WIND TURBINE BLADE WITH DAMPENING OF EDGEWISE VIBRATIONS

Abstract

The present disclosure relates to dampening of predominantly edgewise vibrations in a wind turbine blade. This is achieved by a wind turbine blade comprising one or more bump airfoil sections, each bump airfoil section being characterised in that for any airfoil within the bump airfoil section, the airfoil's pressure side profile y.sub.p has particular geometric properties near the trailing edge of the airfoil. Furthermore, a total length of all bump airfoil sections in the blade is at most 30% of the length of the blade, and at least half of the total length of all bump airfoil sections in the wind turbine blade is provided by one or more bump airfoil sections located spanwise in the outermost 30% of the blade.

Claims

1. A wind turbine blade having a suction side and a pressure side connected at a leading edge of the wind turbine blade and at a trailing edge of the wind turbine blade, the blade extending along a longitudinal axis from a root end of the wind turbine blade at =0 to a tip end of the wind turbine blade at =L, where L is a length of the blade, the blade being characterised in that: (i) the blade comprises one or more bump airfoil sections, each bump airfoil section being characterised in that for any airfoil within the bump airfoil section, the airfoil's pressure side profile y.sub.p(x) has the following geometric properties near the trailing edge of the airfoil, x being an axis defined by a chord of the airfoil, where the chord has a length c and x=0 coincides with a leading edge of the airfoil and x.sub.e=c coincides with a trailing edge of the airfoil: at a point x=x.sub.1, where x.sub.10.6c, the pressure side profile y.sub.p(x) has a slope s.sub.1 and y.sub.p(x.sub.1)>y.sub.s(x.sub.1), where y.sub.s(x) is a suction side profile of the airfoil, there is a point x=x.sub.2, x.sub.2>x.sub.1, where a slope s.sub.2 of the pressure side profile y.sub.p(x) is higher than the slope s.sub.1 at x=x.sub.1, there is a point x=x.sub.3, x.sub.3>x.sub.2, where a slope s.sub.3 of the pressure side profile y.sub.p(x) is lower than the slope s.sub.2 at x=x.sub.2, (ii) a total length, L.sub.B, of all bump airfoil sections in the blade is at most 30% of the length of the blade, L, and (iii) at least half of the total length of all bump airfoil sections in the blade is located within the range =0.7L to =L.

2. A wind turbine blade in accordance with claim 1, wherein x.sub.3 fulfills x.sub.30.97c in at least a part of the one or more bump airfoil sections.

3. A wind turbine blade in accordance with claim 1, wherein x.sub.1 fulfills x.sub.10.8c in at least a part of the one or more bump airfoil sections.

4. A wind turbine blade in accordance with claim 1, wherein x.sub.1 fulfills x.sub.10.7c in at least a part of the one or more bump airfoil sections, and x.sub.2 fulfills x.sub.2>0.8c in at least a part of the one or more bump airfoil sections.

5. A wind turbine blade in accordance with claim 1, there is a point x.sub.2 where s.sub.2 fulfills s.sub.2>0.05 in at least a part of the one or more bump airfoil sections.

6. A wind turbine blade in accordance with claim 1, there is a point x.sub.2 where s.sub.2 fulfills s.sub.2>0.2 in at least a part of the one or more bump airfoil sections.

7. A wind turbine blade in accordance with claim 1, there is a point x.sub.3 where s.sub.3 fulfills s.sub.3<0.05, such as s.sub.3<0.3, in at least a part of the one or more bump airfoil sections.

8. A wind turbine blade in accordance with claim 1, wherein within the range x>0.7c to x=c: a highest slope of the pressure side profile y.sub.p(x) is in the range 0.05 to 0.5, and a lowest slope of the pressure side profile y.sub.p(x) is in the range 0.5 to 0.1.

9. A wind turbine blade in accordance with claim 8, wherein within the range x>0.7c to x=c: a highest slope of the pressure side profile y.sub.p(x) is in the range 0.18 to 0.31, and a lowest slope of the pressure side profile y.sub.p(x) is in the range 0.39 to 0.26.

10. A wind turbine blade in accordance with claim 1, wherein a thickness of any airfoil within a bump airfoil section at x=0.95c is in the range 1.5% to 5% of the chord length c.

11. A wind turbine blade in accordance with claim 1, wherein a camber line of any airfoil within a bump airfoil section has an inflection point within the range x=0.8c to x=0.9c.

12. A wind turbine blade in accordance with claim 11, wherein the slope of the camber line reaches half the maximum value of the slope found in the range 0.8c to 0.9c at a point after the inflection point in the direction of the trailing edge, such as within the range x=0.9c to x=c, such as within the range x=0.9c to x=0.95c.

13. A wind turbine blade in accordance with claim 1, wherein the total length of all bump airfoil sections in the blade is at most 30% of the length of the blade, and wherein at least 80% of the total length of all bump airfoil sections in the blade is located within the range =0.7L to =L.

14. A wind turbine blade in accordance with claim 1, wherein at least a first bump airfoil section of the one or more bump airfoil sections tapers smoothly at a first end of the first bump airfoil section into an airfoil that does not have the geometric properties recited in item (i).

15. A wind turbine blade in accordance with claim 14, wherein the first bump airfoil section tapers smoothly at a second end of the first bump airfoil section opposite the first end of the first bump airfoil section into an airfoil that does not have the geometric properties recited in item (i).

16. A wind turbine blade in accordance with claim 1, wherein a trailing edge enclosing angle .sub.2 is in the range 15-45 degrees in at least a part of the one or more bump airfoil sections, such as in all of the bump airfoil sections.

17. A wind turbine blade in accordance with claim 1, wherein a trailing edge enclosing angle .sub.2 is in the range 20-42 degrees in at least a part of the one or more bump airfoil sections, such as in all of the bump airfoil sections.

18. A wind turbine blade in accordance with claim 1, wherein a trailing edge enclosing angle .sub.2 is in the range 25-40 degrees in at least a part of the one or more bump airfoil sections, such as in all of the bump airfoil sections.

19. A wind turbine blade in accordance with claim 1, wherein at least a part of the one or more bump airfoil sections is obtained by adding a separate element to a wind turbine blade.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The invention is explained in detail below with reference to the embodiments shown in the drawings.

[0077] FIG. 1 is a schematic view of illustrating an exemplary wind turbine.

[0078] FIG. 2 is a schematic view illustrating an exemplary wind turbine blade.

[0079] FIG. 3A is a schematic view of a tip part of a wind turbine blade in accordance with prior art.

[0080] FIG. 3B is a schematic view of a particular airfoil of the tip part shown in FIG. 3A.

[0081] FIG. 4A is a schematic view of a tip part of a wind turbine blade in accordance with an embodiment of the invention.

[0082] FIG. 4B is a schematic view of a particular airfoil of the tip part shown in FIG. 4A.

[0083] FIG. 5 is a schematic view showing an airfoil and various defining characteristics.

[0084] FIG. 6A is a schematic view of a tip part of a wind turbine blade in accordance with an embodiment of the invention.

[0085] FIG. 6B is a schematic view of three particular airfoils along the tip part shown in FIG. 6A.

[0086] FIGS. 7-9 are schematic views of tip parts of wind turbine blades in accordance with various embodiments of the invention.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

[0087] Embodiments of the invention will be described in more detail in the following with reference to the accompanying drawings. Similar reference numerals generally refer to similar elements throughout. The drawings show selected ways of implementing the aspects of the present invention and are not to be construed as limiting. Unless otherwise indicated, the drawings are not necessarily drawn to scale. The relative size of the different elements and their shape may have been chosen to make different elements or details clearly discernible.

[0088] FIG. 1 illustrates a conventional modern upwind wind turbine 2 according to the so-called Danish concept with a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub 8 and three blades 10 extending radially from the hub 8, each blade having a blade root 16 nearest the hub and a blade tip 14 with a tip end 15 furthest from the hub 8. The invention is not limited to wind turbines of this type. Arrows 90 illustrates edgewise vibrations. These may occur both when the blades are in motion and during standstill.

[0089] FIG. 2 shows a schematic view of an exemplary wind turbine blade 10. The wind turbine blade 10 has the shape of a conventional wind turbine blade with a root end 17 and a tip end 15 and comprises a root region 30 closest to the hub, a profiled or airfoil region 34, and a transition region 32 between the root region 30 and the airfoil region 34. The blade 10 comprises a leading edge 18 facing the direction of rotation of the blade 10, when the blade is mounted on the hub, and a trailing edge 20 facing the opposite direction of the leading edge 18.

[0090] The airfoil region 34 (also called the profiled region) preferably has an ideal shape with respect to generating hub rotation, whereas the root region 30 due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade 10 to the hub. The diameter of the root region 30 may be constant along the entire root region 30. The transition region 32 present in the wind turbine blade 10 in this example has a transitional profile gradually changing from the circular shape of the root region 30 to the airfoil profile of the airfoil region 34. The chord length of the transition region 32 typically increases in an outward direction from the hub. The airfoil region 34 has an airfoil profile with a chord extending between the leading edge 18 and the trailing edge 20 of the blade 10.

[0091] Different sections of the blade normally do not have a common plane, since the blade may be twisted and/or curved (i.e. pre-bent) along a direction from the root region to the tip, this being most often the case, for instance to more or less compensate for the local velocity of the blade being dependent on the distance from the hub.

[0092] The wind turbine blade 10 comprises a blade shell which may for instance comprise two blade shell parts, a first blade shell part 24 and a second blade shell part 26, for instance made at least partly of fibre-reinforced polymer. The first blade shell part 24 may for instance be part of a pressure side or upwind blade part. The second blade shell part 26 may for instance be part of a suction side or downwind blade part. The first blade shell part 24 and the second blade shell part 26 are typically joined together, such as glued together, along bond lines or glue joints 28 extending along the trailing edge 20 and the leading edge 18 of the blade 10. Typically, the root ends of the blade shell parts 24, 26 have a semi-circular or semi-oval outer cross-sectional shape that, when the first and second shell parts are joined, forms the root region, such as a circular or oval root region.

[0093] FIG. 3A schematically illustrates the outermost part, referred to herein as tip part, of the blade 10 shown in FIG. 2. The term tip part does not imply a certain relative amount of the entire blade 10, but merely indicates that it includes the tip end 15 of the blade. The other, remaining part, called root part herein, of the blade 10 is not shown in FIG. 3A. More particularly, FIG. 3A illustrates the outermost 30% of the blade 10, indicated as 0.3L in FIG. 3A. FIG. 3A shows the leading edge 18, the trailing edge 20, the tip end 15, and the pressure side 24, and indicates the suction side 26, which is not visible in the view in FIG. 3A where it is hidden by the pressure side 24. The tip part of the prior art blade 10 may for instance correspond to a known NACA profile or a known SERI profile. Arrows 90 illustrates edgewise vibrations, as also illustrated in FIG. 1.

[0094] FIG. 3A furthermore indicates a cross-section B-B of the tip part. The airfoil 315 corresponding to cross-section B-B is shown in FIG. 3B. The leading edge 18, trailing edge 20, pressure side 24, and suction side 26 of the airfoil are indicated. As seen in the highlighted area 320, the airfoil thickness in this known airfoil decreases continuously towards the trailing edge 20. In fact, the airfoil thickness decreases in the final 60% from the leading edge 18.

[0095] FIG. 4A schematically illustrates a tip part of a wind turbine blade 400 in accordance with an embodiment of the invention. As in FIG. 3A, the root part of the blade 400 is not shown in FIG. 4A. That part may for instance be identical to a known corresponding part, such as the root part of the blade 10 shown in FIG. 2, or it may be different.

[0096] As in FIG. 3A, FIG. 4A illustrates the outermost 30% of the blade, indicated as 0.3L in FIG. 4A. FIG. 4A also shows the leading edge 18, the trailing edge 20, the tip end 15, and the pressure side 24. FIG. 4A also indicates the suction side 26, which is not visible in the view in FIG. 4A where it is hidden by the pressure side 24. The tip part of the blade 400 differs from the tip part of blade 10 shown in FIG. 3A by the blade element 410 near the trailing edge 20 of the blade 400. In the present example, the tip part of the blade 400 is shown as a modification, or retrofit, of the blade 10, but it may alternatively be formed as an integrated part of the wind turbine blade when manufactured. The blade element 410 modifies the airfoil along the entire corresponding section, turning the corresponding section of the blade into a bump airfoil section. An example of an airfoil 415 located in the bump airfoil section is shown in FIG. 4B, which shows the airfoil corresponding to the cross-section C-C indicated in FIG. 4A. As shown in FIG. 4B, the addition of the blade element 410 results in a bump near the trailing edge 20. The dashed line in FIG. 4B illustrates the corresponding airfoil of the prior art blade 10 at the same longitudinal position as shown in FIGS. 3A and 3B.

[0097] The airfoil 415 is characterised in part as follows: At x.sub.1=0.75c, the pressure side profile slope, s.sub.1, is 0.10; at x.sub.2=0.82c, the pressure side profile slope, s.sub.2, is 0.21; and at x.sub.3=0.97c, the pressure side profile slope, s.sub.3, is 0.48 (the points are not indicated in the drawings). These values fulfil the properties recited in item (i) of the first aspect of the invention, which means that cross-section C-C (shown in FIG. 4A) is an airfoil of a bump airfoil section in accordance with item (i) of the first aspect of the invention. Although both the blade's cross-section and the blade element 410 change in both shape and size along the longitudinal axis of the blade, all cross-sections, i.e. airfoils, within the region comprising the blade element 410 have the geometric properties recited in item (i) of the first aspect of the invention. Thus, the entire section containing the blade element 410 is a bump airfoil section in the sense of the first aspect of the invention. All values are approximate.

[0098] In line with item (ii) of the first aspect of the invention, the bump airfoil section in FIG. 4A has a length of at most 30% of the entire length of the blade. The bump airfoil section in FIG. 4A has a length which is approximately 23% of the entire length of the blade 400.

[0099] In line with item (iii) of the first aspect of the invention, the bump airfoil section, i.e. the section comprising the blade element 410, is located in the range z=0.7L to z=L, i.e. within the outer 30% of the blade. The shape and location of the bump airfoil section cooperate to provide dampening of edgewise vibrations without adding excessive weight and drag.

[0100] The modified blade 400 shown in FIG. 4B is much less susceptible to the vibrations 90 shown in FIG. 3A for the prior art blade 10.

[0101] It is seen from the properties above that the tip part 400 shown in FIG. 4A belongs to an embodiment of the first aspect of the invention (the root part is not shown).

[0102] In addition, the tip part 400 shown in FIG. 4A has all the characteristics of the second aspect of the invention. Items (ii) and (iii) are the same as in the first aspect of the invention and are therefore met as described above. The pressure side profile y.sub.p(x) of the airfoil 415 has an inflection point at x.sub.i=0.86c and therefore has the geometric properties of item (i) of the second aspect of the invention, which requires that the pressure side profile y.sub.p(x) of the airfoil has an inflection point in the range x=0.7c to x=0.96c. Although both the blade's cross-section and the blade element 410 change in both shape and size along the longitudinal axis of the blade, all cross-sections, i.e. airfoils, within the region comprising the blade element 410 have the geometric properties recited in item (i) of the second aspect of the invention. Thus, the entire section containing the blade element 410 is a bump airfoil section in the sense of the second aspect of the invention.

[0103] It is thus seen from the properties above that the tip part 400 shown in FIG. 4A belongs to an embodiment of the second aspect of the invention.

[0104] In addition, the tip part 400 shown in FIG. 4A has all the characteristics of the third aspect of the invention. Items (ii) and (iii) are the same as in the first aspect of the invention and are therefore met as described above. The pressure side profile y.sub.p(x) of the airfoil 415 in FIG. 4B has a first local extremum (a local minimum) at x=0.80c with a value of 0.005c and a second local extremum (a local maximum) at x=0.925c with a value of 0.046c. All values are approximate. Although both the blade's cross-section and the blade element 410 change in both shape and size along the longitudinal axis of the blade, all cross-sections, i.e. airfoils, within the region comprising the blade element 410 have the geometric properties recited in item (i) of the third aspect of the invention. Thus, the entire section containing the blade element 410 is a bump airfoil section in the sense of the third aspect of the invention.

[0105] It is thus seen from the properties above that the tip part 400 shown in FIG. 4A belongs to an embodiment of the third aspect of the invention.

[0106] On the other hand, the prior art airfoil shown in FIG. 3B does not have the geometric properties recited in item (i) of any of the three aspects of the invention, let alone items (ii) and (iii).

[0107] A wind turbine blade having the tip part shown in FIG. 4A produces less edgewise vibrations, increasing fatigue characteristics, while retaining acceptable noise characteristics. Furthermore, the slope of the lift coefficient for the blade in FIG. 4A is less steep compared to the blade 10 in FIG. 3A, being the main driver for the desired change in aeroelastic characteristics and the resulting avoidance of instabilities and resulting vibrations respectively oscillations.

[0108] FIG. 5, detail 530, also illustrates the enclosing angle .sub.2 of the trailing edge 20. An increase in the enclosing angle between the pressure side and the suction side at the trailing edge can reduce the slope of the lift coefficient, which in cooperation with the features of the first aspect of the invention (and with the second and the third aspects) can be used to tailor stability characteristics of wind blades in operation. The enclosing angle in the airfoil in FIGS. 4B and 5 is approximately 48 degrees. (Note that the axes in for instance FIGS. 4B and 5 are arbitrary, and the figures therefore do not readily reflect the correct enclosing angle.)

[0109] FIG. 6A shows the tip part of another wind turbine blade in accordance with an embodiment of the invention. A blade element 610 providing a bump is located within 30% from the trailing edge, i.e. in the region x0.7c. In this embodiment, the bump is tapered towards the root end of the wind turbine blade, into an airfoil that does not have the geometric properties recited in item (i). This may for instance be a taper into the airfoil at cross-section B-B, which in the present example is identical to airfoil 315 shown in FIG. 3B. Such tapering strongly reduces the noise impact of the blade element 610. The other end of the blade element 610 is not illustrated as being tapered. To further reduce noise, this end may be tapered as well. As in FIG. 4A, the bump in FIG. 6A is illustrated as an add-on to an existing blade, such as blade 10 shown in FIG. 3A.

[0110] In addition to being situated with 30% of the airfoil from the trailing edge, the bump is arranged in the outermost 30% of the blade, similar to the blade element 410 illustrated in FIG. 4A. This is where the bump airfoil section can effectively suppress edgewise vibrations. The length of the bump airfoil section created by the blade element 610 is approximately 18% of the length of the blade.

[0111] It is noted that the bumps shown in the figures illustrate various embodiments of the invention and must not be construed as representing optimal solutions for the blades shown in the figures.

[0112] FIG. 6B shows the airfoils 617, 616, 615 corresponding, respectively, to cross-sections B-B, D-D, and E-E indicated in FIG. 6A. The cross-section B-B is identical to that indicated in FIG. 3A, which shows the tip part of a prior art blade. Thus, as in FIG. 3A, the corresponding airfoil 617 does not have the geometric properties recited in items (i) of any of the aspects of the present invention and therefore is not an airfoil of a bump airfoil section. On the other hand, the airfoil 616 at cross-section D-D meets the features in item (i) of the first aspect of the invention (as well as the second and third aspects). In other words, at cross-section D-D, the bump has the property of a bump airfoil section in the sense of item (i). As such, it contributes to effectively suppress vibrations in the tip part.

[0113] Furthermore, the airfoil 615 at cross-section E-E also has the geometric properties recited in item (i) of the first aspect of the invention (as well as the second and third aspects). Similar to the bump at cross-section D-D, the bump at cross-section E-E has the property of a bump airfoil section in the sense of item (i). Like the bump created by blade element 610 at cross-section D-D, the bump at cross-section E-E therefore also contributes to suppress vibrations in the tip part.

[0114] The airfoils 615, 616, 617 in FIG. 6B also illustrate that the blade element 610 results in a smoothly tapered bump airfoil section. Starting for instance from airfoil 615, corresponding to cross-section E-E, airfoil 616, corresponding to cross-section D-D, has a less pronounced bump, and finally at cross-section 617, corresponding to cross-section B-B, there is no longer a bumpthe airfoil 617 does not have the geometric properties recited in item (i).

[0115] FIG. 7 shows the tip part 700 of another wind turbine blade in accordance with an embodiment of the invention. A blade element 710 providing a bump is located within 30% from the trailing edge, i.e. in the region x0.7c. In this embodiment, the blade element 710 is tapered towards the tip. Although not shown separately, all airfoils along the blade element 710 all have the geometric properties recited in item (i), and the blade section containing the blade element 710 is therefore a bump airfoil section and provides suppression of edgewise vibrations.

[0116] In the embodiment in FIG. 7, the blade element is arranged in the outermost 10% of the blade. In some cases, this may be enough to sufficiently suppress edgewise directions. The shorter bump means less added weight compared to a similar blade without a bump airfoil section. However, bump airfoil sections must have a total length that sufficiently suppresses edgewise vibrations in a blade. This depends on the properties such as the geometry of the blade, its lengths, and its weight. It is noted that for a given blade, edgewise vibrations may be (and often is) a problem only in certain operating conditions. Thus, what constitutes sufficient suppression depends on a number of factors, including not only the blade's properties, but also factors such as the local wind conditions at the site at which the blade is in operation.

[0117] FIG. 8 shows the tip part 800 of another wind turbine blade in accordance with an embodiment of the invention, quite similar to the embodiment in FIG. 7.

[0118] A blade element 810 providing a bump is located within 30% from the trailing edge, i.e. in the region x0.7c. Although not shown separately, all airfoils along the blade element 810 all have the geometric properties recited in item (i), and the blade section containing the blade element 810 is therefore a bump airfoil section and provides suppression of edgewise vibrations. Furthermore, the blade element 810 in this example is tapered towards the tip end of the blade. This reduces the amount of noise generated by the bump airfoil section.

[0119] FIG. 9 illustrates a tip part 900 of another wind turbine blade in accordance with an embodiment of the invention. The embodiment is quite similar to the embodiment in FIG. 4A. However, the embodiment in FIG. 9 contains two separate bump airfoil sections by virtue of blade elements 910 and 911. The blade section between the two blade elements 910 and 911 does not have airfoils with the geometric properties in item (i) of any of the three aspects.

[0120] Various ways of carrying out the invention, in addition to those described above and in the claims, are defined by the following items: [0121] 1. A wind turbine blade having a suction side and a pressure side connected at a leading edge of the wind turbine blade and at a trailing edge of the wind turbine blade, the blade extending along a longitudinal axis z from a root end of the wind turbine blade at z=0 to a tip end of the wind turbine blade at z=L, where L is a length of the blade, the blade being characterised in that: [0122] (i) the blade comprises one or more bump airfoil sections, each bump airfoil section being characterised in that for any airfoil within the bump airfoil section, the airfoil's pressure side profile y.sub.p(x) has the following geometric properties near the trailing edge of the airfoil, x being an axis defined by a chord of the airfoil, where the chord has a length c and x=0 coincides with a leading edge of the airfoil and xc=c coincides with a trailing edge of the airfoil: [0123] at a point x=x.sub.1, where x.sub.10.6c, the pressure side profile y.sub.p(x) has a slope s.sub.1 and y.sub.p(x.sub.1)>y.sub.s(x.sub.1), where y.sub.s(x) is a suction side profile of the airfoil, [0124] there is a point x=x.sub.2, x.sub.2>x.sub.1, where a slope s.sub.2 of the pressure side profile y.sub.p(x) is higher than the slope s.sub.1 at x=x.sub.1, [0125] there is a point x=x.sub.3, x.sub.3>x.sub.2, where a slope s.sub.3 of the pressure side profile y.sub.p(x) is lower than the slope s.sub.2 at x=x.sub.2, [0126] (ii) a total length, L.sub.B, of all bump airfoil sections in the blade is at most 30% of the length of the blade, L, and [0127] (iii) at least half of the total length of all bump airfoil sections in the blade is located within the range z=0.7L to z=L. [0128] 2. A wind turbine blade in accordance with item 1, wherein x.sub.3 fulfills x.sub.30.97c in at least a part of the one or more bump airfoil sections. [0129] 3. A wind turbine blade in accordance with any of items 1-2, wherein x.sub.1 fulfills x.sub.10.8c in at least a part of the one or more bump airfoil sections. [0130] 4. A wind turbine blade in accordance with any of items 1-2, wherein x.sub.1 fulfills x.sub.10.7c in at least a part of the one or more bump airfoil sections, and x.sub.2 fulfills x.sub.2>0.8c in at least a part of the one or more bump airfoil sections. [0131] 5. A wind turbine blade in accordance with any of the preceding items, there is a point x.sub.2 where s.sub.2 fulfills s.sub.2>0.05 in at least a part of the one or more bump airfoil sections. [0132] 6. A wind turbine blade in accordance with any of the preceding items, there is a point x.sub.2 where s.sub.2 fulfills s.sub.2>0.2 in at least a part of the one or more bump airfoil sections. [0133] 7. A wind turbine blade in accordance with any of the preceding items, there is a point x.sub.3 where s.sub.3 fulfills s.sub.3<0.05, such as s.sub.3<0.3, in at least a part of the one or more bump airfoil sections. [0134] 8. A wind turbine blade in accordance with any of items 1-7, wherein within the range x>0.7c to x=c: [0135] a highest slope of the pressure side profile y.sub.p(x) is in the range 0.05 to 0.5, and [0136] a lowest slope of the pressure side profile y.sub.p(x) is in the range 0.5 to 0.1. [0137] 9. A wind turbine blade in accordance with item 8, wherein within the range x>0.7c to x=c: [0138] a highest slope of the pressure side profile y.sub.p(x) is in the range 0.18 to 0.31, and [0139] a lowest slope of the pressure side profile y.sub.p(x) is in the range 0.39 to 0.26. [0140] 10. A wind turbine blade in accordance with any of items 1-9, wherein the total length of all bump airfoil sections in the blade is at most 30% of the length of the blade, and wherein at least 80% of the total length of all bump airfoil sections in the blade is located within the range z=0.7L to z=L. [0141] 11. A wind turbine blade in accordance with any of items 1-10, wherein at least a first bump airfoil section of the one or more bump airfoil sections tapers smoothly at a first end of the first bump airfoil section into an airfoil that does not have the geometric properties recited in item (i). [0142] 12. A wind turbine blade in accordance with item 11, wherein the first bump airfoil section tapers smoothly at a second end of the first bump airfoil section opposite the first end of the first bump airfoil section into an airfoil that does not have the geometric properties recited in item (i). [0143] 13. A wind turbine blade in accordance with any of the preceding items, wherein a trailing edge enclosing angle .sub.2 is in the range 15-45 degrees in at least a part of the one or more bump airfoil sections. [0144] 14. A wind turbine blade in accordance with any of the preceding items, wherein a trailing edge enclosing angle .sub.2 is in the range 20-42 degrees in at least a part of the one or more bump airfoil sections. [0145] 15. A wind turbine blade in accordance with any of the preceding items, wherein at least a part of the one or more bump airfoil sections is obtained by adding a separate element to a wind turbine blade.

LIST OF REFERENCES

[0146] 2 wind turbine [0147] 4 tower [0148] 6 nacelle [0149] 8 hub [0150] 10 blade [0151] 14 blade tip [0152] 15 tip end [0153] 16 blade root [0154] 17 root end [0155] 18 leading edge [0156] 20 trailing edge [0157] 24 first blade shell part (pressure side) [0158] 26 second blade shell part (suction side) [0159] 28 bond lines/glue joints [0160] 30 root region [0161] 32 transition region [0162] 34 airfoil region [0163] 90 edgewise vibrations [0164] 300 tip part of blade [0165] 315 airfoil [0166] 320 trailing edge detail [0167] 400 tip part of blade [0168] 410 trailing edge bump [0169] 415 airfoil [0170] 501 pressure side profile point; bump minimum [0171] 503 pressure side profile point; bump inflection point [0172] 504 pressure side profile point; bump maximum [0173] 505 pressure side profile point [0174] 520 chord [0175] 525 trailing edge bump detail [0176] 530 enclosing angle detail [0177] 600 tip part of blade [0178] 610 blade element providing a bump [0179] 615, 616, 617 airfoil [0180] 700 tip part of blade [0181] 710 blade element providing a bump [0182] 800 tip part of blade [0183] 810 blade element providing a bump [0184] 910, 911 blade element providing a bump [0185] c chord length [0186] L longitudinal axis of blade [0187] .sub.2 enclosing angle of trailing edge [0188] x coordinate axis defined by an airfoil chord [0189] y Cartesian coordinate axis corresponding to an x axis defined by an airfoil chord [0190] y.sub.p(x) pressure side profile [0191] y.sub.s(x) suction side profile