Riblet structure and object

Abstract

To provide a technology of a riblet structure or the like capable of effectively reducing a surface friction resistance while maintaining a certain amount of strength.SOLUTION: A riblet structure 30 is configured such that a plurality of riblets 2 is arranged side by side in a width direction, where the riblet has a first wall part 11 standing substantially vertical to a bottom part 1, and a second wall part 12 connected to the first wall part at a top part 15 and having an inclined part 13 inclined at a predetermined angle with respect to the vertical direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present technology relates to technologies such as riblet structures for reducing surface friction resistance against fluids. [Background technology]

[0002] It has long been known that providing a predetermined riblet pattern on the surface of an aircraft can reduce surface friction resistance against turbulent flows of fluids such as gases and liquids.

[0003] The following Non-Patent Document 1 discloses a structure in which isosceles triangular riblets are arranged along the width direction, and discloses that these isosceles triangular riblets can reduce surface friction resistance by up to approximately 5% to 6% (see Figures 19 and 20 of the following Non-Patent Document 1).

[0004] These isosceles triangular riblets tend to be able to reduce surface friction resistance the more acute the apex angle. Therefore, Non-Patent Document 1 below proposes a riblet formed in the shape of an extremely thin plate (straight when viewed from the front) with an apex angle that is as acute as possible. This extremely thin plate-like riblet can reduce surface friction resistance by a maximum of approximately 10% (see Figures 19 and 20 of Non-Patent Document 1 below).

[0005] These extremely thin plate-like riblets are currently known to be the shape that can most effectively reduce surface friction resistance. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Bechert, D, W., Bruse, M., Hage, W., Van der Hoeven, J, G, T., and Hoppe, G., "Experiments on drag-reducing surfaces and their optimization with an adjustable geometry," Journal of fluid mechanics, vol.338, pp.59-87, 1997. Summary of the Invention [Problem to be solved by the invention]

[0007] However, when plate-like riblets are formed as thin as possible within the processing limits, there is a problem that such riblets are structurally fragile and not suitable for practical use.

[0008] In view of the above circumstances, the object of the present technology is to provide a technology such as a riblet structure that can effectively reduce surface friction resistance while maintaining a certain level of strength. [Means for solving the problem]

[0009] The riblet structure of the present technology is composed of a plurality of riblets arranged in the width direction, each of which has a first wall portion that stands substantially perpendicular to the bottom and a second wall portion that is connected to the first wall portion at the top and has an inclined portion that is inclined at a predetermined angle to the vertical direction.

[0010] It is possible to provide a riblet structure that can effectively reduce surface friction resistance while maintaining a certain level of strength.

[0011] In the above riblet structure, the first wall portions of the riblets adjacent to each other in the width direction may correspond to each other, and the second wall portions thereof may face each other.

[0012] In the above riblet structure, the second wall portion may further have a vertical portion that stands substantially perpendicular to the bottom portion and is connected to the inclined portion.

[0013] In the above-mentioned riblet structure, the inclined portion may have a first inclined portion inclined at the predetermined angle with respect to the vertical direction, and a second inclined portion on the bottom side of the first inclined portion inclined at an angle different from the predetermined angle with respect to the vertical direction.

[0014] In the above riblet structure, the angle at which the second inclined portion is inclined with respect to the vertical direction may be more acute than the angle at which the first inclined portion is inclined with respect to the vertical direction.

[0015] In the above riblet structure, the predetermined angle may be within a range of 20° to 60°.

[0016] In the above riblet structure, the predetermined angle may be within a range of 35° to 50°.

[0017] In the above riblet structure, the distance in the width direction between the two apexes of adjacent riblets is s, and the friction speed is u τ When the dynamic viscosity coefficient is ν, s + =su τ The non-dimensional riblet spacing s is expressed as / ν. + may be in the range of 10 or more and 30 or less.

[0018] In the above riblet structure, the non-dimensionalized riblet spacing s + may be in the range of 10 or more and 25 or less.

[0019] In the above riblet structure, the non-dimensionalized riblet spacing s + However, it may be in the range of 15 or more and 22 or less.

[0020] In the above riblet structure, when the distance in the width direction between two apexes of adjacent riblets is s, the height h of the riblet may be in the range of 0.05 to 1.5 times s.

[0021] In the above-mentioned riblet structure, when the widthwise distance between the two peaks of adjacent riblets is s, the widthwise distance d between the lowest points of the two opposing inclined portions of adjacent riblets may be in the range of 0.1 to 0.98 times s. Libretto structure.

[0022] The object of the present technology has a riblet structure on its surface that is composed of a plurality of riblets arranged in the width direction, each riblet having a first wall portion that stands substantially perpendicular to the bottom and a second wall portion that is connected to the first wall portion at the top and has an inclined portion that is inclined at a predetermined angle to the vertical direction. [Effects of the Invention]

[0023] As described above, according to the present technology, it is possible to provide a technology such as a riblet structure that can effectively reduce surface friction resistance while maintaining a certain level of strength. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 2 is a diagram showing a riblet structure according to the first embodiment. [Figure 2] FIG. 1 is a diagram showing an extremely thin plate-like riblet structure. [Figure 3] FIG. 10 is a diagram showing a riblet structure according to a comparative example. [Figure 4] FIG. 10 is a graph showing the reduction rate of surface friction resistance versus the non-dimensionalized riblet spacing s+ for the riblets according to the present embodiment and the riblets according to the comparative example. [Figure 5] FIG. 10 is a diagram showing a riblet structure according to a second embodiment. [Figure 6]FIG. 10 is a diagram showing a riblet structure according to a third embodiment. [Figure 7] FIG. 11 is a diagram showing an example in which the inclined portion of the second wall portion has two or more steps in the third embodiment. [Figure 8] FIG. 10 is a diagram showing a riblet structure according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0026] First Embodiment <Overall configuration of the riblet structure 30 and configuration of each part> FIG. 1 is a diagram showing a riblet structure 30 according to a first embodiment of the present technology.

[0027] The riblet structure 30 according to this embodiment is capable of reducing surface friction resistance to turbulent fluid flow. The riblet structure 30 is provided on the surface of an object. There are two methods for providing the riblet structure 30 on the surface of an object: a method in which sheet-like riblets are adhered to the surface of the object, and a method in which riblets are directly formed on the surface of the object, and either method may be used.

[0028] Examples of objects on which the riblet structure 30 can be provided include aircraft, ships, cars, turbines, fans, fluid equipment, etc. The riblet structure 30 can be provided on any object as long as it is intended to reduce surface friction resistance against turbulent fluid flow.

[0029] The riblet structure 30 is made of various materials such as resin, metal, and coating, but may be made of any material.

[0030] As shown in FIG. 1, the riblet structure 30 includes a base portion 1 (bottom portion) and a plurality of riblets 2 standing on the base portion 1.

[0031] The base portion 1 is formed in the shape of a flat plate that is thin in the thickness direction (Z-axis direction) and wide in the planar directions (XY directions). The bottom surface of the base portion 1 is adhered to the surface of the object on which the riblets 2 are to be formed, for example, via an adhesive layer. The thickness of the base portion 1 is typically about 5 μm or more and several mm or less. Note that the base portion 1 can also be omitted, in which case the riblets 2 are formed directly on the surface (bottom) of the object.

[0032] Each of the multiple riblets 2 has a shape that is long in the length direction (X-axis direction), short in the width direction (Y-axis direction), and low in the height direction (Z-axis direction).

[0033] The riblets 2 may be formed linearly in the length direction, or may be formed sinusoidally (as viewed from above). When the riblets 2 are formed sinusoidally in the length direction, the height h of the riblets 2 may also be formed sinusoidally (as viewed from the side (Y-axis direction)).

[0034] Each of the multiple riblets 2 has a first wall portion 11 that is substantially perpendicular to the base portion 1, and a second wall portion 12 that is opposite the first wall portion 11 across the top portion 15 and includes an inclined portion 13.

[0035] In addition, the multiple riblets 2 are arranged along the width direction (Y-axis direction) so that the first wall portions 11 sides of adjacent riblets 2 correspond to each other and the second wall portions 12 sides face each other.

[0036] The first wall portion 11 stands upright substantially perpendicular to the base portion 1. Here, "substantially perpendicular" means approximately ±10° from the vertical direction (Z-axis direction). In other words, since the riblet 2 according to this embodiment is small in size as described below, it may not be possible to form the first wall portion 11 strictly in the vertical direction due to processing accuracy issues. For this reason, the first wall portion 11 is considered to be substantially vertical if it is approximately ±10° from the vertical direction.

[0037] The second wall portion 12 has an inclined portion 13 and a vertical portion 14. The inclined portion 13 is connected to the first wall portion 11 at a top 15 and is inclined at a predetermined angle with respect to the vertical direction (Z-axis direction). The vertical portion 14 stands substantially perpendicular to the base portion 1 and is connected to the inclined portion 13. Note that, as with the above, "substantially perpendicular" in the vertical portion 14 also means approximately ±10° with respect to the vertical direction (Z-axis direction).

[0038] In the second wall portion 12, the angle θ at which the inclined portion 13 is inclined relative to the vertical direction (Z-axis direction) (the angle θ of the apex 15) is set from the following perspective. First, the sharper the angle θ, the more the surface friction resistance to the turbulent flow of the fluid can be reduced. On the other hand, the sharper the angle θ, the thinner the riblets 2 become in the width direction, reducing their strength and making them more difficult to process.

[0039] From this perspective, the lower limit of the angle θ is set to 20°. If the angle θ is set to 20° or more, the riblets 2 can be made to have a certain strength or more within the range that processing is possible, and surface friction resistance can be reduced. On the other hand, the upper limit of the angle θ is set to 60°. If the angle θ is set to 60° or less, it is possible to improve the strength while achieving a certain level of reduction in surface friction resistance, and also to reduce the difficulty of processing.

[0040] That is, the angle θ of the inclined portion 13 with respect to the vertical direction is typically in the range of 20° to 60°, and preferably in the range of 35° to 50°. In the example shown in Fig. 1, the angle θ is 45°.

[0041] The distance s in the width direction (Y-axis direction) between two apexes 15 of adjacent riblets 2 is typically set to 10 μm or more and several mm or less. In the example shown in FIG. 1, the distance s is set to 100 μm.

[0042] In this embodiment, there are two types of distance s between the two apexes 15. The first is the distance s at the location where the first wall 11 faces (the location of the U-shaped first groove 22), and the second is the distance s at the location where the second wall 12 faces (the location of the Y-shaped second groove 23). These two distances s are typically the same, but may be different.

[0043] Furthermore, the height h of the riblets 2 is typically set to be 0.05 to 1.5 times the distance s. In the example shown in Fig. 1, the height h is set to 50 µm, which is 0.5 times the distance s.

[0044] Furthermore, the distance d in the width direction between the lowest points of two opposing inclined portions 13 of adjacent riblets 2 (the distance d between the vertical portions 14) is typically set to 0.1 to 0.98 times the distance s. In the example shown in Fig. 1, the distance d is set to 50 µm, which is 0.5 times the distance s.

[0045] Here, if the distance d in the width direction between the lowest points of the two inclined portions 13 (the distance d between the vertical portions 14) is set too wide, the width of the riblet 2 becomes too small, resulting in a decrease in strength. From this perspective, the upper limit of the distance d is set to 0.98 times s. On the other hand, if the distance d is too small, the lower region of the Y-shaped second groove portion 23 becomes small, resulting in an increase in surface friction resistance to turbulent fluid flow. From this perspective, the lower limit of the distance d is set to 0.1 times s.

[0046] The multiple riblets 2 are configured by multiple grooves 21 arranged along the width direction (Y-axis direction). The multiple grooves 21 include first grooves 22 and second grooves 23 arranged alternately in the width direction. In this embodiment, the first grooves 22 and second grooves 23 have different shapes.

[0047] The first groove 22 is formed by a space surrounded by two opposing first walls 11 and the base 1. The shape of the first groove 22 is rectangular, or generally U-shaped.

[0048] The second groove 23 is formed by a space surrounded by two opposing second walls 12 and the base 1. The shape of the second groove 23 is an inverted trapezoid on the top 15 side and a rectangle on the bottom side, and is generally Y-shaped.

[0049] <Basic Concept> Next, the basic concept of this technology will be explained. Fig. 2 is a diagram showing a riblet structure 31 including plate-like riblets 3 that are as thin as possible. As shown in Fig. 2, in this riblet structure 31, the riblets 3 are plate-like with widths that are as thin as possible. It is known that this riblet structure 31 can reduce the surface friction resistance against turbulent fluid flow by up to 10%.

[0050] However, even if the riblets 3 are formed as thin as possible within the processing limit, there is a problem in that such riblets 3 are structurally weak and not suitable for practical use.

[0051] Here, the inventors have considered that the key to improving the surface friction resistance performance of riblets 2 is to sharpen the peaks 15 where the flow due to turbulent fluid is strong, thereby minimizing the surface area of ​​the peaks 15, and to maximize the internal volume of grooves 21 where the flow is weak. On the other hand, for practical use, it is necessary to ensure that the riblets 2 have a certain level of strength, and it is also necessary to consider that this must be within the range that can be processed.

[0052] In other words, the riblet structure 30 according to this embodiment takes into consideration the following four points: 1. Making the peaks 15 as sharp as possible. 2. Making the internal volume of the grooves 21 as large as possible. 3. Ensuring a certain level of strength. 4. Being within the range that can be processed (making processing easy). By satisfying these four points, it is possible to reduce surface friction resistance while ensuring a certain level of strength in the riblets 2 within the range that can be processed.

[0053] Here, the two first walls 11 facing each other in the width direction are substantially perpendicular to each other. Therefore, the first groove 22 is rectangular and its volume is made as large as possible. Therefore, the surface friction resistance can be effectively reduced.

[0054] If possible, it would be desirable to form the second wall portion 12 side substantially vertically and make the riblets 2 as thin as possible like the extremely thin riblets 3, but this would make it impossible to ensure a certain level of strength in the riblets 2. Therefore, in this embodiment, an inclined portion 13 is provided on the second wall portion 12 side.

[0055] The sharper the angle θ at which the inclined portion 13 in the second wall portion 12 is inclined relative to the vertical direction (the angle θ of the apex 15), the more the surface friction resistance can be reduced. However, the sharper the angle θ, the thinner the riblet 2 becomes in the width direction, reducing its strength and making it more difficult to process. For this reason, as described above, the angle θ is within the range of 20° to 60°, and preferably within the range of 35° to 50°.

[0056] The lowest point of the inclined portion 13 in the second wall portion 12 may be in contact with the upper surface of the base portion 1, and the second groove portion 23 may be in an inverted triangular shape (in this case, the second groove portion 23 is generally V-shaped), but in this case the volume of the second groove portion 23 will be slightly smaller. Therefore, in the second wall portion 12, a vertical portion 14 is provided below the inclined portion 13, thereby forming a rectangular portion below the inverted trapezoidal portion in the second groove portion 23. Therefore, the surface friction resistance can also be effectively reduced in the second groove portion 23.

[0057] <Evaluation of surface friction resistance reduction performance> Next, an evaluation of the performance of the riblets 2 in reducing the surface friction resistance against the turbulent flow of a fluid will be described.

[0058] In this evaluation, the surface friction resistance reduction performance of the riblet structure 30 according to this embodiment and the riblet structure 32 according to the comparative example were compared. Fig. 3 is a diagram showing the riblet structure 32 according to the comparative example. The riblets 4 according to the comparative example have an isosceles triangular shape.

[0059] In this evaluation, the dimensionless riblet spacing s + The reduction rate of skin friction resistance was analyzed using a numerical calculation method called DNS (Direct Numerical Simulation). The calculation program used in this DNS analysis was written in Fortran.

[0060] FIG. 4 shows the non-dimensionalized riblet spacing s + FIG. 10 is a graph showing the reduction rate of surface friction resistance with respect to the surface friction resistance.

[0061] where s + means the Reynolds number based on the distance s between the two peaks 15 of the adjacent riblets 2, and s + =su τ / ν is the dimensionless riblet spacing. τ is the friction velocity, and ν is the dynamic viscosity of the fluid. τ is the shear stress τ acting on the wall w and the density of the fluid, ρ, is used to calculate u τ =(τ w / ρ) 1 / 2 It is expressed as:

[0062] In FIG. 4, in this embodiment and the comparative example, the riblet spacing s is non-dimensionalized by changing the distance s between the two apexes 15 of the adjacent riblets 2 and 4. + The value of the reduction rate of surface friction resistance was analyzed by changing the value.

[0063] In this embodiment and the comparative example, the riblets 2, 4 were straight and linear in the length direction. In this embodiment and the comparative example, the height h of the riblets 2, 4 was set to 1 / 2 the distance s (i.e., the height h changed depending on the value of the distance s), and the angle θ of the apex 15 was fixed at 45°. Furthermore, in this embodiment, the distance d between the lowest points of the two inclined portions 13 was set to 1 / 2 the distance s (i.e., the distance d changed depending on the value of the distance s).

[0064] As is clear from FIG. 4, in this embodiment, the dimensionless riblet spacing s + When the ratio is set to about 10 or more and about 30 or less, the reduction rate of the surface friction resistance is improved compared to the comparative example.

[0065] Specifically, in the comparative example (see x marks), the reduction rate of surface friction resistance is at most about 6%, whereas in this embodiment (see ◯ marks), the reduction rate of surface friction resistance is at most about 8%.

[0066] In other words, in this embodiment, although it does not reach the 10% reduction rate of surface friction resistance of the extremely thin plate-shaped riblet 3, it is possible to reduce surface friction resistance by up to approximately 8% while maintaining the strength of the riblet 2 above a certain level within the range that can be processed.

[0067] Here, in this embodiment, the reduction rate of the surface friction resistance is calculated by the non-dimensionalized riblet spacing s + Since it changes according to the change of s + There is a reasonable range for the value of s. + is typically in the range of 10 or more and 30 or less, preferably in the range of 10 or more and 25 or less, and more preferably in the range of 15 or more and 22 or less.

[0068] As shown in Figure 1, if the distance s is 100 μm in the airflow assumed for an aircraft, the non-dimensional riblet spacing s + is approximately 17, and in this case, the surface friction resistance can be reduced by approximately 8%.

[0069] <Effect, etc.> As described above, the riblet structure 30 of this embodiment is composed of a plurality of riblets 2 arranged in the width direction, each of which has a first wall portion 11 that stands substantially perpendicular to the base portion 1 and a second wall portion 12 that is connected to the first wall portion 11 at the top 15 and has an inclined portion 13 that is inclined at a predetermined angle relative to the vertical direction.

[0070] This allows the riblets 2 to have a certain strength or more, and also makes it possible to effectively reduce the surface friction resistance.

[0071] Furthermore, in this embodiment, the first wall portions 11 sides of the riblets 2 adjacent to each other in the width direction correspond to each other, and the second wall portions 12 sides face each other. This makes it possible to increase the area of ​​the first groove portion 22 surrounded by the two opposing first wall portions 11, thereby making it possible to further effectively reduce surface friction resistance.

[0072] In this embodiment, the second wall portion 12 further has a vertical portion 14 that stands substantially perpendicular to the bottom portion and connects to the inclined portion 13. This allows a rectangular portion to be formed in the lower region of the second groove portion 23 that is surrounded by the two opposing second wall portions 12. This allows the area of ​​the second groove portion 23 to be enlarged, thereby making it possible to further effectively reduce surface friction resistance.

[0073] Furthermore, in this embodiment, the angle θ (angle θ of the apex 15) at which the inclined portion 13 of the second wall portion 12 is inclined relative to the vertical direction (Z-axis direction) is typically within a range of 20° to 60°, and preferably within a range of 35° to 50°. This allows the riblets 2 to have a certain strength or more within the range that can be processed, and enables the surface friction resistance to be effectively reduced.

[0074] In this embodiment, the height h of the riblets 2 is set within a range of 0.05 to 1.5 times the distance s in the width direction between two adjacent peaks 15. This makes it possible to further effectively reduce the surface friction resistance.

[0075] Furthermore, in this embodiment, the widthwise distance d between the lowest points of two opposing inclined portions 13 (the width d of the rectangular portion in the second groove portion 23) is set within a range of 0.1 to 0.98 times the distance s. This makes it possible to further effectively reduce surface friction resistance by making the width of the riblets 2 at least a certain value and ensuring that the riblets 2 have a certain strength or greater, while widening the width d of the rectangular portion in the second groove portion 23 as much as possible.

[0076] In this embodiment, the dimensionless riblet spacing s + is typically within the range of 10 or more and 30 or less, preferably within the range of 10 or more and 25 or less, and more preferably within the range of 15 or more and 22 or less. This makes it possible to effectively reduce the surface friction resistance, and the surface friction resistance can be reduced by up to about 8%.

[0077] Second Embodiment Next, a second embodiment of the present technology will be described. In the descriptions of the second and subsequent embodiments, parts having the same configurations and functions as those in the first embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted or simplified.

[0078] 5 is a diagram showing a riblet structure 33 according to the second embodiment. The second embodiment differs from the first embodiment in that the inclined portion 13 in the second wall portion 12 has two stages. Therefore, this point will be mainly described.

[0079] In the riblet 5, the inclined portion 13 of the second wall portion 12 includes a first inclined portion 13a on the top 15 side and a second inclined portion 13b on the bottom side. The first inclined portion 13a is inclined at a first angle θ1 with respect to the vertical direction. The second inclined portion 13b is inclined at a second angle θ2 with respect to the vertical direction that is different from the first angle θ1. The second angle θ2 is more acute than the first angle θ1.

[0080] The first angle θ1 is typically within a range of 20° to 60°, and preferably within a range of 35° to 50°, similar to the range of the angle θ (angle of the apex 15) at which the inclined portion 13 is inclined in the first embodiment described above. The second angle θ2 is typically 20° or greater and less than the first angle θ1.

[0081] In the example shown in FIG. 5, the first angle θ1 is set to 45° and the second angle θ2 is set to 30°.

[0082] In the second embodiment, since the inclined portion 13 of the second wall portion 12 has two steps, the shape of the second groove portion 24 surrounded by the second wall portion 12 is different from that of the first embodiment. Specifically, the second groove portion 24 is formed by a first inverted trapezoidal portion on the top 15 side, a second inverted trapezoidal portion in the middle, and a rectangular portion on the bottom side.

[0083] In the second embodiment, as in the first embodiment described above, the riblets 5 can be made to have a certain strength or more within the range that can be processed, and surface friction resistance can be effectively reduced. Furthermore, in the second embodiment, the inclined portion 13 is configured in two stages, which further increases the volume of the second groove portion 24, thereby further reducing surface friction resistance.

[0084] In the second embodiment, the inclined portion 13 has been described as having two steps, but the inclined portion 13 may be configured as having three or more steps.

[0085] Third Embodiment Next, a third embodiment of the present technology will be described. In the above-described embodiments, the case where the vertical portion 14 is provided in the second wall portion 12 has been described, but the vertical portion 14 may also be omitted. In the third embodiment, a shape in which the vertical portion 14 is omitted from the second wall portion 12 will be described.

[0086] Fig. 6 is a diagram showing a riblet structure 34 according to the third embodiment. As shown in Fig. 6, in the third embodiment, in the second wall portion 12 of the riblet 6, the inclined portion 13 is connected to the base portion 1, and no vertical portion 14 is provided.

[0087] In the third embodiment, the angle θ of the inclined portion 13 with respect to the vertical direction is typically in the range of 20° to 60°, and preferably in the range of 35° to 50°, as in the above-described embodiments. In the example shown in Fig. 6, the angle θ is 30°.

[0088] 6, the distance d in the width direction between the lowest points of two opposing inclined portions 13 is set to 50 μm. Note that this distance d can be changed as appropriate (it may be set to 0.1 to 0.98 times the distance s).

[0089] In the third embodiment, the vertical portion 14 is omitted from the second wall portion 12, and therefore the shape of the second groove portion 25 surrounded by the opposing second wall portions 12 is different from that of the above-described embodiments. Specifically, the shape of the second groove portion 25 is an inverted trapezoid.

[0090] In the third embodiment, as in the above-described embodiments, the riblets 6 can be made to have a certain strength or more within the range that can be processed, and the surface friction resistance can be effectively reduced.

[0091] Here, in the third embodiment, the inclined portion 13 of the second wall portion 12 may have two or more steps, as in the second embodiment. Fig. 7 is a diagram showing an example in which the inclined portion 13 of the second wall portion 12 has two or more steps in the third embodiment.

[0092] In the riblet structure 35 shown in Figure 7, the vertical portion 14 is omitted from the second wall portion 12 of the riblet 7, and the inclined portion 13 is connected to the base portion 1. The inclined portion 13 of the second wall portion 12 includes a first inclined portion 13a on the apex 15 side that is inclined at a first angle θ1 with respect to the vertical direction, and a second inclined portion 13b that is inclined at a second angle θ2 with respect to the vertical direction.

[0093] The ranges of the first angle θ1 and the second angle θ2 are the same as those in the second embodiment. Note that the example shown in Fig. 7 illustrates a case where the first angle θ1 is set to 45° and the second angle θ2 is set to 20°.

[0094] Fourth Embodiment Next, a fourth embodiment of the present technology will be described. In each of the above-described embodiments, the first wall portions 11 of two adjacent riblets 2 correspond to each other and the second wall portions 12 of the two adjacent riblets 2 face each other. However, the first wall portions 11 and the second wall portions 12 of two adjacent riblets 2 may face each other.

[0095] 8 is a diagram showing a riblet structure 36 according to the fourth embodiment. In this riblet structure 36, the first wall portion 11 and the second wall portion 12 of two adjacent riblets 8 face each other.

[0096] In the example shown in Fig. 8, the distance s between two adjacent peaks 15 is 100 µm, the height h of the riblets 2 is 50 µm, and the angle θ of the peaks 15 is 45°, but these values ​​can be changed as appropriate within the ranges described above. Also, in the example shown in Fig. 8, the distance d between the first wall portion 11 and the lowest point of the inclined portion 13 of the second wall portion 12 facing the first wall portion 11 is 75 µm, but this value can also be changed as appropriate.

[0097] In the fourth embodiment, the grooves 27 have the same shape. In the example shown in Fig. 8, the grooves 27 include an inverted trapezoidal portion on the top 15 side and a rectangular portion on the bottom side.

[0098] In the fourth embodiment, as in the above-described embodiments, the riblets 8 can be made to have a certain strength or more within the range that can be processed, and the surface friction resistance can be effectively reduced.

[0099] In the fourth embodiment, the inclined portion 13 may have two or more steps (see the second embodiment), and the vertical portion 14 may be omitted (see the third embodiment). [Explanation of symbols]

[0100] 1...Base 2~8...Libretto 11...First wall portion 12...Second wall 13…Slope part 14...Vertical part 15...Top 30~36...Riblet structure

Claims

1. A riblet structure comprising a plurality of riblets, each having a longitudinal direction in a direction along the flow of a fluid and a width direction perpendicular to the longitudinal direction, arranged at intervals in the width direction, Each of the plurality of riblets has a first wall portion corresponding to one wall portion in the width direction, standing substantially perpendicular to the bottom portion, and a second wall portion corresponding to the other wall portion in the width direction, connected to the first wall portion at the top portion, having an inclined portion inclined at a predetermined angle with respect to the vertical direction. Libretto structure.

2. 2. The riblet structure according to claim 1, The first wall portions of the riblets adjacent to each other in the width direction face each other, and the second wall portions thereof face each other. Libretto structure.

3. The riblet structure according to claim 1 or 2, The second wall portion further has a vertical portion that stands substantially perpendicular to the bottom portion and is connected to the inclined portion. Libretto structure.

4. The riblet structure according to any one of claims 1 to 3, The inclined portion has a first inclined portion inclined at the predetermined angle with respect to the vertical direction, and a second inclined portion inclined at an angle different from the predetermined angle with respect to the vertical direction and located closer to the bottom than the first inclined portion. Libretto structure.

5. 5. The riblet structure according to claim 4, The angle at which the second inclined portion is inclined with respect to the vertical direction is more acute than the angle at which the first inclined portion is inclined with respect to the vertical direction. Libretto structure.

6. The riblet structure according to any one of claims 1 to 5, The predetermined angle is in the range of 20° to 60°. Libretto structure.

7. 7. The riblet structure according to claim 6, The predetermined angle is in the range of 35° to 50°. Libretto structure.

8. The riblet structure according to any one of claims 1 to 7, The distance in the width direction between the two peaks of adjacent riblets is s, and the friction speed is u τ When the dynamic viscosity coefficient is ν, s ＋ =su τ / ν ＋ is in the range of 10 to 30 Libretto structure.

9. 9. The riblet structure according to claim 8, The non-dimensionalized riblet spacing s ＋ is in the range of 10 to 25 Libretto structure.

10. 10. The riblet structure of claim 9, The non-dimensionalized riblet spacing s ＋ is in the range of 15 to 22 Libretto structure.

11. The riblet structure according to any one of claims 1 to 10, When the distance in the width direction between two apexes of the adjacent riblets is s, the height h of the riblet is in the range of 0.05 to 1.5 times s. Libretto structure.

12. The riblet structure according to any one of claims 2 to 11, When the distance in the width direction between the two peaks of the adjacent riblets is s, the distance in the width direction between the lowest points of the two inclined portions facing each other in the adjacent riblets is in the range of 0.1 to 0.98 times s. Libretto structure.

13. An object having a riblet structure on its surface, the riblets each having a longitudinal direction in a direction along the flow of a fluid and a width direction perpendicular to the longitudinal direction, arranged at intervals in the width direction, Each of the plurality of riblets has a first wall portion corresponding to one wall portion in the width direction, standing substantially perpendicular to the bottom portion, and a second wall portion corresponding to the other wall portion in the width direction, connected to the first wall portion at the top portion, having an inclined portion inclined at a predetermined angle with respect to the vertical direction. Object.

Images