High efficiency riblet structure and method for producing such a structure

Abstract

The invention relates to a riblet structure (1), comprising longitudinal channels (2) oriented in a main direction (H). According to the invention, each longitudinal channel (2) has a substructure (3) with structural elements (4) arranged in, in particular periodic, succession for a fluid flow flowing in the longitudinal channel (2) in the main direction (H), wherein a ratio of an average spacing (d) of the structural elements (4) in the main direction (H) to an average spacing of channel edges of the longitudinal channel (2) is less than 1, in particular is 0.09 to 0.33. The invention further relates to a method for production and a method for computer-implemented simulation of a surface with such a riblet structure (1).

Description

[0001] Highly efficient riblet structure and method for producing such a

[0002] The invention relates to a riblet structure having longitudinal channels oriented in a main direction.

[0003] The invention further relates to a method for producing such a riblet structure.

[0004] Furthermore, the invention relates to a method for the computer-implemented simulation of a surface with a riblet structure, wherein the riblet structure has longitudinal channels oriented in a main direction.

[0005] Riblet structures are known from the prior art. These are surface structures, typically in the sub-millimeter range, designed to reduce the frictional resistance of a surface having the surface structure against a fluid flowing along the surface, such as air or water. Riblet structures are used, for example, on aircraft fuselages or wings to reduce the frictional resistance of the aircraft against an air flow flowing along the aircraft.

[0006] Riblet structures are usually formed with spaced-apart longitudinal ribs arranged on a base plane along a main direction, which in use corresponds to a main flow direction of the fluid, so that two adjacent longitudinal ribs each form a longitudinal channel.

[0007] For example, document EP 3 170743 A1 discloses a riblet structure with a plurality of ribs formed in this way. The base surface arranged between the ribs can be flat or curved.

[0008] Scientific studies show that frictional resistance on a surface can be reduced by up to approximately 8% if the surface is equipped with riblets. This is considered the theoretical maximum; in practice, lower values ​​are usually achieved. Higher efficiency coupled with simpler production and / or application of riblets would be desirable. This is where the invention comes in. The object of the invention is to provide a riblet structure of the type mentioned above that exhibits high efficiency and, preferably, high practical implementation.

[0009] A further aim is to provide a method for producing such a riblet structure.

[0010] Another aim is to provide a method for simulating such a riblet structure.

[0011] The object is achieved according to the invention in that, in a riblet structure of the type mentioned at the outset, the respective longitudinal channel has a substructure with structural elements arranged one after the other, in particular periodically, for a fluid flow flowing in the longitudinal channel in the main direction, wherein a ratio of an average distance of the structural elements in the main direction to an average distance of the channel edges of the longitudinal channel is less than 1, in particular from 0.09 to 0.33.

[0012] As a rule, the riblet structure or the longitudinal channels are formed with a plurality of spaced-apart longitudinal ribs that protrude from a base surface and are oriented in the main direction. The respective longitudinal channel is usually formed by, in particular, two adjacent longitudinal ribs. The longitudinal ribs are usually oriented in the main direction. A part of the base surface located between two longitudinal ribs, in particular of the respective longitudinal channel, is usually referred to as the base surface section. A longitudinal channel wall, in particular longitudinal channel walls, of the respective longitudinal channel are usually formed with, in particular, side surfaces of the respective longitudinal ribs and the base surface section located between the longitudinal ribs. The side surfaces of the respective longitudinal ribs, which are usually adjacent in the secondary direction and with which side surfaces the longitudinal channel is formed, are also referred to as the longitudinal rib side surfaces of the longitudinal channel.The base surface is generally formed as a base plane, and the base surface section as a base plane section. The longitudinal ribs are generally oriented in the main direction, with the longitudinal ribs being arranged next to one another at a distance from one another in a direction orthogonal to the main direction, which is particularly referred to as the secondary direction. The longitudinal ribs of the respective longitudinal channel can be wave-shaped or, preferably, rectilinear, particularly in a plan view of the riblet structure. A vertical direction of the riblet structure is usually oriented orthogonal to the main direction and orthogonal to the secondary direction.

[0013] The invention is based on the finding that, in a conventional riblet structure without a substructure, which riblet structure is formed with longitudinal channels oriented in a main direction, it is possible to influence the flow behavior of a fluid flowing along the riblet structure directly at the riblet structure, in particular in a so-called viscous sublayer of a fluid-dynamic boundary layer of the fluid flow, by introducing a substructure into a longitudinal channel wall forming a respective longitudinal channel, without, however, significantly influencing or disrupting a boundary layer profile of the fluid flow somewhat further away from the riblet structure, in particular a flow turbulence above the longitudinal channels. In this way, the potential exists to improve the flow behavior directly at the riblet structure by structurally developing a substructure of the respective longitudinal channel.

[0014] Conventional riblet structures without a substructure and with a generally flat base surface section between the longitudinal ribs of the respective longitudinal channel generate turbulent viscous drag, caused primarily by an interaction surface of the respective longitudinal channel wall, in particular of the respective base surface section located between the ribs. It has been shown that the viscous drag can be reduced by providing a substructure in the longitudinal channel wall, in particular in the base surface section and / or the longitudinal rib side surfaces of the longitudinal channel. The substructure can be implemented in the respective longitudinal channel with structural elements arranged one after the other in the main direction, which are preferably formed by elevations and / or depressions arranged one after the other in the main direction. The structural elements in the respective longitudinal channel are preferably arranged periodically one after the other in the main direction.The respective longitudinal channel usually forms the substructure in such a way that a fluid flow flowing in the main direction in the longitudinal channel encounters structural elements one after the other. The reduction in viscous drag can be explained by a reduced interaction area of ​​the longitudinal channel wall. Due to the introduced substructure, usually in the form of elevations and / or depressions, an area of ​​the longitudinal channel wall exposed to a flow flowing along the longitudinal channel wall can be reduced, or a part of the longitudinal channel wall can be lowered by the depressions and thus contributes only to a reduced extent to the viscous drag. This is particularly true in a plan view of the riblet structure. For example, an area of ​​the base surface section located at the same height can be reduced, or a part of the base surface or base surface section can be lowered by depressions, thus contributing only to a reduced extent to the viscous drag.Conversely, however, it has been shown that the substructure, in particular the depressions, generates additional pressure resistance. The total resistance used for comparison is thus the sum of viscous resistance and pressure resistance for a riblet structure, in particular the respective longitudinal channel, having the substructure, compared to the purely viscous resistance for a riblet structure, in particular the respective longitudinal channel, without the substructure. An advantageously reduced total resistance or frictional resistance can therefore be achieved with a riblet structure with the substructure if, on the one hand, the substructure reduces the viscous resistance as much as possible and, on the other hand, the pressure resistance generated in return is kept as small as possible.The area of ​​the longitudinal channel wall, in particular the base surface and / or the longitudinal rib side surfaces of the respective longitudinal channel, which area is not part of the structural elements, is also referred to as the interaction surface or reference surface of the base surface, or, in particular, analogously, as the interaction plane or reference plane of the base plane. The reference surface typically represents the base surface without substructure. It is preferred if the structural elements are formed with, in particular, depressions.

[0015] Investigations have shown that for each longitudinal channel, by setting an average distance between the structural elements in the main direction, particularly when the structural elements are formed by depressions, the total resistance or frictional resistance, in particular a reduction in viscous resistance and the extent of pressure resistance, can be varied in relation to an average distance between the channel edges of the longitudinal channel. An advantageously low total resistance can be achieved if the ratio of an average distance between the structural elements in the main direction to an average distance between the channel edges of the longitudinal channel is less than 1.0, in particular a maximum of 0.8, preferably a maximum of 0.5, particularly preferably a maximum of 0.33, especially preferably a maximum of 0.22. The ratio is generally greater than 0.05, in particular at least 0.09, preferably at least 0.13.The physical cause is assumed to be the generation of a rotation in a flow field within a region of the structural elements. Simulations of flow behavior have shown that when the flow flows in the main direction through the structural elements, a partial lateral discharge or deflection of a portion of the flow from the respective longitudinal channel is generated or amplified, particularly with rotation of the flow component, so that resistance in the main direction is reduced. This can be referred to as the "cross-channel effect." Typically, a rotation of the flow component is generated transversely, particularly orthogonally, to the main direction.It is advantageous if the ratio of the average distance of the structural elements in the main direction to the average distance of the channel edges of the longitudinal channel is from 0.05 to 0.8, in particular from 0.09 to 0.5, particularly preferably from 0.09 to 0.33, particularly preferably from 0.13 to 0.22.

[0016] In this way, an advantageous riblet structure for a surface can be achieved to reduce frictional resistance against a fluid flowing along the surface in a main direction. Typically, the longitudinal channels are oriented in the main direction and arranged next to one another in a secondary direction, which is oriented orthogonally to the main direction. The fluid can be formed by a gas, such as air, and / or a liquid, such as water.

[0017] It is advantageous if the respective longitudinal channel has structural elements arranged one after the other in the main direction. In a plan view of the riblet structure, the structural elements can be arranged one after the other in a secondary direction oriented orthogonally to the main direction. The average spacing of the structural elements in the main direction usually refers to a spacing in the main direction regardless of whether the structural elements arranged one after the other are arranged one after the other in the secondary direction. For example, the structural elements arranged one after the other in the main direction can completely overlap, partially overlap, or not overlap at all in the main direction. The riblet structure can be viewed in plan view. A fluid or fluid flow flowing in the main direction in the longitudinal channel can strike the structural elements one after the other and be influenced by the structural elements, in particular at least partially discharged laterally ordeflected, as explained above in particular. The above applies in particular to the respective longitudinal channel. Typically, several, in particular a majority, preferably substantially all, of the longitudinal channels of the riblet structure are implemented with a substructure, in particular of this type. Preferably, the longitudinal channels are separated by the longitudinal ribs in such a way that adjacent longitudinal channels are formed with side surfaces of the longitudinal rib opposite one another on the respective longitudinal rib.

[0018] As a rule, the longitudinal channels are formed with a plurality of spaced-apart longitudinal ribs which project from a base surface and are oriented in the main direction, wherein the average distance between the channel edges is an average rib tip distance of the longitudinal ribs forming the longitudinal channel. Each longitudinal channel is usually formed with two immediately adjacent longitudinal ribs, usually in the secondary direction, also referred to as a pair of longitudinal ribs. A longitudinal channel bottom surface of the longitudinal channel wall can be formed by the base surface or a base surface section of the base surface located between the longitudinal ribs, and / or longitudinal channel side surfaces of the longitudinal channel wall can be formed by longitudinal rib side surfaces of the longitudinal channel. The structural elements are usually formed with the base surface section and / or the longitudinal rib side surfaces, in particular inserted into them. The longitudinal ribs are usually wedge-shaped orhave a triangular or trapezoidal basic shape in a cross-section orthogonal to the longitudinal extension of the longitudinal ribs. Side walls or longitudinal rib side surfaces of the longitudinal ribs can be flat, concave or convex. The longitudinal ribs are usually arranged on the base surface or protrude from it. The base surface, in particular the base surface section of the base surface located between two longitudinal ribs, can be essentially flat. The structural elements, in particular in the form of elevations and / or depressions, can be inserted into the base surface and / or the longitudinal rib side surfaces of the longitudinal channel or formed with them. In a cross-section orthogonal to the main direction or a longitudinal direction of the longitudinal channel, the longitudinal channel often forms essentially an upside-down trapezoidal or parabolic recess over which the substructure is superimposed.The respective base surface section can be concave, convex, or preferably flat. If the base surface is flat, it is often referred to as the base plane. The respective longitudinal rib can have a wavy or preferably straight profile in the longitudinal direction of the longitudinal rib, particularly regardless of the substructure. The longitudinal direction of the longitudinal rib is usually oriented essentially parallel to the main direction.

[0019] Top view, especially of the riblet structure and / or the longitudinal channels, typically refers to a view orthogonal to the main direction and orthogonal to a secondary direction. The top view is typically taken along the height direction.

[0020] Typically, the longitudinal channels are arranged side by side in the secondary direction. As a rule, the longitudinal ribs, especially of the respective longitudinal channel, are spaced apart from one another in the secondary direction. The secondary direction is usually oriented orthogonally to the main direction.

[0021] As a rule, a rib height of the ribs or the rib pair of the longitudinal channel is higher than the substructure, in particular higher than a topology formed between the ribs with the base surface or the respective base surface section.

[0022] Preferably, an average height of the base surface section is less than, in particular half, preferably a quarter, an average rib height of the two ribs of the longitudinal channel that delimit the base surface section. It is understood that a specified height, in particular the rib height, is usually defined in the height direction. The height direction is generally orthogonal to the base surface or, in particular, both orthogonal to the main direction and orthogonal to the secondary direction.

[0023] The respective longitudinal rib side surface of the longitudinal channel is usually oriented substantially parallel to the main direction. It is practical if the longitudinal rib side surface is designed such that, particularly in a cross-section orthogonal to the main direction, a tangent applied to the longitudinal rib side surface has an angle of 15° to 25°, preferably approximately 20°, to the height direction or to a direction orthogonal to the base surface, particularly the base surface section adjacent to the longitudinal rib side surface. This usually applies to one or both longitudinal rib side surfaces of the longitudinal channel. Both side surfaces of the respective longitudinal rib can expediently be designed in this way. An opening angle of a rib tip of the respective longitudinal rib can expediently be from 30° to 50°, preferably approximately 40°.An efficient reduction in overall resistance can be achieved when the ratio of the average rib height of the longitudinal ribs forming the longitudinal channel to the average rib tip spacing of the longitudinal ribs is at least 0.1 and / or at most 0.8, in particular at most 0.6. This applies particularly to the respective longitudinal channel. High practicality and high efficiency can be achieved when the ratio is from 0.1 to 0.8, in particular from 0.2 to 0.5, preferably from 0.25 to 0.4, particularly preferably from 0.28 to 0.37, and especially preferably from 0.32 to 0.35.

[0024] The substructure, in particular the structural elements, can be arranged at least partially on the base surface, in particular the base surface section, of the respective longitudinal channel and / or at least partially on at least one of the longitudinal rib side surfaces of the respective longitudinal channel, in particular formed with these. A low overall resistance can be achieved if the substructure, in particular the structural elements, are formed at least partially with at least one longitudinal rib side surface and at least partially with the base surface of the longitudinal channel. The substructure can be formed at least partially with the longitudinal rib side surfaces of both longitudinal ribs of the longitudinal channel. The longitudinal rib side surfaces of the longitudinal channel usually refer to those side surfaces of the respective longitudinal rib with which side surface the longitudinal channel is formed, wherein the longitudinal rib side surfaces of the longitudinal channel usually face one another.For example, structural elements can be arranged on the base surface, in particular the base surface section, of the respective longitudinal ribs and preferably on one or both longitudinal rib sides of the longitudinal channel. This applies in particular to the recesses if the substructure, in particular the structural elements, are formed with or by recesses.

[0025] The substructure, in particular the structural elements, of the respective longitudinal channel typically have a height of at least 2% and / or a height of at most 60% of a, in particular average, rib height of the longitudinal ribs of the respective longitudinal channel. The height of the substructure, in particular structural elements, can be from 5% to 50%, in particular from 10% to 40%, preferably from 15% to 30%, of the, in particular average, rib height of the longitudinal ribs of the respective longitudinal channel. Typically, an average apex region, in particular peak, of the substructure, in particular of the structural elements, of the respective longitudinal channel is located at a height level of at least 2% and / or a maximum of 60% of a, in particular average, rib height of the ribs of the respective longitudinal channel.The height level can be from 5% to 50%, in particular from 10% to 40%, preferably from 15% to 30%, of the, in particular average, rib height of the longitudinal ribs of the respective longitudinal channel. The rib height can be an average rib height of the two longitudinal ribs of the respective longitudinal channel. The rib height of the respective longitudinal rib can refer to an average rib height of the longitudinal rib. The above applies in particular to the depressions if the substructure, in particular the structural elements, is formed with or by depressions. The height can then be a depth of the respective depression.

[0026] The structural elements can be formed with elevations and / or depressions. Typically, the respective structural element is formed with, in particular by, an elevation and / or with, in particular by, a depression. To reduce resistance, it is advantageous if the structural elements are formed with wall segments arranged one after the other in the main direction and running at least partially transversely, in particular orthogonally, to the main direction. It is assumed that this makes it possible to at least partially guide or deflect a fluid flow flowing in the longitudinal channel with the wall segments transversely to the main direction or to generate a rotational movement of the fluid flow oriented transversely to the main direction. The respective structural element can be implemented with, in particular by, a depression.

[0027] It is particularly advantageous for reducing the overall resistance if the substructure, in particular the structural elements, are formed with, in particular by, depressions. The cause is assumed to be an at least partial penetration of flow components of the fluid flow into the depressions and in particular an associated reduction in viscous resistance. In particular, a lateral rotational movement of the flow components appears to develop particularly stable as a result. The wall segments can then be parts of the depression walls forming the depressions. In particular, the wall segments can be formed by depression wall sections of the depression walls facing the main direction. Typically, the respective structural element is implemented with, in particular by, a depression.The respective longitudinal channel can then have a substructure with recesses arranged one after the other for a fluid flow flowing in the main direction in the longitudinal channel, in particular recess walls or recess wall sections of the recesses. Typically, the respective longitudinal channel can have the recesses in such a way that a fluid flow flowing in the main direction in the longitudinal channel successively encounters recesses, in particular recess walls or

[0028] It is advantageous if a ratio of an average distance between the depressions, in particular depression walls or depression wall sections, in the main direction to an average distance between the channel edges, in particular an average rib tip distance of the longitudinal ribs, of the longitudinal channel is less than 1, in particular at most 0.8, preferably at most 0.5, particularly preferably at most 0.33, especially preferably at most 0.22. The ratio is generally greater than 0.05, in particular at least 0.09. It is advantageous if the ratio is from 0.05 to 0.8, in particular from 0.09 to 0.5, particularly preferably from 0.09 to 0.33, particularly preferably from 0.13 to 0.22. The ratio can advantageously be designed as defined in this document, in particular with regard to structural elements.The respective longitudinal channel, in particular its longitudinal channel wall, can then have depressions spaced apart from one another in the main direction. What is particularly relevant is that a fluid flow flowing in the main direction successively encounters depressions, in particular depression walls or depression wall sections, which are spaced apart from one another according to the aforementioned ratio. This applies in particular in a plan view of the riblet structure, in particular the respective longitudinal channel. A depression wall section is usually a region of a depression wall of a depression facing the main direction. The depression wall section is usually oriented transversely, in particular orthogonally to the main direction. In particular, the features explained for the substructure, in particular for structural elements or wall segments, apply analogously to the depressions.The depressions can be connected to one another, in particular forming a depression network, or can be formed separately from one another.

[0029] It is expedient if the respective longitudinal channel has structural elements, in particular depressions, on the side surface forming the longitudinal channel of at least one, in particular both, of the longitudinal ribs of the longitudinal channel and / or in the base surface, in particular the base surface section, of the longitudinal channel. In particular, a respective structural element, in particular a respective depression, can be arranged both on at least one, in particular both, longitudinal rib side surfaces of the longitudinal channel and on the base surface, in particular the base surface section, of the longitudinal channel.

[0030] It is advantageous if structural elements, in particular depressions, extend from one of the longitudinal ribs to the other longitudinal rib of the longitudinal channel, wherein the structural elements, in particular depressions, preferably run along at least one of the longitudinal rib side surfaces of the longitudinal channel up to at least half the rib height of the respective longitudinal rib. This preferably applies to the side surfaces of both longitudinal ribs forming the longitudinal channel, with which side surfaces the longitudinal channel is formed. In particular, the structural elements, in particular the depressions, can extend essentially to the rib tip or the longitudinal channel edge. It is particularly advantageous if the respective structural element, in particular the respective depression, is designed in this way.

[0031] It can be advantageous for reducing pressure resistance if the structural elements, in particular depressions, are spaced at least partially, preferably entirely, from at least one or both of the longitudinal ribs of the respective longitudinal channel in a direction orthogonal to the main direction or in the secondary direction. This can be achieved by a pronounced demarcation of the structural elements, in particular depressions, in the base surface or by a pronounced at least partial spacing of the respective structural element, in particular the respective depression, from the longitudinal ribs. This can advantageously be achieved by a base surface region adjacent to the respective structural element, in particular between at least one of the longitudinal ribs and the structural element.It is advantageous for reduced viscous resistance if the structural elements are spaced at least partially, preferably completely, from both longitudinal ribs. It is understood that the two longitudinal ribs here refer to the two immediately adjacent longitudinal ribs between which the structural elements are arranged. The structural elements or the respective structural element can in particular be implemented by depressions or a respective depression. It is advantageous if the respective depression is designed as a transverse groove running transversely, in particular orthogonally, to the main direction. It is particularly practical if the substructure is formed with transverse ribs arranged one behind the other in the main direction, oriented transversely to the main direction, and in particular wedge-shaped, so that depressions are formed between the transverse ribs. The transverse ribs can be oriented orthogonally to the main direction.The depressions between the transverse ribs can then represent the depressions or structural elements described in this document, in particular those described above. The transverse ribs are usually wedge-shaped or have a triangular or trapezoidal basic shape in a cross-section orthogonal to the longitudinal extent of the transverse ribs. Wall surfaces of the depressions or transverse ribs can be flat, concave, or convex. In particular, the structural elements can be formed by wall surfaces, in particular side surfaces, of the transverse ribs. These are usually those wall surfaces of the transverse ribs that face the main direction. The wall surfaces usually run transversely, in particular orthogonally to the main direction.

[0032] Typically, an average depth of the longitudinal channel and / or an average rib height of the longitudinal ribs is less than 250 pm, in particular less than 100 pm, preferably less than 80 pm. It is advantageous if the average depth and / or average rib height is between 250 pm and 1 pm, in particular between 100 pm and 5 pm, preferably between 25 pm and 40 pm, particularly preferably between 30 pm and 37 pm, particularly preferably about 34 pm. Typically, an average distance between the channel edges and / or an average rib tip distance of the longitudinal ribs of the longitudinal channel is less than 100 pm, in particular less than 80 pm. It has proven useful if the average depth or the average rib tip distance of the longitudinal channel is between 60 pm and 85 pm, in particular between 70 pm and 80 pm, preferably about 74 pm.Accordingly, it has proven particularly effective if the average rib height of the longitudinal ribs is between 30 pm and 37 pm, preferably approximately 34 pm, and the average rib tip spacing of the longitudinal ribs is between 70 pm and 80 pm, preferably approximately 74 pm. The rib tip spacing of the longitudinal ribs is usually measured in a cross-section orthogonal to the main direction between the highest points of the longitudinal ribs or rib tips of the longitudinal ribs. The rib tip spacing of the longitudinal ribs is usually a distance between tips or peaks of immediately adjacent longitudinal ribs or pairs of longitudinal ribs of the respective longitudinal channel.

[0033] Typically, in a cross-section orthogonal to the main direction, the base surface section has a width between 50 pm and 70 pm, in particular approximately 60 pm, or the rib spacing of the longitudinal ribs in this cross-section has such a size. The base surface section typically borders the longitudinal ribs or their side walls.

[0034] It is advantageous if, particularly in the main direction, the structural elements, preferably the wall segments and / or the depressions, are formed with a repeating ordered structure. This applies in particular to the respective longitudinal channel. The structural elements are preferably arranged regularly in the main direction. This applies in particular in a plan view of the riblet structure. This allows resistance-reduced properties to be practically implemented over a large area. This can be implemented in the main direction and / or orthogonal to the main direction, in particular in the secondary direction. This applies in particular to depressions if the structural elements are formed by depressions. For this purpose, the structural elements, in particular depressions, can form a periodic structure, in particular in the main direction, generally with regular, preferably constant, distances between the structural elements, in particular depressions.This applies in particular to the respective longitudinal channel.

[0035] In particular, there may be several structural element classes represented by differently designed structural elements, depressions, wherein structural elements, in particular depressions, of different structural element classes are arranged next to one another according to a predetermined ordering scheme. Typically, structural elements of the same design, in particular depressions, are assigned to the same structural element class. It is expedient if structural elements of different structural element classes are arranged alternately next to one another, in particular periodically. For this purpose, individual or groups of immediately adjacent structural elements of each structural element class can alternate with one another. It is possible to arrange the structural elements irregularly, but a regular arrangement or an arrangement with an ordering structure or ordering scheme is preferred.The above applies in particular in the main direction and / or orthogonal to the main direction, in particular in the secondary direction. This applies in particular in a plan view of the riblet structure. Preferably, the respective structural element is a depression. The structural element class can then be referred to as a depression class. The structural elements, in particular depressions, of different structural element classes, in particular depression classes, can differ, for example, in their shape, depth, distances between two immediately adjacent structural elements, in particular depressions, or another of the features described in this document. The above can be implemented for a respective longitudinal channel.

[0036] It may be advantageous if the respective longitudinal channel is formed with directly adjacent, in particular directly merging, side surfaces of the longitudinal ribs of the longitudinal channel, in particular those adjacent in the secondary direction. The longitudinal channel wall of the longitudinal channel can then be formed by the side surfaces of the longitudinal ribs or the longitudinal rib side surfaces, in particular without a base surface section. The longitudinal rib side surfaces can then have the substructure, in particular the structural elements. The longitudinal rib side surfaces can expediently adjoin one another, forming an angle.

[0037] Depending on the application flow conditions, it may be advantageous if the longitudinal rib side surfaces in the respective longitudinal channel are continuously connected to one another or continuously connected to the base surface section of the longitudinal channel. The base surface section can be flat or curved, in particular concave or convex.

[0038] It has been shown that a pronounced spacing of the respective structural element from at least one, preferably both, of the longitudinal ribs can usually be favorable for low pressure resistance. It is advantageous if, in a plan view of the riblet structure in a direction orthogonal to the main direction, an average distance between at least one of the two longitudinal ribs and a structural element edge section facing this longitudinal rib of a structural element edge bounding the respective structural element is more than 5%, in particular more than 10%, preferably more than 20%, particularly preferably more than 25%, of an average rib spacing between the longitudinal ribs. The rib spacing of the longitudinal ribs usually refers, in a cross-section orthogonal to the main direction, to a spacing of the longitudinal ribs at the level of the base surface or its reference surface or, as a rule, corresponding to a width of the respective base surface section.The rib spacing of the longitudinal ribs is usually measured orthogonally to the rib height of the longitudinal ribs. As explained above, a spacing of the structural element or a structural element edge section of a structural element edge of the respective structural element can be used to influence the characteristics of the resistance behavior, in particular of viscous resistance or pressure resistance, in order to optimize this. It is preferred if such a spacing is present from both of the longitudinal ribs, i.e. in particular if both longitudinal ribs each have such a distance from the structural element edge section facing the respective longitudinal rib and delimiting the respective structural element. It is understood that the distance to the respective longitudinal rib can be different or the same.The average distance between at least one of the two longitudinal ribs and the structural element edge section facing this longitudinal rib and delimiting the respective structural element is generally less than 90%, in particular less than 80%, preferably less than 65%, particularly preferably less than 50%, particularly preferably less than 45%, of an average rib spacing between the longitudinal ribs. It is advantageous if the structural element, preferably a center of gravity of a surface formed by the structural element edge, is arranged substantially centrally between the two longitudinal ribs, i.e. in particular in a direction orthogonal to the main direction substantially centrally between the two longitudinal ribs.The aforementioned distances between the longitudinal rib and the structural element edge section are usually determined between a point at which the respective longitudinal rib borders the base surface section and a point on the structural element edge section. This applies in particular to a plan view of the riblet structure. Preferably, the respective structural element is a depression. The structural element edge or structural element edge section can then be a depression edge or

[0039] Recess edge section. The above applies in particular to the respective longitudinal channel. It is expedient if the structural element, in particular a structural element edge segment of the structural element, borders on a base surface region located, in particular in a direction orthogonal to the main direction or in the secondary direction, between the recess and one of the longitudinal ribs. For optimized resistance behavior, it is advantageous if the base surface region is essentially flat. The base surface region can adjoin both the structural element or the structural element edge segment and the at least one longitudinal rib, in particular its longitudinal rib side surface. Preferably, such a base surface region is present between both of the longitudinal ribs and each of the structural element or each of a structural element edge segment of the structural element. In practice, the structural element edge segment can be the structural element edge section.It is preferred if the structural element or the structural element edge is largely, particularly preferably substantially entirely, surrounded by such a base surface region. The above applies in particular to the respective longitudinal channel.

[0040] A particularly pronounced reduction in frictional resistance can be achieved if, in a cross-section parallel to the base surface, in particular its reference surface, an average longitudinal extent of the structural element, in particular of the recess, in the main direction is a maximum of 1.5 times, in particular a maximum of 1.2 times, as large as an average width extent of the structural element, in particular of the recess, in a direction orthogonal to the main direction or in the secondary direction. This applies particularly in a plan view of the base surface. In particular, it is advantageous if an opening area of ​​the recess defined by the recess edge is designed in this way.It is advantageous if the average longitudinal extent of the depression in the main direction is a maximum of 0.9 times, in particular a maximum of 0.5 times, preferably a maximum of 0.3 times, particularly preferably a maximum of 0.2 times, as large as an average width extent of the depression in a direction orthogonal to the main direction or in the secondary direction.

[0041] To reduce pressure resistance, it is advantageous if the average depth of the depressions is less than 75%, in particular between 5% and 70%, of the average rib height of the longitudinal ribs of the respective longitudinal channel. It has been shown that at greater depths, turbulence can occur, which leads to an increase in pressure resistance. The average depth of the depression is preferably between 5% and 60%, in particular between 10% and 45%, preferably between 10% and 30%, particularly preferably between 10% and 20%, of the average rib height of the longitudinal ribs of the longitudinal channel.

[0042] To achieve viscous resistance, it may be advantageous if several of the structural elements, in particular depressions, between the two longitudinal ribs of the longitudinal channel overlap each other in a direction orthogonal to the main direction, particularly in a secondary direction. This applies particularly in a top view of the riblet structure.

[0043] The structural elements, in particular the wall segments and / or the recesses, of the longitudinal channel can form a row of structural elements, in particular wall segments and / or the recesses, oriented in the main direction. It is advantageous if the structural elements, in particular the wall segments and / or the recesses, of the longitudinal channel form several rows of structural elements, in particular wall segments and / or the recesses, oriented in the main direction. For example, two rows of structural elements, in particular recesses, running in the main direction can be present in the longitudinal channel. It is advantageous if immediately adjacent rows of arrangements are offset from one another in the main direction, preferably by half the distance between two immediately adjacent structural elements of one of the rows of arrangements. This makes it possible to implement flow behavior that is advantageous with regard to the overall resistance.

[0044] The structural elements between the rows of arrangements can overlap each other in a direction orthogonal to the main direction, especially in a secondary direction. This applies particularly in a plan view of the riblet structure. The above applies analogously to wall segments and / or recesses if the structural elements are implemented as wall segments or recesses.

[0045] It is practicable if the respective depression has a predominantly, in particular substantially, constant cross-sectional area along a depth direction of the depression. For example, the depression can be formed with or by a substantially cylindrical or prism-shaped, in particular cuboid-shaped, cube-shaped or pyramid-shaped, recess. In this case, an edge of the depression and / or a base area of ​​the prism shape can be polygonal, for example triangular, square, pentagonal or hexagonal. It can be advantageous for turbulent flow behavior if the depression has a decreasing cross-sectional area at least in sections along the depth direction of the depression, in particular predominantly, preferably substantially. The depression can be formed, for example, with or by a substantially conical or pyramid-shaped recess.A tip of the conical or pyramidal shape usually has, in particular, longitudinal ribs running counter to the height direction of the riblet structure. A base area of ​​the pyramidal shape can be, for example, triangular, quadrangular, pentagonal, or hexagonal. It is advantageous if, in a cross-section orthogonal to the main direction, a recess wall of the recess has, at least in sections, in particular substantially, a gradient that corresponds to a gradient of the longitudinal rib side surface of one of the longitudinal ribs of the longitudinal channel. The longitudinal rib is usually the longitudinal rib that lies closer to the recess wall. The recess wall is usually a side wall of the recess oriented in the main direction. The recess wall is preferably substantially parallel to the longitudinal rib side surface of the longitudinal rib. Opposite recess walls or side walls of the recess are preferably designed in this way.It is expedient if the depression has a first depression segment with a cross-sectional area that remains constant along the depth direction of the depression and a second depression segment, in particular arranged downstream of the first depression segment in the depth direction, with a cross-sectional area that decreases along the depth direction. Preferably, the second depression segment forms an end segment of the depression. The cross-sectional area of ​​the depression is generally oriented orthogonally to the depth direction of the depression. The first or second depression segment can be formed in the aforementioned manner.

[0046] Depending on the intended use, different shapes of recesses, especially cross-sectional areas of recesses, can be particularly advantageous. For example, the respective recess in a cross-section or the recess edge can be shaped without corners, particularly oval or round, or polygonal, for example, triangular, square, pentagonal, or hexagonal.

[0047] To reduce pressure resistance, it is advantageous if the respective depression has a depression wall with which the depression is formed, which is designed at least in sections, in particular substantially, as a surface of revolution, in particular with a rotation axis in the depth direction of the depression. Thus, the depression wall can be formed with a surface segment of an ellipsoid of revolution, in particular a spherical surface, a paraboloid of revolution, or a hyperboloid of revolution. It can be advantageous if the depression wall section is formed with a segment of a conical or cylindrical surface. For example, the depression can have a depression wall that is substantially dome-shaped.

[0048] It is advantageous if the respective depression is formed in the depth direction with a first depression region and a second depression region downstream of this, wherein the first depression region has a smaller diameter than the second depression region. It is preferred if a diameter, in particular a surface cross-sectional area, of the first depression region is less than 80% of a diameter, in particular a surface cross-sectional area, of the second depression region. It has been shown that this is advantageous for reducing the pressure resistance. It is preferred if the diameter or the surface cross-sectional area of ​​the first depression region is less than 65%, in particular between 5% and 50%, preferably between 10% and 30%, particularly preferably less than 20% of the diameter or the surface cross-sectional area of ​​the second depression region.The reason for the advantageous effect on pressure resistance in this application appears to be an interaction between flows or turbulences in the different depression regions. Advantageously, several, in particular a majority, preferably essentially all, of the depressions in the respective longitudinal channel can be designed in this way. The depression regions can be differently shaped or each designed according to the characteristics specified for the shape and size of the depression, in particular differently from one another.

[0049] It can be advantageous for flow behavior in the depressions if a plurality of the depressions are connected to one another via at least one fluid-conductive connecting channel. Advantageously, a plurality of the depressions in the respective longitudinal channel, which depressions are arranged next to one another, in particular in the main direction and / or orthogonal to this or in the secondary direction, can be connected to one another via at least one connecting channel. It has proven useful if these are directly adjacent depressions. Although more complex in design, it can be advantageous for flow behavior if depressions of different longitudinal channels, which longitudinal channels are preferably directly adjacent, are connected to one another via at least one fluid-conductive connecting channel.The connecting channel preferably runs beneath one or more longitudinal ribs of the longitudinal channels in order to connect the depressions of different longitudinal channels to one another. In practical terms, depressions in several, a majority, preferably substantially all, of the longitudinal channels can be connected to one another via at least one or more connecting channels. Preferably, depressions orthogonal to the main direction or in the secondary direction, in particular directly adjacent, are connected via the at least one connecting channel, in particular if they are depressions of different longitudinal channels. It is understood that it is advantageous if several connecting channels are provided in order to connect the depressions to one another. In this case, several connecting channels can be present between each two depressions. The connecting channel can, for example, be formed as a bore within a material forming the base surface.It has proven effective if the connecting channel connects to the respective depression in an area of ​​the lower third of the depression, especially in an area of ​​the depression's base, usually laterally. This applies particularly in a top view.

[0050] To adjust the pressure resistance, it is advantageous if several longitudinal channel classes are present, wherein the longitudinal channel classes represent longitudinal channels with different recess configurations, wherein longitudinal channels and respective recesses of different longitudinal channel classes are arranged next to one another according to a predetermined ordering scheme, usually in a direction orthogonal to the main direction, in particular in the secondary direction. For example, a longitudinal channel with recesses having a first recess depth and a longitudinal channel with recesses having a second recess depth can be arranged next to one another in alternating fashion.Different longitudinal channel classes can be distinguished from one another, for example, by the shape of the recesses, the depth of the recesses, the spacing between the recesses, and / or the arrangement of the recesses within the respective longitudinal channel. Typically, longitudinal channels of the same design are assigned to the same longitudinal channel class. This is especially true in plan view.

[0051] It is advantageous if the base surface has a basic shape in a direction transverse, in particular orthogonal, to the main direction across a plurality of longitudinal channels, which basic shape is formed with a plurality of alternating, in particular wave-shaped, height changes. This applies in particular to the reference surface or the base surface without the substructure. The direction can be the secondary direction. An average distance, in particular a periodicity, of immediately adjacent height changes can be greater than an average rib tip distance, in particular of the respective longitudinal channels. The average distance, in particular the periodicity, can in particular be at least twice, preferably at least three times, particularly preferably at least five times, as large as an average rib tip distance, in particular of the respective longitudinal channels. The basic shape is preferably wave-shaped, in particular sinusoidal.

[0052] It is advantageous if a surface of a component is present, wherein the surface has a riblet structure, in particular as described in this document. Depending on the features and effects of the riblet structure, the surface of the component can be designed with low frictional resistance with respect to a fluid flowing along the surface or a corresponding fluid flow. The component can expediently be part of a means of transport, for example an aircraft, ship or car, or part of a fluid conducting element, for example a pipe, or a piece of clothing, for example a protective suit or sports suit, in order to reduce frictional resistance of the surface of the part having the riblet structure with respect to a fluid flowing along the surface.The fluid flow can be, for example, a gas flow, in particular air flow, and / or liquid flow, in particular water flow.

[0053] According to the above statements, the invention relates in a further aspect to a component with a surface, wherein the surface has a riblet structure which, when a fluid flows around the surface, reduces a frictional resistance for the fluid by more than 8.5%, preferably more than 9.0%, in particular more than 9.5%, for example more than 10.0% or more than 10.5% or more than 11.0% or more than 11.5% or more than 12.0% or more than 12.5%. Typically, the reduction is between 8.5% and 13%. This applies in particular in connection with a riblet structure described in this document. The component can be the aforementioned component. The riblet structure can be a riblet structure described in this document or can be implemented according to the features and effects described in this document.The riblet structure may be applied to a component base of the component with a foil to form the surface and / or embossed or otherwise incorporated into the surface to form the surface.

[0054] The further objective can be achieved if a method for producing a riblet structure is implemented, wherein the riblet structure is designed as described in this document, in particular as described above, and the riblet structure is introduced into the surface by embossing or abrasive machining of a surface. The surface can be the surface of the aforementioned component. In this way, a riblet structure as described in this document can advantageously be produced with low frictional resistance to a fluid flowing along the riblet structure. The riblet structure can be produced with the features and effects described in this document. It is practical if a negative mold of the riblet structure is used for production in order to introduce the riblet structure into the surface by pressing the negative mold onto the surface.

[0055] The manufacturing method can be designed according to the features and effects described in this document in the context of a riblet structure or a surface of a component with a riblet structure, in particular above. The same applies in reverse for the riblet structure or a surface of a component with a riblet structure with regard to the manufacturing method.

[0056] The further aim mentioned at the beginning can be achieved if a method for the computer-implemented simulation of a surface with a riblet structure is implemented to generate the riblet structure, wherein the riblet structure has longitudinal channels oriented in a main direction, wherein a substructure is assigned to the respective longitudinal channel such that a flow resistance compared to a fluid flow flowing along the riblet structure in the main direction is reduced by more than 8.5%, in particular more than 9%, preferably more than 10%, particularly preferably more than 11%, especially particularly preferably more than 12%. In this way, a form of the riblet structure can be generated, in particular computer-implemented. As explained above, it has been shown that by forming the respective longitudinal channel with a substructure, a particularly reduced overall resistance orFlow resistance can be achieved with respect to a fluid flow flowing in the main direction. The riblet structure or substructure can be the riblet structure or substructure described in this document and, in particular, can be implemented accordingly. Subsequently, a real surface with a riblet structure can be implemented according to the generated riblet structure, in particular the shape of the generated riblet structure. This can be implemented according to the method for producing a riblet structure, in particular as described in this document.

[0057] A reduction in total drag, frictional drag, or flow resistance typically refers to a surface without a riblet structure. The surface without a riblet structure can, for example, be a surface, particularly a flat one. It is generally assumed that with a riblet structure having longitudinal channels, a reduction in flow resistance of up to a theoretical value of 8% can be achieved. By forming the respective longitudinal channel with a substructure, particularly as described in this document, a greater reduction in flow resistance or greater efficiency of the riblet structure can be achieved.

[0058] It is advantageous if the substructure is simulated, in particular generated, with structural elements arranged one after the other, in particular periodically, for a fluid flow in the main direction in the longitudinal channel. As described in particular in this document, a high level of efficiency can be achieved in a practical manner with structural elements arranged one after the other in the respective longitudinal channel. The substructure can be implemented as described in this document. It is advantageous if a ratio of an average spacing of the structural elements in the main direction to an average spacing of channel edges, in particular an average rib tip spacing of the longitudinal ribs, of the longitudinal channel is less than 1, in particular from 0.09 to 0.33. The ratio can be less than 1, in particular a maximum of 0.8, preferably a maximum of 0.5, particularly preferably a maximum of 0.33, especially preferably a maximum of 0.22.The ratio is generally greater than 0.05, in particular at least 0.09. It is advantageous if the ratio is from 0.05 to 0.8, in particular from 0.09 to 0.5, particularly preferably from 0.09 to 0.33, particularly preferably from 0.13 to 0.22.

[0059] It is expedient if the simulation, in particular generation, of the substructure of the respective longitudinal channel is carried out with an iterative change, in particular iterative adjustment, of distances between the structural elements in the respective longitudinal channel.

[0060] A high degree of application practicality can be achieved if the substructure, in particular the respective longitudinal channel, is generated using machine learning, in particular using a computer-implemented neural network.

[0061] The method for computer-implemented simulation can be designed according to the features and effects described in this document, particularly above, in the context of a riblet structure or a surface of a component with a riblet structure or a method for producing a riblet structure. The same applies in reverse to the riblet structure or a surface of a component with a riblet structure or a method for producing a riblet structure with regard to the method for computer-implemented simulation.

[0062] It is understood that specified height and depth specifications usually refer to a direction orthogonal to the base surface or its reference surface, and specifications regarding length and width generally refer to a direction parallel to the base surface or its reference surface. Further features, advantages, and effects of the invention will become apparent from the following illustration of an exemplary embodiment. The drawings, to which reference is made, show:

[0063] Fig. 1 is a schematic representation of a conventional riblet structure of the prior art with longitudinal channels oriented in the main direction, having flat base surface sections between the longitudinal ribs;

[0064] Fig. 2 and Fig. 3 schematic sections of the riblet structure of Fig. 1 in view from obliquely above and in plan view;

[0065] Fig. 4 a schematic profile view of a section of the riblet structure of the

[0066] Fig. 1;

[0067] Fig. 5 to Fig. 10 show schematic sections of a longitudinal channel of riblet structures, wherein the longitudinal channel has a substructure;

[0068] Fig. 11 and Fig. 12 are schematic representations of a riblet structure, wherein the respective longitudinal channel in the base surface section has depressions running transversely to the main direction as structural elements;

[0069] Fig. 13 is a schematic representation of a riblet structure, wherein the respective longitudinal channel has depressions extending transversely to the main direction in the base surface section and on the longitudinal rib side surfaces of the longitudinal ribs as structural elements;

[0070] Fig. 14 shows a graph showing a determined resistance change AR in % for 530 different riblet structures;

[0071] Fig. 15 is a graph showing results of a resistance reduction AR for riblet structures according to Fig. 1 or Fig. 2, according to Fig. 5 and according to Fig. 6;

[0072] Fig. 16 is a graph showing averaged boundary layer profiles for the riblet structures according to Fig. 1 and Fig. 2, according to Fig. 5 and according to Fig. 6;

[0073] Fig. 17 is a graph showing time-averaged vertical velocities v + for the riblet structures according to Fig. 1 and Fig. 2, according to Fig. 5 and according to Fig. 6;

[0074] Fig. 18 shows the time-averaged speeds v + flow pattern corresponding to Fig. 17;

[0075] Fig. 19 shows a representation of a riblet structure, wherein the respective longitudinal channel has two rows of structural elements formed as depressions extending in the main direction; Figs. 20 to 47 show representations of riblet structures, comprising longitudinal channels with substructures with different ratios of an average spacing d of the structural elements in the main direction to an average rib tip spacing Rs;

[0076] Fig. 48 to Fig. 75 show further riblet structures, particularly investigated by means of computer simulation, whose longitudinal channels have a substructure;

[0077] Fig. 76 to Fig. 77 show schematic sections of a longitudinal channel of a riblet structure, wherein the longitudinal channel has a substructure formed with transverse ribs;

[0078] Fig. 78 to Fig. 80 representations of a riblet structure, wherein the respective longitudinal channel in the base surface section has transverse ribs running transversely to the main direction as structural elements.

[0079] Fig. 1 shows a schematic representation of a conventional riblet structure 1 of the prior art. The riblet structure 1 has longitudinal channels 2 oriented in the main direction H, wherein the longitudinal channels 2 are formed by spaced-apart wedge-shaped longitudinal ribs 5 projecting from a base surface 6 in the main direction H. The base surface 6 forms a flat base surface section 7 between two immediately adjacent longitudinal ribs 5 or pairs of longitudinal ribs. The respective longitudinal channel 2 is generally formed by two immediately adjacent longitudinal ribs 5, usually in a secondary direction N orthogonal to the main direction, and the base surface section 7 arranged between the longitudinal ribs 5. The longitudinal ribs 5 usually have flat longitudinal rib side walls or longitudinal rib side surfaces 8. Fig. 2 and Fig. 3 show schematic representations of a section of the riblet structure 1 of Fig. 1, in order to illustrate the riblet structure 1 of Fig.1, wherein Fig. 2 shows a view from above and Fig. 3 shows a plan view of the riblet structure 1 of Fig. 1.

[0080] Fig. 4 shows a schematic profile representation of a section of the riblet structure 1 of Fig. 1 , which shows a longitudinal channel 2. The characteristic data used to describe riblet structures 1 are usually a rib tip distance Rs, a rib spacing RD and a rib height R z which are measured in a cross-section orthogonal to the main direction H or orthogonal to the base surface 6, in particular a respective base surface section 7. The rib tip spacing Rs denotes a distance between the longitudinal ribs 5 or longitudinal rib pairs of the longitudinal channel 2, the rib spacing RD denotes a width of the base surface section 7 between the longitudinal ribs 5 of the longitudinal channel 2, and the rib height R za height of the longitudinal ribs 3, measured from the base surface 2 or the respective base surface section 4. This also applies analogously to the other riblet structures 1 specified here, in particular those below.

[0081] Such a structure of Fig. 1 to Fig. 4 has proven suitable for reducing frictional resistance against a fluid flowing in the main direction H along the riblet structure 1. For such a riblet structure 1, a resistance reduction of 8% in the frictional resistance or flow resistance is usually considered theoretically achievable.

[0082] During the development of the riblet structure 1 described in this document, investigations and computer simulations were conducted with a large number of different riblet structures 1. In particular, a substructure 3 introduced into the longitudinal channels 2, in particular its extension, shape, and / or size, was varied and investigated. Fig. 14 shows a graph representing a determined resistance change AR in % for 530 riblet structures 1 with different substructures 3, with each of the riblet structures 1 being represented as a point in the graph. Development progress led to riblet structures 1 with a resistance reduction in the flow resistance in a range greater than 8% to 13%.

[0083] An advantageous riblet structure 1 has longitudinal channels 2 oriented in a main direction H, wherein the respective longitudinal channel 2 has a substructure 3 with structural elements 4 arranged one after the other for a fluid flow flowing in the longitudinal channel 2 in the main direction H, wherein a ratio of an average distance d of the structural elements 4 in the main direction H to an average distance between channel edges of the longitudinal channel 2 or an average rib tip distance Rs is less than 1. Advantageously, the ratio can be from 0.05 to 0.8, in particular from 0.09 to 0.5, particularly preferably from 0.09 to 0.33, particularly preferably from 0.13 to 0.22. The embodiments described below can be implemented with such a ratio. Such an implementation of the riblet structure 1 enables an advantageous reduction in frictional resistance and, in particular, a pronounced implementation practicality.In particular, the resistance reduction can be practically varied by varying a distance, in particular the average distance d, between the structural elements 4, as well as typically an arrangement and / or a shape of the structural elements 4. Depending on the specific design, a balance can be adjusted between a reduction in the viscous resistance of an interaction surface of a respective longitudinal channel 2 and a pressure resistance additionally generated by the substructure 3—as explained in particular above in this document.

[0084] In the examples given here, an average rib height R zof the longitudinal ribs 5 may be less than 250 pm, preferably approximately 34 pm. The average rib tip spacing Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 may be between 70 pm and 80 pm, preferably approximately 74 pm. An opening angle of a rib tip of the respective longitudinal rib may expediently be from 30° to 50°, preferably approximately 40°.

[0085] Fig. 5 to Fig. 10 show schematic representations of sections of a longitudinal channel 2 of a riblet structure 1. The longitudinal channel 2, in particular its longitudinal channel wall, is formed with longitudinal rib side surfaces 8 of longitudinal ribs 5 and usually a base surface section 7 located between the longitudinal ribs 5. The respective longitudinal channel 2 has a substructure 3, so that for a fluid flow flowing in the longitudinal channel 2 in the main direction H, structural elements 4 of the substructure 3, designed as depressions, are arranged one after the other. The respective depression can be arranged in the base surface section 7 and / or the longitudinal rib side surfaces 8 of the longitudinal channel 2. The respective section of the longitudinal channel 2 is usually representative of the substructure 3 in the respective longitudinal channel 2 or the longitudinal channels 2 of the riblet structure 1.It is advantageous if the respective depression is designed as a transverse channel, also referred to as a transverse groove, running transversely, in particular orthogonally, to the main direction H. This is shown by way of example in Fig. 5 to Fig. 8. The substructure 3 of the longitudinal channel 2 can have transverse ribs 9 arranged one behind the other in the main direction H and running transversely to the main direction H, so that the depressions, in particular transverse grooves, are formed between the transverse ribs 9, in particular by the transverse ribs 9. The transverse ribs 9 can run orthogonally to the main direction H, in particular in a secondary direction N. Secondary direction N usually refers to a direction orthogonal to the main direction H and orthogonal to a height direction z of the riblet structure 1. The depressions or transverse ribs 9 are preferably wedge-shaped, usually in a cross-section with a triangular or trapezoidal basic shape. Wall surfaces of the depressions orTransverse ribs 9 can be flat, concave, or convex, particularly if the wall surfaces are oriented transversely or orthogonally to the main direction. Depending on whether the depressions or transverse ribs 9 are arranged in the base surface section 7 and / or one or both longitudinal rib side surfaces 8 of the longitudinal channel 2, different resistance reductions can be achieved. In the longitudinal channel 2 of Fig. 5, the depressions, in particular transverse grooves, are arranged in the base surface section 7, in particular introduced therein, and longitudinal rib side surfaces 8 of the longitudinal ribs 5 are formed without a substructure 3, in particular without structural elements 4. The longitudinal rib side surfaces 8 of the longitudinal channel 2 are usually flat, but can alternatively also be concave or convex. Such a design of the longitudinal channels 2 is usually associated, by way of example, with a resistance reduction of 11.5%. In Fig. 11 and Fig. 12, the section of Fig.5 corresponding riblet structures 1 are shown schematically. The ratio of an average distance d of the structural elements 4 in the main direction H to the average distance of the channel edges or to the average rib tip distance Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 is greater in the longitudinal channels 2 of Fig. 11 than in the longitudinal channels 2 of Fig. 12. Fig. 11 represents a representation with a ratio between 0.5 and 1. Fig. 12 represents a representation with a ratio of 0.09 to 0.5, in particular from 0.09 to 0.33. Depending on a selected ratio, the resistance reduction can be varied. In the longitudinal channel 2 of Fig. 6, the depressions, in particular transverse furrows, are arranged in the base surface section 7 and on the longitudinal rib side surfaces 8 of the longitudinal channel 2, in particular are introduced into these.Such a design of the longitudinal channels 2 is usually associated, for example, with a resistance reduction of 11.5%. Fig. 13 shows a schematic representation of a riblet structure corresponding to the section in Fig. 6. In a manner analogous to Fig. 5 to Fig. 10, Fig. 76 and Fig. 77 show schematic representations of a section of a longitudinal channel of a riblet structure 1, wherein Fig. 76 shows the section in a perspective representation and Fig. 77 shows the section in plan view. In the longitudinal channel 2 of Figs. 76 and Fig. 77, the substructure 3 is formed with transverse ribs arranged in the base surface section 7, in particular introduced therein, as structural elements 4, wherein, for example, the longitudinal rib side surfaces 8 of the longitudinal ribs 5 are formed without a substructure 3, in particular without structural elements 4. It is intended that the respective transverse rib, as shown exemplarily in the section of Fig. 76 and Fig.77, runs transversely, in particular orthogonally to the main direction and has a gradient, usually in the main direction, of a maximum of 50°, in particular a maximum of 45°, preferably a maximum of 35°, particularly preferably a maximum of 25°. The gradient generally relates to a flat reference surface from which the transverse rib protrudes. The transverse rib is generally wedge-shaped and preferably symmetrical. The transverse rib can be designed as explained, in particular above, with regard to transverse ribs. Fig. 78 to Fig. 80 show a riblet structure 1 corresponding to the section in Fig. 76 and Fig. 77. Fig. 79 shows the riblet structure 1 of Fig. 78 in a cross-section orthogonal to the main direction H. Fig. 80 represents a height profile of the riblet structure 1 of Fig. 78 in a cross-section oriented parallel to the main direction H.

[0086] Table 1 shows an example of the resistance reduction of a riblet structure 1 according to Fig. 1 or Fig. 2, which does not have a substructure 3, compared with the resistance reductions of the riblet structure 1 according to Fig. 5 and the riblet structure 1 according to Fig. 6. It can be seen that by introducing the structural elements 4, in particular transverse grooves, into the respective base surface section 7 of the longitudinal channels 2, a considerable resistance reduction can be achieved compared to a riblet structure 1 without substructure 3, which resistance reduction can be further increased if the longitudinal rib side surfaces 8 of the longitudinal channels 2 are also formed with structural elements 4, in particular transverse grooves.

[0087] Table 1 : Resistance changes AR in % of riblet structures 1 , whose longitudinal channels 2 are formed according to Fig. 2, Fig. 5 and Fig. 6. Fig. 15 shows a graph with measurement results and computer simulation results for a riblet structure 1 according to Fig. 1 and Fig. 2, for a riblet structure 1 according to Fig. 5, and for a riblet structure 1 according to Fig. 6, wherein a drag reduction AR in % is plotted against a local Reynolds number s+. Results for the riblet structure 1 according to Fig. 2 are plotted as dash-dotted lines with round measurement points. Results for the riblet structure according to Fig. 5 are plotted as solid lines with triangular measurement points. Results for the riblet structure 1 according to Fig. 6 are plotted as dashed lines and square measurement points. Results for the riblet structures of Fig. 5 and Fig. 6 show higher drag reductions than the results for the standard riblets of Fig. 2.

[0088] Fig. 16 shows a graph which represents averaged boundary layer profiles for the riblet structure 1 according to Fig. 1 and Fig. 2, for the riblet structure 1 according to Fig. 5 and for the riblet structure 1 according to Fig. 6. In the graph, a dimensionless wall distance y+ is plotted against a dimensionless velocity u+. Results for the riblet structure 1 according to Fig. 2 are shown as dash-dotted and dash-two-dotted lines. Results for the riblet structure 1 according to Fig. 5 and Fig. 6 are shown as dashed and solid lines, with results for the riblet structure 1 according to Fig. 5 being shown in grey. The dash-dotted line and the dashed lines show a velocity in a valley of the longitudinal channels 2 and the dash-two-dotted line and the solid lines show a velocity in a peak region of the longitudinal channels 2 or longitudinal ribs 5. The results show for the riblet structures of Fig.5 and Fig. 6 significantly higher speeds.

[0089] Fig. 17 shows a graph which represents a dimensionless wall distance y+ against time-averaged vertical velocities v+ or velocities parallel to the height direction z for the riblet structures 1 according to Fig. 1 and Fig. 2, according to Fig. 5 and according to Fig. 6. Results for the riblet structure 1 according to Fig. 2 are shown as dash-dotted and dash-two-dotted lines. Results for the riblet structure according to Fig. 5 and Fig. 6 are shown as dashed and solid lines, with results for the riblet structure 1 according to Fig. 5 being shown in grey. It can be seen that the results for riblet structures 1 according to Fig. 5 and Fig. 6 have reduced vertical velocities compared to results for a riblet structure 1 according to Fig. 2. A corresponding flow pattern is shown in Fig.18, wherein a flow field in the region of a valley of the respective flow channel has a vertically downward velocity and at the tips of the longitudinal ribs 5 of the respective longitudinal channel 2 has a vertically upward velocity.

[0090] 11 and 12 show a schematic representation of a riblet structure 1, wherein the respective longitudinal channel 2 in the base surface section 7 has depressions running as structural elements 4 transversely to the main direction H, in particular in the secondary direction N. The longitudinal rib side surfaces 8 of the respective longitudinal channel 2 do not have a substructure 3 or structural elements 4. The respective depression extends from one of the longitudinal ribs 5 to the other longitudinal rib of the longitudinal channel 2. The respective depression has a substantially strip-shaped, in particular rectangular, opening area. The depression can have a substantially constant cross-sectional shape along a depth direction T of the depression. The depression can be tapered in the depth direction T. In particular, the substructure 3, in particular the structural elements 4, can be implemented according to the substructure 3 described for FIG. 5.The ratio of an average distance d of the structural elements 4 in the main direction H to the average distance of the channel edges or to the average rib tip distance Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 is greater in the longitudinal channels 2 of Fig. 11 than in the longitudinal channels 2 of Fig. 12. Fig. 11 represents a representation with a ratio between 0.5 and 1. Fig. 12 represents a representation with a ratio of 0.09 to 0.5, in particular from 0.09 to 0.33.

[0091] Fig. 13 shows a schematic representation of a riblet structure 1, wherein the respective longitudinal channel 2 has a substructure 3, in particular structural elements 4, in the base surface section 7 and on the longitudinal rib side surfaces 8. The substructure 3, in particular structural elements 4, can be implemented analogously to Fig. 11 and / or Fig. 12. The respective recess can extend substantially along the base surface section 7 and the longitudinal rib side surfaces 8 of the two longitudinal ribs 5 of the longitudinal channel 2. In particular, the substructure 3, in particular the structural elements 4, can be implemented according to the substructure 3 described for Fig. 6. Fig. 7 to Fig. 8 show further embodiments of longitudinal channels 2 with substructures 3, which can be advantageous depending on the intended application and / or the resistance reduction to be set. Fig.7 shows a section of a longitudinal channel 2, in which the substructure 3, in particular the structural elements 4, are arranged on the longitudinal rib side surfaces 8 of the longitudinal channel 2, in particular are introduced into these, and the base surface section 7 of the longitudinal channel 2 is designed essentially without a substructure 3, in particular without structural elements 4. The base surface section 7 is then usually flat, but can alternatively also have a concave or convex shape. It can be advantageous to design the respective longitudinal channel 2 with longitudinal rib side surfaces 8 of the longitudinal channel 2 that directly adjoin one another, in particular that directly merge into one another. The longitudinal channel wall of the longitudinal channel 2 can then be formed by the longitudinal rib side surfaces 8 of the longitudinal channel 2, in particular without a base surface section 7. This is shown as an example in Fig. 8. The longitudinal rib side surfaces 8 can then have the structural elements 4, in particular transverse grooves orTransverse ribs 9. The longitudinal rib side surfaces 8 of the longitudinal channel 2 can expediently adjoin one another forming an angle. Depending on the application flow conditions, it can be advantageous if the longitudinal rib side surfaces 8 in the respective longitudinal channel 2 continuously adjoin one another or continuously adjoin the base surface section 7 of the longitudinal channel 2. The base surface section 7 can be flat or curved, in particular concave or convex. This is shown as an example in Fig. 9. It can be advantageous if the structural elements 4 are spaced apart in the secondary direction N, at least in sections, preferably entirely, from at least one or both longitudinal ribs 5 of the longitudinal channel 2. The structural elements 4 are then usually arranged in the base surface section 7 of the longitudinal channel 2 or formed with it. Fig.10 shows an example of a section of a flow channel, wherein the structural element 4 is spaced apart from both longitudinal ribs 5 of the longitudinal channel 2. The structural element 4 is designed, for example, as a rectangular or square recess.

[0092] Depending on the application objective, the features presented above, in particular, for forming the longitudinal channel 2 or the structural elements 4 can be freely combined with one another in different ways and, as will be understood by a person skilled in the art, are in particular not limited to the exemplary implementation examples. It has been shown that, to reduce resistance, a multitude of different forms of structural elements 4 are possible, which can differ from one another in shape and / or depth and / or spatial arrangement in the longitudinal channel 2. The structural elements 4 can be depressions and / or elevations. The respective longitudinal channel 2 can have several rows of structural elements 4 running in the main direction H. Fig.19 a riblet structure 1, wherein the respective longitudinal channel 2 has two rows of arrangements of structural elements 4 designed as depressions running in the main direction H, wherein the rows of arrangements are arranged in the base surface section 7.

[0093] Fig. 20 to Fig. 47 show examples of different riblet structures, investigated in particular by means of computer simulation, whose longitudinal channels 2 have a substructure 3. The substructure 3 is formed with different ratios of an average distance d of the structural elements 4 in the main direction H to an average rib tip distance Rs of channel edges or tip distances of the longitudinal ribs 5 of the respective longitudinal channel 2. Table 2 shows the respective determined resistance reductions AR.

[0094] Table 2: Resistance changes AR in % for riblet structures whose longitudinal channels 2 are formed according to Fig. 20 to Fig. 47, wherein the substructures 3 of the longitudinal channels 2 a ratio — of an average distance d of the structural elements 4 in «s

[0095] Main direction H to an average rib tip distance Rs of the longitudinal ribs 5 of the respective longitudinal channel 2.

[0096] The riblet structures 1 shown in Fig. 20 to Fig. 23 have a ratio of the average spacing d of the structural elements 4 in the main direction H to the average rib tip spacing Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 of 0.67 in the respective longitudinal channel 2. Fig. 20 and Fig. 22 show the respective riblet structure 1 in a perspective view, Fig. 21 and Fig. 23 show the respective riblet structure 1 in a cross-section orthogonal to the main direction H. A grayscale bar indicated in the figures, in particular also in the following figures, corresponds to a gray value change in the height direction z. The riblet structure 1 shown in Fig. 20 and Fig. 21 has a substructure 3 in the respective longitudinal channel 2, which substructure 3 is implemented as structural elements 4 by an arrangement row of rectangular depressions oriented in the main direction H.The respective recess is tapered in the depth direction T of the recess, in particular corresponding to a slope of the longitudinal rib side surfaces 8 of the longitudinal channel 2. Fig. 22 and Fig. 23 show a riblet structure 1, which is designed analogously to the riblet structure 1 of Fig. 20 and Fig. 21, wherein the longitudinal ribs 5 are designed with a smaller height.

[0097] The riblet structures 1 shown in Fig. 24 to Fig. 31 have, in the respective longitudinal channel 2, a ratio of the average distance d of the structural elements 4 in the main direction H to the average rib tip distance Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 of 0.33. Fig. 24, Fig. 26, Fig. 28 and Fig. 30 show the respective riblet structure in a perspective view, Fig. 25, Fig. 27, Fig. 29 and Fig. 31 show the respective riblet structure 1 in a cross-section orthogonal to the main direction H. The riblet structure 1 shown in Fig. 24 and Fig. 25 has, in the respective longitudinal channel 2, a substructure 3 which is implemented as structural elements 4 by an arrangement row of rectangular depressions oriented in the main direction H. The respective recess is tapered in the depth direction T of the recess, in particular corresponding to a slope of the longitudinal rib side surfaces 8 of the longitudinal channel 2. Fig. 26 and Fig.27 show a riblet structure 1, wherein the substructure 3 in the respective longitudinal channel 2 is implemented by two rows of rectangular or square depressions oriented in the main direction H as structural elements 4. Fig. 30 and Fig. 31 show a riblet structure, wherein the substructure 3 in the respective longitudinal channel 2 is implemented by a row of rectangular or square depressions oriented in the main direction H as structural elements 4, wherein the structural elements 4 are spaced from both longitudinal ribs 5 of the respective longitudinal channel 2.

[0098] The riblet structures 1 shown in Fig. 32 to Fig. 37 have in the respective longitudinal channel 2 a ratio of the average distance d of the structural elements 4 in the main direction H to the average rib tip distance Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 of 0.22. Fig. 32, Fig. 34 and Fig. 36 show the respective riblet structure in a perspective view, Fig. 33, Fig. 35 and Fig. 37 show the respective riblet structure in a cross-section orthogonal to the main direction H. The riblet structure 1 shown in Fig. 32 and Fig. 33 has in the respective longitudinal channel 2 a substructure 3 which is implemented by an arrangement row of rectangular depressions oriented in the main direction H as structural elements 4, wherein the respective depression extends over both longitudinal rib side surfaces 8 of the longitudinal channel 2 and the base surface section 7 of the respective longitudinal rib. Fig. 34 and Fig.35 show a riblet structure, wherein the substructure 3 in the respective longitudinal channel 2 is implemented by a row of rectangular or square depressions oriented in a main direction H as structural elements 4, wherein the structural elements 4 are spaced from both longitudinal ribs 5 of the respective longitudinal channel 2. Fig. 36 and Fig. 37 show a riblet structure 1, wherein the substructure 3 in the respective longitudinal channel 2 is implemented by a row of rectangular depressions oriented in the main direction H as structural elements 4. The respective depression is tapered in the depth direction T of the depression, in particular corresponding to a slope of the longitudinal rib side surfaces 8 of the longitudinal channel 2.

[0099] The riblet structures 1 shown in Fig. 38 to Fig. 43 have, in the respective longitudinal channel 2, a ratio of the average distance d of the structural elements 4 in the main direction H to the average rib tip distance Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 of 0.17. Fig. 38, Fig. 40 and Fig. 42 show the respective riblet structure in a perspective view, Fig. 39, Fig. 41 and Fig. 43 show the respective riblet structure in a cross-section orthogonal to the main direction H. The riblet structure 1 shown in Fig. 38 and Fig. 39 has, in the respective longitudinal channel 2, a substructure 3 which is implemented as structural elements 4 by an arrangement row of rectangular depressions oriented in the main direction H, wherein the respective depression extends over both longitudinal rib side surfaces 8 and the base surface section 7 of the respective longitudinal channel 2. Fig. 40 and Fig.41 show a riblet structure, wherein the substructure 3 in the respective longitudinal channel 2 is implemented by an arrangement row of rectangular depressions oriented in the main direction H as structural elements 4. The respective depression is tapered in the depth direction T of the depression, in particular corresponding to a slope of the longitudinal rib side surfaces 8 of the longitudinal channel 2. Fig. 42 and Fig. 43 show a riblet structure, wherein the substructure 3 in the respective longitudinal channel 2 is implemented by an arrangement row of rectangular or square depressions oriented in the main direction H as structural elements 4, wherein the structural elements 4 are spaced from both longitudinal ribs 5 of the respective longitudinal channel 2.

[0100] The riblet structures 1 shown in Fig. 44 to Fig. 47 have a ratio of the average spacing d of the structural elements 4 in the main direction H to the average rib tip spacing Rs of the longitudinal ribs 5 of the respective longitudinal channel 2 of 0.13 in the respective longitudinal channel 2. Fig. 44 and Fig. 46 show the respective riblet structure in a perspective view, Fig. 45 and Fig. 47 show the respective riblet structure in a cross-section orthogonal to the main direction H. The riblet structure 1 shown in Fig. 44 and Fig. 45 has a substructure 3 in the respective longitudinal channel 2, which is implemented by an arrangement row of rectangular depressions oriented in the main direction H as structural elements 4, wherein the respective depression extends over both longitudinal rib side surfaces 8 of the longitudinal channel 2 and the base surface section 7 of the respective longitudinal rib. Fig. 46 and Fig.47 show a riblet structure, wherein the substructure 3 in the respective longitudinal channel 2 is implemented by an arrangement row of rectangular depressions oriented in the main direction H as structural elements 4. The respective depression is tapered in the depth direction T of the depression, in particular corresponding to a slope of the longitudinal rib side surfaces 8 of the longitudinal channel 2. In Fig. 48 to Fig. 75, further examples are shown, in particular by means of.

[0101] Computer simulation shows riblet structures investigated whose longitudinal channels 2 have a substructure 3. The substructure 3 was investigated with an advantageous ratio of an average spacing d of the structural elements 4 in the main direction H to an average rib tip spacing Rs of the longitudinal ribs 5 of the respective longitudinal channel 2. The corresponding determined resistance reductions AR are shown in Table 3.

[0102] Table 3: Resistance changes AR in % for riblet structures whose longitudinal channels 2 are formed according to Fig. 50 to Fig. 76, wherein the longitudinal channels 2 have different

[0103] Substructures 3. A riblet structure 1 shown in Fig. 48 has a substructure 3 in the respective longitudinal channel 2, which is formed with two rows of, in particular, trapezoidal, depressions oriented in the main direction H, wherein the depressions are connected to one another by a longitudinal depression running in the main direction H. A riblet structure 1 shown in Fig. 49 has a substructure 3 in the respective longitudinal channel 2, which is formed with two rows of elevations oriented in the main direction H as a structural element 4, wherein the rows of elevations are arranged offset from one another, in particular by half a distance between two elevations of one of the rows of elevations arranged directly one behind the other.A riblet structure 1 shown in Fig. 50 has a substructure 3 in the respective longitudinal channel 2, which is formed with two rows of, in particular, rectangular or square, depressions as structural elements 4 oriented in the main direction H, wherein the rows of arrangements are offset from one another, in particular by half a distance of two directly successive depressions of one of the rows of arrangements. A riblet structure 1 shown in Fig. 51 has a substructure 3 in the respective longitudinal channel 2, which is formed with two rows of arrangements of, in particular, rectangular or square, depressions as structural elements 4 oriented in the main direction H, wherein depressions arranged directly behind one another in the respective row of arrangements are offset from one another in the secondary direction N. A riblet structure 1 shown in Fig.The riblet structure 1 shown in Fig. 52 has a substructure 3 in the respective longitudinal channel 2, which substructure is formed with two rows of arrangements of, in particular, rectangular or square, depressions oriented in the main direction H as structural elements 4, wherein the rows of arrangements are offset from one another, in particular by half a distance of two depressions of one of the rows of arrangements arranged directly one behind the other, wherein the depressions of the rows of arrangements partially overlap one another in the main direction H. A substructure 3 shown in Fig. 53 and Fig.The riblet structure 1 shown in Fig. 54 has a substructure 3 in the respective longitudinal channel 2, which substructure is formed with an arrangement row of, in particular, rectangular or square, depressions as structural elements 4 oriented in the main direction H, wherein the depressions have a tapered depression cross-section in the depth direction T, which depression cross-section decreases both in the main direction H and in the secondary direction N with increasing depth direction T. In particular, the depression can have a pyramid-like depression cross-section in the depth direction T, in particular with a pyramid tip oriented opposite to the height direction z. A riblet structure 1 shown in Fig. 55 is formed analogously to Fig. 50 with greater depth of the depressions. A riblet structure 1 shown in Fig.The riblet structure 1 shown in Fig. 56 has a substructure 3 in the respective longitudinal channel 2, which is formed with three rows of, in particular rectangular or square, depressions as structural elements 4 oriented in the main direction H, the depressions of the rows of arrangements being arranged one behind the other in the secondary direction N, wherein in the respective row of arrangements the depressions in the secondary direction N are arranged alternately offset from one another. A riblet structure 1 shown in Fig. 57 and Fig. 58 has a substructure 3 in the respective longitudinal channel 2, which is formed with a row of depressions as structural elements 4 oriented in the main direction H, wherein the depressions in the base surface section 7 extend diagonally from one of the longitudinal ribs 5 to the other longitudinal rib. The riblet structure 1 can preferably have longitudinal channels 2 with depressions running diagonally in the secondary direction N and diagonally opposite to the secondary direction N.A riblet structure 1 shown in Fig. 59 has a substructure 3 in the respective longitudinal channel 2, which is formed with an arrangement row of, in particular triangular, depressions as structural elements 4 oriented in the main direction H. In particular, in the secondary direction N along several longitudinal channels 2, bottom surfaces of the depressions can have a, in particular periodically, changing depth. A riblet structure 1 shown in Fig. 61 has a substructure 3 in the respective longitudinal channel 2, which is formed with an arrangement row of depressions as structural elements 4 oriented in the main direction H, wherein the respective depression is implemented by several triangular depressions, which triangular depressions are arranged in particular to form a rectangle. A riblet structure 1 shown in Fig.The riblet structure 1 shown in Fig. 62 has a substructure 3 in the respective longitudinal channel 2, which is formed with several rows of spherical, in particular circular or elliptical, depressions oriented in the main direction H as structural elements 4. In particular, one or more rows of arrangements can have depressions which have depression volumes trimmed by the longitudinal rib side surfaces 8 or an extension of the longitudinal rib side surfaces 8. One of the rows of arrangements can expediently be arranged adjacent to each of the longitudinal ribs 5, so that the depression volumes of the depressions of the respective row of arrangements are trimmed by the respective longitudinal rib or the longitudinal rib side surface 8, as explained above. A substructure 3 shown in Fig. 63 and Fig.The riblet structure shown in Figure 64 has a substructure 3 in the respective longitudinal channel 2, which is formed with an arrangement row of spherical, in particular circular or elliptical, elevations oriented in the main direction H as structural elements 4. In particular, in the secondary direction N, the

[0104] Base surface 6 along several longitudinal channels 2 have a, in particular periodically, varying height. In particular, the elevations of the substructure 3 can be formed such that fictitious spherical elevations are positioned on an imaginary arrangement plane, wherein a part of the fictitious spherical elevations projecting beyond the base surface 6 forms the elevations of the substructure 3. This is shown in Fig. 64. A riblet structure 1 shown in Fig. 65 has in the respective longitudinal channel 2 a

[0105] Substructure 3, which is formed with an arrangement row of arrow-shaped elevations oriented in the main direction H, in particular pointing in the main direction H, as structural elements 4, wherein in particular arrowheads of an arrow shape of the elevations are connected to one another by a longitudinal elevation running in the main direction H. The longitudinal elevation and two consecutive arrow-shaped elevations can each form a depression between them. The respective depression can be formed so that the depth of the depression increases in the direction of the longitudinal rib closest to it, as shown in Fig. 66. A riblet structure 1 shown in Fig. 67 has a substructure 3 in the respective longitudinal channel 2, which is formed with an arrangement row of arrow-shaped depressions oriented in the main direction H, in particular pointing in the main direction H. The depressions can be spaced from both longitudinal ribs 5 of the longitudinal channel 2.The depressions can have a constant depth, in particular in the secondary direction N. A riblet structure 1 shown in Fig. 68 has a substructure 3 implemented analogously to the riblet structure 1 of Fig. 67, wherein the depressions have a smaller average distance from one another in the main direction H compared to the riblet structure 1 of Fig. 67. A riblet structure shown in Fig. 69 and Fig. 70 has a substructure 3 implemented analogously to the riblet structure 1 of Fig. 67, wherein the depressions have a larger average distance in the main direction H compared to the riblet structure 1 of Fig. 67. Riblet structures 1 shown in Fig. 71 and Fig. 72 have a substructure 3 in the respective longitudinal channel 2, which is formed with a lattice-like arrangement of depressions running in the main direction H. The respective lattice-like arrangement can have depressions with opening areas of different sizes.Depressions of the respective grid-like arrangement can have the same depth or different depths from one another. It is advantageous if depressions arranged closer to a center of the depression have a greater depth than depressions arranged further away from the center. A riblet structure shown in Fig. 74 and Fig. 75 has a substructure 3 in the respective longitudinal channel 2, which is formed with one or more rows of depressions oriented in the main direction H, wherein the respective depression has a width that increases in the main direction H. The width of the depression is usually measured in the secondary direction N. The width usually refers to an opening area of ​​the depression. A generation of a substructure 3 of the riblet structure, in particular of the respective longitudinal channel 2, can be carried out using machine learning. In this way, for example, the substructures 3 of Fig. 71, Fig. 72 and Fig.73 were identified.

[0106] A specified shape of a structural element 4, in particular a protrusion or depression, typically refers to a shape of a cross-section of the depression, usually oriented orthogonally to the height direction z or depth direction T, and / or an opening area of ​​the depression. The depression can be implemented with a constant cross-section in the depth direction T of the depression or with a varying, usually tapered, cross-section.

Claims

Patent claims 1. Riblet structure (1), having longitudinal channels (2) oriented in a main direction (H), characterized in that the respective longitudinal channel (2) has a substructure (3) with structural elements (4) arranged one after the other, in particular periodically, for a fluid flow flowing in the longitudinal channel (2) in the main direction (H), wherein a ratio of an average distance (d) of the structural elements (4) in the main direction (H) to an average distance of channel edges of the longitudinal channel (2) is less than 1, in particular from 0.09 to 0.

33.

2. Riblet structure (1) according to claim 1, characterized in that the longitudinal channels (2) are formed with a plurality of spaced-apart longitudinal ribs (5) projecting from a base surface (6) and oriented in the main direction (H), wherein the average distance between the channel edges is an average rib tip distance (Rs) of the longitudinal ribs (5) forming the longitudinal channel (2).

3. Riblet structure (1) according to claim 2, characterized in that a ratio of an average rib height (R z ) from the longitudinal ribs (5) forming the longitudinal channel (2) to an average rib tip distance (Rs) of the longitudinal ribs (5) is a maximum of 0.8, preferably from 0.25 to 0.

4.

4. Riblet structure (1) according to claim 2 or 3, characterized in that the substructure (3), in particular the structural elements (4), are arranged at least partially on the base surface (6) of the respective longitudinal channel and / or at least partially on at least one longitudinal rib side surface (8) of the respective longitudinal channel.

5. Riblet structure (1) according to one of claims 1 to 4, characterized in that the structural elements (4) are formed with elevations and / or depressions.

6. Riblet structure (1) according to one of claims 2 to 5, characterized in that structural elements (4) of the substructure (3) extend from one of the longitudinal ribs (5) to the other longitudinal rib of the longitudinal channel (2), wherein preferably the Structural elements (4) along at least one of the longitudinal rib side surfaces (8) up to half a rib height (R z ) of the respective longitudinal rib.

7. Riblet structure (1) according to one of claims 2 to 6, characterized in that structural elements (4) of the substructure (3) are spaced apart from at least one or both of the longitudinal ribs (5) in a direction orthogonal to the main direction (H) at least in sections, preferably entirely.

8. Riblet structure (1) according to one of claims 1 to 7, characterized in that the substructure (3) is formed with transverse ribs (9) arranged one behind the other in the main direction (H) and oriented transversely to the main direction (H), in particular wedge-shaped, so that depressions are formed between the transverse ribs (9).

9. Riblet structure (1) according to one of claims 1 to 8, characterized in that, in particular in the main direction (H), the structural elements (4) are formed with a repeating order structure.

10. Riblet structure (1) according to one of claims 1 to 9, characterized in that the structural elements (4) in the longitudinal channel (2) form a plurality of rows of structural elements (4) oriented in the main direction (H).

11. Riblet structure (1) according to claim 10, characterized in that immediately adjacent rows of arrangements are arranged offset from one another in the main direction (H), preferably in each case by half a distance (d) between two immediately adjacent structural elements (4) of one of the rows of arrangements.

12. Surface of a component, characterized in that the surface has a riblet structure (1) according to one of claims 1 to 11.

13. A method for producing a riblet structure (1), characterized in that a riblet structure (1) according to one of claims 1 to 11 is introduced into the surface by embossing or abrasive machining of a surface.

14. Method for the computer-implemented simulation of a surface with a riblet structure (1), in particular a riblet structure (1) according to one of claims 1 to 11, for generating the riblet structure (1), wherein the riblet structure (1) has longitudinal channels (2) oriented in a main direction (H), characterized in that a substructure (3) is assigned to the respective longitudinal channel (2), so that a flow resistance with respect to a fluid flow flowing along the riblet structure (1) in the main direction (H) is reduced by more than 8.5%, preferably more than 10%.

15. The method according to claim 14, characterized in that the substructure (3) is simulated with structural elements (4) arranged one after the other, in particular periodically, for a fluid flow flowing in the longitudinal channel (2) in the main direction (H).

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