Wind turbine blade with vortex generator

Abstract

The present disclosure relates to a wind turbine blade provided with one or more vortex generators at respective one or more locations on the surface of the blade. Each vortex generator comprises a fin with a leading edge and a trailing edge, the leading edge of the fin being closer to the leading edge of the wind turbine blade than the trailing edge of the fin, and wherein the leading edge of the fin extends from a bottom portion proximal to the surface of the blade to a top portion distal from the surface of the blade, the leading edge of the fin being configured such that at least a portion of the leading edge of the fin between the bottom portion and the top portion is located closer to the leading edge of the wind turbine blade than the bottom portion of the fin.

Description

[0001] Title

[0002] Wind turbine blade with vortex generator

[0003] Technical field

[0004] The present invention relates to a wind turbine blade provided with vortex generators as well as vortex generators for wind turbine blades.

[0005] Background of the invention

[0006] In wind turbine blade aerodynamics, boundary layer separation occurs when an airfoil profile experiences a relatively high angle of attack, which can cause a separation of attached airflow from the suction side of the airfoil. Such boundary layer separation results in a reduction in lift generated by the airfoil.

[0007] Vortex generators are used on wind turbine blades in order to induce turbulent airflow vortices as the aerodynamic profile of the blade is impacted upon by an incident airflow. The creation of these vortices acts to delay separation of the attached airflow from the aerodynamic profile, thereby improving blade performance for a wide range of angles of attack.

[0008] However, there is a continued need for improving the efficiency of vortex generators, in particular to create stable vortices in order to lower the induced drag.

[0009] Summary of the invention

[0010] A first aspect of the invention provides a wind turbine blade having a leading edge and a trailing edge, wherein the wind turbine blade is provided with one or more vortex generators at respective one or more locations on the surface of the blade on a surface of the wind turbine blade, wherein each vortex generator comprises a fin having a leading edge and a trailing edge, the leading edge of the fin being closer to the leading edge of the wind turbine blade than the trailing edge of the fin, and wherein the leading edge of the fin extends from a bottom portion proximal to the surface of the blade to a top portion distal from the surface of the blade, the leading edge of the fin being configured such that at least a portion of the leading edge of the fin between the bottom portion and the top portion is located closer to the leading edge of the wind turbine blade than the bottom portion of the fin. In other words, the leading edge of the fin is inclined forwards, towards the leading edge of the wind turbine blade, such that at least a portion of the leading edge of the fin is arranged with a spacing to the surface of the wind turbine blade. This means that the leading edge of the fin is configured such that at least a portion of the leading edge of the fin between the bottom portion and the top portion is located closer to the leading edge of the wind turbine blade than any part of the bottom portion of the fin.

[0011] Thereby, incoming airflow (during operation of the wind turbine blade) from the leading edge towards the trailing edge of the blade can flow past the upper edge of the fin and below the leading edge of the fin and across the surface of the wind turbine blade. By letting the fin be inclined forwards, into the incoming flow, it has been found that this provides more stable vortices generated by the fins of the vortex generators. Thereby, the boundary layer can be reenergised while the induced drag from the vortex generators is minimised. This in turn improves the overall lift-to-drag ratio of the blade and / or minimises noise.

[0012] In some embodiments, the fin is arranged at an angle acrelative to a local chord of the wind turbine blade at the location of the vortex generator. Preferably, the angle acis at least 6 degrees, and more preferably at least 8 or 9 degrees. Preferably, the angle acis less than 15 degrees.

[0013] Alternatively, the angle acis measured relative to a first normal that is normal to the leading edge of the blade, wherein the first normal is a normal that intersects the fin. The chord at the location of the vortex generator and a normal to the leading edge of the blade that intersects the fin are usually within a few degrees of each other. The angles cited in the previous paragraph apply also in embodiments where the angle acis measured relative to the blade leading edge normal that intersects the fin.

[0014] By angling the vortex generator relative to the chord, the fin can provide stronger vortices.

[0015] In some embodiments, the entire leading edge of the fin or at least substantially the entire leading edge of the fin is located closer to the leading edge of the wind turbine blade than the bottom portion of the fin.

[0016] In other words, the entire leading edge of the fin is inclined forwards substantially its entire length. Preferably, at least 90% of the leading edge of the fin is located nearer the leading edge of the blade than the bottom portion of the fin. In some embodiments, the leading edge of the fin on average forms an average angle az with the tangent plane of the blade surface at the location of the vortex generator. The average angle az is at most 85 degrees, such as at most 60 degrees, such as at most 45 degrees, such as at most 30 degrees, such as in the range 5-30 degrees, such as in the range 10-30 degrees. The leading edge average angle az can be seen as an angle between 1) a straight line from the bottom portion of the leading edge of the fin to the top portion of the leading edge of the fin and 2) a line that is parallel with a tangent plane of the blade surface at the location of the fin. Further, the average angle az is the angle forward of the bottom portion of the leading edge in the direction of the leading edge of the blade. In other words, it is the angle defined by the "opening" that exists between the leading edge of the fin and the surface of the blade at the location of the fin. In alternative formulation, the leading edge is angled forward such that it forms an average angle of at least 5 degrees, preferably at least 15 degrees, such as at least 30 degrees, such as at least 45 degrees, such as 60-85 degrees, such as 60-80 degrees to a surface normal at location of the vortex generator.

[0017] In case the leading edge is not straight, the "angle" changes along the leading edge. The average angle of the leading edge is therefore a measure of the overall angle of the leading edge of the fin.

[0018] In some embodiments, the one or more vortex generators are arranged in pairs with their respective fins being arranged with opposite angles (+ / -ac) relative to the local chord of the wind turbine blade at the location of each pair of vortex generators. In this arrangement, the fins produce counterrotating vortices. Preferably, the angle acis at least 5 degrees relative to the local chord, preferably at least 6 degrees, and more preferably at least 8 or 9 degrees. Preferably, the angle acis less than 15 degrees.

[0019] The pairs of fins are configured to have parameters as defined in the claims of WO 2013 / 014015 or with relative parameters as defined in table 1 of WO 2013 / 014080. The fins may also be arranged in accordance with the counter-propagating configuration explained in "Control of a decelerating boundary layer. Part 1: Optimization of passive vortex generators", by Godard and Stanislav, Aerospace Science and Technology 10 (2006), 181-191, e.g. as defined in Table 5 of this paper.

[0020] In some embodiments, a height Azof each fin is at most 0.05 times a length of a local chord of the wind turbine blade at the location of the vortex generator. The height may even be 2% or less of the local chord length. Alternatively, or in addition thereto, the height of each fin may be between 10% and a 100% of the local boundary layer thickness for a smooth profile at the design point of the wind turbine blade. In some embodiments, a length of the bottom portion of the fin is at most 0.1 times, such as at most 0.05 times, a length of a local chord of the wind turbine blade at the location of the vortex generator.

[0021] In some embodiments, an entire length Z / of the fin is at most 0.1 times a length of a local chord of the wind turbine blade at the location of the vortex generator, such as at most 0.05 times a length of a local chord of the wind turbine blade at the location of the vortex generator.

[0022] In some embodiments, an upper edge of the fin is rounded. This can result in more stable vortices being generated at the upper edge.

[0023] In some embodiments, the one or more vortex generators are provided as at least one of: a single fin extending from a base, a fin pair extending from corresponding two bases, or a fin pair extending from a common base, or a plurality of fins extending from one common base.

[0024] In some embodiments, each of the vortex generators comprises only a base and one or more fins.

[0025] Different geometries lead to different airflow properties, as will be described in more detail in the detailed description. In some embodiments, the leading edge of the fin is straight or at least substantially straight.

[0026] In some embodiments, the trailing edge of the fin is straight or at least substantially straight.

[0027] In some embodiments, the trailing edge of the fin on average forms an average angle rwith the tangent plane of the blade surface at the location of the vortex generator. In some embodiments, the average angle ar is at least 95 degrees, such as at least 100 degrees, such as at least 110 degrees, such as at least 120 degrees. The trailing edge average angle a^can be seen as an angle between 1) a straight line from the bottom portion of the trailing edge of the fin to the top portion of the trailing edge of the fin and 2) a line that is parallel with a tangent plane of the blade surface at the location of the fin. Further, the average angle ar is the angle forward of the bottom portion of the trailing edge in the direction of the trailing edge of the blade. In other words, it is the angle that comprises the fin.

[0028] In some embodiments: the leading edge of the fin on average forms an average angle (az) with the surface of the blade at the location of the vortex generator in the range 10-50 degrees, and the trailing edge of the fin on average forms an average angle («T) with the surface of the blade at the location of the vortex generator less than the average angle (az) of the leading edge of the fin, such as 2-15 degrees less, such as 5-10 degrees less.

[0029] In some embodiments: the leading edge of the fin on average forms an average angle (az) in the range 10-50 degrees with the surface of the blade at the location of the vortex generator, and the trailing edge of the fin on average forms an average angle (az) of at least 95 degrees with the surface of the blade at the location of the vortex generator.

[0030] In some embodiments, a height of the fin along an entire upper edge of the fin monotonically decreases in a direction from the leading edge of the fin to the trailing edge of the fin. This reduces drag while still allowing strong vortex creating along the upper edge. Preferably, an average slope of the upper edge is in the range 5 degrees to 40 degrees.

[0031] In some embodiments, the leading edge of the fin, the trailing edge of the fin, and the bottom portion of the fin form a substantially quadrilateral shape.

[0032] In some embodiments the leading edge of the fin is convex seen from the leading edge of the wind turbine blade and the trailing edge of the fin is convex seen from the trailing edge of the wind turbine blade. That is, the fin has a "bulge" when viewed from its side.

[0033] In some embodiments, the leading edge of the fin is concave seen from the leading edge of the wind turbine blade and wherein the trailing edge of the fin is convex seen from the trailing edge of the wind turbine blade. This geometry is similar to a shark's fin, although oriented in the different direction as compared to the incoming flow.

[0034] In some embodiments, the leading edge of the fin and / or the trailing edge of the fin is corrugated or undulates along the leading edge of the fin.

[0035] In some embodiments, a height hf of the fin the is at most 0.05 times a length of a local chord of the wind turbine blade at the location of the vortex generator, and a length of the bottom portion of the fin is at most 0.1 times a length of a local chord of the wind turbine blade at the location of the vortex generator. In some embodiments, an entire length Z of the fin is at most 0.1 times a length of a local chord of the wind turbine blade at the location of the vortex generator, such as at most 0.05 times a length of a local chord of the wind turbine blade at the location of the vortex generator.

[0036] A second aspect of the invention provides a wind turbine blade having a leading edge and a trailing edge and comprising one or more vortex generators on a surface of the wind turbine blade, each vortex generator comprising a fin having a leading edge and a trailing edge, the leading edge of the fin being closer to the leading edge of the wind turbine blade than the trailing edge of the fin, a first portion of the fin, including at least a part of the leading edge of the fin, is located a distance from the surface of the wind turbine blade.

[0037] Similarly to the first aspect, the vortex generator on the wind turbine blade allows air to flow past the first portion of the fin both at an upper edge of the fin and below the leading edge of the fin in a region between the leading edge of the fin and the wind turbine blade surface. This allows for creation of vortices not only at the upper edge of the fin but also under the leading edge. Known fins do not allow generation of such additional vortices.

[0038] A third aspect of the invention provides a wind turbine blade having a leading edge and a trailing edge and comprising one or more vortex generators on a surface of the wind turbine blade, each vortex generator comprising a fin having a leading edge and a trailing edge, the leading edge of the fin being closer to the leading edge of the wind turbine blade than the trailing edge of the fin, wherein the leading edge has a first part where a distance between the leading edge of the fin and the surface of the wind turbine blade increases in a direction toward the leading edge of the wind turbine blade.

[0039] Similarly to the first and second aspects, the vortex generator on the wind turbine blade allows air to flow past the first portion of the fin both at an upper edge of the fin and below the leading edge of the fin in a region between the leading edge of the fin and the wind turbine blade surface.

[0040] A fourth aspect of the invention provides a vortex generator for a wind turbine blade. The vortex generator has a leading edge and a trailing edge, the vortex generator comprising a fin having a leading edge and a trailing edge, the vortex generator being configured to be arranged on a surface of the wind turbine blade such that the leading edge of the fin is closer to the leading edge of the wind turbine blade than the trailing edge of the fin, and wherein the leading edge of the fin extends from a bottom portion proximal to the surface of the blade to a top portion distal from the surface of the blade, the leading edge of the fin being configured such that at least a portion of the leading edge of the fin between the bottom portion and the top portion is located closer to the leading edge of the wind turbine blade than the bottom portion of the fin. Brief description of the drawings

[0041] The invention is described in detail below with reference to the drawings.

[0042] Fig. 1 is a schematic view illustrating an exemplary wind turbine.

[0043] Fig. 2 is a schematic view illustrating an exemplary wind turbine blade.

[0044] Figs. 3a-3d shows various views of a vortex generator for use in the invention.

[0045] Figs. 4a-4b illustrate wind turbine blades in accordance with the invention.

[0046] Figs. 5a-5f illustrate various embodiments of the invention.

[0047] Detailed description of selected embodiments

[0048] Embodiments of the invention will be described in more detail in the following with reference to the accompanying drawings. Similar reference numbers 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.

[0049] 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.

[0050] 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 and a tip end 15 and comprises a root region 30 closest to the hub, a profiled or airfoil region 34 having an aerodynamic profile, 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.

[0051] 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 or may taper. The transition region 32 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 in 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. The chord length in the airfoil region typically decreases in the direction of the tip end 15.

[0052] 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.

[0053] The wind turbine blade 10 may for instance comprise two blade shell parts, a first blade shell part 36 and a second blade shell part 38, for instance made at least partly of fibre-reinforced polymer. The first blade shell part 36 may for instance be part of a pressure side or upwind blade part. The second blade shell part 38 may for instance be part of a suction side or downwind blade part. The first blade shell part 36 and the second blade shell part 38 are typically joined together, such as glued together, along bond lines or glue joints extending along the trailing edge 20 and the leading edge 18 of the blade 10.

[0054] Figs. 3a-3d shows different views of a vortex generator 40 suitable for use in the present invention. As shown in Figs. 3a-3c, the vortex generator 40 has a fin 43 with a leading edge 41 and a trailing edge 42, the fin having a bottom portion 51 closest to the surface of the wind turbine blade, when the vortex generator is placed thereon, the fin 43 further having a top portion 52 opposite the bottom portion 51. The fin in this example is connected to a base 45 that is suitable for attaching the vortex generator to the surface of a wind turbine blade, for instance near the root region 30, in the transition region 32, at the suction side of the airfoil region 34, or at pressure side of the airfoil region.

[0055] Fig. 3b illustrates the vortex generator seen in the direction from the trailing edge 42 of the fin 43 towards the leading edge 41 of the fin 43. Fig. 3c illustrates the vortex generator seen in the direction from the leading edge 41 of the fin 43 towards the trailing edge 42 of the fin 43, i.e. opposite the view in Fig. 3b. Reference 43 refers to the fin as such but is also used below to refer to the low- pressure side of the fin when positioned at an angle with respect to the local chord of the wind turbine blade at the location of the fin.

[0056] Fig. 3d illustrates the vortex generator in a top view. The vortex generator is arranged on a surface

[0057] 46 of a wind turbine blade. As shown in Fig. 3a and 3d, the fin 43 extends forward and into the airflow, with a space between the front-most part of the fin 43 and the surface 46 of the blade. At least a portion of the leading edge 41 of the fin 43 between the bottom portion 51 and the top portion 52 of the fin 43 is located closer to the leading edge 18 of the wind turbine blade than any part of the bottom portion 51 of the fin 43. This is in contrast to known vortex generators. In particular, the present invention provides a wind turbine blade with a vortex generator having a fin arranged such that incoming air may flow not only over the upper edge of the fin 43, but also under the leading edge 41 of the fin 43. When arranged at an angle with respect to the airflow (this angle changes dynamically with wind direction and wind speed), the fin is in a sense forward-swept, having a portion that is located a distance from the surface 46 of the blade in the direction towards the leading edge 18 of the blade. In this configuration, a low pressure is created on one side 43 of the fin relative to the pressure on the opposite side of the fin, as is also known from current vortex generators. However, by virtue of the geometry of the fin 43 and the way it is arranged on the wind turbine blade, vortices are created not only at the upper edge of the fin, as shown in Fig. 3a, but additionally, secondary vortices are created closer to the surface 46 of the blade than the traditional vortices, as shown in Fig. 3a. This happens because air is able to flow from the high pressure side to the low pressure side under the leading edge 41 of the fin 43. This is not possible on known wind turbine blades with known vortex generators arranged on the wind turbine blade in known configurations. The vortices are referred to as secondary vortices to distinguish them from the known vortices created at the upper edge of known vortex generators arranged in known ways.

[0058] Surprisingly, reference tests have shown that for a fin producing the same amount of drag as a typical known vortex generator, the fins arranged in accordance with the invention produce more stable vortices compared to vortices produced at the same wind turbine blade but using typical known vortex generators arranged in known configurations. In more general embodiments, the fin on the blade in accordance with the invention can be thought of as having an "overhang" in the direction towards the leading edge 18 of the wind turbine blade 10. It is this overhang under which air can flow from the high pressure side of the fin to the low pressure side of the fin.

[0059] For known vortex generators, the height of the fin increases in the direction from the leading edge of the fin to the trailing edge of the fin or have a uniform height. These fins produce a high drag near the surface of the wind turbine blade. This may also be the case in some embodiments of the present invention. However, the secondary vortices help energise the air slowed down by the higher drag near the surface of the wind turbine blade. Thus, the surface drag in isolation may be the same as for known vortex generators, but the secondary vortices counteract the surface drag produced by the fins when air flows past them. As a result, the effective drag is reduced while stable vortices are still produced. In other words, the present invention produces the same amount of vorticity, but with reduced drag. Thus, the same wind turbine blade can produce more power for the same wind speed and wind direction when configured in accordance with the present invention. The same wind turbine blade, but with fins configured in accordance with the invention, can therefore exhibit a lower stall speed. This, in turn, allows the blade to operate during lower wind speeds than with traditional vortex generators on the same wind turbine blade.

[0060] As shown in Fig. 3d, and as described above, the airflow hits the fin at an angle ac. The airflow, coming from the leading edge 18 of the wind turbine blade, is substantially parallel to the chord at the location of the vortex generator 40. The arrow 48 in Fig. 3d is parallel to the chord.

[0061] As mentioned above, reference 43 in Fig. 3d refers not only to the fin as such, but also points specifically to the low pressure side of the fin. As described above, vortices are created not only at the upper edge of the fin, but also under the leading edge of the fin, the leading edge effectively being forward-swept, resulting in a distance between the forward-most part of the fin and the surface 46 of the blade.

[0062] Fig. 4a illustrates vortex generators with two fins 43a and 43b arranged on a wind turbine blade surface 46, in accordance with the invention. Fig. 4a illustrates two of the vortex generators 40a and 40b identical to the vortex generator 40 shown in Figs. 3a-3d. The fins 43a and 43b are therefore identical to fin 43 illustrated in Figs. 3a-3d. The leading edges of the fins are nearer one another than their respective trailing edges. Air flowing over the vortex generators 40a and 40b from the leading edge 18 of the blade 10 encounters a fin that the air can flow not only over, but also under, as described above and as illustrated in Fig. 3a. References 43a and 43b illustrate the low pressure sides of the fins of the vortex generators 40a and 40b during operation. Vortices are created on the low pressure sides, i.e. between the two vortex generators 40a and 40b.

[0063] It is noted that the size of the fins in Figs. 4a-4b is exaggerated to make it easier to discern the features discussed in relation to the general aspect of the invention. The drawing is not to scale. Further, it is noted that a blade is typically provided with a plurality of vortex generator fin pairs. The pairs of fins may for instance be arranged and configured to have parameters as defined in the claims of WO 2013 / 014015 or with relative parameters as defined in table 1 of WO 2013 / 014080. The fins may also be arranged in accordance with the counter-propagating configuration explained in "Control of a decelerating boundary layer. Part 1: Optimization of passive vortex generators", by Godard and Stanislav, Aerospace Science and Technology 10 (2006), 181-191, e.g. as defined in Table 5 of this paper. Fig. 4b is similar to Fig. 4a, but with a single vortex generator 50 having two fins 53a, 53b arranged on a common base 55. The fins 53a and 53b may be otherwise identical to the fins 43a and 43b, i.e. identical to the fin 43 shown in Figs. 3a-3d. The use of a common base 55 provides for an easier way of placing two fins 53a and 53b in a precise relationship to each other on the surface 46 of the blade 10. In case the fins 53a and 53b are arranged with the same relationship as 40a and 40b in Fig. 4a, the airflow properties are essentially identical to the airflow around the two separate vortex generators in Fig. 4a, being different only due to aerodynamic differences between two individual bases. That is, the difference resulting from having two bases as in Fig. 4a as opposed to the common base 55 in Fig. 4b. The leading edges of the fins 53a and 53b still allow air to flow under the leading edge to create secondary vortices.

[0064] In both Fig. 4a and Fig. 4b, the result is that more stable vortices are created and the boundary layer is therefore energised to a higher degree for the same drag as discussed above. Thus, the blades in Figs. 4a and 4b will have a lower stall speed compared to the same blades with known vortex generators arranged in known configurations. In addition, the overall induced drag from the vortex generators is lower. Thereby, the lift-to-drag ratio is improved compared to existing wind turbine blades provided with known vortex generators in known configurations.

[0065] Figs. 5a-5f show examples of vortex generators with various fin geometries suitable for obtaining a lower drag for the same vortex strength as known vortex generators arranged in known configurations on otherwise identical blades. All the embodiments are advantageous, but the discussion below highlights some considerations that should be considered to obtain the best results, i.e. a satisfactory production of vortices at the lowest possible drag, or an improved production of vortices at a drag similar to the drag produced by known vortex generators arranged in known ways. In accordance with the invention, the fins in all of Figs. 5a-5f have a forward-swept leading edge 41 allowing the airflow to generate vortices on the leading edge.

[0066] In the embodiment in Fig. 5a, the leading edge 41 forms an (average) angle az of 26 degrees with respect to the surface 46 of the wind turbine blade at the location of the vortex generator ("horizontal"). The trailing edge forms an angle ar of 19 degrees with respect to horizontal. All numbers are approximate and exemplary. The fin has a length / . / (seen from above along a surface normal) and a top portion 44 with a length Lt. The bottom portion of the fin has a length Lb. The fin has a height ht.

[0067] The rather similar, low angles and the relatively short top portion 44 relative to the base portion results in relatively long edges 41, 42, that are "relatively parallel". This leads two "long" interaction areas between the airflow at the leading and trailing edges 41, 42, but with a relatively low drag. For all embodiments of the invention, the length of the fin, Lf, is advantageously in the range 0.05 to 0.1 times the length of the local chord (i.e. the chord at the location of the vortex generator). Preferably under the latter constraint, the height of the fin is advantageously at most 0.05 times the length of the local chord. All taken into consideration, the ratio hf / Lf is advantageously less than 0.5. Advantageously, the ratio is at least 0.2, especially in cases where the leading edge angle az is within 5 degrees of the trailing edge angle ar

[0068] A difference of 5-10 degrees between the leading edge angle az and the trailing edge angle ar provides good performance.

[0069] Together with the straight leading and trailing edges, the flat portion 44 on the fin in Fig. 5a gives the fin a quadrilateral shape, which is relatively simple to manufacture. The flat portion 44, in addition to producing vortices, provides strength to the fin. This is in contrast to a substantially triangular shape in which the front-most part of the fin is more pointed which may result in vibrations that can reduce the stability of the vortices.

[0070] Fig. 5b illustrates a fin in which the leading edge angle az is 45 degrees and the trailing edge angle ar is 32 degrees. The height Azof the fin is the same as that in Fig. 5a. The fin in Fig. 5b generally exhibits similar behaviour and produces about the same drag, but the higher angles result in a lower efficiency in producing vortices. The difference between the leading and trailing edge angles is 13 degrees, which is also less advantageous. In many cases, it is advantageous that the difference in the angles is at most 10 degrees. To avoid very long fins, which may exhibit some instability, the difference in angles is advantageously at least 5 degrees.

[0071] For the same height A / and same bottom length Lb, but a larger difference in angles, as in Fig. 5b, the top portion 44 becomes longer than in the embodiment in Fig. 5a, which may result in production of stronger vortices at the upper edge of the fin, which is an advantage of the fin in Fig. 5b compared to the fin in Fig. 5a.

[0072] The fin in Fig. 5c is similar to the fin in Fig. 5b. Many of the same considerations therefore apply. Fig. 5c is presented to illustrate that a rounded upper edge 47 can improve performance. The smooth transition between a horizontal upper edge at the front of the fin and the final angle a rat the bottom portion of the fin, i.e. where the fin meets the base 45, can result in slightly stronger vortices. Compared to the fin in Fig. 5b, the smooth transition of the fin in Fig. 5c more gradually increases the strength of vortices, whereas the kink between the top portion 44 and the trailing edge 42 in Fig. 5b can result in a reduction of the strength of the vortices. Note that even though there is a smooth transition in Fig. 5c, the trailing edge 42 in Fig. 5c is still substantially straight and will otherwise have the same effect on the airflow at that portion of the trailing edge.

[0073] The vortex generator in Fig. 5c still exhibits good performance, but as discussed in relation to Fig. 5b, the large angles of the leading and trailing edges reduce the performance compared to the fin shown in Fig. 5a having smaller angles. This trade-off should be considered.

[0074] The fin in Fig. 5d is similar to the fin in Fig. 5c and many of the same considerations therefore apply. For exemplification of the advantage of the fin in Fig. 5d, a fin with relatively large leading edge average angle and trailing edge average angles is described.

[0075] The main difference from the fin in Fig. 5c is the concave shape of the leading edge 41 (seen from the leading edge 18 of the blade). This shape has two effects. Firstly, it reduces the drag across the fin compared to the fin in Fig. 5c. Secondly, the fin in Fig. 5d will provide vortices that are a little further from the surface 46 compared to the vortices created by the fin in Fig. 5c. In some cases, this can improve the performance of the vortex generator, such as when located closer to the leading edge 18 of the blade than the point of maximum thickness of the airfoil at the spanwise location of the vortex generator.

[0076] The design parameter discussed in relation to Fig. 5d also applies for lower leading edge and trailing edge angles az and ar. Thus, different elements can be combined when optimizing the fin for the particular blade.

[0077] Fig. 5e illustrates a fin having a convex trailing edge (seen from the trailing edge 20 of the blade), similar to the trailing edges in Figs. 5c and 5d. Instead of a concave leading edge, as the fin in Fig. 5d, the leading edge in Fig. 5e is convex (seen from the leading edge 18 of the blade). Further, it is rounded in its front-most part.

[0078] The convex shape of the leading edge 41 has the effect of creating vortices closer to the surface 46 of the blade compared to the leading edges in Figs. 5c and 5d. This can be desirable at vortex generator positions past the point of maximum thickness of the airfoil at the spanwise location of the vortex generator, especially if the airfoil at the same time has a high curvature.

[0079] The top portion has a more uniform width compared to the embodiments in Fig. 5c and 5d. This has the advantage that the airflow at the trailing edge 42 is more uniform, which in turn leads to stronger vortices. This will have the additional effect that vortices are generated on the trailing edge 42 closer to the surface 46 of the blade, compared to the embodiments in Figs. 5c and 5d. As a result, both the vortices generated at the upper edge and the secondary vortices (generated by the leading edge) are closer to the surface of the blade, improving the energising effect near the blade surface. Both effects are advantageous for vortex generator positions past the point of maximum thickness of the airfoil at the spanwise location of the vortex generator, especially if the airfoil at the same time has a high curvature.

[0080] Fig. 5f illustrates a further embodiment of the invention. The leading edge forms an angle of 37 degrees with horizontal. The trailing edge forms an angle of 110 degrees with horizontal.

[0081] The leading edge 41 of the fin 43 in Fig. 5f can be seen as "forward-swept" and the trailing edge 42 of the fin 43, having a trailing edge angle greater than 90 degrees, such as at least 95 degrees, can be seen as "rearward-swept". In other words, the leading edge and the trailing edge are swept in opposite directions. The trailing edge angle in this example is 110 degrees.

[0082] The fin in Fig. 5f has a shorter bottom portion compared to the bottom portion if the previous examples. More precisely, the length is about 46 % of the bottom portions of the fins in Figs. 5a-5e. An advantage of the overall design in Fig. 5f is that the drag near the bottom portion of the fin is lower than for the fins in Figs. 5a-5e. This is at the cost of a higher drag higher up on the fin. The upper edge 47 is slanted slightly downward, which leads to generation of strong vortices at the upper edge. The leading edge angle of 37 degrees in Fig. 5f is a good compromise between producing strong secondary vortices while not adding too much drag. The fin in Fig. 5f is particularly advantageous in regions where it is almost always windy, but where winds are unstable, for instance changing directions frequently. Further, the vortex generator in Fig. 5f may improve performance when winds are frequently gusty. The vortex generator in Fig. 5f produces more drag compared to the vortex generators shown in Figs. 5a-5e, but the longer upper edge produces vortices that can better withstand relatively strong, but unstable winds and gusts, which may otherwise disturb the airflow. The vortex generator in fig. 5f thus allows improved performance over a large span of wind speeds, but at the cost of a higher stall speed. However, in locations where it is usually windy, stalling is rarely a problem. List of references

[0083] 2 wind turbine

[0084] 4 tower

[0085] 6 nacelle

[0086] 8 hub

[0087] 10 blade

[0088] 14 blade tip

[0089] 15 tip end

[0090] 16 blade root

[0091] 18 leading edge

[0092] 20 trailing edge

[0093] 30 root region

[0094] 32 transition region

[0095] 34 airfoil region

[0096] 36 pressure side shell portion of blade

[0097] 40 vortex generator

[0098] 40a, b vortex generator

[0099] 41 leading edge of vortex generator

[0100] 42 trailing edge of vortex generator

[0101] 43 fin, low-pressure side of fin

[0102] 43a, b fin, low-pressure side of fin

[0103] 44 top portion of fin

[0104] 45 base of vortex generator

[0105] 46 surface of wind turbine blade

[0106] 47 top portion of fin

[0107] 48 local chord at vortex generator

[0108] 48a, b local chord at vortex generator

[0109] 49 straight line from bottom to top of leading edge of fin

[0110] 50 vortex generator

[0111] 51 bottom portion of fin

[0112] 52 top portion of fin

[0113] 53a, b fin, low-pressure side of fin

[0114] 55 base of vortex generator acinclination of fin relative to local chord cu average angle of leading edge of fin ar average angle of trailing edge of fin hf free distance of front portion of fin

[0115] L longitudinal axis of blade Lb length of bottom portion

[0116] Lf length of fin

[0117] Lt length of top portion

Claims

Claims1. A wind turbine blade (10) having a leading edge (18) and a trailing edge (20), wherein the wind turbine blade is provided with one or more vortex generators (40, 40a, 40b) at respective one or more locations on a surface (46) of the wind turbine blade, wherein each vortex generator comprises a fin (43, 43a, 43b, 53a, 53b) having a leading edge (41) and a trailing edge (42), the leading edge (41) of the fin (43, 43a, 43b, 53a, 53b) being closer to the leading edge (18) of the wind turbine blade (10) than the trailing edge (42) of the fin (43, 43a, 43b, 53a, 53b), and wherein the leading edge of the fin extends from a bottom portion (51) proximal to the surface (46) of the blade to a top portion (52) distal from the surface (46) of the blade, the leading edge (41) of the fin being configured such that at least a portion of the leading edge (41) of the fin between the bottom portion (51) and the top portion (52) is located closer to the leading edge (18) of the wind turbine blade than the bottom portion (51) of the fin.

2. A wind turbine blade in accordance with claim 1, wherein the fin (43, 43a, 43b, 53a, 53b) is arranged at an angle (ac) relative to a local chord (48) of the wind turbine blade at the location of the vortex generator.

3. A wind turbine blade in accordance with claim 1 or 2, wherein the entire leading edge (41) or at least substantially the entire leading edge (41) of the fin is located closer to the leading edge (18) of the wind turbine blade than the bottom portion (51) of the fin (43, 43a, 43b, 53a, 53b).

4. A wind turbine blade in accordance with any of claims 1-3, wherein the leading edge of the fin on average forms an average angle (az) with the surface (46) of the blade at the location of the vortex generator, wherein the average angle is at most 85 degrees, such as at most 60 degrees, such as at most 45 degrees, such as at most 30 degrees, such as in the range 5-30 degrees.

5. A wind turbine blade in accordance with any of claims 1-4, wherein the one or more vortex generators are arranged in pairs with their respective fins being arranged with opposite angles (+ / -ac) relative to the local chord (48) of the wind turbine blade at the location of each pair of vortex generators (43a, 43b, 53a, 53b).

6. A wind turbine blade in accordance with any of claims 1-5, wherein the one or more vortex generators are provided as at least one of: a single fin (43) extending from a base (45), a finpair (43a, 43b) extending from corresponding two bases, or a fin pair (53a, 53b) extending from a common base (55), or a plurality of fins extending from one common base.

7. A wind turbine blade in accordance with any of claims 1-6, wherein the leading edge (41) of the fin is straight or at least substantially straight.

8. A wind turbine blade in accordance with any of claims 1-7, wherein the trailing edge (42) of the fin is straight or at least substantially straight.

9. A wind turbine blade in accordance with any of claims 1-8, wherein: the leading edge of the fin on average forms an average angle (az) with the surface (46) of the blade at the location of the vortex generator in the range 10-50 degrees, and the trailing edge (42) of the fin on average forms an average angle (az) with the surface (46) of the blade at the location of the vortex generator less than the average angle (az) of the leading edge (41), such as 2-15 degrees less, such as 5-10 degrees less.

10. A wind turbine blade in accordance with any of claims 1-8, wherein: the leading edge of the fin on average forms an average angle (az) in the range 10-50 degrees with the surface (46) of the blade at the location of the vortex generator, and the trailing edge (42) of the fin on average forms an average angle (az) of at least 95 degrees with the surface (46) of the blade at the location of the vortex generator.

11. A wind turbine blade in accordance with any of claims 1-10, wherein a height of the fin (43) along an entire upper edge of the fin (43) monotonically decreases in a direction from the leading edge (41) of the fin to the trailing edge (42) of the fin.

12. A wind turbine blade in accordance with any of the preceding claims, wherein the leading edge (41) of the fin, the trailing edge (42) of the fin, and the bottom portion (51) of the fin form a substantially quadrilateral shape.

13. A wind turbine blade in accordance with any of claims 1-12, wherein the leading edge (41) of the fin is convex seen from the leading edge (18) of the wind turbine blade (10) and wherein the trailing edge (42) of the fin is convex seen from the trailing edge (20) of the wind turbine blade (10).

14. A wind turbine blade in accordance with any of claims 1-12, wherein the leading edge (41) of the fin is concave seen from the leading edge (18) of the wind turbine blade (10) and wherein the trailing edge (42) of the fin is convex seen from the trailing edge (20) of the wind turbine blade (10).

15. A wind turbine blade in accordance with any of claims 1-14, wherein a height hi of the fin is at most 0.05 times a length of a local chord (48) of the wind turbine blade at the location of the vortex generator, and wherein a length of the bottom portion (51) of the fin (43) is at most 0.1 times a length of a local chord (48) of the wind turbine blade at the location of the vortex generator.

16. A wind turbine blade in accordance with any of claims 1-15, wherein the leading edge of the fin is corrugated or undulates along the leading edge of the fin.

17. A wind turbine blade in accordance with any of claims 1-16, wherein the trailing edge of the fin is corrugated or undulates along the leading edge of the fin.

18. A wind turbine blade in accordance with any of claims 1-17, wherein an upper edge of the fin is rounded.

19. A vortex generator for a wind turbine blade (10) having a leading edge (18) and a trailing edge (20), wherein the vortex generator comprises a fin (43, 43a, 43b, 53a, 53b) having a leading edge (41) and a trailing edge (42), and wherein the vortex generator is configured to be arranged on a surface of the wind turbine blade (10), such that the leading edge (41) of the fin (43, 43a, 43b, 53a, 53b) is closer to the leading edge (18) of the wind turbine blade (10) than the trailing edge (42) of the fin (43, 43a, 43b, 53a, 53b), and wherein the leading edge of the fin extends from a bottom portion (51) proximal to the surface (46) of the blade to a top portion (52) distal from the surface (46) of the blade, the leading edge (41) of the fin being configured such that at least a portion of the leading edge (41) of the fin between the bottom portion (51) and the top portion (52) is located closer to the leading edge (18) of the wind turbine blade than the bottom portion (51) of the fin.

Images