Sound absorbing panel with riblet

Abstract

This sound absorbing panel with a riblet is used by attaching the same at a position facing an air flow path. The sound absorbing panel comprises a porous plate that is provided with the riblet and through holes and that faces the air flow path, and a cell that has internal space which is linked with the air flow path via the through holes. The riblet is provided on the surface of the porous plate that faces the air flow path, and is not provided to the back surface of the porous plate or the interior surfaces of the through holes.

Description

Acoustic panel with riblets

[0001] The present disclosure relates to ribletted acoustic panels.

[0002] Patent Document 1 discloses an acoustic panel for a turbine engine. The acoustic panel includes a support layer including a set of divided cavities having open faces, an exterior sheet bonded to cover and close the open faces, perforations, and inlets of at least a subset of the perforations oriented axially relative to the axial flow path. The perforations are located in the opposing sheet and are in fluid communication with corresponding ones of the cavities to form a set of acoustic resonators.

[0003] U.S. Patent No. 1,096,1913

[0004] According to the technology described in Patent Document 1, non-circular perforations along the axial direction are used to reduce air resistance and friction loss that occurs at and behind the perforations. However, there is a problem in that it is not possible to achieve both reduction in friction loss and noise absorption over the entire surface of the perforated panel that faces the air flow path of the sound-absorbing panel.

[0005] The present disclosure has been made in view of the above-mentioned problems, and has an object to provide a sound-absorbing panel with riblets that can absorb noise generated in an air flow path while reducing friction loss caused by the flow in the air flow path.

[0006] The riblet-equipped sound-absorbing panel according to the present disclosure is attached to a position facing an air flow path when used. It comprises a perforated plate that faces the air flow path and is provided with riblets and through holes, and cells having internal spaces that communicate with the air flow path via the through holes. The riblets are provided on the surface of the perforated plate that faces the air flow path, and are not provided on the back surface of the perforated plate or on the inner surfaces of the through holes.

[0007] The center-to-center spacing of the multiple through holes along the main flow on the surface may be defined as a first distance, and the center-to-center spacing of the multiple through holes along a direction perpendicular to the main flow may be defined as a second distance, and the first distance and the second distance may be determined based on the proportion of the area of ​​the through holes to the surface.

[0008] The first distance may be different from the second distance.

[0009] The first distance may be less than the second distance.

[0010] The first distance may be less than 1.16 times the second distance.

[0011] The product of the first distance and the second distance may be determined based on a ratio.

[0012] The product of the first distance and the second distance may be determined inversely proportional to the ratio.

[0013] The first distance may be greater than or equal to three times the radius of the through hole.

[0014] A riblet arranged at a position at least a predetermined distance from the center of a through hole along a direction perpendicular to the main flow on the surface is referred to as a first riblet. A riblet arranged at a position less than the predetermined distance from the center of a through hole is referred to as a second riblet. Here, the pattern of the second riblet may be different from the pattern of the first riblet.

[0015] The height of the second riblet may be less than the height of the first riblet.

[0016] The width of the second riblet may be less than the width of the first riblet.

[0017] The height of the second riblets may approach the height of the first riblets along the main flow path as they approach other through-holes adjacent to the through-hole downstream of the main flow path.

[0018] The width of the second riblet may approach the width of the first riblet along the main flow path as it approaches another through-hole adjacent to the through-hole downstream of the main flow path.

[0019] The predetermined distance may be equal to or greater than the radius of the through-hole.

[0020] The predetermined distance may be equal to or less than 1.4 times the radius of the through hole.

[0021] According to the present disclosure, it is possible to provide a sound-absorbing panel with riblets that can absorb noise generated within an air flow path while reducing friction loss caused by the flow within the air flow path.

[0022] Fig. 1 is an external view of a sound-absorbing panel with riblets according to a first embodiment of the present disclosure; Fig. 2 is a cross-sectional view of a sound-absorbing panel with riblets in the vicinity of a through-hole; Fig. 3 is a cross-sectional view showing an example of a riblet; Fig. 4 is an external view of a sound-absorbing panel with riblets according to a second embodiment of the present disclosure; Fig. 5 is an external view of a sound-absorbing panel with riblets according to a third embodiment of the present disclosure; Fig. 6 is a diagram showing an example of the relationship between sound pressure and the increase in friction loss;

[0023] Hereinafter, several exemplary embodiments will be described with reference to the drawings. Note that common parts in the drawings are given the same reference numerals, and duplicated explanations will be omitted.

[0024] [Configuration of riblet-equipped sound-absorbing panel according to the first embodiment] Figure 1 is an external view of a riblet-equipped sound-absorbing panel according to the first embodiment of the present disclosure. The riblet-equipped sound-absorbing panel 10 is attached to an external space through which noise propagates, for example, facing the air flow path, in order to absorb noise generated by a device having an air flow path, such as a jet engine. In other words, the riblet-equipped sound-absorbing panel 10 is attached in a position facing the air flow path when in use.

[0025] Fig. 1 shows the external appearance (part) of a riblet-equipped sound-absorbing panel 10 as seen from the air flow path side. As shown in Fig. 1, the riblet-equipped sound-absorbing panel 10 has a surface ST on which a plurality of through holes HL are formed. Furthermore, riblets RB are formed in areas AR of the surface ST where no through holes HL are arranged.

[0026] Figure 2 is a cross-sectional view of a riblet-equipped sound-absorbing panel near a through hole. The riblet-equipped sound-absorbing panel 10 includes a perforated plate PT that is provided with riblets RB and through holes HL and faces the air flow path, and cells CV that have internal spaces that communicate with the air flow path via the through holes HL. Figure 2 shows that the cells CV are formed by the perforated plate PT, a bottom wall PB, and partition walls PI, and that the internal spaces of the cells CV communicate with the air flow path above the perforated plate PT via the through holes HL.

[0027] 1 and 2, the riblet-equipped sound-absorbing panel 10 has a large number of through-holes HL that connect the cells CV to the air flow path, allowing the cells CV to function as Helmholtz resonators. As a result, the riblet-equipped sound-absorbing panel 10 absorbs noise in the air flow path.

[0028] The riblets RB are provided on the surface ST of the perforated plate PT that faces the air flow path, and are not provided on the back surface SB of the perforated plate PT or the inner surfaces SI of the through holes HL. This means that the riblets RB are provided only on the surface ST of the perforated plate PT, which reduces the cost of forming the riblets RB. The riblets RB are formed by various methods, such as a transfer sheet molding method or a laser processing method. The method of forming the riblets RB on the surface ST is not limited to the examples given here.

[0029] Figure 3 is a cross-sectional view showing an example of a riblet. The riblet RB is a fine vertical groove structure (pattern) applied to the surface ST of the perforated plate PT in order to reduce the turbulent frictional resistance of the fluid. Various types of riblets RB can be used for the riblet-equipped sound-absorbing panel 10. Parameters that characterize the pattern of the riblet RB include the width RW between the apexes of the riblet RB and the height RH of the riblet RB. The parameters that characterize the pattern of the riblet RB are not limited to the examples given here.

[0030] Forming riblets RB on the surface ST reduces noise in the airflow path and also reduces the area where the fast airflow in the airflow directly hits the surface ST. As a result, friction loss caused by the airflow in the airflow path can be reduced. Surface resistance friction on the surface ST can also be reduced.

[0031] Next, the arrangement of the through holes HL will be described. As shown in Fig. 1, the center-to-center spacing between the multiple through holes HL along the main flow direction MD of the airflow path is defined as a first distance S1. The center-to-center spacing between the through holes HL along a direction perpendicular to the main flow direction MD is defined as a second distance S2.

[0032] The first distance S1 and the second distance S2 are determined based on the ratio of the area of ​​the through holes HL to the surface ST (opening ratio), which is set based on various conditions such as the sound-absorbing performance to be imparted to the riblet-equipped sound-absorbing panel 10.

[0033] The first distance S1 may be set to a value different from the second distance S2. In particular, the first distance S1 may be set to a value smaller than the second distance S2. Directly below the main flow direction MD of the through hole HL, the main flow is influenced by the through hole HL, and frictional stress tends to increase. On the other hand, to the side of the through hole HL (in a direction perpendicular to the main flow direction MD), the increase in frictional stress due to the main flow being influenced by the through hole HL is small.

[0034] By setting the first distance S1 to be smaller than the second distance S2, it is possible to expand the flow range that is less susceptible to the influence of the through holes HL. Here, the width RW of the riblets RB that are effective for the mainstream is larger than the width RW of the riblets RB that are effective for the flow downstream of the through holes HL. Therefore, it is possible to reduce the cost of forming the riblets RB.

[0035] The first distance S1 may be set to be smaller than 1.16 times the second distance S2. For example, when the through holes HL are arranged in a triangular lattice pattern, the first distance S1 is 2√3 / 3 times the second distance S2 (approximately 1.16 times the second distance S2). Compared to when the through holes HL are arranged in a triangular lattice pattern, setting the first distance S1 to be smaller than 1.16 times the second distance S2 also makes it possible to expand the flow range that is less susceptible to the influence of the through holes HL. As a result, the cost of forming the riblets RB can be reduced.

[0036] Alternatively, the product of the first distance S1 and the second distance S2 may be determined based on the ratio (opening rate) of the area of ​​the through holes HL to the surface ST. In particular, the product of the first distance S1 and the second distance S2 may be determined inversely proportional to the ratio of the area of ​​the through holes HL to the surface ST. In designing the riblet-equipped sound-absorbing panel 10, the radius R of the through holes HL and the open area rate are determined, and then the first distance S1 and the second distance S2 are set to determine the arrangement of the through holes HL. By determining the product of the first distance S1 and the second distance S2 based on a ratio, it is possible to easily expand the flow range that is less affected by the through holes HL and reduce the cost of forming the riblets RB.

[0037] Furthermore, the first distance S1 may be three times or more the radius R of the through holes HL. This prevents the through holes HL from getting too close to each other, and ensures the strength of the porous plate PT.

[0038] [Configuration of riblet-equipped sound-absorbing panel according to second embodiment] Figure 4 is an external view of a riblet-equipped sound-absorbing panel according to a second embodiment of the present disclosure. Figure 4 shows the external appearance (part) of the riblet-equipped sound-absorbing panel 10 as viewed from the air flow path side. As shown in Figure 4, the riblet-equipped sound-absorbing panel 10 has a plurality of through holes HL formed on the surface ST. Furthermore, the area of ​​the surface ST where no through holes HL are arranged is made up of areas AR1 and AR2.

[0039] Here, an area that is a predetermined distance or more from the center of the through hole HL along a direction perpendicular to the main flow direction MD is defined as area AR1. An area that is less than a predetermined distance from the center of the through hole HL along a direction perpendicular to the main flow direction MD is defined as area AR2. For the sake of explanation, the riblets RB arranged in area AR1 are referred to as first riblets, and the riblets RB arranged in area AR2 are referred to as second riblets.

[0040] In Figure 4, the pattern of the second riblets is different from the pattern of the first riblets. For example, the height RH of the second riblets may be smaller than the height RH of the first riblets. The width RW of the second riblets may be smaller than the width RW of the first riblets.

[0041] Region AR1 can be said to be a flow range that is less affected by the through holes HL. Therefore, it is desirable that the pattern of the first riblets arranged in region AR1 be a pattern that is effective for the main flow. On the other hand, it is desirable that the pattern of the second riblets arranged in region AR2 be a pattern that is effective for the flow after it has been affected by the through holes HL. Therefore, the height RH of the second riblets is made smaller than the height RH of the first riblets, and the width RW of the second riblets is made smaller than the width RW of the first riblets. This makes it possible to reduce the cost of forming the riblets RB.

[0042] The pattern of the second riblets arranged in region AR2 may be set based on their positional relationship with the through holes HL. For example, the height RH of the second riblet may be set to approach the height RH of the first riblet as it approaches another through hole HL adjacent to the through hole HL on the downstream side of the mainstream along the mainstream direction MD. Similarly, the width RW of the second riblet may be set to approach the width RW of the first riblet as it approaches another through hole HL adjacent to the through hole HL on the downstream side of the mainstream along the mainstream direction MD.

[0043] For example, let us compare the pattern of second riblets at point P1 and the pattern of second riblets at point P2 with the pattern of first riblets at point P3. Points P1 and P2 are arranged in order in the mainstream flow direction MD. That is, point P2 is located downstream of point P1.

[0044] Point P1 is close to the upstream through-hole HL and is a point where the flow is more influenced by the upstream through-hole HL. On the other hand, point P2 is farther from the upstream through-hole HL and is a point where the flow is less influenced by the upstream through-hole HL. Therefore, compared to the flow at point P1, the flow at point P2 is closer to the flow at point P3. In other words, the further away from the upstream through-hole HL and the closer to the downstream through-hole HL, the smaller the influence of the upstream through-hole HL on the flow.

[0045] Therefore, the pattern of the second riblets may be set to approach the pattern of the first riblets as they approach other through holes HL adjacent to the through hole HL downstream of the mainstream along the mainstream direction MD, thereby increasing the number of riblets that are effective for the mainstream.

[0046] Alternatively, the predetermined distance used to define the regions AR1 and AR2 may be equal to or greater than the radius R of the through hole HL. Alternatively, the predetermined distance may be equal to or less than 1.4 times the radius R1. This limits the region where riblets effective for the flow affected by the through hole HL are arranged, and increases the region where riblets effective for the main flow are arranged. As a result, it is possible to more effectively reduce friction loss caused by the flow in the airflow passage.

[0047] [Configuration of riblet-equipped sound-absorbing panel according to third embodiment] Figure 5 is an external view of a riblet-equipped sound-absorbing panel according to a third embodiment of the present disclosure. Figure 5 shows the external appearance (part) of the riblet-equipped sound-absorbing panel 10 as viewed from the air flow path side. As shown in Figure 5, the riblet-equipped sound-absorbing panel 10 has a plurality of through holes HL formed in the surface ST. Furthermore, within the surface ST, an area AR3 exists downstream of an obstacle OB along the main flow direction MD.

[0048] As a result of the obstruction of the mainstream flow by the obstacle OB, the speed of the mainstream in the area AR3 is less than a predetermined speed, and the speed of the mainstream on the surface ST other than the area AR3 is equal to or greater than the predetermined speed. Here, the width of the riblets (riblets for high-speed flow areas) arranged on the surface ST other than the area AR3 may be smaller than the width of the riblets (riblets for low-speed flow areas) arranged in the area AR3.

[0049] For example, the width of the riblets at point P5 (riblets for the high-speed flow region) is smaller than the width of the riblets at point P4 (riblets for the low-speed flow region). This makes it possible to provide both a pattern that is effective for the high-speed main flow of the main flow in the airflow path and a pattern that is effective for the low-speed main flow of the main flow on the surface ST of the porous plate PT. As a result, it becomes possible to more effectively reduce friction loss caused by the flow in the airflow path.

[0050] [Configuration of riblet-equipped sound-absorbing panel according to the fourth embodiment] Fig. 6 is a diagram showing an example of the relationship between sound pressure and the increase in friction loss. In order to investigate the sound absorption effect of the riblet-equipped sound-absorbing panel, a test was conducted to investigate the change in friction loss between the panel and the air flow path by changing the sound pressure.

[0051] As a result of the test, the friction loss increased by 0.3% when an applied sound pressure of 135 dB was compared to the friction loss when no sound pressure was applied. Furthermore, the friction loss increased by 3.1% when an applied sound pressure of 145 dB was compared to the friction loss when no sound pressure was applied. In other words, the increase in friction loss when an applied sound pressure of 145 dB was approximately 10 times the increase in friction loss when an applied sound pressure of 135 dB was compared to the increase in friction loss when an applied sound pressure of 135 dB was applied. This is presumably due to an increase in loss due to blowing out of the through holes as the applied sound pressure increased.

[0052] Riblets are designed to reduce losses in the mainstream boundary layer by the flow emitted from the through-holes, but they cannot achieve sufficient loss reduction as they are. Therefore, in areas where sound pressure is high, it is necessary to suppress the flow emitted from the through-holes.

[0053] In this embodiment, through holes arranged at positions where the sound pressure on the surface is equal to or greater than a predetermined sound pressure are referred to as through holes for a high sound pressure range. Through holes arranged at positions where the sound pressure on the surface is less than the predetermined sound pressure are referred to as through holes for a low sound pressure range. The diameter of the through holes for a high sound pressure range is made smaller than the diameter of the through holes for a low sound pressure range. This makes it possible to provide both through holes effective for high sound pressures and through holes effective for low sound pressures on the surface ST of the perforated plate PT. As a result, it is possible to more effectively reduce friction loss caused by the flow in the air flow path.

[0054] [Effects of the embodiment] As described in detail above, the riblet-equipped sound-absorbing panel according to the present disclosure is attached to a position facing the air flow path when used. It comprises a perforated plate that is provided with riblets and through holes and faces the air flow path, and cells that have internal spaces that communicate with the air flow path via the through holes. The riblets are provided on the surface of the perforated plate that faces the air flow path, and are not provided on the back surface of the perforated plate or on the inner surfaces of the through holes.

[0055] This makes it possible to absorb noise generated within the airflow path while reducing friction loss caused by the flow within the airflow path, thereby achieving both a reduction in noise emitted by aircraft and other devices equipped with airflow paths and improved energy efficiency.

[0056] The center-to-center spacing of the plurality of through holes along the main flow on the surface may be defined as a first distance, and the center-to-center spacing of the plurality of through holes along a direction perpendicular to the main flow may be defined as a second distance, and the first and second distances may be determined based on the proportion of the area of ​​the through holes to the surface. This makes it possible to easily expand the flow area that is less affected by the through holes in the design of the sound-absorbing panel with riblets. As a result, the cost of forming the riblets can be reduced.

[0057] The first distance may be different from the second distance, or may be smaller than the second distance, which can easily widen the flow area that is less affected by the through holes and reduce the cost of forming the riblets.

[0058] The first distance may be smaller than 1.16 times the second distance. This allows design changes to be made based on the case where the through holes HL are arranged in a triangular lattice pattern. This makes it possible to easily expand the flow area that is less affected by the through holes and reduce the cost of forming riblets.

[0059] The product of the first distance and the second distance may be determined based on a ratio. Alternatively, the product of the first distance and the second distance may be determined inversely proportional to the ratio. In designing a sound-absorbing panel with riblets, after determining the radius and open area ratio of the through holes, the first distance and the second distance are set to determine the arrangement of the through holes. By determining the product of the first distance and the second distance based on a ratio, it is possible to easily expand the flow area that is less affected by the through holes and reduce the cost of forming the riblets.

[0060] The first distance may be three times or more the radius of the through holes, which prevents the through holes from coming too close to each other and ensures the strength of the perforated plate.

[0061] Riblets arranged at a position at least a predetermined distance from the center of a through hole along a direction perpendicular to the main flow on the surface are referred to as first riblets. Riblets arranged at a position less than a predetermined distance from the center of a through hole are referred to as second riblets. The pattern of the second riblets may be different from the pattern of the first riblets. This makes it possible to provide both a pattern that is effective for the main flow in the air flow path and a pattern that is effective for the flow after being affected by the through holes on the surface ST of the porous plate PT. As a result, friction loss caused by the flow in the air flow path can be more effectively reduced.

[0062] The height of the second riblets may be smaller than the height of the first riblets. Also, the width of the second riblets may be smaller than the width of the first riblets. This makes it possible to arrange the riblets in a pattern that is effective for the flow after being affected by the through holes. As a result, it is possible to reduce friction loss caused by the flow in the air flow path.

[0063] The height of the second riblets may approach the height of the first riblets as they approach other through-holes adjacent to the through-hole on the downstream side of the main flow along the main flow. Furthermore, the width of the second riblets may approach the width of the first riblet as they approach other through-holes adjacent to the through-hole on the downstream side of the main flow along the main flow. This allows the number of riblets arranged in a pattern that is effective with respect to the main flow in the airflow path to be increased. As a result, the effect of reducing friction loss caused by the flow in the airflow path can be enhanced. Furthermore, the cost of forming the riblets can be reduced.

[0064] The predetermined distance may be equal to or greater than the radius of the through hole. Alternatively, the predetermined distance may be equal to or less than 1.4 times the radius of the through hole. This limits the area where riblets that are effective for the flow affected by the through hole are arranged, and increases the area where riblets that are effective for the main flow are arranged. As a result, it is possible to more effectively reduce friction loss caused by the flow in the air flow path. Furthermore, it is possible to reduce the cost of forming riblets.

[0065] Riblets arranged at positions where the speed of the main flow on the surface is equal to or greater than a predetermined speed are referred to as riblets for the high flow rate region. Riblets arranged at positions where the speed of the main flow on the surface is less than a predetermined speed are referred to as riblets for the low flow rate region. The width of the riblets for the high flow rate region may be smaller than the width of the riblets for the low flow rate region. This makes it possible to provide both a pattern that is effective for the high-speed main flow of the main flow in the air flow path and a pattern that is effective for the low-speed main flow on the surface ST of the perforated plate PT. As a result, it is possible to more effectively reduce friction loss caused by the flow in the air flow path.

[0066] A through hole arranged at a position where the sound pressure on the surface is equal to or greater than a predetermined sound pressure is defined as a through hole for a high sound pressure range. A through hole arranged at a position where the sound pressure on the surface is less than a predetermined sound pressure is defined as a through hole for a low sound pressure range. The diameter of the through hole for a high sound pressure range may be smaller than the diameter of the through hole for a low sound pressure range. This allows both through holes effective for high sound pressures and through holes effective for low sound pressures to be provided on the surface ST of the perforated plate PT. As a result, it is possible to more effectively reduce friction loss caused by the flow in the air flow path.

[0067] According to the present disclosure, it is possible to reduce noise emitted by aircraft and the like equipped with devices having air flow paths while improving energy efficiency, which can contribute to, for example, Goal 13 of the Sustainable Development Goals (SDGs) led by the United Nations, "Take urgent action to combat climate change and its impacts."

[0068] Although several embodiments have been described, the embodiments can be modified or varied based on the above disclosure. All components of the above embodiments and all features described in the claims may be individually extracted and combined, unless they contradict each other.

[0069] The entire contents of Japanese Patent Application No. 2023-122640 (filing date: July 27, 2023) are incorporated herein by reference.

[0070] 10 Sound absorbing panel with riblets AR1, AR2, AR3 Area CV Cell HL Through hole MD Main flow direction PT Perforated plate R Radius RB Riblet RH Height RW Width S1 First distance S2 Second distance ST Surface

Claims

1. A sound-absorbing panel with riblets that is attached to a position facing an air flow path and used, comprising: a perforated plate that is provided with riblets and through holes and faces the air flow path; and cells having internal spaces that communicate with the air flow path via the through holes, wherein the riblets are provided on the surface of the perforated plate that faces the air flow path, and are not provided on the back surface of the perforated plate or on the inner surface of the through holes.

2. A sound-absorbing panel with riblets as described in claim 1, wherein a first distance is a center-to-center distance between the multiple through holes along a main flow on the surface, and a second distance is a center-to-center distance between the multiple through holes along a direction perpendicular to the main flow, and the first distance and the second distance are determined based on a proportion of an area of ​​the through holes to the surface.

3. The ribletted acoustic panel of claim 2, wherein the first distance is different from the second distance.

4. The ribletted acoustic panel of claim 3, wherein the first distance is less than the second distance.

5. The ribletted acoustic panel of claim 2, wherein the first distance is less than 1.16 times the second distance.

6. The sound-absorbing panel with riblets according to claim 2, wherein the product of the first distance and the second distance is determined based on the ratio.

7. The sound-absorbing panel with riblets according to claim 6, wherein the product of the first distance and the second distance is determined inversely proportional to the ratio.

8. The sound-absorbing panel with riblets according to claim 2, wherein the first distance is greater than or equal to three times the radius of the through hole.

9. A sound-absorbing panel with riblets as described in any one of claims 1 to 8, wherein the riblets arranged at a position more than a predetermined distance from the center of the through hole along a direction perpendicular to the main flow on the surface are defined as first riblets, and the riblets arranged at a position less than a predetermined distance from the center of the through hole are defined as second riblets, and the pattern of the second riblets is different from the pattern of the first riblets.

10. The ribletted acoustic panel according to claim 9, wherein the height of the second riblet is smaller than the height of the first riblet.

11. The ribletted acoustic panel according to claim 9, wherein the width of the second riblet is smaller than the width of the first riblet.

12. The sound-absorbing panel with riblets as described in claim 9, wherein the height of the second riblet approaches the height of the first riblet as it approaches another through hole adjacent to the through hole downstream of the main flow along the main flow.

13. The sound-absorbing panel with riblets as described in claim 9, wherein the width of the second riblet approaches the width of the first riblet as it approaches another through hole adjacent to the through hole downstream of the main flow along the main flow.

14. The sound-absorbing panel with riblets according to claim 9, wherein the predetermined distance is equal to or greater than the radius of the through hole.

15. The sound-absorbing panel with riblets according to claim 9, wherein the predetermined distance is 1.4 times or less the radius of the through hole.

16. A sound-absorbing panel with riblets as described in any one of claims 1 to 8, wherein the riblets arranged at a position where the speed is equal to or greater than a predetermined speed based on the speed of the main flow on the surface are riblets for a high flow velocity range, and the riblets arranged at a position where the speed is less than the predetermined speed are riblets for a low flow velocity range, and the width of the riblets for the high flow velocity range is smaller than the width of the riblets for the low flow velocity range.

17. A sound-absorbing panel with riblets as described in any one of claims 1 to 8, wherein the through holes arranged at positions where the sound pressure on the surface is equal to or greater than a predetermined sound pressure are defined as through holes for a high sound pressure range, and the through holes arranged at positions where the sound pressure is less than the predetermined sound pressure are defined as through holes for a low sound pressure range, and the diameter of the through holes for the high sound pressure range is smaller than the diameter of the through holes for the low sound pressure range.

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