Riblet structure, and method for producing such a riblet structure

Abstract

The invention relates to a riblet structure (1) for a surface for reducing a friction resistance with respect to a fluid which flows in a main direction (H) along the surface, wherein the riblet structure (1) has a plurality of, in particular, wedge-shaped longitudinal ribs (3) which protrude from a base surface (2), are oriented in the main direction (H) and are spaced apart from one another. In order to achieve a low friction resistance, it is provided according to the invention that, between in each case two adjacent longitudinal ribs (3), the base surface (2) has depressions (5) which are spaced apart from one another in the main direction (H), which depressions (5) are spaced apart in a direction orthogonal with respect to the main direction (H) at least in sections, preferably completely, from at least one or both of the longitudinal ribs (3). The invention also relates to the use of and to a method for producing such a riblet structure (1).

Description

[0001] Riblet structure and method for producing such a

[0002] The invention relates to a riblet structure for a surface for reducing frictional resistance with respect to a fluid flowing in a main direction along the surface, wherein the riblet structure has a plurality of spaced-apart, in particular wedge-shaped, longitudinal ribs projecting from a base surface and oriented in the main direction.

[0003] Furthermore, the invention relates to a use of such a riblet structure.

[0004] Furthermore, the invention relates to a method for producing such a riblet structure.

[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 typically formed with spaced-apart longitudinal ribs arranged on a base plane along a main direction, which in use corresponds to the main flow direction of the fluid. The longitudinal ribs are often wedge-shaped.

[0007] For example, document EP 3 170 743 A1 discloses a riblet structure with a plurality of wedge-shaped longitudinal ribs formed in this way. The base surface arranged between the longitudinal ribs can be flat or curved.

[0008] This is where the invention comes in. The object of the invention is to provide a riblet structure of the type mentioned above, which exhibits low frictional resistance to a fluid flowing along the riblet structure or a surface having the riblet structure.

[0009] Furthermore, it is an aim to specify a use of such a riblet structure.

[0010] Furthermore, a further aim is to provide a method for producing 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 base surface has depressions spaced apart from one another in the main direction between each two adjacent longitudinal ribs, which depressions are spaced apart from one another in a direction orthogonal to the main direction at least in sections, preferably entirely, from at least one or both of the longitudinal ribs.

[0012] The invention is based on the finding that, in a conventional riblet structure formed with spaced-apart longitudinal ribs arranged on a base surface, 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 depressions into a base surface section located between two adjacent longitudinal ribs, without, however, significantly influencing or disrupting a boundary layer profile of the fluid flow somewhat further away from the riblet structure, in particular flow turbulence over the longitudinal ribs. In this way, there is the potential to improve the flow behavior directly at the riblet structure with a structural development of the base surface section between the longitudinal ribs.The adjacent longitudinal ribs usually refer to two directly adjacent longitudinal ribs, in particular in a direction orthogonal to the main direction. The base surface is usually designed as a base plane and the base surface section as a base plane section. The longitudinal ribs are usually oriented in the main direction, with the longitudinal ribs arranged next to one another at a distance from one another in a direction orthogonal to the main direction. Conventional riblet structures with a usually flat base surface section between the longitudinal ribs generate turbulent viscous drag, primarily caused by an interaction surface of the base surface section located between the longitudinal ribs. It has been shown that the viscous drag can be reduced by introducing depressions into the base surface section.This can be explained by a reduced interaction area of ​​the base surface section, since the introduced depressions reduce an area of ​​the base surface section at the same height, or rather, a part of the base surface is lowered by the depression and thus only contributes to the viscous resistance to a reduced extent. Conversely, however, it has been shown that the depressions generate additional pressure resistance. The total resistance used for comparison thus results from the sum of the viscous resistance and pressure resistance in a riblet structure with depressions, compared to the purely viscous resistance in a riblet structure without depressions. An advantageously reduced total resistance, orFrictional resistance can therefore be achieved in a riblet structure with depressions if, on the one hand, the depressions reduce the viscous resistance as much as possible and, on the other hand, the pressure resistance generated in return is kept as small as possible.

[0013] Investigations have shown that this can be achieved with a pronounced demarcation of the depressions in the base surface or a pronounced at least partial spacing of the respective depression from the longitudinal ribs. This can advantageously be achieved with a base surface region adjacent to the respective depression, in particular between at least one of the longitudinal ribs and the depression, or with an at least partial, preferably complete, spacing of the depression from the longitudinal ribs. The cause appears to be the generation of turbulent flow at the edge sections of the depression, in particular when a flow component penetrates into the respective depression. This is related to the pressure resistance generated by the depression.This can be achieved if the respective depression is formed at least partially, preferably entirely, spaced from at least one of the two longitudinal ribs in a direction orthogonal to the main direction. It is advantageous for reduced viscous resistance if the depression is at least partially, preferably entirely, spaced from both longitudinal ribs. It is understood that the two longitudinal ribs here refer to the two immediately adjacent longitudinal ribs between which the respective depression or depressions are arranged. Base surface section usually refers to the part of the base surface located between the two longitudinal ribs or two immediately adjacent longitudinal ribs. That part of the base surface section which has no depressions or in which no depressions are inserted is often referred to as the interaction surface section or reference surface section of the base surface.The fluid can be formed with a gas, for example air, and / or a liquid, such as water.

[0014] Two directly adjacent longitudinal ribs, also referred to as a pair of longitudinal ribs, generally form a groove between them, also referred to as a longitudinal channel. A groove bottom surface is usually formed by the base surface or a base surface section of the base surface located between the longitudinal ribs, and in particular groove side surfaces or longitudinal channel side surfaces are formed by side surfaces of the longitudinal ribs. The depressions are inserted into the base surface or the base surface section or are formed with it / this. The longitudinal ribs are usually wedge-shaped or have a triangular or trapezoidal basic shape in a cross-section orthogonal to the longitudinal extent of the longitudinal ribs. Side walls of the longitudinal ribs can be flat. 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 each two longitudinal ribs, can be essentially flat, with the depressions additionally being inserted into the base surface or formed with it. The groove often forms essentially an upside-down trapezoidal recess, with the recess formed by the respective depression additionally adjoining this. If the base surface is flat, it is often referred to as the base plane. That area of ​​the base surface which is not part of the depressions 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. As a rule, the longitudinal rib height of the two longitudinal ribs or the pair of longitudinal ribs is higher than a substructure or topology formed between them with the base surface or the respective base surface section.Preferably, an average height of the base surface section is less than, in particular half, preferably a quarter, of an average longitudinal rib height of the two longitudinal ribs delimiting the base surface section. Top view, in particular of the riblet structure and / or the longitudinal channels, usually refers to a view orthogonal to the main direction and orthogonal to a secondary direction. The top view is usually taken along the height direction. Typically, the longitudinal ribs or longitudinal channels are arranged next to one another in the secondary direction. As a rule, the longitudinal ribs, in particular of the respective longitudinal channel, are arranged at a distance from one another in the secondary direction. The secondary direction is usually oriented orthogonal to the main direction. It is understood that a specified height, in particular the longitudinal rib height, is usually defined in the height direction.

[0015] The height direction is usually orthogonal to the base surface or, in particular, both orthogonal to the main direction and orthogonal to the secondary direction.

[0016] The lateral surface of a respective longitudinal rib is usually oriented substantially parallel to the main direction. It is practical if the lateral surface is designed such that, particularly in a cross-section orthogonal to the main direction, a tangent applied to the lateral surface forms an angle between 30° and 50°, preferably approximately 40°, with the base surface or the base surface section adjacent to the lateral surface. This preferably applies to both lateral surfaces of the respective rib.

[0017] It is usually advantageous for the depressions to be formed with a repeating order structure or to be regularly arranged, particularly in the main direction. 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. For this purpose, the depressions can form a periodic structure, in particular with regular, preferably constant, spacing between them. In particular, several depression classes represented by differently formed depressions can be present, with depressions of different depression classes being arranged next to one another according to a predetermined ordering scheme. Typically, depressions of the same design are assigned to the same depression class.It is expedient if depressions of different depression classes are arranged alternately next to one another, in particular periodically. For this purpose, individual depressions or groups of immediately adjacent depressions of each depression class can alternate with one another. It is possible to arrange the depressions irregularly, but a regular arrangement or an arrangement with an ordered structure or ordering scheme is preferred. The above applies in particular in the main direction and / or orthogonal to the main direction. This applies in particular in a cross-section essentially parallel to the base surface or in a plan view of the base surface. The depressions of different depression classes can differ, for example, in their shape, depth of the depressions, distances between two immediately adjacent depressions or one of the other features described in this document, in particular below.The above can be implemented for a respective base area section.

[0018] It has been shown that a very pronounced spacing of the respective depressions from at least one, preferably both, of the longitudinal ribs is favorable for low pressure resistance. It is advantageous if, in a plan view of the base surface in a direction orthogonal to the main direction, an average distance between at least one of the two longitudinal ribs and a depression edge section facing this longitudinal rib and delimiting the respective depression is more than 5%, in particular more than 10%, preferably more than 20%, particularly preferably more than 25%, of an average longitudinal rib spacing between the longitudinal ribs. The longitudinal rib spacing usually refers, in a cross-section orthogonal to the main direction H, 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 longitudinal rib spacing is usually measured orthogonally to the longitudinal rib height. As explained above, the resistance behavior, in particular viscous resistance or pressure resistance, can be influenced by spacing the depression or a depression edge section of a depression edge of the respective depression 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 depression edge section facing the respective longitudinal rib and delimiting the respective depression. 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 recess edge section facing this longitudinal rib and delimiting the respective recess 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 longitudinal rib spacing between the longitudinal ribs. It is advantageous if the recess, preferably a centroid of a surface formed by the recess 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 recess edge section are usually determined between a point at which the respective longitudinal rib borders the base surface section and a point on the recess edge section.

[0019] It is expedient if the depression, in particular a depression edge segment of the depression, borders on a base surface region located, in particular in a direction orthogonal to the main direction, between the depression 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 depression or the depression edge segment and the at least one longitudinal rib, in particular its lateral surface. Preferably, such a base surface region is present between both of the longitudinal ribs and each of the depression or each of a depression edge segment of the depression. In practice, the depression edge segment can be the depression edge section. It is preferred if the depression or the depression edge is largely, in particular preferably essentially entirely, surrounded by such a base surface region.

[0020] 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, the average longitudinal extent of the depression in the main direction is a maximum of 1.5 times, in particular a maximum of 1.2 times, as large as the average width of the depression in a direction orthogonal to the main direction. This applies in particular in a plan view of the base surface. It is particularly advantageous if an area defined by the edge of the depression is designed in this way. It is advantageous for reduced pressure resistance if the average depth of the depressions is less than 75%, in particular between 5% and 70%, of an average longitudinal rib height. It has been shown that at greater depths turbulence can occur, which leads to an increase in pressure resistance.Preferably, the average depth of the depression is between 5% and 60%, in particular between 10% and 45%, preferably less than 30%, particularly preferably less than 15%, of the average longitudinal rib height of the longitudinal ribs.

[0021] To adjust a viscous resistance, it is advantageous if several of the recesses between the two longitudinal ribs overlap each other in the direction orthogonal to the main direction.

[0022] To achieve a balance between viscous drag and pressure drag, it is advantageous if the depressions between the longitudinal ribs or the pair of longitudinal ribs form several rows of depressions oriented in the main direction. For example, two rows of depressions running in the main direction can be present between the two longitudinal ribs. It is advantageous if immediately adjacent rows of depressions are offset from one another in the main direction, preferably by half the distance between two immediately adjacent depressions in one of the rows. This makes it possible to achieve flow behavior that is advantageous with regard to the overall drag. The depressions between the rows can overlap one another in the direction orthogonal to the main direction.

[0023] 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 or cube-shaped, recess. A base area of ​​the prism shape can be polygonal, for example triangular, square, pentagonal or hexagonal. However, it can be advantageous for turbulent flow behavior if the depression has a decreasing cross-sectional area at least in sections, in particular predominantly, preferably substantially, along the depth direction of the depression. The depression can, for example, be formed with or by a substantially conical or pyramid-shaped recess. A base area of ​​the pyramid shape can, for example, be triangular, square, pentagonal or hexagonal.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.

[0024] 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.

[0025] 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 part 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 part of a conical or cylindrical surface. For example, the depression can have a depression wall that is substantially dome-shaped.

[0026] 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 recessed areas. Advantageously, several, in particular a majority, preferably essentially all, of the recesses between the two longitudinal ribs can be designed in this way. The recessed areas can be differently shaped or each designed according to the characteristics specified for the shape and size of the recess, in particular differently from one another.

[0027] It can be advantageous for flow behavior in the depressions if several of the depressions are connected to one another via at least one fluid-conductive connecting channel. Expediently, several of the depressions between the two longitudinal ribs, which are arranged next to one another, in particular in the main direction and / or orthogonal to this, 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 between different of two directly adjacent longitudinal ribs or pairs of longitudinal ribs are connected to one another via at least one fluid-conductive connecting channel. The connecting channel usually runs beneath the longitudinal ribs in order to connect the depressions between different pairs of longitudinal ribs.In practice, recesses between several, a majority, preferably essentially all, of the longitudinal rib pairs can be connected to one another via at least one or more connecting channels. Preferably, recesses orthogonal to the main direction, in particular directly adjacent, are connected via the at least one connecting channel, in particular if they are recesses of different longitudinal rib pairs. It is understood that it is advantageous if several connecting channels are provided to connect the recesses to one another. In this case, several connecting channels can be present between each two recesses. The connecting channel can, for example, be formed as a bore within a material forming the base surface.It has proven to be effective if the connecting channel is connected to the respective depression in an area of ​​a lower third of the depression, in particular in an area of ​​a depression bottom, usually laterally.

[0028] To adjust the pressure resistance, it is advantageous if several longitudinal rib pair classes are present, wherein the longitudinal rib pair classes represent longitudinal rib pairs with mutually different configurations of depressions arranged between the respective longitudinal rib pair, wherein longitudinal rib pairs and respective depressions of different longitudinal rib pair classes are arranged next to one another according to a predetermined ordering scheme, usually in a direction orthogonal to the main direction. For example, a longitudinal rib pair with depressions having a first depth of the depressions and a longitudinal rib pair with depressions having a second depth of the depressions can be arranged next to one another alternately.Different longitudinal rib pair classes can differ 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 between the longitudinal ribs of the respective longitudinal rib pair. Typically, identically shaped longitudinal rib pairs and the recesses arranged between their longitudinal ribs are assigned to the same longitudinal rib pair class.

[0029] It is advantageous for flow behavior if transverse elevations are arranged on the base surface, each of which adjoins one of the side surfaces of one of the longitudinal ribs, wherein in a cross-section orthogonal to the main direction, the transverse elevation has a downward-sloping, in particular flat, surface contour in a direction facing away from the side wall and preferably merges into the base surface. It is understood that this is a side surface of the longitudinal rib facing the depressions. Typically, a plurality of spaced-apart transverse elevations are provided in the main direction, which adjoin the side surface of the respective longitudinal rib. Preferably, such transverse elevations are arranged on the side surfaces of both longitudinal ribs. The side surfaces of the two longitudinal ribs usually face one another. The respective transverse elevation usually merges into the base surface or preferably adjoins one of the depressions orthe edge of the recess.

[0030] It has proven to be effective if the transverse elevations are present on mutually facing side surfaces of the two longitudinal ribs in such a way that, in the cross-section, tangents applied to the surface contours of the transverse elevations orthogonal to the main direction form an angle greater than 90°, in particular greater than 120°, to each other.

[0031] It is expedient if one of the depressions is present between each two immediately adjacent transverse elevations. These can be immediately adjacent transverse elevations in the main direction and / or orthogonal to the main direction. These can be immediately adjacent transverse elevations which adjoin the same longitudinal rib or its side surface, and / or immediately adjacent transverse elevations which adjoin different of the two longitudinal ribs or their side surfaces, in particular those facing each other. It is advantageous for a flow pattern if the respective transverse elevation, in plan view onto the base plane, has a longitudinal extension oriented obliquely to the longitudinal rib longitudinal direction of the respective longitudinal rib to which the transverse elevation adjoins, wherein a longitudinal direction of the transverse elevation forms an acute angle to the longitudinal rib longitudinal direction.The angle may suitably be between 10° and 80°, in particular less than 60°, preferably about 45°.

[0032] Typically, the longitudinal rib height of the longitudinal ribs and / or a longitudinal rib tip spacing between the longitudinal rib tips is less than 250 pm, in particular less than 100 pm, preferably less than 80 pm. It is advantageous if the longitudinal 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. It has proven useful if the longitudinal rib tip spacing, i.e. a distance between tips or peaks of immediately adjacent longitudinal ribs or pairs of longitudinal ribs, is between 60 pm and 85 pm, in particular between 70 pm and 80 pm, preferably about 74 pm. It has proven particularly useful if the longitudinal rib height is between 30 pm and 37 pm, preferably about 34 pm, and the longitudinal rib tip distance is between 70 pm and 80 pm, preferably about 74 pm.The longitudinal rib tip spacing is usually measured in a cross-section orthogonal to the main direction between the highest points of the longitudinal ribs or longitudinal rib tips.

[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 longitudinal rib spacing 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 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. 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.

[0035] The aforementioned objective is achieved by using a riblet structure, particularly described in this document, as a surface on a component to reduce friction between the surface and a gas flow, in particular an air flow, and / or a liquid flow, in particular a water flow. According to the features and effects of the riblet structure, the surface of the component can be designed with low frictional resistance to a fluid flowing along the surface.

[0036] The further objective stated at the outset is achieved if a method for producing a riblet structure is provided, wherein the riblet structure is designed as described in this document and the riblet structure is introduced into the surface by embossing or abrasive machining of a surface. 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.

[0037] It is understood that height and depth specifications usually refer to a direction orthogonal to the base surface or its reference surface, and length and width specifications usually refer to a direction parallel to the base surface or its reference surface.

[0038] Further features, advantages, and effects will become apparent from the following exemplary embodiments. The drawings, to which reference is made, show:

[0039] Fig. 1 shows a conventional riblet structure of the prior art with longitudinal ribs oriented in the main direction and flat base surface sections between pairs of longitudinal ribs;

[0040] Fig. 2 and Fig. 3 sections of the riblet structure of Fig. 1 in view from obliquely above and in plan view;

[0041] Fig. 4 is a profile view of a section of the riblet structure of Fig. 1;

[0042] Fig. 5 shows a riblet structure with depressions spaced apart from one another in the main direction in base surface sections located between pairs of longitudinal ribs, the depressions having a rectangular cross-section;

[0043] Fig. 6 and Fig. 7 sections of the riblet structure of Fig. 5 in view from obliquely above and in plan view;

[0044] Fig. 8 shows a riblet structure with depressions spaced apart in the main direction in base surface sections located between pairs of longitudinal ribs, wherein the depressions have a triangular cross-section; Fig. 9 shows a riblet structure with depressions spaced apart in the main direction in base surface sections located between pairs of longitudinal ribs, wherein the depression walls of the depressions are designed as surfaces of revolution;

[0045] Fig. 10 and Fig. 11 sections of the riblet structure of Fig. 9 in view from obliquely above and in plan view;

[0046] Fig. 12 shows a riblet structure with depressions spaced apart in the main direction in base surface sections located between pairs of longitudinal ribs, the depressions having a triangular cross-section and forming two rows of depressions oriented in the main direction;

[0047] Fig. 13 and Fig. 14 sections of the riblet structure of Fig. 12 in view from obliquely above and in plan view;

[0048] Fig. 15 shows a riblet structure with depressions spaced apart from one another in the main direction in base surface sections located between pairs of longitudinal ribs, the depressions each extending to both longitudinal ribs of the respective pair of longitudinal ribs;

[0049] Fig. 16 and Fig. 17 sections of the riblet structure of Fig. 15 in view from obliquely above and in plan view;

[0050] Fig. 18 is a graph showing position-dependent frictional resistance for different riblet structures;

[0051] Fig. 19 to Fig. 21 Contour diagrams for a riblet structure without depressions, a riblet structure with rectangular depressions and a riblet structure with spherical depressions, with an average frictional resistance shown in grayscale.

[0052] Fig. 1 shows a schematic representation of a conventional riblet structure 1 of the prior art. The riblet structure 1 is formed with a plurality of wedge-shaped longitudinal ribs 3 which project from a base surface 2 and run in the main direction H and are spaced apart from one another. The base surface 2 forms a flat base surface section 4 between two directly adjacent longitudinal ribs 3 or pairs of longitudinal ribs which are usually arranged in a secondary direction N orthogonal to the main direction. The longitudinal ribs 3 preferably have flat longitudinal rib side walls 8. Such a structure has proven suitable for reducing frictional resistance to a fluid flowing in the main direction H along the riblet structure 1. Fig. 2 and Fig. 3 show schematic representations of a section of the riblet structure 1 of Fig. 1 in order to clarify the riblet structure 1 of Fig. 1, wherein Fig. 2 shows a view obliquely from above and Fig.3 shows a plan view of the riblet structure 1 of Fig. 1.

[0053] Fig. 4 shows a schematic profile representation of a section of the riblet structure 1 of Fig. 1. The characteristic data used to describe riblet structures 1 are usually a longitudinal rib tip distance Rs, a longitudinal rib distance RD and a longitudinal rib height R z which are measured in a cross-section orthogonal to the main direction H or orthogonal to the base surface 2, in particular to a height of a respective base surface section 4. The longitudinal rib tip spacing Rs denotes a distance between two immediately adjacent longitudinal ribs 3 or pairs of longitudinal ribs, the longitudinal rib spacing RD denotes a width of the base surface section 4 between two immediately adjacent longitudinal ribs 3 or pairs of longitudinal ribs, and the longitudinal 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. A height direction z is usually oriented orthogonal to the main direction H and orthogonal to the secondary direction N.

[0054] By providing that the base surface 2 has recesses 5 spaced apart from one another in the main direction H between each two immediately adjacent longitudinal ribs 3 or pairs of longitudinal ribs, wherein the recesses 5 are spaced apart from one another in a direction orthogonal to the main direction H at least in sections, preferably entirely, from at least one or both of the longitudinal ribs 3, a further reduction in the frictional resistance can advantageously be achieved.

[0055] As explained below, investigations and computer simulations were conducted with a variety of different riblet structures 1, with particular attention being paid to the extent, shape, and / or size of depressions 5 introduced into the base surface 2 or base surface sections 4. A selection of investigated riblet structures 1 is explained below.

[0056] Fig. 5 shows a schematic representation of a riblet structure 1 with a basic design according to Fig. 1, wherein additionally in the respective base surface section 4 located between the longitudinal ribs 3 of a respective pair of longitudinal ribs there are recesses 5 spaced from one another in the main direction H. The recesses 5 have a rectangular cross-section or a rectangular recess edge 6. The recesses 5 are arranged at a distance from the longitudinal ribs 3 of the pair of longitudinal ribs. Such a configuration has proven to be very effective for reducing frictional resistance. The recesses 5 can expediently each have a substantially constant cross-section or a cross-section that decreases in size along their depth direction T. The recesses 5 then form recesses in the shape of a cuboid. Fig. 6 and Fig. 7 show schematic representations of a section of the riblet structure 1 of Fig. 5 in order to illustrate the riblet structure 1 of Fig.5, wherein Fig. 6 shows a view from above and Fig. 7 shows a plan view of the riblet structure 1 of Fig. 5.

[0057] A design of a riblet structure 1 analogous to the riblet structure 1 of Fig. 5 is shown schematically in Fig. 8, wherein the recesses 5 each have a triangular cross-section or triangular recess edge 6. The recesses 5 in Fig. 8 are also spaced apart from the two longitudinal ribs 3 of the pair of longitudinal ribs and can, in particular, be designed with a constant cross-section or a cross-section that decreases along their depth direction T.

[0058] Fig. 9 shows a schematic representation of another riblet structure 1 with a basic design according to Fig. 1 or Fig. 5, wherein the depressions 5 are formed with corner-free depression edges. The depressions 5 can advantageously have depression walls 7 which are designed as surfaces of revolution, usually with axes of rotation in the depth direction T of the depressions 5. It has proven useful if the depression walls 7 of the depressions 5 are formed as part of an ellipsoid of revolution, preferably a spherical surface or spherically. Depressions 5 designed in this way are particularly suitable for implementing reduced viscous resistance while simultaneously keeping pressure resistance low. In Fig. 9, the depressions 5 are essentially formed as part of a sphere or spherical surface. Fig. 10 and Fig. 11 show schematic representations of a section of the riblet structure 1 of Fig. 9, in order to illustrate the riblet structure 1 of Fig.9, wherein Fig. 10 shows a view obliquely from above and Fig. 11 a plan view of the riblet structure 1 of Fig. 9. Fig. 12 shows a schematic representation of a riblet structure 1 with a basic structure of Fig. 1, wherein the respective base surface section 4 located between the pairs of longitudinal ribs has depressions 5 spaced from one another in the main direction H, wherein the depressions 5 form two rows of depressions oriented in the main direction H. The rows of depressions are arranged next to one another or offset in the direction orthogonal to the main direction H, as can be seen in particular in a view in the main direction H, wherein the rows of depressions are arranged offset or shifted from one another in the main direction H, usually by half a distance between two immediately adjacent depressions 5 of one of the rows of depressions.As can be seen in Fig. 12, a reference surface of the base surface section 4, i.e., a part of the base surface section 4 without depressions 5, can thereby form a zigzag structure running back and forth between the two rows. Fig. 13 and Fig. 14 show schematic representations of a section of the riblet structure 1 of Fig. 12 to illustrate the riblet structure 1 of Fig. 12, wherein Fig. 13 shows a view obliquely from above and Fig. 14 shows a plan view of the riblet structure 1 of Fig. 12. With such a configuration, a reduced viscous resistance and, at the same time, a low pressure resistance can be implemented, so that the frictional resistance can be advantageously reduced, particularly compared to the configuration according to Fig. 1. The pressure resistance cannot usually be kept as low as with a configuration of the depressions 5 with recess walls 7 in the shape of a surface of revolution, such as according to Fig. 9.

[0059] Fig. 15 shows a schematic representation of a riblet structure 1 with a basic construction according to Fig. 1, wherein the base surface section 4 has depressions 5 spaced apart from one another in the main direction H. However, it is provided here that the depressions 5 extend as far as both longitudinal ribs 3 of the respective pair of longitudinal ribs, so that a type of sinusoidal shape of the base surface section 4 results in the main direction H. Fig. 16 and Fig. 17 show schematic representations of a section of the riblet structure 1 of Fig. 15 in order to clarify the riblet structure 1 of Fig. 15, wherein Fig. 16 shows a view obliquely from above and Fig. 17 shows a plan view of the riblet structure 1 of Fig. 15. With such a shape, a reduction in frictional resistance was achieved compared to a flat surface without riblet structure 1, but the frictional resistance was higher than with a riblet structure 1 with a flat base surface 2 without depressions 5 according to Fig. 1.Simulation results for the aforementioned design variants are presented below. For the data presented below, 84 longitudinal ribs 3 were arranged on a base surface 2, with a longitudinal rib tip spacing Rs between each two immediately adjacent longitudinal ribs 3 of 74 pm and a longitudinal rib height R. z of 34 pm. The fluid used was water at a temperature of 25 °C.

[0060] For the riblet structures 1 of Fig. 1 - without recesses 5 Fig. 5 - rectangular recesses 5 Fig. 8 - triangular recesses 5, Fig. 9 - spherical recesses 5 Fig. 12 - triangular recesses 5 in two rows - and Fig. 15 - recesses 5 up to both longitudinal ribs 3 - the results for resistance values ​​with respect to the fluid flowing along the riblet structure 1 in the main direction H are given below in Table 1. This results in a total resistance Ps as the sum of pressure resistance PP and viscous resistance Pv Finally, Table 1 shows the percentage reduction in frictional resistance compared to a flat surface without riblet structure 1.

[0061] As shown in Table 1, a typical riblet structure 1 of Fig. 1 without provided depressions 5 results in a reduction in frictional resistance of 8.10% compared to a flat surface without a riblet structure 1. In comparison, greater reductions in frictional resistance are achieved by the riblet structures 1 of Fig. 5, Fig. 8, Fig. 9, and Fig. 12. While a riblet structure 1 of Fig. 1 has viscous resistance and virtually no pressure resistance, the provision of depressions 5 in the base surface sections 4 reduces the viscous resistance but, in turn, generates pressure resistance. If both of these resistances are kept small, a reduction in frictional resistance of a riblet structure 1 with depressions 5 compared to a surface without a riblet structure 1 can be achieved, which reduction is greater than for a riblet structure 1 of Fig. 1 without depressions 5.

[0062] A particularly advantageous reduction in frictional resistance of 9.94% is achieved with the riblet structure 1 in Fig. 8 - which has triangular depressions 5 in cross-section, which are spaced apart from the two longitudinal ribs 3. With the riblet structure 1 in Fig. 5 - which has rectangular depressions 5 in cross-section - a reduction in frictional resistance of 8.57% is achieved, and with the riblet structure in Fig. 9 - depressions 5 which have depression walls 7 formed as part of a spherical surface - a reduction in frictional resistance of 8.56% is achieved compared to a surface without riblet structure 1. With the riblet structure 1 in Fig. 12 - triangular depressions 5 in two rows - a reduction in frictional resistance of 9.53% is achieved.15 - depressions 5, which extend to both longitudinal ribs 3 - only a 7.22% reduction in frictional resistance could be achieved compared to a surface without riblet structure 1, i.e. less reduction than with a riblet structure 1 of Fig. 1 without depressions 5.

[0063] Table 1: Simulation results of a frictional resistance of the riblet structures 1 of Fig. 1 - without depressions 5 -, Fig. 5 - rectangular depressions 5 -, Fig. 8 - triangular depressions 5 -, Fig. 9 - spherical depressions 5 -, Fig. 12 - triangular depressions 5 in two rows - and Fig. 15 - depressions 5 up to both longitudinal ribs 3 - with PP the pressure resistance, Pv the viscous resistance, Ps the total resistance (PP+P V ) and AR the percentage change in frictional resistance compared to a flat surface without riblet structure 1.

[0064] Fig. 18 shows a graph in which the frictional resistance for different riblet structures 1 is represented as a position-dependent one in a direction orthogonal to the main direction H. Line 10, shown as a dash-dotted line, shows the frictional resistance for a riblet structure 1 without depressions 5 according to Fig. 1. Periodic peaks of line 10 are visible, which correspond to the longitudinal ribs 3 of the riblet structure 1. Between each peak, line 10 forms a valley with a slight parabolic increase in frictional resistance centrally between adjacent peaks. Line 11, shown as a dotted line, shows the frictional resistance for a riblet structure 1 with rectangular depressions 5 according to Fig. 5, and line 12, shown as a solid line, shows the frictional resistance for a riblet structure 1 with spherical depressions 5 according to Fig. 9.Lines 11 and 12 also have a basic shape of line 10, whereby, in contrast to line 10, the tips of lines 11 and 12 have a lower height and, in the middle between the tips, there is a drop or depression in the frictional resistance in line 11 and line 12. This corresponds to the depressions 5 of the riblet structures 1 in Fig. 5 and Fig. 9, respectively, and the reduction in overall resistance caused by the depressions 5. For comparison, the graph in Fig. 18 with line 9, shown as a dashed line, also shows frictional resistance on a flat surface without riblet structure 1. Line 9 shows a constant frictional resistance along the surface.

[0065] Fig. 19 to Fig. 21 show contour diagrams for an average frictional resistance of different riblet structures 1, with the frictional resistance being represented by grayscale. Fig. 19 shows a riblet structure 1 according to Fig. 1—without depressions 5—, Fig. 20 shows a riblet structure according to Fig. 5—rectangular depressions 5—and Fig. 9 shows a riblet structure 1 according to Fig. 9—spherical depressions 5. The contour diagrams show—corresponding to the graph in Fig. 18—that a reduced average frictional resistance exists between the longitudinal ribs 3. The rectangular depressions 5 of the riblet structure 1 according to Fig. 5 correspond in Fig. 20 to rectangular dark areas of particularly reduced frictional resistance, and the spherical depressions 5 of Fig. 9 correspond in Fig. 21 to circular dark areas of particularly reduced frictional resistance. The areas shown in Fig.21 The light areas lying between the circular dark areas also correspond to a smaller frictional resistance than the light areas lying between the rectangular dark areas in Fig. 20. This corresponds to the finding of a particularly reduced frictional resistance in spherical depressions 5 or depressions 5 with corner-free depression edge 6.

[0066] With a riblet structure 1, wherein the riblet structure 1 has a plurality of spaced-apart longitudinal ribs 3 projecting from a base surface 2 and oriented in the main direction H, an advantageously low frictional resistance of the riblet structure 1 with respect to a fluid flowing along the riblet structure 1 in the main direction H can be achieved in that the base surface 2 or the base surface section 4 has spaced-apart depressions 5 between each two adjacent longitudinal ribs 3 or pairs of longitudinal ribs, wherein the depressions 5 are spaced apart in a direction orthogonal to the main direction H from at least one, preferably both, longitudinal ribs 3. As a result, both viscous resistance can be reduced and pressure resistance can be kept low, so that a pronounced reduction in frictional resistance of the riblet structure 1 can be achieved.This is especially true if the recesses 5 are spaced at a significant distance from both longitudinal ribs 3 and are preferably positioned centrally between the longitudinal ribs 3. Configuring the recesses 5 with a rectangular cross-section or with a recess wall 7 that is part of a rotational ellipsoid, preferably a sphere or spherical surface, has proven particularly advantageous for achieving low frictional resistance.

Claims

Patent claims 1. Riblet structure (1) for a surface for reducing frictional resistance to a fluid flowing along the surface in a main direction (H), wherein the riblet structure (1) has a plurality of spaced-apart, in particular wedge-shaped, longitudinal ribs (3) projecting from a base surface (2) and oriented in the main direction (H), characterized in that between each two adjacent longitudinal ribs (3), the base surface (2) has depressions (5) spaced from each other in the main direction (H), which depressions (5) are spaced apart from at least one or both of the longitudinal ribs (3) in a direction orthogonal to the main direction (H), at least in sections, preferably entirely.

2. Riblet structure (1) according to claim 1, characterized in that the depressions (5), in particular in the main direction (H), are formed with a repeating order structure.

3. Riblet structure (1) according to claim 1 or 2, characterized in that in a plan view of the base surface (2) in a direction orthogonal to the main direction (H), an average distance between at least one of the two longitudinal ribs (3) and a recess edge section facing this longitudinal rib (3) and delimiting the respective recess (5) is more than 5% of an average distance between the longitudinal ribs (3).

4. Riblet structure (1) according to one of claims 1 to 3, characterized in that in a cross section parallel to the base surface, an average longitudinal extent of the depression in the main direction is at most 1.5 times, in particular at most 1.2 times, as large as an average width extent of the depression in a direction orthogonal to the main direction.

5. Riblet structure (1) according to one of claims 1 to 4, characterized in that the depressions (5) between the two longitudinal ribs form several rows of depressions oriented in the main direction (H).

6. Riblet structure (1) according to claim 5, characterized in that the rows of depressions are arranged offset from one another in the main direction (H), preferably by half a distance between two adjacent depressions (5) of an adjacent row.

7. Riblet structure (1) according to one of claims 1 to 6, characterized in that the respective depression (5) has a predominantly constant cross-sectional area along a depth direction (T) of the depression (5).

8. Riblet structure (1) according to one of claims 1 to 7, characterized in that the respective depression (5) is shaped in a cross-section without corners, in particular oval or round, or is shaped polygonally, in particular triangular or square.

9. Riblet structure (1) according to one of claims 1 to 8, characterized in that the respective depression (5) has a depression wall (7) forming the depression (5), which is designed at least in sections as a surface of rotation, in particular with a rotation axis in the depth direction (T) of the depression (5).

10. Riblet structure (1) according to one of claims 1 to 9, characterized in that the respective depression (5) is formed in the depth direction (T) with a first depression region and a second depression region arranged downstream of the first depression region, wherein the first depression region has a smaller diameter than the second depression region, preferably a diameter of the first depression region is smaller than 80% of a diameter of the second depression region.

11. Riblet structure according to one of claims 1 to 10, characterized in that several of the recesses (5) are connected to one another via at least one fluid-conductive connecting channel.

12. Riblet structure (1) according to one of claims 1 to 11, characterized in that a longitudinal rib height (R z ) of the longitudinal ribs (3) and / or a Longitudinal rib spacing (RD) between the longitudinal ribs (3) is less than 250 pm, in particular less than 100 pm.

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

14. Use of a riblet structure (1) according to one of claims 1 to 12 as a surface in a component, in particular a surface according to claim 13, in order to reduce friction between the surface and a gas flow, in particular air flow, and / or liquid flow, in particular water flow.

15. A method for producing a riblet structure (1), characterized in that the riblet structure (1) is designed according to one of claims 1 to 12, wherein the riblet structure (1) is introduced into the surface by embossing or abrasive machining of a surface.

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