, and h2. Through iterative optimization, the design parameters for the small-size low-frequency sound-absorbing structure that meets the requirements can be obtained.

[0026] III. Design Method for Ducted Tail Rotor Acoustic Liner Based on Acoustic Metamaterials: Traditional acoustic liner design methods simply involve placing sound-absorbing structures on the inner side of the ducted tail rotor, forming a complete ring of sound-absorbing structures that constitute the acoustic liner. This approach has a narrow sound absorption bandwidth and poor performance. Therefore, this invention proposes a helicopter ducted tail rotor acoustic liner with broadband line spectrum noise suppression based on the bandgap theory of acoustic metamaterials. First, a single-layer acoustic liner structure is designed based on the designed small-size low-frequency line spectrum sound-absorbing structure. This single-layer acoustic liner structure is used as the unit cell structure of the acoustic metamaterial. By applying the periodic boundary conditions of the acoustic metamaterial, its bandgap characteristics and transmissibility characteristics are obtained. This allows for the determination of the number of layers in the acoustic liner structure and the geometric installation position and arrangement of each layer, thereby achieving optimal noise reduction. The designed acoustic liner structure is shown below. Figure 1 As shown, it consists of K (K>3) layers of single acoustic liner structure, each layer composed of a designed small-sized low-frequency line spectrum sound-absorbing structure (with the same configuration but possibly different parameters). However, the structural parameters of each acoustic liner layer can be different, which needs to be determined based on the bandgap calculation results. Finally, the entire acoustic liner structure is embedded inside the helicopter tail rotor duct. When the helicopter ducted tail rotor is working, the line spectrum noise it generates will decrease rapidly with the increase of the number of acoustic liner layers, thereby achieving a good noise suppression effect.

[0027] The advantages of this invention compared to the prior art are as follows:

[0028] This invention broadens the sound absorption frequency band, ensuring noise reduction even with fluctuations in the tail rotor speed. It suppresses the first two harmonic noises with the highest energy. The specific effect can be determined by comparing the impedance tube simulation results of this invention with those of existing ducted tail rotor sound-absorbing structures. Assuming a ducted tail rotor speed of 3660 rpm and 13 blades, the first and second harmonic noise frequencies are 793 Hz and 1586 Hz, respectively. Using the Nelder-Mead optimization algorithm, it was determined that the sound-absorbing structure can absorb the first two harmonic noises when the micro-perforated plate aperture d = 0.4 mm, perforation rate p = 4.0%, first layer channel height h1 = 18.1 mm, second layer channel height h2 = 3.3 mm, first diaphragm length l1 = 22.5 mm, and second diaphragm length l2 = 22.9 mm. However, existing ducted tail rotor sound-absorbing structures are composed of a Helmholtz resonator and a quarter-wavelength tube. The quarter-wavelength tube can only absorb high-frequency noise, and it cannot be designed so that the two sound-absorbing resonant peak frequencies correspond to the frequencies of the first two harmonic noises. Therefore, while absorbing the first harmonic noise, it can only absorb the 5th and 6th harmonic noises. Simulation results verify that the sound-absorbing structure of this invention is more suitable for ducted tail rotor noise reduction. Attached Figure Description

[0029] Figure 1 This is a cross-sectional schematic diagram of the sound-absorbing unit and an overall schematic diagram of the duct acoustic liner of the present invention;

[0030] Figure 2 This is a simulation result diagram of the noise reduction of the duct acoustic liner of the present invention;

[0031] Figure 3 This is a simulation comparison diagram of the sound absorption effect of the sound absorption structure of the present invention and existing sound absorption structures;

[0032] Among them, 1-micro-perforated plate, 2-outer wall, 3-first partition 3, 4-second partition 4, 5-first acoustic liner, 6-second acoustic liner, 7-third acoustic liner, 8-fourth acoustic liner. Detailed Implementation

[0033] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the following examples provide a more detailed description of the invention. It should be noted that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0034] This embodiment specifically provides a low-frequency multi-line spectrum noise reduction structure suitable for helicopter ducted tail rotor noise reduction, such as... Figure 1 As shown. In practice, it can be divided into two parts: duct acoustic liner structure design and duct acoustic liner fabrication and installation. Specifically:

[0035] Part 1: Duct Acoustic Liner Structure Design

[0036] 1.1 Calculation of the first two harmonic noise frequencies of the ducted tail rotor: Since the noise energy of the ducted tail rotor is mainly concentrated in the first two harmonics, this invention focuses on reducing the noise of the first two harmonics. First, the operating speed Ω (rpm) of the ducted tail rotor and the number of tail rotor blades N are determined, and then the frequency is calculated according to the formula f... base =Ω / 60×N to calculate the fundamental frequency f of the ducted tail rotor noise. base The first harmonic noise frequency f1 = f base ×1, the second harmonic noise frequency f2=f base ×2.

[0037] 1.2 Sound Absorbing Unit Structure Selection: To suppress the first two harmonic noises of the ducted tail rotor, this invention combines a micro-perforated plate with a folded back cavity. The folded back cavity is created by adding a partition to the original cavity. By separating the cavity, the sound absorbing unit has additional absorption peaks, and the partition extends the noise propagation distance within the cavity. Therefore, this composite structure ensures suppression of low-frequency multi-line spectrum noise while reducing the overall thickness of the sound absorbing unit. For ease of structural design, the number of folded layers (n) of the folded back cavity is set to 3 layers. A schematic diagram of the sound absorbing unit structure is shown below. Figure 1 Left.

[0038] 1.3 Optimization Design of Sound Absorbing Unit Structural Parameters: The parameter optimization design of this invention was completed in COMSOL finite element simulation software. First, finite element models of the perfectly matched layer, background pressure field, micro-perforated plate, and folded back cavity were established in COMSOL in sequence. The type of the perfectly matched layer is Cartesian. The pressure amplitude of the background pressure field is set to 1 Pa, the sound velocity is set to 343 m / s, and the wave direction is perpendicular to the micro-perforated plate. The micro-perforated plate is directly generated by the internal perforated plate module. The aperture d, plate thickness t, and perforation rate p of the micro-perforated plate can be defined. The parameters of the folded back cavity are controlled by custom variables, namely the length l1 of the first partition 3, the length l2 of the second partition 4, the height h1 of the first layer channel, and the height h2 of the second layer channel. The depth of the entire folded back cavity is limited to 30 mm. The optimization algorithm uses the Nelder-Mead downhill simplex method built into COMSOL for parameter design. The optimization objective is to maximize the sum of the sound absorption coefficients of the sound-absorbing unit at the first and second harmonic frequencies. The optimization variables are the seven parameters: d, t, p, l1, l2, h1, and h2.

[0039] 1.4 Duct Acoustic Liner Layout Design: Based on the designed small-size low-frequency line spectrum sound-absorbing structure, a single-layer acoustic liner structure was designed. This single-layer acoustic liner structure was used as the unit cell structure of the acoustic metamaterial. The periodic boundary conditions of the acoustic metamaterial were applied to obtain its bandgap characteristics and transmissibility characteristics, thereby determining the number of layers in the acoustic liner structure. A cavity of corresponding depth was designed in the inner wall of the duct, and the duct acoustic liner was embedded in the cavity, thus completing the duct acoustic liner model construction.

[0040] 1.5 Simulation Verification of Noise Reduction for Duct Acoustic Liner: The tail rotor noise was replaced by N monopole sources. The simulation module was selected in the frequency domain, and the calculation frequency was chosen to be the first and second harmonic noise frequencies. Monitoring points were placed in the near field (2R from the duct center, where R is the duct radius) and the far field (4R from the duct center) to obtain noise data. The specific location distribution is shown in [reference needed]. Figure 2 After completing the calculations, the sound pressure levels at monitoring points were compared with and without the duct acoustic liner to verify the feasibility of the invention.

[0041] The structural design of the noise reduction liner for the ducted tail rotor is now complete.

[0042] Part Two: Preparation and Assembly of Duct Acoustic Liners

[0043] 2.1 Fabrication of the micro-perforated plate: The micro-perforated plate is made of stainless steel plate. First, it is machined to a thickness of t and an area of ​​S (S=(π / 36*R)). 2 The stainless steel plate is used as an example. Then, the number of holes M = 4Sp / πd is calculated based on the perforation rate p and the hole diameter d. 2 Next, a corresponding number of evenly distributed micro-holes are cut on the surface of the stainless steel plate using a laser. Finally, a wall with a radius of R is processed. The previously processed flat micro-perforated plate is placed on the wall and bent to form a curved micro-perforated plate with the same curvature as the duct wall.

[0044] 2.2 Fabrication of the Bending Back Cavity: Based on the optimized structural parameters, a three-dimensional model was created in Solidworks and exported as an STL file. The STL file was then imported into a 3D printer, and photosensitive resin was selected as the material to complete the fabrication of the bending back cavity.

[0045] 2.3 Preparation of duct acoustic liners: First, the micro-perforated plate is glued to the curved back cavity to form a sound-absorbing unit. To facilitate installation, the sound-absorbing units are glued together in sequence to form a quarter-ring structure. This step is repeated 4K times to obtain 4K identical quarter-ring structures.

[0046] 2.4 Installation of the culvert acoustic liner: Cut off a portion of the culvert inner wall, leaving a space consistent with the thickness and depth of the acoustic liner. Apply glue to the inner wall of the culvert and attach the quarter-ring structures from the above steps one by one into the culvert until it is full.

[0047] The preparation and installation of the noise reduction liner for the ducted tail rotor are now complete.

[0048] Taking the Z-9 ducted tail rotor as an example, the Z-9 ducted tail rotor operates at 3660 rpm and has 13 blades. Therefore, its first harmonic noise frequency is 3660 / 60*13 = 793 Hz, and its second harmonic frequency is 1586 Hz. In the COMSOL finite element simulation software, finite element models of a Cartesian perfectly matched layer, a background pressure field, a micro-perforated plate, and a folded back cavity are established sequentially. The background pressure field amplitude is set to 1 Pa, the sound velocity is set to 343 m / s, and the wave direction is perpendicular to the micro-perforated plate. To ensure that the sound-absorbing unit can simultaneously suppress noise at 793 Hz and 1586 Hz, the optimization objective is to maximize the sum of the sound absorption coefficients of the sound-absorbing unit at 793 Hz and 1586 Hz. The Nelder-Mead downhill simplex method is used to optimize the micro-perforated plate, considering the aperture d (0.4 mm-1 mm), plate thickness t (1 mm-3 mm), perforation rate p (1%-5%), and the length of the first partition 3. The sound-absorbing unit achieved a sound absorption coefficient of 1 at 793Hz and 1589Hz when the following parameters were considered: thickness l1 (20mm-25mm), length l2 (20mm-25mm) of the second partition 4, height h1 (1mm-20mm) of the first channel, and height h2 (1mm-20mm) of the second channel. The optimization results showed that when the thickness t of the micro-perforated plate was 2mm, the aperture d was 0.4mm, the perforation rate p was 4.0%, the height h1 of the first channel was 18.1mm, the height h2 of the second channel was 3.3mm, the length l1 of the first partition 3 was 22.5mm, and the length l2 of the second partition 4 was 22.9mm, the sound absorption coefficient was 1 at 793Hz and 1589Hz, which is perfect sound absorption. Subsequently, a single-layer acoustic liner structure was designed based on the sound-absorbing unit. This single-layer acoustic liner structure was used as the unit cell structure of the acoustic metamaterial. The periodic boundary conditions of the acoustic metamaterial were applied to obtain its bandgap characteristics and transmissivity characteristics. Then, the number of layers of the acoustic liner structure was determined to be 4 (first acoustic liner 5, second acoustic liner 6, third acoustic liner 7, fourth acoustic liner 8).

[0049] To facilitate simulation and verification of the noise reduction effect of the acoustic liner, this invention scales down the Z-9 duct to 0.3 times its original size. A 3D model of the optimized sound-absorbing unit is created in Solidworks. Following a 0.3-fold scaling down of the Z-9 duct's inner diameter, 36 sound-absorbing units are first arranged in a circular array, then linearly arranged four times along the axial direction to form a duct acoustic liner with a total of 144 sound-absorbing units. A space of equal thickness and depth to the acoustic liner is cut into the inner wall of the duct. The 3D models of the duct and the acoustic liner are imported into COMSOL and assembled. A monopole source is then placed at the characteristic profile of the tail rotor blade (i.e., 0.7 times the blade length) to simulate the noise source during high-speed tail rotor rotation. Since the Z-9 has 13 tail rotor blades, 13 monopole sources are required. Noise monitoring points are located in the near field (2R from the duct center, where R is the duct radius) and the far field (4R from the duct center). Specific location distribution is shown in [details omitted]. Figure 2The simulation study focused on the frequency domain, with calculation frequencies of 793Hz and 1586Hz. Simulation results show that, after using the acoustic liner of this invention, the noise reduction effect on the exhaust side of the ducted tail rotor is 1.2-8.1dB at different locations at the first and second harmonic frequencies.

[0050] The specific effects can be determined by comparing and analyzing the impedance tube simulation results of the present invention and existing ducted tail rotor sound-absorbing structures. Assuming the ducted tail rotor rotates at 3660 rpm and has 13 blades, its first and second harmonic noise frequencies are 793 Hz and 1586 Hz, respectively. Using the Nelder-Mead optimization algorithm, it was determined that the sound-absorbing structure can absorb the first two harmonic noises when the micro-perforated plate aperture d = 0.4 mm, perforation rate p = 4.0%, first layer channel height h1 = 18.1 mm, second layer channel height h2 = 3.3 mm, first diaphragm 3 length l1 = 22.5 mm, and second diaphragm 4 length l2 = 22.9 mm. However, existing ducted tail rotor sound-absorbing structures are composed of a Helmholtz resonator and a quarter-wavelength tube. The quarter-wavelength tube can only absorb high-frequency noise and cannot design the two sound-absorbing resonance peak frequencies to correspond to the first two harmonic noise frequencies. Therefore, while absorbing the first harmonic noise, it can only absorb the 5th and 6th harmonic noises. Simulation comparison results are shown below Figure 3 As shown. This verifies that the sound-absorbing structure of the present invention is more suitable for noise reduction of ducted tail rotors.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A noise reduction liner for a helicopter ducted tail rotor designed for low-frequency line spectrum noise suppression, characterized in that, Including the ducted acoustic liner structure, the noise reduction liner for the helicopter ducted tail rotor targets the first two harmonic noises; the calculation of the first two harmonic noise frequencies of the ducted tail rotor involves first determining the operating speed Ω (rpm) of the ducted tail rotor and the number of tail rotor blades. N According to the formula f base =Ω / 60× N The fundamental frequency of the ducted tail rotor noise was calculated. f base First harmonic noise frequency f 1= f base ×1, Second harmonic noise frequency f 2= f base ×2; Sound absorption unit structure selection: In order to suppress the first two harmonic noises of the ducted tail rotor, the micro-perforated plate is combined with the folded back cavity. The folded back cavity is a cavity with a partition added behind the micro-perforated plate. By separating the cavity, the sound absorption unit has additional sound absorption peaks, and the partition extends the propagation distance of noise in the cavity. The composite structure ensures that low-frequency multi-line spectrum noise can be suppressed while reducing the overall thickness of the sound absorption unit. The sound-absorbing unit structure includes a micro-perforated plate (1) and an outer wall (2); the micro-perforated plate (1) is located at the top of the sound-absorbing structure, the micro-perforated plate (1) faces the inner side of the duct, the micro-perforated plate (1) is connected to the air, and its thickness is [missing information]. t , aperture is d The perforation rate is p The outer wall (2) of the sound-absorbing structure is the bottom plate and side plate of the entire structure, wherein the bottom plate is connected to the inner wall of the duct, and the four side plates are connected to the nearby sound-absorbing structure; the first partition (3) and the second partition (4) are installed inside the cavity formed by the micro-perforated plate (1) and the outer wall (2), and the thickness of the first partition (3) and the second partition (4) is the same. t The length of the first partition (3) is l 1. The length of the second partition (4) is l 2, l 1 and l 2 must be less than the length of the cavity inside the sound-absorbing unit; the width of the first partition (3) and the second partition (4) is consistent with the width of the sound cavity inside the sound-absorbing unit; optimize the geometric parameters of the cavity, micro-perforated plate and partition of the sound-absorbing unit structure, change the sound absorption performance of the sound-absorbing unit, and achieve the purpose of suppressing the line spectrum noise of the helicopter duct tail rotor.

2. The helicopter ducted tail rotor noise reduction liner for low-frequency line spectrum noise suppression according to claim 1, characterized in that, The structural parameters of the sound-absorbing unit are optimized as follows: the structural parameters of the sound-absorbing unit are optimized through finite element simulation so that the first two sound absorption resonance peaks of the sound-absorbing unit are consistent with the first two harmonic noise frequencies of the tail rotor. First, an impedance tube model is established in the finite element simulation software. The direction of the acoustic wave in the impedance tube is perpendicular to the micro-perforated plate (1). The parameters of the micro-perforated plate and the folded back cavity structure are controlled by user-defined variables, namely the aperture. d , plate thickness t Perforation rate p Length of the first partition (3) l 1. Length of the second partition (4) l 2. Height of the first-floor passage h 1. Second-level passage height h 2; The micropore shape, micropore layout, and the thickness of the first and second partitions can also be used as design variables; An optimization algorithm is used to design the parameters of the sound-absorbing unit. The optimization objective is to maximize the sum of the sound absorption coefficients of the sound-absorbing unit at the first and second harmonic noise frequencies of the ducted tail rotor. The optimization variables are: d, t, p, l 1. l 2. h 1. h 2 These 7 parameters.

3. A helicopter ducted tail rotor noise reduction liner for low-frequency line spectrum noise suppression according to claim 1, characterized in that, The optimized sound-absorbing unit is designed as a single-layer acoustic liner structure. This single-layer acoustic liner structure is used as the unit cell structure of the acoustic metamaterial. The periodic boundary conditions of the acoustic metamaterial are used to obtain its bandgap characteristics and transmissivity characteristics and compare them with the first two harmonic noise frequencies of the ducted tail rotor. Then, the number of layers of the acoustic liner structure and the geometric installation position and arrangement of each layer of acoustic liner are determined to achieve the best noise reduction effect. The ducted tail rotor acoustic liner structure is composed of K Composed of a single-layer acoustic liner structure, the aforementioned K >3. Each acoustic liner layer is composed of a designed small-sized low-frequency line spectrum sound-absorbing structure; finally, the entire acoustic liner structure is embedded inside the helicopter tail rotor duct; when the helicopter tail rotor is working, the line spectrum noise it generates will decrease rapidly with the increase of the number of acoustic liner layers, thereby achieving a good noise suppression effect.

4. A helicopter ducted tail rotor noise reduction liner for low-frequency line spectrum noise suppression according to claim 3, characterized in that, The configurations of the small-sized low-frequency line spectrum sound-absorbing structures inside the duct acoustic liner are the same, and the unit parameters of the small-sized low-frequency line spectrum sound-absorbing structures are the same or different; the structural parameters of each layer of acoustic liner are the same or different, and are determined based on the bandgap calculation results; if the operating speed fluctuation of the duct tail rotor is within ±5% during operation, the same sound-absorbing unit is used to make the duct acoustic liner. Conversely, it is necessary to optimize the design of sound-absorbing unit structural parameters with a wide first-order and second-order sound-absorbing frequency band, and combine different sound-absorbing units to create ducted sound liners to achieve good noise reduction effects.

5. A helicopter ducted tail rotor noise reduction liner for low-frequency line spectrum noise suppression according to any one of claims 1 to 4, wherein the fabrication and assembly of the ducted acoustic liner are as follows: Preparation of micro-perforated plate (1): The micro-perforated plate (1) is made of metal or composite material. First, it is machined to a thickness of [missing information]. t The area is S The surface of the plate, the aforementioned S =(π / 36*R) 2 Then based on the perforation rate p and aperture d Calculate the number of holes M =4 Sp / π d 2 Next, a corresponding number of evenly distributed microholes are cut on the surface of the plate using a laser. Finally, a wall with a radius of R is processed. The previously processed planar micro-perforated plate is placed on the wall and bent to form a curved micro-perforated plate with the same curvature as the duct wall. Fabrication of the curved back cavity: Based on the optimized structural parameters, a three-dimensional model is created in the three-dimensional modeling software and exported in a file format that can be recognized by the 3D printer. The file is then imported into the 3D printer, and photosensitive resin is selected as the material to complete the fabrication of the curved back cavity. Duct acoustic liner fabrication: First, the micro-perforated plate is glued to the curved back cavity to form a sound-absorbing unit. For ease of installation, the sound-absorbing units are glued together in sequence to form a quarter-ring structure. This step is repeated 4 times. K 4 times K A single quarter-ring structure; Installation of duct acoustic liner: Cut off part of the inner wall of the duct and leave a space with the same thickness and depth as the acoustic liner; apply glue to the inner wall of the duct and glue the quarter-ring structure from the above steps to the inside of the duct one by one until it is filled. The preparation and installation of the noise reduction liner for the ducted tail rotor are now complete.

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