Permeable acoustic metamaterial panels to reduce blade passing frequency noise

Abstract

The present invention provides a wing pass frequency noise reduction panel for reducing noise generated when a fluid moves due to the rotation of a fan, comprising: a main body provided with at least one fluid passage hole through which the fluid passes; and a plurality of wavelength tubes disposed inside the main body and communicating with the fluid passage hole, wherein the plurality of wavelength tubes have different lengths.

Description

Technology Field

[0001] The present invention relates to a blade pass frequency noise reduction panel that reduces blade pass frequency (BPF) noise generated in a fan of a cooling system. Background Technology

[0002] In general, air conditioning fans are used for cooling in machine systems where the temperature rises during operation, and as these fans rotate at a constant speed, peak noise is generated at the blade passing frequency (BPF).

[0003] Such wing-pass frequency noise has a narrow band but a high sound pressure level, which degrades the sound quality of general household appliances and acts as a significant disadvantage in terms of acoustic stealth for combat vehicles.

[0004] To reduce such noise, mechanical products generating fan noise can be completely enclosed with panels such as rigid walls, but this can cause the internal temperature to rise rapidly, leading to a decrease in operating performance or failure of the product.

[0005] For this reason, research on multi-perforated plates (MPP) that allow high-temperature fluids to pass through but block noise has been actively conducted recently.

[0006] For example, various types of acoustic metapanel designs are proposed, such as attaching a membrane with fine pores in the direction of fluid flow to allow fluid to pass through while blocking noise, or inserting a labyrinth structure, such as a coiling path, inside a panel with a certain thickness instead of a membrane to reduce noise transmitted through the panel.

[0007] However, in order to enhance noise reduction performance, conventional acoustic metapanels often have the sum of the cross-sectional areas of the fluid-permeable holes excessively small compared to the total panel cross-section, or the labyrinthine structure through which the fluid passes becomes complex. While a reduction in transmitted noise can be expected when the ratio of the hole area to the total panel cross-sectional area decreases or the flow path becomes complex, the flow rate of high-temperature fluid released to the outside is reduced due to increased flow resistance.

[0008] To overcome these drawbacks, an acoustic metapanel design is proposed that increases the cross-sectional ratio to 33% using an acoustic cage structure. However, the interior remains complex and the thickness increases, and noise reduction performance is shown only in a single target frequency band, so there is a limitation in that it is difficult to expect reduction of transmitted noise occurring at frequencies that are multiples of the wing pass-through frequency.

[0009] A technology related to such wing pass-through frequency noise reduction is presented in Korean Published Patent No. 10-1998-0060499 (October 7, 1998). The problem to be solved

[0010] The present invention aims to provide a wing pass-through frequency noise reduction panel for noise reduction across various target frequency bands, which has a simple structure. means of solving the problem

[0011] The present invention provides a wing pass frequency noise reduction panel for reducing noise generated when a fluid moves due to the rotation of a fan, comprising: a main body provided with at least one fluid passage hole through which the fluid passes; and a plurality of wavelength tubes disposed inside the main body and communicating with the fluid passage hole, wherein the plurality of wavelength tubes have different lengths.

[0012] In addition, a plurality of the above-mentioned wavelength tubes may be spaced apart from each other in the circumferential direction of the fluid passage hole.

[0013] In addition, a plurality of the above-mentioned wavelength tubes may be spaced apart from each other in the direction of fluid passage.

[0014] In addition, the plurality of the above-mentioned wavelength tubes include a first wavelength tube, a second wavelength tube, and a third wavelength tube, and the first wavelength tube, the second wavelength tube, and the third wavelength tube may be provided with sequentially shorter lengths.

[0015] In addition, the first wavelength tube, the second wavelength tube, and the third wavelength tube may be sequentially arranged in the circumferential direction of the fluid passage hole.

[0016] In addition, the plurality of the above-mentioned wavelength tubes may be arranged so that their lengths become shorter towards the rear of the main body based on the direction of fluid passage.

[0017] In addition, the fluid passage hole has a regular hexagonal cross-sectional shape, and the wave tube can be arranged along the cross-sectional shape of the fluid passage hole.

[0018] In addition, the length of the above wavelength tube can be determined by determining the target frequency band by considering multiples of the wing passing frequency generated by the fan, and identifying the starting frequency of the first bandgap from the dispersion curve obtained through acoustic analysis of the main body. Effects of the invention

[0019] The wing passage frequency noise reduction panel according to the present invention is arranged inside the main body such that a plurality of wavelength tubes having different lengths are connected to a fluid passage hole through which fluid can pass. As such, it has a much wider effective frequency band in each target frequency band and a high transmission loss value, thereby enabling the reduction of transmission noise even when minute changes in motor rotation speed occur at frequencies that are multiples of the wing passage frequency while securing a sufficient flow rate. Brief explanation of the drawing

[0020] FIG. 1 is a perspective view of a wing pass-through frequency noise reduction panel according to one embodiment of the present invention. Figure 2 is an enlarged view of part A shown in Figure 1. Figure 3 is a partial cutaway perspective view of part A shown in Figure 1. FIG. 4 is a perspective view of a part of section A shown in FIG. 1. Figure 5 is a perspective view of part A shown in Figure 1. Figure 6 is a partial enlarged view of Figure 5. Figure 7 is a cross-sectional view along the line AA shown in Figure 4. Figure 8 is a cross-sectional view along the BB line shown in Figure 4. Fig. 9 is a front view of the unit shown in Fig. 4. FIG. 10 is a partial perspective view of a wing pass-through frequency noise reduction panel according to another embodiment of the present invention. FIG. 11 is a front view of the first and last unit of the wing pass-frequency noise reduction panel shown in FIG. 10. FIG. 12 is a dispersion curve graph obtained by performing acoustic analysis to determine the length of a first wavelength tube connected to the first unit of a wing pass-through frequency noise reduction panel according to another embodiment of the present invention. FIG. 13 is a graph of the transmission loss curve calculated by connecting the unit bodies of a wing pass-frequency noise reduction panel according to another embodiment of the present invention. FIG. 14 is a graph of the transmission loss curve of a unit connected to a designed second wavelength tube and a third wavelength tube of a wing pass-through frequency noise reduction panel according to another embodiment of the present invention. FIG. 15 is a frequency band comparison graph of a wing pass-through frequency noise reduction panel formed with a unit having the same wavelength tube length as a design model that forms a unit array connecting the designed first, second, and third wavelength tubes of a wing pass-through frequency noise reduction panel according to another embodiment of the present invention. FIG. 16 is a sound pressure distribution diagram of the third unit finally designed at the center frequency of the target frequency band of the wing pass-through frequency noise reduction panel according to one embodiment of the present invention. Specific details for implementing the invention

[0021] Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the attached drawings. Prior to this, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, and based on the principle that the inventor can appropriately define the concepts of the terms to best describe his invention, they should be interpreted in a meaning and concept consistent with the technical spirit of the present invention.

[0022] Therefore, the embodiments described in this specification and the configurations illustrated in the drawings are merely the most preferred embodiments of the present invention and do not represent all of the technical ideas of the present invention; thus, it should be understood that there may be equivalent variations that can replace them at the time of filing this application.

[0023] First, metamaterials are materials artificially designed to possess unique properties not found in nature, and they are composed of repeating arrangements of monomers.

[0024] Here, acoustic metamaterials can control wave energy such as sound waves, and the acoustic properties of the acoustic metamaterials can be determined from the dispersion curve obtained through acoustic analysis of the unit, where the band gap, the region where the wavenumber becomes imaginary, coincides with the noise reduction band.

[0025] In other words, the bandgap region is the frequency range where transmitted noise is reduced compared to incident noise when an acoustic metamaterial is implemented by repeating unit cells.

[0026] The present invention relates to a wing-passing frequency noise reduction panel for reducing fluid noise, wherein a plurality of wavelength tubes for reducing fluid noise in a corresponding frequency band are arranged in communication with a fluid passage hole through which fluid passes, thereby reducing fluid noise in a corresponding frequency band without obstructing the fluid flow passing through the plurality of wavelength tubes. Referring to the drawings, the invention is as follows.

[0027] Referring to FIGS. 1 to 16, a wing pass frequency noise reduction panel according to one embodiment of the present invention may include a main body (100) and a wavelength tube (200). The wing pass frequency noise reduction panel of one embodiment is placed on a fluid path where fluid moves due to the rotation of a fan, and reduces noise generated at the wing pass frequency (BPF) as the fan rotates.

[0028] The above main body (100) can be placed on a fluid path where fluid travels.

[0029] The above-mentioned main body (100) has a plate shape and may be provided with at least one fluid passage hole (110) through which fluid moved by the rotation of the fan passes.

[0030] The above fluid passage hole (110) may be provided to have a regular hexagonal cross-sectional shape, but is not limited thereto. When the fluid passage hole (110) is formed with a regular hexagonal cross-sectional shape in this way, a sufficient flow rate of the fluid passing through can be secured.

[0031] The above wavelength tube (200) can be placed inside the main body (100).

[0032] One side of the above-mentioned wavelength tube (200) in the longitudinal direction is connected to a fluid passage hole (110), allowing some of the fluid passing through the fluid passage hole (110) to flow in, thereby serving to reduce noise caused by the blade passing frequency due to the rotation of the fan.

[0033] The above-mentioned wavelength tube (200) may be configured in multiple numbers on the inside of the main body (100).

[0034] In addition, multiple wavelength tubes (200) may be provided with different lengths to increase the noise reduction effect according to the wing pass frequency band.

[0035] For example, a plurality of the above-mentioned wavelength tubes (200) may be spaced apart from each other in the circumferential direction of the fluid passage hole (110).

[0036] Here, the plurality of wavelength tubes (200) may include a first wavelength tube (210), a second wavelength tube (220), and a third wavelength tube (230).

[0037] Additionally, the first wavelength tube (210), the second wavelength tube (220), and the third wavelength tube (230) may be provided with sequentially shorter lengths. For example, the first wavelength tube (210) may have the longest length, the second wavelength tube (220) may have the second longest length, and the third wavelength tube (230) may have the shortest length.

[0038] The first wavelength tube (210), the second wavelength tube (220), and the third wavelength tube (230) can be arranged sequentially in the circumferential direction of the fluid passage hole (110).

[0039] At this time, the first wavelength tube (210), the second wavelength tube (220), and the third wavelength tube (230) are each arranged along the cross-sectional shape of the fluid passage hole (110), thereby minimizing the installation space within the main body (100).

[0040] In addition, as shown in FIGS. 5 and 6, a plurality of the above-mentioned wavelength tubes (200) may be spaced apart from each other in the direction of passage of the fluid passing through the fluid passage hole (110).

[0041] And, as shown in FIG. 10, a plurality of the above-mentioned wavelength tubes (200) may be provided such that their lengths become shorter as they move toward the rear side of the main body (100) with respect to the direction of fluid passage. For example, the first wavelength tube (210), the second wavelength tube (220), and the third wavelength tube (230) positioned at the foremost side of the main body (100) with respect to the direction of fluid passage have the longest lengths, and the first wavelength tube (210), the second wavelength tube (220), and the third wavelength tube (230) may be provided such that their lengths become shorter sequentially as they move toward the rear side of the main body (100).

[0042] Here, the length of the above-mentioned wavelength tube (200) is determined by determining the target frequency band by considering multiples of the wing passing frequency generated by the fan, and by identifying the band gap from the dispersion curve obtained by acoustic analysis of the main body (100).

[0043] The determination of the length of the above-mentioned wavelength tube (200) is explained in detail as follows. Here, the explanation regarding the determination of the length of the above-mentioned wavelength tube (200) is based on the unit body (U), which is a part of the wing pass-through frequency noise reduction panel shown in FIG. 4.

[0044] First, the motor rotating the cooling fan with m blades rotates at a constant speed, and the blade passing frequency (BPF: ) determined by this constant speed rotation angular velocity (Ω [rpm]) [Hz]) is expressed as in Equation 1. However, since the motor connected to the fan operates under open-loop control, the actual rotational speed increases or decreases slightly around the known rotational angular velocity. Additionally, noise peak frequencies exist at multiples of this blade pass-through frequency. Considering these two factors, the target frequency for reducing peak noise generated by the fan ( ) is expressed as in mathematical equation 2. In the corresponding equation is a term included to account for minute changes in the rotational angular velocity of the motor. Therefore, the nth target frequency band can be expressed as in Equation 3.

[0045]

[0046]

[0047]

[0048] Considering that peak noise caused by the rotation of the fan is generated at integer multiples of the BPF (1, 2, 3, 4, 5 times, etc.) and that the quarter-wave tube can reduce noise at 1, 3, 5 times the fundamental frequency, etc., a design strategy for a unit composed of three wavelength tubes (200), namely the first, second, and third wavelength tubes (210, 220, 230, Side branch), as shown in FIG. 9, is established. If the fundamental frequency of the first wavelength tube (210, Side branch A) is designed to match the blade passing frequency (BPF), noise can be reduced at frequencies 1, 3, and 5 times the BPF, and if the fundamental frequency of the second wavelength tube (220, Side branch B) is designed to match 2 times the BPF, noise can be reduced at frequencies 2, 6, and 10 times the BPF. Likewise, if the fundamental frequency of the third wavelength tube (230, Side branch C) is designed to match four times that of the BPF, noise can be reduced at frequencies four, twelve, and twenty times that of the BPF.

[0049] In order to account for changes in the noise peak frequency due to minute changes in the rotational speed of a rotating motor, an analysis model in which multiple units are arranged in the direction of fluid flow must have a noise reduction frequency band of a certain width or larger. Generally, it is known that arranging about 5 to 6 identical units provides a noise reduction frequency band of some degree, but methods to modify some of the structure of the arranged units to further expand the width are also being introduced. Accordingly, in the present invention, the length of each wavelength tube (200) of the first and last units arranged is determined to match the frequencies at both ends of the corresponding target frequency band, and the length of each wavelength tube (200) of the units in between is determined by linear interpolation.

[0050] For example, as shown in FIG. 10, when N units composed solely of a first wavelength tube (210, Side branch A) are arranged, as shown in FIG. 11 (a) and (b), the lengths of the first wavelength tubes (210, Side branch A) of the first and last units (each and After determining the value (noted as ) to satisfy Equation 4, the length of the first wavelength tube (210, Side branch A) of the n-th unit existing therein ( ) is reduced linearly using Equation 5. The lengths of the second wavelength tube (220, Side branches B) and the third wavelength tube (230, Side branches C) of each unit are also determined in the same way. In the actual acoustic design process, since the peak frequency of transmission loss does not exist exactly at the value determined by Equation 4, the value determined by Equation 4 is selected as the initial value, and some tuning is required during the acoustic analysis process.

[0051]

[0052]

[0053] And, the motor's rotational speed (Ω), number of blades (m), and the bandwidth of the target frequency ( ), panel thickness and external dimensions of the unit (thickness, height, width), regular hexagonal fluid passage hole (110) and one side length of the cross-section of the unit ( and This is a design problem to determine the lengths of the three tubes constituting the unit body with the condition determined. The values ​​used for this purpose are shown in Table 1. Using these values, the porosity of the unit body cross-section in Figure 4 is set to approximately 40.31%. In other words, not only is transmitted noise reduced, but the flow rate through it also has a value greater than that of conventional acoustic metamaterial panels.

[0054] motor rotation speed Ω 6800 rpm Number of motor blades m 6 Width of target frequency 40 Hz Thickness of the entire panel 90 mm Thickness of the unit 15.0 mm Height of the unit 70.7 mm Width of the unit 122.5 mm The length of one side of the cross-section of a regular hexagonal unit 69.3 mm Side length of the cross-section of a regular hexagonal fluid passage hole 44.0 mm Thickness of the panel bulkhead w 2.5 mm Thickness of the internal cavity 10 mm

[0055] When 6800 rpm for the motor's rotational speed (Ω) and 6 for the motor's blade count (m) are substituted into Equation 1, the BPF becomes 680 Hz. Furthermore, if the frequency band of interest is limited to 4000 Hz, it can be anticipated that there may be peaks in fan noise at frequencies 1 to 5 times the BPF. Therefore, a design that increases the transmission loss value around these 5 frequencies must be implemented. Based on this, the calculated target frequency bands (I ~ V) are shown in Table 2. Accordingly, the noise reduction frequency band of the first wavelength tube (210, Side branch A) should be designed to include target frequency bands I, III, and V, while the target frequency bands of the second wavelength tube (220, Side branches B) and the third wavelength tube (230, Side branches C) should be designed to include target frequencies II and IV, respectively.

[0056] Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ

[0057] Using the starting frequency (660 Hz) of the target frequency band I and Equation 4) After selecting the initial value of , the dispersion curve of the unit connected to the first wavelength tube (210, Side branch A) is obtained. When obtaining the dispersion curve for a single unit, Bloch Floquet boundary conditions are applied to both boundaries, and acoustic analysis is performed by assuming that all remaining boundaries are rigid wall conditions. (At this time, the density physical property of air at a temperature of 300℃ , )

[0058] FIG. 12(a) shows a dispersion curve graph obtained by performing acoustic analysis to determine the length of the first wavelength tube (210, Side branch A) connected to the first unit. It can be seen that a band gap exists between the first and second dispersion curves. It is known that this frequency band almost coincides with the noise reduction frequency band of an acoustic metamaterial arranged with such units. Therefore, several iterative calculations are performed so that the starting frequency of this band gap matches the starting frequency of the target frequency. The length of the first wavelength tube (210, Side branch A) of the first unit obtained in this way ( ) is 195 mm, and in the same way, considering the end frequency (700 Hz) of the target frequency band I as in FIG. 12 (b), the length of the first wavelength tube (210, Side branch A) of the last unit ( : N=6) is determined. And, the length of the first wavelength tube (210, Side branch A) connected to the 2nd to 5th unit ( ~ ) is calculated using mathematical formula 5. As an example , The transmission loss curve calculated by connecting the unit designed in this way is as shown in Fig. 13. Referring to Fig. 13, it can be seen that there are high transmission loss values ​​in the target frequency bands I, III, and V.

[0059] Then, the lengths of the second wavelength tube (220, Side branch B) and the third wavelength tube (230, Side branch B) of each unit are determined using the method for determining the length of the first wavelength tube (210, Side branch A) described above.

[0060] The above second wavelength tube (220, Side Branch B) is designed considering the target frequency band II. For example, , , , , .

[0061] In addition, the length of the third wavelength tube (230, Side branch C) of each unit is determined by considering the target frequency band IV. For example, , , , , .

[0062] FIG. 14 (a) shows the transmission loss curve of a unit connected to a second wavelength tube (220, Side branch B) designed in this way, and FIG. 14 (b) shows the transmission loss curve of a unit connected to a third wavelength tube (230, Side branch C). Referring to FIG. 14 (a) and (b), it can be seen that high transmission loss values ​​are shown in the target frequency band, respectively.

[0063] Thus, the design model forming a unit array by connecting the first, second, and third wavelength tubes (210, 220, 230) designed in this way is as shown in FIG. 15 (a). Here, FIG. 15 (b) shows an acoustic metal material designed by repeating the same unit. That is, the unit arrays arranged in FIG. 15 (b) have the same length for each wavelength tube (200).

[0064] For example, : , , The transmission loss curves of these two design models can be seen in Fig. 15(c). In Fig. 15(d), the previously set target frequency band (Table 2) and the actual noise reduction frequency band are compared. On the transmission loss curve, the noise reduction frequency band is selected from the frequency at which the signal rises by more than 5 dB and the frequency at which it decreases by less than 5 dB. As a result, it can be seen that a much wider noise reduction frequency band is formed centered around the target frequency band.

[0065] Figure 16 shows the sound pressure distribution of the third unit of the final designed acoustic metamaterial panel at the center frequency of the target frequency band. It can be easily seen that, in accordance with the design purpose, the first wavelength tube (210, Side branch A) performs an acoustic function for noise reduction at frequencies 1 / 3 / 5 times that of the BPF (first (Fig. 16 (a)), third (Fig. 16 (c)), and fifth (Fig. 16 (e)). Of course, it can also be seen that the second wavelength tube (220, Side branches B) and the third wavelength tube (230, Side branches C) also perform their acoustic roles for noise reduction at the second (Fig. 16 (b)) and fourth (Fig. 16 (d)) of the BPF as originally intended.

[0066] The wing pass-frequency noise reduction panel of the present invention reduces noise generated by a cooling fan used to dissipate heat generated inside a mechanical system to the outside.

[0067] In this embodiment, a wing-passing frequency noise reduction panel has multiple wavelength tubes (200) of different lengths connected to a regular hexagonal fluid passage hole (110) through which fluid can pass. At this time, the dimensions of the fluid passage hole (110) are determined to ensure sufficient flow rate. In each unit, the multiple wavelength tubes (200) connected to the fluid passage hole (110) each act as independent quarter wavelength tubes, and the length of the wavelength tubes (200) is determined by considering multiples of the wing-passing frequency. Additionally, in order to reduce transmitted noise even with changes in peak noise frequency caused by minute changes in the rotational speed of the motor rotating the cooling fan, the length of each of the multiple wavelength tubes (200) constituting each unit in the unit array is varied linearly. Since the breathable acoustic metamaterial panel designed in this way has a much wider effective frequency band in each target frequency band and a high transmission loss value, it is possible to reduce transmitted noise even when minute changes in the motor rotational speed occur at frequencies that are multiples of the wing-passing frequency while ensuring sufficient flow rate.

[0068] The present invention has been described with reference to the embodiments illustrated in the drawings, but this is merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the true technical scope of protection of the present invention should be determined by the technical spirit of the appended claims. Explanation of the symbols

[0069] 100: Main body 110: Fluid passage hole 200: Wave Section 210: 1st Wave Section 220: Second Wave Minister 230: Third Wave Minister

Claims

Claim 1 A wing passage frequency noise reduction panel for reducing noise generated when a fluid moves due to the rotation of a fan comprises: a main body (100) provided with at least one fluid passage hole (110) through which a fluid passes; and a plurality of wavelength tubes (200) disposed inside the main body (100) and communicating with the fluid passage hole (110), wherein the fluid passage hole (110) has a regular hexagonal cross-sectional shape, and the wavelength tubes (200) are arranged along the cross-sectional shape of the fluid passage hole (110), and the plurality of wavelength tubes (200) include a first wavelength tube (210), a second wavelength tube (220), and a third wavelength tube (230) having different lengths, and the first wavelength tube (210), the second wavelength tube (220), and the third wavelength tube (230) are provided with sequentially shorter lengths, and the length of the wavelength tubes (200) is a multiple of the wing passage frequency generated by the fan. A wing pass-through frequency noise reduction panel determined by determining the target frequency band by considering and identifying the starting frequency of the first band gap from the dispersion curve obtained by acoustic analysis of the main body (100). Claim 2 In claim 1, the plurality of wavelength tubes (200) are wing-passing frequency noise reduction panels spaced apart from each other in the circumferential direction of the fluid passage hole (110). Claim 3 In claim 1, the plurality of wavelength tubes (200) are wing-pass frequency noise reduction panels spaced apart from each other in the direction of fluid passage. Claim 4 delete Claim 5 In claim 1, the first wavelength tube (210), the second wavelength tube (220), and the third wavelength tube (230) are sequentially arranged in the circumferential direction of the fluid passage hole (110) to form a wing passage frequency noise reduction panel. Claim 6 In claim 3, the plurality of wavelength tubes (200) are arranged such that their lengths become shorter as they move toward the rear of the main body (100) with respect to the direction of fluid passage, forming a wing passage frequency noise reduction panel. Claim 7 delete Claim 8 delete

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