Abstract
A method for producing an object with riblets on and/or in the surface, around which object a fluid flows during use. A frictional resistance acting on a surface region along a flow direction during a flow around the object in the fluid is calculated and added up to a cumulative frictional resistance over a length of the surface region in the flow direction, after which the riblets are provided on and/or in a partial region of the surface in which an increase in the cumulative frictional resistance is at least 0.9, in particular greater than 1.0 to 0.9. Moreover, a method is provided for modifying a surface of an object around which a fluid flows during use, such as a foil. A structure having riblets is created on and/or in the surface, which structure reduces flow resistance of the object. Furthermore, a fluid flows around an object during use.
Claims
1. A method for producing an object provided with riblets on and/or in the surface, around which object a fluid flows during use, wherein a frictional resistance acting on a surface region along a flow direction during a flow around the object in a fluid is calculated and added up to a cumulative frictional resistance over a length of the surface region in the flow direction, after which the riblets are provided on and/or in a partial region of the surface in which an increase in the cumulative frictional resistance is at least 0.9, in particular greater than 1.0 to 0.9, wherein the cumulative frictional resistance is stated in % in relation to the total frictional resistance of 100% and the length is stated in % in relation to the total length.
2. The method according to claim 1, wherein the frictional resistance acting along the flow direction is calculated at certain points.
3. The method according to claim 1, wherein the frictional resistance acting along the flow direction is calculated at multiple points in a plane transverse to the flow direction, particularly points lying along a straight line in a plane transverse to the flow direction, after which the cumulative frictional resistance is added up from the frictional resistances calculated in such a manner.
4. The method according to claim 1, wherein a region of the surface beginning downstream remains disregarded in the calculation.
5. The method according to claim 4, wherein the region of the surface that begins downstream accounts for maximally 20%, preferably maximally 15%, in particular maximally 10%, of the total area of the surface.
6. The method according to claim 1, wherein the riblets are provided in a range of 20% to 90%, preferably 25% to 85%, in particular 30% to 80%, of the longitudinal extension of the surface in the flow direction.
7. The method according to claim 1, wherein the riblets are provided only in a partial region of the surface.
8. The method according to claim 1, wherein the riblets are provided directly or indirectly on a foil.
9. The method according to claim 1, wherein a, in particular bendable, film is used as an object.
10. The method according to claim 1, wherein a foil is used as an object.
11. An object, obtainable according to claim 1, wherein the riblets are provided only in the partial region of the surface.
12. A method for modifying a surface of an object around which a fluid flows during use, such as a foil, wherein a structure having riblets is created on and/or in the surface, which structure reduces a flow resistance of the object, wherein, during the creation of the structure with riblets Can and/or in the surface, regions having a higher wall shear stress on the profile when there is a flow around said profile are provided with riblets, and regions having a lower wall shear stress on the profile are embodied without riblets, Wherein the wall shear stress is placed in relation to a cumulative frictional resistance, and wherein riblets are provided on and/or in a partial region of the surface in which an increase in a cumulative frictional resistance is at least 0.9, in particular greater than 1.0 to 0.9, wherein a frictional resistance acting on a surface region along a flow direction during a flow around the object in a fluid is calculated and added up to the cumulative frictional resistance over a length of the surface region in the flow direction, wherein the cumulative frictional resistance is stated in °/0″ in relation to the total frictional resistance of 100% and the length is stated in % in relation to the total length.
13. The method according to claim 12, wherein the structure with riblets is created on an upper side of the profile, in particular a suction side of an airfoil for an aircraft.
14. The method according to claim 12, wherein the structure with riblets is attached to the surface of the object as a film.
15. The method according to claim 14, wherein the film is bonded to the surface.
16. (canceled)
Description
[0035] Furthermore, the invention is explained below in greater detail with the aid of exemplary embodiments. In the drawings which are thereby referenced:
[0036] FIG. 1a shows a schematic illustration of an airfoil in cross section;
[0037] FIG. 1b shows a schematic illustration of an airfoil in cross section with a natural transition of the flowing air, or without measures;
[0038] FIG. 2 shows a schematic illustration of flow conditions for an airfoil according to FIG. 1a when a wire is affixed;
[0039] FIG. 3 shows a schematic illustration of the flow conditions when a rough surface is affixed on the front side on an airfoil according to FIG. 1a;
[0040] FIG. 4 shows simulation results for a wall friction (left) as well as a velocity distribution (right) for an airfoil according to FIG. 1a with an angle of attack of 8′;
[0041] FIG. 5 through FIG. 8 show simulation results for a smooth surface of an airfoil according to FIG. 1a with different angles of attack;
[0042] FIG. 9 shows wall friction distributions on a suction side of the airfoil according to FIG. 1a with different angles of attack;
[0043] FIG. 10 shows a diagram concerning wind tunnel results when a wire is affixed according to FIG. 2, for different riblet structures;
[0044] FIG. 11 shows an affixing of riblets in different regions for the diagram according to FIG. 10;
[0045] FIG. 12 shows a diagram concerning wind tunnel results when a rough surface is affixed according to FIG. 3, for different riblet structures;
[0046] FIG. 13 shows an affixing driblets in different regions for the diagram according to FIG. 12;
[0047] FIG. 14 shows a diagram concerning wind tunnel results for a natural transition according to FIG. 1b;
[0048] FIG. 15 shows an affixing driblets in different regions for the diagram according to FIG. 14;
[0049] FIG. 16 shows a diagram concerning a total frictional resistance over a length of the airfoil according to FIG. 1a.
[0050] In FIG. 1a, an airfoil is illustrated in cross section. The airfoil is an asymmetrical profile. Visible in FIG. 1a are the upper suction side and the lower pressure side of the airfoil. A corresponding airfoil was used for wind tunnel tests. In practice, a transition from a laminar to a turbulent flow is present on the suction side in the range between approximately 0.4 and 0.6 on the X-axis. For the lower or pressure side, the corresponding region lies roughly in the range from 0.7 to 0.9 on the X-axis. This is illustrated by way of example in FIG. 1b.
[0051] In FIG. 2 and FIG. 3, flow conditions on an airfoil according to FIG. 1a are illustrated, when disruption points are deliberately installed in order to produce a turbulent flow on the airfoil. The situation in FIG. 2, for example, simulates real conditions or transitions in which a boundary layer transition from laminar to turbulent occurs (cf. FIG. 1a). In FIG. 2, what are referred to as trip wires are affixed to the airfoil. The trip wires are indicated by an arrow. In wind tunnel tests, these trip wires result in a turbulent flow occurring on the airfoil in the regions after the trip wires. As illustrated, a trip wire can thereby be arranged both on the suction side and also on the pressure side. In both cases, it is found that the flow after the trip wire (that is, when there is a flow into the airfoil from the front in the flow direction) is turbulent. Before the trip wire, a laminar flow is present, as is also illustrated in FIG. 2.
[0052] In FIG. 3, a disruption point is likewise affixed to the airfoil, in this case, however, this is not a linear disruption point as with a trip wire, but rather a front-side region that is provided with a certain roughness. This results, when there is an inflow from the front, that is, from the left side in FIG. 3, in the presence of a turbulent flow on the airfoil along the flow direction after the disruption region with increased roughness, which region is again indicated by an arrow. A corresponding roughness can, for example, be achieved by means of an adhesive tape that has a certain roughness on the surface.
[0053] Tests were conducted in a wind tunnel with corresponding airfoils which, as will be explained below, have been modified with riblets. The testing line was 2000 mm wide, 1460 mm high, and 3200 mm long. The measuring table was positioned 1,500 mm from the leading edge.
[0054] An ambient pressure was 97000 Pa for the tests in the wind tunnel. The ambient temperature was 28° C. An with a density of 1.122 kg/m.sup.3 was used as fluid. The dynamic viscosity vas 18.62.Math.10.sup.−6 Pas.
[0055] FIG. 4 shows simulation results for a smooth surface (without riblets) of an airfoil according to to FIG. 1a and FIG. 1b. In FIG. 4, the velocity distribution is shown on the left. In the region of the suction side, and there in the initial region of the airfoil, the velocity is the greatest and reaches up to approximately 60 m/s. On the right side in FIG. 4, the wall friction is depicted, with the wall shear stress on the suction side being illustrated. The corresponding simulation relates to a scenario in which the airfoil has an angle of attack of 8°.
[0056] In FIG. 5 through FIG. 8, simulation results are also depicted for a smooth surface of the same airfoil. The wall shear stress is once again indicated, which, as can be seen, depends on the angle of attack. It can likewise be seen, by reference to the succession of angles of attack from 6° (FIG. 5) up to 12° (FIG. 8) in 2° increments, that a region with particularly high wall shear stress inherently always appears, in which regions riblets could have the greatest effect. In the illustrations in FIG. 5 through FIG. 8, which have been produced in black and white, these regions correspond to those which are indicated by arrows and, in a color illustration with a scale from blue to red for low to high wall shear stress, would appear in red. The corresponding points are indicated by an arrow.
[0057] FIG. 9, wall friction distributions are illustrated, namely for the suction side of the airfoil according to FIG. 1a and individual simulations according to FIG. 5 through FIG. 8. As can be seen, there are in all cases regions in which the wall shear stress, in relative terms, is particularly high, or in which a high wall friction exists. As the angle of attack increases, a starting point of the regions of turbulent flow moves forward, or towards a smaller longitudinal extension. Larger angles of attack thus result in greater wall friction.
[0058] If a trip wire is affixed, as is illustrated in FIG. 2, attaching riblets results in an improvement in the flow in the sense of a reduction in turbulent flow.
[0059] On the basis of these simulations, practical tests were conducted using profiles with and without disruption points or disruption regions according to FIG. 2 and FIG. 3. In FIG. 10, corresponding measurement results for an adapted surface according to FIG. 2 are illustrated. In FIG. 10, it can be seen that the efficiency, and therefore the ratio of lift to drag (due to the affixing of riblets) on the suction side of the airfoil according to FIG. 1a increases compared to a smooth surface. If the riblets are attached as illustrated in FIG. 11, that is, over different lengths of the airfoil, it becomes apparent that the results with riblets V_s3 according to FIG. 10 are nearly as good as the riblets V_s3. In other words: Even if the riblets V_s3 are only affixed in a much smaller partial region, namely in a range of approximately 0.2 to 0.6 of the total longitudinal extension of the airfoil (20% to 60%), a virtually equally good effect results. V_r1, an arrangement with trip wire, but without riblets, serves as a reference; due to the relative calculation (c.sub.eff=lift force/resistance force) and Δc.sub.eff=−100+00.Math.c.sub.eff_riblets/ceff_smooth), a zero line results for this reference.
[0060] In FIG. 12 and FIG. 13, analogous test results are illustrated for the arrangement according to FIG. 3, that is, the situation of a subsequent affixing of a predetermined roughness on a front side of the airfoil, into which front side a flow occurs during use and which side is located in the frontmost position in a downstream direction. In this case, it is also apparent that the resistance can be reduced by riblets, wherein the riblets V_r3 are sufficient to achieve the necessary reduction of the frictional resistance in ample regions. Taken overall, the riblets V_r2 are somewhat superior, but they also require much greater proportional coverage of the airfoil surface, and thus a significantly greater material cost. Whereas the riblets V_r3 are only arranged in a partial region (in relation to a longitudinal extension) of approximately 50%, the riblets V_r2 are arranged over nearly 90% of a chord length. V_r1 represents, like V_r1 in FIG. 10, the reference, namely the zero line, which in this case is an airfoil according to FIG. 1a equipped with the roughness on which the experiment is based, but without riblets.
[0061] In FIG. 14 and FIG. 15, corresponding results for a natural transition, that is, real conditions, are shown. As can be seen, marked increases in efficiency result even under real conditions when riblets are arranged merely in certain zones or regions. Only with very small angles of attack is there a slight loss; but as soon as angles of attack that are relevant in practice are worked with, the advantages of riblets become apparent, wherein a coverage in certain zones is already sufficient to achieve a large increase in efficiency.
[0062] The corresponding results can be depicted very clearly, in particular on the basis of FIG. 16. As is illustrated in FIG. 16, the total frictional resistance along a longitudinal axis of an airfoil ultimately adds up to 100%. In those regions in which the frictional resistance is particularly large, a slope of >1 results for the corresponding curve of the cumulative frictional resistance. Precisely in those regions is it necessary to affix the riblets in order to achieve a greatest possible effect of the same. This can preferably take place in a continuous region until the slope reaches 0.9. Under this slope (that is, with a flat curve for the cumulative frictional resistance) it is no longer necessary to provide riblets.
[0063] Though the affixing of riblets in certain regions or zones does not necessarily need to be continuous, it is nevertheless preferred that a partial area of an airfoil or of another object is continuously covered with riblets. In the remaining regions, in which the conditions for a high effectiveness of the riblets are not present, there is an omission, or no riblets are provided. Thus, with an airfoil according to FIG. 1a, it can occur in practice that a front-side region and back-side region are embodied free of riblets, whereas riblets are provided in a middle region, namely in a continuous manner. In this case, continuous does not refer to the actual structure of the riblets, but rather to the planar region that is covered with riblets.
[0064] A method according to the invention and a correspondingly produced object are characterized in that there is a largest possible increase in efficiency with a minimized expenditure. Through a full-area coverage with riblets, it could be possible to achieve an even greater increase in efficiency, but this is counteracted by considerably larger production expenditures, in particular a significantly greater quantity of riblets or riblet structures and the necessary production thereof.
