Riblet structure and method for generating such a riblet structure

Abstract

The present invention relates to a riblet structure (1) for a surface for the purpose of reducing frictional resistance with respect to a fluid flowing in a major direction (H) along the surface. The riblet structure (1) includes a plurality of longitudinal ribs (3) that project from a base surface (2), are oriented in the major direction (H), and spaced apart from each other, particularly wedge-shaped. To achieve low frictional resistance, according to the present invention, the base surface (2) includes a recess (5) between each pair of adjacent longitudinal ribs (3) that is spaced apart from each other in the major direction (H), and the recess (5) is spaced at least partially, preferably completely, from at least one or both longitudinal ribs (3) in a direction perpendicular to the major direction (H). Furthermore, the present invention relates to the use and method of manufacturing such a riblet structure (1).

Description

[Technical Field]

[0001] The present invention relates to a riblet structure for a surface, for the purpose of reducing frictional resistance with respect to a fluid flowing in the principal direction along the surface. This riblet structure includes a plurality of longitudinal ribs, particularly wedge-shaped, that project from the base surface, are oriented in the principal direction, and are spaced apart from each other.

[0002] The present invention further relates to the use of such a riblet structure.

[0003] The present invention also relates to a method for manufacturing such a riblet structure. [Background technology]

[0004] Riblet structures are known from the prior art. These riblet structures are typically surface structures in the sub-millimeter range, designed to reduce the frictional resistance of the surface containing the surface with respect to a fluid flowing along the surface, such as air or water. Riblet structures are used, for example, in the fuselage or wings of aircraft to reduce the frictional resistance of the aircraft against the airflow along the aircraft.

[0005] The riblet structure is typically formed by longitudinal ribs positioned at the base level, extending along a primary direction corresponding to the main fluid flow direction during use, and spaced apart from one another. The longitudinal ribs are often embodied in a wedge shape.

[0006] For example, the document EP3 170 743 A1 discloses a riblet structure having multiple wedge-shaped longitudinal ribs embodied in this manner. The base surface positioned between the longitudinal ribs may be flat or may be embodied in a curved form. [Overview of the project]

[0007] This is the starting point of the present invention. The object of the present invention is to identify a riblet structure of the first named type that has low frictional resistance to a fluid flowing along the riblet structure or a surface containing a riblet structure.

[0008] Furthermore, the aim is to identify the use of such riblet structures.

[0009] Furthermore, a further objective is to identify a method for manufacturing such riblet structures.

[0010] According to the present invention, in the case of the riblet structure of the first named type, the base surface includes recesses spaced apart from each other along the principal direction between each of two adjacent longitudinal ribs, and the objective is achieved by these recesses being spaced apart from at least one or both longitudinal ribs in a direction perpendicular to the principal direction, at least partially, preferably completely.

[0011] The basis of this invention is the finding that, in the case of a typical riblet structure, i.e., a structure formed by arranging longitudinal ribs spaced apart from each other on a base surface, it is possible for a fluid to influence the flow behavior along the riblet structure, particularly in the so-called viscous sublayer of the hydrodynamic boundary layer of the fluid flow, directly above the riblet structure, by creating depressions in the base surface portions located between two adjacent longitudinal ribs, while doing so without causing significant influence or disturbance to the boundary layer profile of the fluid flow somewhat away from the riblet structure, particularly the turbulence of the flow on the longitudinal ribs. Therefore, it is possible to directly improve the flow behavior in a riblet structure by using structural improvements to the base surface portions between longitudinal ribs. Adjacent longitudinal ribs typically mean two longitudinal ribs that are directly adjacent to each other, particularly in a direction perpendicular to the main direction. The base surface is usually embodied as the base level, and the base surface portion is embodied as the base level portion. The longitudinal ribs are usually oriented in the main direction, and the longitudinal ribs are arranged next to each other and spaced apart in a direction perpendicular to the main direction.

[0012] Typical riblet structures usually have flat base surface portions between longitudinal ribs, which generate turbulent viscous drag primarily due to the interaction region of the base surface portions located between the longitudinal ribs. It has been shown that viscous drag can be reduced by introducing depressions in the base surface portions. This can be explained by the reduction of the interaction region of the base surface portions, which is due to the reduction of the area of ​​the base surface portions at the same height due to the introduced depression, or the base surface portion being lowered by the depression, thereby contributing only in a small way to the viscous drag. However, it has then been shown that depressions generate additional pressure drag. In a riblet structure with depressions, the total drag used for comparison is the sum of viscous and pressure drags compared to the simple viscous drag in the case of a riblet structure without depressions. Therefore, in a riblet structure with depressions, a favorably reduced total drag, or friction drag, can be achieved if the reduction in viscous drag due to the depressions is as significant as possible, while the pressure drag generated in response is kept as small as possible.

[0013] Tests have shown that this can be achieved by a distinct demarcation of the depressions in the base surface, or at least partially, by a distinct spacing between each depression and the longitudinal ribs. This can be advantageously implemented by a base surface region adjacent to each of the depressions, and in particular between at least one longitudinal rib and the depressions, or by the depressions being at least partially, preferably completely, spaced apart from the longitudinal ribs. The cause appears to be the generation of turbulence at the depression edges of the depressions, particularly when a portion of the flow penetrates each depression. This relates to the pressure resistance generated by the depressions. This is achievable when each depression is embodied at least partially, preferably completely, spaced apart from at least one of the two longitudinal ribs in a direction perpendicular to the principal direction. When each depression is spaced at least partially, preferably completely, from both longitudinal ribs, it is advantageous for reduced viscous resistance. In this case, the two longitudinal ribs should be understood to mean each depression or two directly adjacent longitudinal ribs between which those depressions are located. The base surface portion typically refers to the part of the base surface located between two longitudinal ribs or between two directly adjacent longitudinal ribs. The portion of the base surface portion that does not contain or into which no recesses are inserted is typically called the interaction region portion or reference region portion of the base surface. The fluid can be formed by gases such as air and / or liquids such as water.

[0014] Each of two directly adjacent longitudinal ribs, also called a longitudinal rib pair, typically forms a groove between the ribs, which is also called a longitudinal channel. In this case, the groove base surface is typically formed by a base surface or a base surface portion of a base surface located between each longitudinal rib, and in particular, the groove side surface or longitudinal channel side surface is formed by the side surface of the longitudinal rib. The recess is inserted into or formed using the base surface or base surface portion. The longitudinal rib is typically embodied in a wedge shape or in a basic triangular or trapezoidal shape in a cross section perpendicular to the longitudinal extension of the longitudinal rib. The side walls of the longitudinal rib may be embodied flat. The longitudinal rib is typically located on or projecting from a base surface. The base surface, in particular the base surface portion of a base surface located between each of two longitudinal ribs, may be embodied substantially flat, in which case the recess may be further inserted into or formed using the base surface. In this way, the grooves often form substantially inverted trapezoidal recesses, to which recesses formed with further indentations are connected. When the base surface is realized in a flat manner, it is often called the base level. The portion of the base surface that is not part of the indentations is also called the interaction region or base surface reference region, or more specifically, likewise the interaction level or reference level of the base level. Typically, the height of the longitudinal ribs of two longitudinal ribs or a pair of longitudinal ribs is greater than the substructure or topology formed between the ribs using the base surface or each portion of the base surface. Preferably, the average height of the base surface portion is less than the average height of the longitudinal ribs of the two longitudinal ribs that are adjacent to the base surface portion, more specifically less than half of it, and preferably one-quarter of it. In particular, a plan view of a riblet structure and / or longitudinal channel typically means a view perpendicular to the primary direction and also perpendicular to the secondary direction. The plan view typically appears along the height direction. Typically, the longitudinal ribs or longitudinal channels are arranged adjacent to each other in the secondary direction. Typically, longitudinal ribs, particularly the ribs of each longitudinal channel, are arranged to be spaced apart from each other in the secondary directions.Secondary directions are typically oriented perpendicular to the primary directions. It should be understood that the heights mentioned, particularly the heights of the longitudinal ribs, are typically defined in the height direction.

[0015] The height direction is usually perpendicular to the base plane, or, in particular, perpendicular to both the primary and secondary directions.

[0016] The sides of each longitudinal rib are typically oriented substantially parallel to the principal direction. It is feasible for the sides to be embodied such that, particularly in cross-sections perpendicular to the principal direction, the tangent to the side has an angle between 30° and 50°, preferably approximately 40°, with respect to the base surface or the base surface portion adjacent to the side. Preferably, this applies to both sides of each rib.

[0017] Advantageously, typically, the depressions are provided to be realized in a repeating structural order or arranged in a repeating manner, particularly in the main direction. As a result, it is possible to achieve low resistance characteristics over a wide area in a practical manner. This can be implemented in the main direction and / or orthogonal to the main direction. For this purpose, the depressions can form a periodic structure, in particular, which is regular, and preferably the distance between the depressions may be constant. Specifically, there may be multiple depression classes represented by differently realized depressions, and depressions of different depression classes may be arranged next to one another in a predefined ordering pattern. Typically, identically realized depressions are each assigned to the same depression class. It is advantageous that depressions from different depression classes are arranged next to one another, in particular, so as to periodically substitute for one another. For this purpose, individual depressions or groups of directly adjacent depressions may alternate from one depression class each. It is also possible to arrange the depressions irregularly, but a regular arrangement or an arrangement having a structural order or ordering pattern is preferred. The above applies particularly in the principal direction and / or perpendicular to the principal direction. This applies especially in sections substantially parallel to the base plane or in plan views of the base plane. Recesses belonging to different recess classes may differ, for example, in terms of their shape, depth, the spacing between two directly adjacent recesses, or any of the other features described herein, particularly below. The above can also be implemented to apply to each base plane portion.

[0018] Typically, a very significant spacing of each depression from at least one, preferably both, longitudinal ribs has been shown to be beneficial for low pressure resistance. In a plan view of the base plane, it is advantageous, particularly preferable, more preferably greater than 5%, of the average longitudinal rib spacing between the longitudinal ribs, in a direction perpendicular to the main direction, between at least one of the two longitudinal ribs and the depression edges adjacent to each depression and facing the longitudinal rib, to be greater than 5% of the average longitudinal rib spacing between the longitudinal ribs. The longitudinal rib spacing typically refers to the spacing of the longitudinal ribs at the height of the base plane in a cross section perpendicular to the main direction H, or the spacing in their reference area, or typically the spacing corresponding to the width of each base plane portion. The longitudinal rib spacing is typically measured in a direction perpendicular to the height of the longitudinal ribs. As mentioned above, the spacing between the depressions, or the spacing between the depression edges of each depression, can affect the marked resistance behavior, particularly viscous resistance or pressure resistance, thereby optimizing the aforementioned behavior. It is preferable that such spacing exists from both longitudinal ribs, that is, in particular, that each of both longitudinal ribs has such spacing from the depression edge that is in contact with and facing the respective longitudinal rib. It should be understood that the spacing from each longitudinal rib may be different or equal. The average spacing between at least one of the two longitudinal ribs and the depression edge that is in contact with and facing the respective depression is usually less than 90% of the average longitudinal rib spacing between the longitudinal ribs, particularly less than 80%, particularly preferably less than 65%, preferably less than 50%, and preferably less than 45%. It is advantageous that the centroid of the region formed by the depression, preferably the depression edge, is substantially located midway between the two longitudinal ribs, i.e., substantially midway between the two longitudinal ribs, particularly in the direction perpendicular to the principal direction. The spacing between the longitudinal ribs and the depression edge is thus typically determined between the point where each longitudinal rib is adjacent to the base surface portion and the point of the depression edge.

[0019] Particularly in the depression edge section of the depression, it is advantageous if the depression is adjacent to the base surface area located between one of the depression and the longitudinal rib, especially when it is in a direction orthogonal to its main direction. For optimized resistance behavior, it is beneficial that the base surface area is embodied substantially flat. The base surface area can be connected to both the depression or both the depression edge section, and at least one longitudinal rib, especially to its side surface. Preferably, such a base surface area exists between both longitudinal ribs and each depression, or each depression edge section of the depression. Practically, the depression edge section can be a depression edge. It is preferred that the depression or the depression edge is incorporated almost, particularly preferably substantially completely, by such a base surface area.

[0020] Particularly significant reduction of frictional resistance can be achieved when, in a cross-section parallel to the reference area of the base surface, the average longitudinal expansion in the main direction of the depression is at most 1.5 times, especially at most 1.2 times, the average width expansion in the direction orthogonal to the main direction of the depression. This particularly applies in the plan view of the base surface. It is advantageous especially when the area defined by the depression edge is embodied in such a way.

[0021] It is advantageous for reduced pressure resistance if the average depth of the depression is less than 75% of the average height of the longitudinal rib of the longitudinal rib, especially between 5% and 70%. It has been shown that at greater depths, turbulence can occur, thereby resulting in an increase in pressure resistance. Preferably, the average depth of the depression is between 5% and 60% of the average height of the longitudinal rib of the longitudinal rib, especially between 10 and 45%, preferably less than 30%, particularly preferably less than 15%.

[0022] For the purpose of setting viscous resistance, it is advantageous that a plurality of depressions between two longitudinal ribs overlap each other in a direction orthogonal to the main direction.

[0023] For the harmony between the viscous resistance and the pressure resistance, it is beneficial for the depressions to form a plurality of rows of depressions oriented in the main direction between the longitudinal ribs or pairs of longitudinal ribs. For example, between two longitudinal ribs, there may be two rows of depressions extending along the main direction. It is advantageous that the directly adjacent rows of depressions are arranged to be offset from each other in the main direction, and preferably, they may be offset by half of the distance between two directly adjacent depressions in each row of depressions. Thus, a favorable flow behavior regarding the total resistance can be realized. The depressions between the rows can thereby overlap each other in a direction orthogonal to the main direction.

[0024] Each depression is practicable if it mainly, particularly substantially, has a constant cross-sectional surface along the depth direction of the depression. For example, the depression can be formed by or have a substantially cylindrical or prismatic recess, particularly a square or cubic recess. Thereby, the prismatic base surface can be a polygon, such as a triangle, quadrilateral, pentagon, or hexagon. However, if the depression has a cross-sectional surface that decreases, at least partially, particularly in most cases, preferably substantially, along the depth direction of the depression, it can be beneficial for the turbulent flow behavior. The depression can be formed by or have a substantially conical or pyramidal recess, for example. Thereby, the pyramidal base surface can be a triangle, quadrilateral, pentagon, or hexagon. It is convenient if, along the depth direction of the depression, the depression includes a first depression section having a constant cross-sectional surface along the depth direction and a second depression section, particularly located downstream of the first depression section in the depth direction and having a cross-sectional surface that decreases along the depth direction. Preferably, the second depression section constitutes the end section of the depression. The cross-sectional surface of the depression is usually oriented perpendicular to the depth direction of the depression. Thereby, the first or second depression section can be embodied in the above manner.

[0025] Depending on the intended use, different recess shapes, particularly the cross-sectional surface of the recess, may be particularly beneficial. For example, each recess or recess edge in the cross-section may be formed in a particularly elliptical or rounded shape, such as having no corners, or it may be formed as a polygon, such as a triangle, square, pentagon, or hexagon.

[0026] To reduce pressure resistance, it is beneficial that each depression includes a depression wall into which the depression is formed, and that the depression wall may be embodied at least partially, and especially substantially, as a plane of rotation, and particularly have an axis of rotation in the depth direction of the depression. Thus, the depression wall may be formed of a portion of a revolutionary ellipsoid, especially a spherical surface, of a revolutionary parabola or revolutionary hyperboloid. It may be beneficial if the depression wall portion is formed of a portion of a cone envelope or cylindrical envelope. For example, the depression may include a depression wall that is embodied substantially in a dome shape.

[0027] In the depth direction, each depression is formed by a first depression region and a second depression region located behind the first depression region, and it is advantageous that the first depression region has a smaller diameter than the second depression region. Thus, it is preferable that the diameter, particularly the cross-sectional surface area, of the first depression region is less than 80% of the diameter, particularly the cross-sectional surface area, of the second depression region. This has been shown to be advantageous in reducing pressure resistance. It is preferable that the diameter or cross-sectional surface area of ​​the first depression region is less than 65% of the diameter or cross-sectional surface area of ​​the second depression region, particularly between 5% and 50%, more preferably between 10% and 30%, and most preferably less than 20%. In use, the advantageous effect on pressure resistance appears to be due to the interaction between flow or turbulence in the different depression regions. For convenience, multiple, particularly most, preferably substantially all, depressions between two longitudinal ribs can be embodied in this manner. The recessed areas may have different shapes, or may be embodied by features specified for the shape and size of each recess, and may be particularly different from each other.

[0028] Multiple depressions may be connected to one another by at least one fluidically conductive connecting channel, which may be beneficial for the fluid behavior in the depressions. For convenience, multiple depressions between two longitudinal ribs may be arranged adjacent to each other, particularly in the principal direction and / or in a direction perpendicular to the principal direction, and may be connected to each other by at least one connecting channel. This has been found to be effective if the depressions are directly adjacent. In a more complex design, depressions between two directly adjacent longitudinal ribs or different instances of pairs of longitudinal ribs may be connected to each other by at least one fluidically conductive connecting channel, which may be beneficial for the fluid behavior. Thus, the connecting channel typically operates beneath the longitudinal ribs and is intended to connect depressions between different pairs of longitudinal ribs to each other. Feasibly, multiple, or most, preferably substantially all, depressions between pairs of longitudinal ribs may be connected to each other by at least one or more connecting channels. Preferably, and especially preferably, directly adjacent depressions perpendicular to the main direction are connected via at least one connecting channel, and particularly preferably when the depressions belong to different longitudinal rib pairs. It should be understood that it is beneficial to provide multiple connecting channels to connect those depressions to one another. Thus, there may be multiple connecting channels between each of two depressions. The connecting channels may be formed, for example, as holes in the material forming the base surface. It has been found that the connecting channels are typically effective when they connect horizontally to a third region of the depression bottom of each depression, particularly to the region of the depression base.

[0029] For the purpose of setting pressure resistance, it is advantageous to have multiple longitudinal rib pair classes, where a longitudinal rib pair class represents longitudinal rib pairs having recesses of different embodiments arranged between each longitudinal rib pair, and longitudinal rib pairs and their respective recesses belonging to different longitudinal rib pair classes are typically arranged adjacent to each other in a predetermined sequential pattern in a direction orthogonal to the main direction. Thus, for example, longitudinal rib pairs having recesses with a first depth and longitudinal rib pairs having recesses with a second depth can be arranged adjacent to each other in an alternating manner. For example, different longitudinal rib pair classes may differ from each other in terms of the shape of the recesses, the depth of the recesses, the spacing between the recesses, and / or the arrangement of the recesses between the longitudinal ribs of each longitudinal rib pair. Typically, each identically embodied longitudinal rib pair and the recesses arranged between their longitudinal ribs are assigned to the same longitudinal rib pair class.

[0030] Advantageous for flow behavior is when transverse ridges are arranged on the base plane, each connected to one side of one of the longitudinal ribs, where, in a cross section perpendicular to the main direction, the transverse ridges have a surface contour that is particularly flat, with its height decreasing in the direction away from the sidewall, and preferably transitions to the base plane. This should be understood as the side of the longitudinal rib, which faces a depression. Typically, a plurality of transverse ridges are provided that are spaced apart from each other in the main direction, and these transverse ridges are connected to the side of each longitudinal rib. Preferably, such transverse ridges are arranged on the side of both longitudinal ribs. The side of the two longitudinal ribs thus typically faces each other. Each transverse ridge typically transitions to the base plane, or preferably connects to one of the depressions or one of their depression edges.

[0031] Transverse ridges on the sides of two longitudinal ribs, where they face each other, have been found to be effective when they are connected to the aforementioned sides in a cross section perpendicular to the principal direction in such a way that the tangents applied to the surface contours of the transverse ridges are greater than 90°, particularly greater than 120°.

[0032] It is advantageous if one of the depressions exists between each of two directly adjacent transverse ridges. The transverse ridges can thus be directly adjacent in the principal direction and / or adjacent perpendicular to the principal direction. These can be directly adjacent transverse ridges connected to the same longitudinal rib or its side surface, and / or directly adjacent transverse ridges connected to different longitudinal ribs of two longitudinal ribs or their side surfaces, particularly side surfaces facing each other. It is beneficial for flow behavior if each transverse ridge has a longitudinal extension in the base-level plan view that is oriented obliquely to the longitudinal direction of the longitudinal rib to which the transverse ridge is connected, where the longitudinal direction of the transverse ridge forms an acute angle with the longitudinal direction of the longitudinal rib. The angle can be between 10° and 80° for convenience, and may be less than 60° in particular, preferably about 45°.

[0033] Typically, the longitudinal rib height and / or longitudinal rib tip spacing between longitudinal rib tips is provided to be less than 250 μm, particularly less than 100 μm, and preferably less than 80 μm. It is beneficial when the longitudinal rib height is between 250 μm and 1 μm, particularly between 100 μm and 5 μm, preferably between 25 μm and 40 μm, specifically preferably between 30 μm and 37 μm, and particularly preferably approximately 34 μm. It has been found to be beneficial when the longitudinal rib tip spacing, i.e., the distance between the tips or peaks of directly adjacent longitudinal ribs or pairs of longitudinal ribs, is between 60 μm and 85 μm, particularly between 70 μm and 80 μm, and preferably approximately 74 μm. Therefore, it has been found to be particularly effective when the height of the longitudinal ribs is between 30 μm and 37 μm, preferably approximately 34 μm, and the distance between the longitudinal rib tips is between 70 μm and 80 μm, preferably approximately 74 μm. The distance between the longitudinal rib tips is typically measured between the highest points of the longitudinal ribs or between the longitudinal rib tips in a cross section perpendicular to the main direction.

[0034] Typically, in a cross section perpendicular to the main direction, the base surface portion has a width between 50 μm and 70 μm, particularly approximately 60 μm, or the longitudinal rib spacing in the above cross section is of this size. The base surface portion is typically adjacent to the longitudinal ribs or their side walls.

[0035] It is advantageous if a surface exists in the component, and such surface includes a riblet structure, particularly as described herein. Due to the features and effects of the riblet structure, the surface of the component can be embodied with lower frictional resistance for fluids flowing along that surface. For convenience, the component may be, for example, part of a means of transport such as an aircraft, ship, or automobile; part of a fluid-guiding element such as a pipe; or part of an article of clothing such as a protective suit or athletic suit, all of which are intended to reduce frictional resistance for fluids flowing along a part of the surface having a riblet structure.

[0036] The initially named purpose is realized when the use of riblet structures, particularly the riblet structures described herein, is provided as a surface on a component and used to reduce friction between that surface and gaseous flows, particularly air flows, and / or liquid flows, particularly water flows. According to the features and effects of riblet structures, the surface of a component can be embodied with lower frictional resistance with respect to the fluid flowing along that surface.

[0037] Another objective, initially named, is realized when a method is provided for generating a riblet structure, which is embodied as described herein, and the riblet structure is brought to the surface by embossing or material removal processes. Such a method makes it advantageous to generate the riblet structure described herein, which has low frictional resistance to fluids flowing along the riblet structure. The riblet structure can be manufactured to have the characteristics and effects corresponding to those described herein. For production, it is feasible to use the negative shape of the riblet structure and bring the riblet structure to the surface by pressing the negative shape onto the surface.

[0038] It should be understood that the height and depth information described typically refers to directions perpendicular to the base plane or their reference areas, while descriptions of length and width usually refer to directions parallel to the base plane or their reference areas. [Brief explanation of the drawing]

[0039] Additional features, advantages, and effects can be seen from the exemplary embodiments described below, as referenced in the drawings:

[0040] [Figure 1] This figure shows a typical riblet structure in the prior art, in which longitudinal ribs are oriented in the main direction and flat base surface portions are provided between pairs of longitudinal ribs. [Figure 2]This figure shows a detailed oblique top view of the riblet structure shown in Figure 1. [Figure 3] This figure shows a detailed plan view of the riblet structure shown in Figure 1. [Figure 4] This figure shows a detailed profile diagram of the riblet structure shown in Figure 1. [Figure 5] This figure shows a riblet structure in the base surface portion located between pairs of longitudinal ribs, with recesses spaced apart in the main direction, and the recesses having a rectangular cross-section. [Figure 6] Figure 5 shows a detailed oblique top view of the riblet structure. [Figure 7] This figure shows a detailed plan view of the riblet structure shown in Figure 5. [Figure 8] This figure shows a riblet structure in the base surface portion located between pairs of longitudinal ribs, with depressions spaced apart in the main direction, each depression having a triangular cross-section. [Figure 9] This figure shows a riblet structure in the base surface portion located between pairs of longitudinal ribs, with depressions spaced apart in the main direction, where the depression walls are realized as surfaces of rotation. [Figure 10] This figure shows a detailed, oblique top view of the riblet structure shown in Figure 9. [Figure 11] This figure shows a detailed plan view of the riblet structure shown in Figure 9. [Figure 12] This figure shows a riblet structure in the base surface portion located between pairs of longitudinal ribs, with depressions spaced apart in the main direction. The depressions have a triangular cross-section and form two rows of depressions oriented in the main direction. [Figure 13] This figure shows a detailed, oblique top view of the riblet structure shown in Figure 12. [Figure 14] This figure shows a detailed plan view of the riblet structure shown in Figure 12. [Figure 15]This figure shows a riblet structure in the base surface portion located between pairs of longitudinal ribs, with depressions spaced apart in the main direction, each depression extending to both longitudinal ribs of the respective pair of longitudinal ribs. [Figure 16] This figure shows a detailed oblique top view of the riblet structure shown in Figure 15. [Figure 17] This figure shows a detailed plan view of the riblet structure shown in Figure 15. [Figure 18] This is a graph showing frictional resistance as a function of position relative to different riblet structures. [Figure 19] This diagram shows the contour of a riblet structure that has a riblet structure without depressions, and the average frictional resistance is shown in grayscale. [Figure 20] This diagram shows the contour of a riblet structure having rectangular depressions, with the average frictional resistance shown in grayscale. [Figure 21] This diagram shows the outline of a riblet structure with spherical depressions, and the average frictional resistance is shown in grayscale. [Modes for carrying out the invention]

[0041] Figure 1 shows a schematic example of a typical riblet structure 1 from the prior art. The riblet structure 1 is formed by having a plurality of wedge-shaped longitudinal ribs 3 that project from a base surface 2, operate along a main direction H, and are spaced apart from one another. The base surface 2 forms flat base surface portions 4 between two longitudinal ribs 3 or pairs of longitudinal ribs, which are typically directly adjacent in a secondary direction N perpendicular to the main direction. The longitudinal ribs 3 preferably include flat longitudinal rib sidewalls 8. Such structures have been found to be suitable for reducing frictional resistance with respect to fluids flowing along the riblet structure 1 in the main direction H. Figures 2 and 3 show schematic details of the riblet structure 1 shown in Figure 1 to illustrate it more clearly, with Figure 2 showing an oblique top view of the riblet structure 1 shown in Figure 1 and Figure 3 showing a plan view of the riblet structure 1 shown in Figure 1.

[0042] Figure 4 shows a schematic profile description of the details of the riblet structure 1 shown in Figure 1. Typically, the longitudinal rib tip spacing R S , longitudinal rib spacing R D , and longitudinal rib height R z These are identified as characteristic data for describing the riblet structure 1, and are measured in cross-sections perpendicular to the main direction H and perpendicular to the base surface 2, in particular with respect to the height of each base surface portion 4. Longitudinal rib tip spacing R S This refers to the distance between two directly adjacent longitudinal ribs 3 or pairs of longitudinal ribs, where R is the longitudinal rib spacing. D This refers to the width of the base surface portion 4 between two directly adjacent longitudinal ribs 3 or a pair of longitudinal ribs, and the longitudinal rib height R. z This refers to the height of the longitudinal rib 3, measured starting from the base surface 2 or each base surface portion 4. This also applies to other riblet structures 1 specified herein. The height direction z is typically oriented perpendicular to the primary direction H and also perpendicular to the secondary direction N.

[0043] The base surface 2 includes recesses 5 spaced apart in the main direction H between each of two directly adjacent longitudinal ribs 3 or pairs of longitudinal ribs, wherein the recesses 5 are spaced apart from at least one or both longitudinal ribs 3 in a direction perpendicular to the main direction H, which is advantageous in achieving a further reduction in frictional resistance.

[0044] As described below, several different riblet structures 1 were tested and computer simulations were performed, in particular the expansion, shaping, and / or enlargement of the depressions 5 created in the base surface 2 or base surface portion 4. The selection of riblet structures 1 tested is described below.

[0045] Figure 5 shows a schematic diagram of the riblet structure 1 of the basic design according to Figure 1, where the recesses 5 spaced apart in the main direction H are further present in the respective base surface portions 4 located between the longitudinal ribs 3 of each longitudinal rib pair. The recesses 5 have a rectangular cross section or a rectangular recess edge 6. The recesses 5 are positioned spaced apart from the longitudinal ribs 3 of each longitudinal rib pair. Such a design has been found to be very efficient in reducing frictional resistance. For convenience, the recesses 5 may each have a substantially constant cross section or a lower cross section along their depth direction T. The recesses 5 then form a cubic recess. Figures 6 and 7 show schematic diagrams of the details of the riblet structure 1 shown in Figure 5 to illustrate the riblet structure 1 shown in Figure 5 more clearly, with Figure 6 showing an oblique top view of the riblet structure 1 shown in Figure 5 and Figure 7 showing a plan view of the riblet structure 1 shown in Figure 5.

[0046] A design for a riblet structure 1 similar to the riblet structure 1 shown in Figure 5 is schematically shown in Figure 8, where each recess 5 has a triangular cross-section or a triangular recess edge 6. The recesses 5 shown in Figure 8 are also realized so as to be spaced apart from the two longitudinal ribs 3 of a pair of longitudinal ribs, and in particular, they can be realized with a constant cross-section or a lower cross-section along their depth direction T.

[0047] Figure 9 shows a schematic diagram of a further riblet structure 1 having the basic design according to Figures 1 and 5, where the recess 5 is embodied with a recess edge without corners. The recess 5 may advantageously include a recess wall 7 embodied as a plane of rotation, typically having an axis of rotation in the depth direction T of the recess 5. This has been found to be effective when the recess wall 7 of the recess 5 is embodied as part of a spheroidal, preferably spherical, surface, or spherically. A recess 5 embodied in this manner is particularly suitable for achieving reduced viscous resistance while having minimized pressure resistance. In Figure 9, the recess 5 is embodied substantially as part of a sphere or spherical surface. Figures 10 and 11 show schematic diagrams of the details of the riblet structure 1 shown in Figure 9 to illustrate it more clearly, with Figure 10 showing an oblique top view of the riblet structure 1 shown in Figure 9 and Figure 11 showing a plan view of the riblet structure 1 shown in Figure 9.

[0048] Figure 12 shows a schematic diagram of a riblet structure 1 having the basic design according to Figure 1, where each base surface portion 4 located between longitudinal rib pairs includes recesses 5 spaced apart from each other in the main direction H, where the recesses 5 form two rows of recesses oriented in the main direction H. The rows of recesses are arranged to be adjacent or offset from each other in a direction perpendicular to the main direction H, and as seen particularly in the figure for the main direction H, the rows of recesses are offset or displaced from each other in the main direction H, typically by half the distance between two directly adjacent recesses 5 in one of the rows of recesses. As seen in Figure 12, the reference area of ​​the base surface portion 4, i.e., the portion of the base surface portion 4 without recesses 5, can thus form a zigzag structure that reciprocates between the two rows. Figures 13 and 14 show schematic diagrams of the details of the riblet structure 1 shown in Figure 12, in order to more clearly illustrate the riblet structure 1 shown in Figure 12. Figure 13 shows an oblique top view of the riblet structure 1 shown in Figure 12, and Figure 14 shows a plan view of the riblet structure 1 shown in Figure 12. This type of design can achieve simultaneously reduced viscous resistance and low pressure resistance, thus advantageously reducing frictional resistance, especially compared to the design shown in Figure 1. Therefore, pressure resistance cannot typically be minimized to the same extent as in one embodiment of a recess 5 having a recessed wall 7 with the shape of a rotating surface, for example, as shown in Figure 9.

[0049] Figure 15 shows a schematic view of the riblet structure 1 having the basic design according to Figure 1, where the base surface portion 4 includes depressions 5 spaced apart from each other in the main direction H. However, in this case, the depressions 5 are assumed to be provided so as to extend to the longitudinal ribs 3 of both of the respective longitudinal rib pairs, resulting in a sinusoidal-like profile of the base surface portion 4 in the main direction H. Figures 16 and 17 show schematic views of details of the riblet structure 1 shown in Figure 15 to show the riblet structure 1 more clearly, Figure 16 shows a perspective top view of the riblet structure 1 shown in Figure 15, and Figure 17 shows a plan view of the riblet structure 1 shown in Figure 15. In the shape of this type, reduction of frictional resistance was achieved as compared with a plane having no riblet structure 1, but the frictional resistance was higher than that of the riblet structure 1 (according to Figure 1) having a flat base surface 2 without the depressions 5.

[0050] The simulation results of the modification example of the above design are presented below. Regarding the data provided below, the longitudinal rib tip spacing R between two directly adjacent longitudinal ribs 3 arranged on the base surface 2 S is 74 μm each, and 84 longitudinal ribs 3 with a longitudinal rib height R z of 34 μm were used. Water at a temperature of 25 °C was used as the fluid.

[0051] Regarding the riblet structures 1 shown in Figure 1 (without depressions 5), Figure 5 (rectangular depressions 5), Figure 8 (triangular depressions 5), Figure 9 (spherical depressions 5), Figure 12 (triangular depressions 5 in two rows), and Figure 15 (depressions 5 extending to both longitudinal ribs 3), the results of the resistance parameters for the fluid flowing in the main direction H along the riblet structure 1 are described in Table 1 below. The total resistance P S is hereby shown as the sum of the pressure resistance P P and the viscous resistance P V . Finally, the percentage of reduction of frictional resistance when compared with a plane having no riblet structure 1 is also described in Table 1.

[0052] As shown in Table 1, for the typical riblet structure 1 shown in Figure 1, when the recess 5 is not provided, the frictional resistance is reduced by 8.10% compared to a plane without the riblet structure 1. By comparison, a more significant reduction in frictional resistance is achieved by the riblet structures 1 from Figures 5, 8, 9, and 12. The riblet structure 1 in Figure 1 has viscous resistance and virtually no pressure resistance, but by providing the recess 5 in the base surface portion 4, the viscous resistance is reduced, while pressure resistance is then generated. If both of these resistances are minimized, the reduction in frictional resistance of the riblet structure 1 with the recess 5 can be achieved compared to a plane without the riblet structure 1, and this reduction is greater than in the case of the riblet structure 1 in Figure 1 without the recess 5.

[0053] When the riblet structure 1 shown in Figure 8 (recess 5 with a triangular cross-section) included recesses 5 spaced apart from both longitudinal ribs 3, it resulted in a particularly favorable reduction of 9.94% in frictional resistance. In the case of the riblet structure 1 shown in Figure 5 (recess 5 with a rectangular cross-section), frictional resistance was reduced by 8.57%, and in the case of the riblet structure 1 shown in Figure 9 (recess 5 including recess walls 7 embodied as part of a spherical surface), a reduction of 8.56% in frictional resistance was achieved compared to a surface without the riblet structure 1. In the case of the riblet structure 1 shown in Figure 12 (triangular recess 5 in two rows), frictional resistance was reduced by 9.53%. In contrast, in the case of the riblet structure 1 shown in Figure 15 (recess 5 extending to both longitudinal ribs 3), frictional resistance was reduced by only 7.22% compared to a surface without the riblet structure 1, which was a smaller reduction than the case of the riblet structure 1 shown in Figure 1 without recess 5.

[0054] Table 1: Simulation results for the frictional resistance of the riblet structure 1 shown in Figure 1 (no depression 5), Figure 5 (rectangular depression 5), Figure 8 (triangular depression 5), Figure 9 (spherical depression 5), Figure 12 (triangular depression 5 in two rows), and Figure 15 (depression 5 extending to both longitudinal ribs 3), P P P is pressure resistance. V is viscous resistance, PS The total resistance is (P P +P V ), and ΔR represent the percentage change in frictional resistance compared to a plane without the riblet structure 1. [Table 1]

[0055] Figure 18 shows graphs illustrating the frictional resistance of different riblet structures 1 as a function of position in a direction perpendicular to the main direction H. Line 10, shown as a dashed line, shows the frictional resistance of riblet structure 1 without the depressions 5 according to Figure 1. Periodic peaks are visible in line 10, and they correspond to the longitudinal ribs 3 of riblet structure 1. Between the peaks, line 10 each forms a progression of valleys in which the frictional resistance increases slightly parabolic in the middle between adjacent peaks. Line 11, shown as a dotted line, shows the frictional resistance of riblet structure 1 with rectangular depressions 5 according to Figure 5, and line 12, shown as a solid line, shows the frictional resistance of riblet structure 1 with spherical depressions 5 according to Figure 9. Lines 11 and 12 similarly have the basic shape of line 10, but in contrast to line 10, the peak heights of lines 11 and 12 are lower, and furthermore, in the middle between the peaks, there is a reduction or depression in the frictional resistance of lines 11 and 12. This corresponds to the depressions 5 in the riblet structure 1 in Figures 5 and 9, respectively, and to the reduction in total resistance caused by the depressions 5. For comparison, the graph in Figure 18 also shows the frictional resistance of a plane without the riblet structure 1, indicated by the dotted line 9. Line 9 represents a constant frictional resistance along the surface.

[0056] Figures 19 to 21 show contour diagrams of the average frictional resistance of different riblet structures 1, where the frictional resistance is represented in grayscale. Figure 19 shows riblet structure 1 according to Figure 1 (without depressions 5), Figure 20 shows riblet structure according to Figure 5 (with rectangular depressions 5), and Figure 9 shows riblet structure 1 according to Figure 9 (with spherical depressions 5). What is obvious in the contour diagrams is that, corresponding to the graph in Figure 18, the reduced average frictional resistance lies between the longitudinal ribs 3. The rectangular depressions 5 in riblet structure 1 according to Figure 5 correspond to the rectangular dark regions of particularly reduced frictional resistance in Figure 20, and the spherical depressions 5 in Figure 9 correspond to the circular dark regions of particularly reduced frictional resistance in Figure 21. The light regions located between the circular dark regions in Figure 21 also correspond to a smaller frictional resistance than the light regions located between the rectangular dark regions in Figure 20. This corresponds to the finding of particularly reduced frictional resistance in the case of a spherical recess 5 or a recess 5 having a recess edge 6 without corners.

[0057] In the riblet structure 1, the riblet structure 1 includes a plurality of longitudinal ribs 3 that project from the base surface 2, are oriented in the main direction H, and are spaced apart from each other, and includes a recess 5 in the base surface 2 or base surface portion 4 that is spaced apart from each other between two adjacent longitudinal ribs 3 or pairs of longitudinal ribs, and the recess 5 is spaced apart from at least one, preferably both longitudinal ribs 3, in a direction perpendicular to the main direction H, so that the riblet structure 1 can achieve advantageously low frictional resistance to fluid flowing along the riblet structure 1 in the main direction H. As a result, it is possible to reduce viscous resistance and minimize pressure resistance, and thus a significant reduction in the frictional resistance of the riblet structure 1 can be achieved. This is particularly applicable when the recess 5 has a significant distance from both longitudinal ribs 3 and is preferably located in the center between the longitudinal ribs 3. One embodiment of the recess 5 having a rectangular cross-section, or a recess wall 7 which is preferably a sphere or part of a spheroidal ellipsoid, has been found to be particularly beneficial for achieving low frictional resistance. [Other possible items] [Item 1] A riblet structure (1) relating to a surface, for the purpose of reducing frictional resistance to a fluid flowing along the surface in a principal direction (H), wherein the riblet structure (1) comprises a plurality of longitudinal ribs (3) particularly wedge-shaped, protruding from a base surface (2), oriented in a principal direction (H), and spaced apart from each other, wherein between each of two adjacent longitudinal ribs (3), the base surface (2) has a recess (5) spaced apart from each other along the principal direction (H), and the recess (5) is spaced apart from at least one or both of the longitudinal ribs (3) in a direction perpendicular to the principal direction (H), at least partially, preferably entirely. [Item 2] The riblet structure (1) according to item 1, characterized in that the recesses (5) are realized in a repeating structural sequence, particularly in the main direction (H). [Item 3] The riblet structure (1) according to item 1 or 2, characterized in that, in a plan view of the base surface (2), the average spacing in a direction perpendicular to the main direction (H) between at least one of the two longitudinal ribs (3) and the recess edges that are in contact with each of the recesses (5) and facing the longitudinal ribs (3) is greater than 5% of the average spacing between the longitudinal ribs (3). [Item 4] A riblet structure (1) according to any one of items 1 to 3, characterized in that, in a cross section parallel to the base surface, the average longitudinal expansion of the recess in the main direction is up to 1.5 times, and particularly up to 1.2 times, the average width expansion of the recess in the direction perpendicular to the main direction. [Item 5] The riblet structure (1) according to any one of items 1 to 4, characterized in that the recess (5) forms a plurality of rows of recesses oriented in the main direction (H) between the two longitudinal ribs. [Item 6] The riblet structure (1) according to item 5, characterized in that the rows of recesses are offset from one another in the main direction (H), preferably by half the distance between two adjacent recesses (5) in adjacent rows. [Item 7] The riblet structure (1) according to any one of items 1 to 6, characterized in that each of the recesses (5) has a constant cross-sectional surface mainly along the depth direction (T) of the recess (5). [Item 8] A riblet structure (1) according to any one of items 1 to 7, characterized in that, in cross-section, each of the recesses (5) is formed to be particularly elliptical or round and without corners, or particularly triangular or quadrilateral and polygonal. [Item 9] The riblet structure (1) according to any one of items 1 to 8, wherein each of the recesses (5) includes a recess wall (7) that forms the recess (5), and the recess wall (7) is at least partially embodied as a plane of rotation including an axis of rotation in the depth direction (T) of the recess (5). [Item 10] In the depth direction (T), each of the recesses (5) is formed with a first recess region and a second recess region located behind the first recess region, the first recess region having a smaller diameter than the second recess region, preferably the diameter of the first recess region being less than 80% of the diameter of the second recess region, as described in any one of items 1 to 9. [Item 11] The riblet structure according to any one of items 1 to 10, characterized in that the plurality of recesses (5) are connected to one another via at least one fluidically conductive connection channel. [Item 12] The height of the longitudinal rib (R) of the longitudinal rib (3) z ), and / or the longitudinal rib spacing (R) between the longitudinal ribs (3) DA riblet structure (1) according to any one of items 1 to 11, characterized in that the diameter is less than 250 μm, and especially less than 100 μm. [Item 13] The surface of the component is characterized by having a riblet structure (1) as described in any one of items 1 to 12. [Item 14] To reduce friction between a surface and a gas flow, particularly an air flow and / or a liquid flow, particularly a water flow, use of a riblet structure (1) as described in any one of items 1 to 12, on the surface of the component, in particular as the surface described in item 13. [Item 15] A method for manufacturing a riblet structure (1), characterized in that a riblet structure (1) described in any one of items 1 to 12 is realized, and the riblet structure (1) is brought to the surface by a surface embossing treatment or a material removal treatment.

Claims

1. A riblet structure relating to a surface, intended to reduce frictional resistance to a fluid flowing along the surface in a principal direction, wherein the riblet structure comprises a plurality of longitudinal ribs, particularly wedge-shaped, protruding from a base surface, oriented in a principal direction, and spaced apart from each other, wherein between each of two adjacent longitudinal ribs, the base surface has a recess spaced apart from each other along the principal direction, and the recess is spaced apart from at least one or both of the longitudinal ribs in a direction perpendicular to the principal direction, at least partially, preferably entirely.

2. The riblet structure according to claim 1, wherein the recesses are embodied in a repeating structural sequence, particularly in the main direction.

3. The riblet structure according to claim 1 or 2, wherein, in a plan view of the base surface, the average spacing between at least one of the two longitudinal ribs and the recess edges that are in contact with each recess and facing the longitudinal rib, in a direction perpendicular to the main direction, is greater than 5% of the average spacing between the longitudinal ribs.

4. The riblet structure according to claim 1 or 2, wherein in a cross section parallel to the base surface, the average longitudinal expansion of the recess in the main direction is up to 1.5 times, particularly up to 1.2 times, the average width expansion of the recess in the direction perpendicular to the main direction.

5. The riblet structure according to claim 1 or 2, wherein the recesses form a plurality of rows of recesses oriented in the main direction between the two longitudinal ribs.

6. The riblet structure according to claim 5, wherein the rows of recesses are offset from one another in the main direction, preferably by half the distance between two adjacent recesses in an adjacent row.

7. The riblet structure according to claim 1 or 2, wherein each of the recesses has a constant cross-sectional surface mainly along the depth direction of the recess.

8. The riblet structure according to claim 1 or 2, wherein in cross-section, each of the recesses is formed to be particularly elliptical or round and without corners, or particularly triangular or quadrilateral and polygonal.

9. The riblet structure according to claim 1 or 2, wherein each of the recesses includes a recess wall forming the recess, and the recess wall is at least partially embodied as a plane of rotation including an axis of rotation in the depth direction of the recess.

10. The riblet structure according to claim 1 or 2, wherein, in the depth direction of the recess, each recess is formed with a first recess region and a second recess region located behind the first recess region, the first recess region having a smaller diameter than the second recess region, preferably the diameter of the first recess region being less than 80% of the diameter of the second recess region.

11. The riblet structure according to claim 1 or 2, wherein the plurality of recesses are connected to one another via at least one fluidically conductive connecting channel.

12. The riblet structure according to claim 1 or 2, wherein the height of the longitudinal ribs and / or the longitudinal spacing between the longitudinal ribs is less than 250 μm, and in particular less than 100 μm.

13. The surface of the component is a surface having the riblet structure described in claim 1 or 2.

14. To reduce friction between a surface and a gas flow, particularly an air flow and / or a liquid flow, particularly a water flow, use of the riblet structure according to claim 1 or 2 as the surface of the component, in particular as the surface according to claim 13.

15. A method for manufacturing a riblet structure, wherein the riblet structure described in claim 1 or 2 is realized, and the riblet structure is brought to the surface by a surface embossing treatment or material removal treatment.

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