Technical field

The present invention relates to a noise reducing device for a wind turbine blade, having a first end, a second end, a first side surface, and a second side surface, wherein the noise reducing device comprises a base part extending from the first end to a proximal end, at least one array of first noise reducing elements extending from the proximal end to the second end, and at least one array of second noise reducing elements arranged relative to the first noise reducing elements.

The present invention further relates to a wind turbine blade with at least one noise reducing device as mentioned above. Background

It is well-known that large wind turbine blades for modern wind turbines suffer from trailing edge noise generated by the airflow passing over the aerodynamic profile of the wind turbine blade. The airflow transforms from a substantially laminar airflow into a turbulent airflow over the suction and pressure sides which separates from the local boundary layer at a separation point. This generates vortices behind the trailing edge which generate a wake effect and associated hissing sounds at about one kilohertz (kHz).

It is known to integrate a noise reducing profile into the trailing edge section of the wind turbine blade. However, this increases the complexity of the lay-up process and adds additional steps to the manufacturing process. Another way of solving this problem is to attach one or more noise reducing devices at or near the trailing edge of the wind turbine blade. These noise reducing devices normally comprise a plurality of serrations projecting from one or more base plates. However, they suffer from loss of noise reduc- tion performance, particularly when the serrations are not aligned with the main flow direction.

WO 2016/001420 A1 discloses a wind turbine blade with a plurality of serrations projecting outwards from the trailing edge of the wind turbine blade. A pair of vanes, i.e. two vanes, is positioned on each serration, symmetrically arranged relative to the cen- tre line of the serration. The vanes have a U-shaped profile placed over the peripheral edge of the serration, wherein each leg of this profile extends partly along the side surface of the serration towards the trailing edge. The local height of each leg is equal to or is greater than the thickness of the local boundary layer on that side at the trailing edge. The two vanes are spaced apart by half of the width of the serration. The respective vanes are aligned with the main flow direction and thus project perpendicularly relative to the trailing edge.

US 2015/078896 A1 also discloses a wind turbine blade with a plurality of serrations projecting outwards from the trailing edge of the wind turbine blade. The serrations are bent towards a pressure or suction side relative to the chord line of the wind turbine blade. Two types of vanes are positioned on each serration; two small vanes and two large vanes are symmetrically arranged relative to the centre line of the serration. Both types of vanes have a triangular or elliptical shaped cross-sectional profile and project from the same pressure or suction side of the serration.

US 2015/0247487 A1 discloses another wind turbine blade with a plurality of serrations projecting outwards from the trailing edge of the wind turbine blade. The serrations each has an integrated rib element projecting from one side surface of the serration. The ribs and serrations are made using a three-dimensional printer. The rib is aligned with the centreline of the serration and thus projects perpendicularly from the trailing edge. It is stated that the rib is specifically designed to add structural stiffness to the serrations to prevent oscillation of the serrations. EP 2940292 A1 discloses a vortex generator device having an l-shaped or U-shaped cross-sectional profile, wherein the vortex generator device is arranged on the suction side at a chordwise distance from the leading edge. The maximum height found at the local leading edge of the vortex generator device exceeds the local boundary layer thickness.

US 2010/0143144 A1 discloses fins having an cured profile in the chordwise direction, wherein the fins are arranged on the suction side and the local leading edge of the fins are preferably placed at a chordwise distance of 50% to 90% from the leading edge. In a preferred embodiment, the intermediate distance between adjacent fins is 2 to 8 times the local boundary layer thickness and the height of the fins is 50% to 75% of the local boundary layer thickness. WO 2015/192915 A1 discloses an array of noise reducing devices arranged on the suction side at the trailing edge. Each device has a cumb or needle shaped upper part connected to a spacer part which is further connected to a base part. The base part is attached to the wind turbine blade while the spacer part ensures that the upper part is placed at a distance of 1 cm to 4 cm from the blade surface.

Object of the invention

An object of the invention is to provide a noise reducing device and a wind turbine blade that solves the abovementioned problems.

Another object of the invention is to provide a noise reducing device and a wind turbine blade that lowers the scattering efficiency at trailing edge of wind turbine blade.

Yet another object of the invention is to provide a noise reducing device and a wind turbine blade that minimises the loss of noise reduction performance.

Another further object of the invention is to provide a noise reducing device and a wind turbine blade that provides an improved noise reduction at the trailing edge. Detailed description of the invention

An object of the invention is achieved by a noise reducing device for a wind turbine blade, having a first end, a second end, a first side surface and a second side surface, the noise reducing device comprises a base part and at least one noise reducing element, the base part extends from the first end to a proximal end and the at least one noise reducing element extends from said proximal end to the second end, the base part is configured for attachment to a side surface adjacent to a trailing edge of the wind turbine blade or for attachment to a trailing edge surface of the wind turbine blade, wherein at least one airflow modifying element projects from at least one side surface of the at least one noise reducing element, the at least one airflow modifying element is configured to interact with a local boundary layer attached to said at least one side surface of the noise reducing device when installed, wherein the at least one airflow modifying element extends substantially in a transverse direction from a local first end to a local second end, characterised in that the at least one airflow modifying element has a local height measured between said at least one side surface and a top of said airflow modifying element, the local height is equal to or less than two-thirds of a local boundary layer thickness at the noise reducing device when installed, wherein the local boundary layer thickness is determined when the wind turbine blade is operated at a rated rotational speed.

This provides a noise reducing device which combines the effects of noise reducing el- ements with the effect of airflow modifying elements. This configuration lowers the scattering efficiency of the original trailing edge of the wind turbine blade as well as minimises the loss of noise reduction performance caused by the misalignment of the modified airflow. This configuration further provides an improved noise reduction effect compared to conventional noise reducing devices as the airflow modifying elements modify the turbulence in the boundary layer.

The dimensions of the projecting U-shaped vane pair of WO 2016/001420 A1 provide two relative large vanes which affect the modified airflow in the entire thickness of the local boundary layer. These large vanes increase the drag and thus have a negative impact on the aerodynamic performance of the wind turbine blade. Wind tunnel tests have shown that the majority of the energy of the vortices affecting the unsteady surface pressure at the trailing edge and thus the generation of trailing edge noise are concentrated in the lower half of the boundary layer, i.e. towards the side surface of the wind turbine blade. Therefore, it has surprisingly been found that the height of the air- flow modifying element can be reduced without adversely affecting the effect of the airflow modifying element.

A local height of the airflow modifying element is measured from a side surface of the noise reducing element to a top of the airflow modifying element. A total height of the airflow modifying elements is measured between opposite facing tops of airflow modifying elements located on opposite side surfaces of the noise reducing element. The lo- cal and/or total height may be selected dependent on the thickness of the local boundary layer at a longitudinal position and/or chordwise position on the wind turbine blade at which the noise reducing device is installed and/or dependent on the aerodynamic profile of the wind turbine blade. In example, the local height may be equal to or less than two-thirds of the local boundary layer thickness. In example, but not limited to, the local height may be between one-third and two-thirds of the local boundary layer thickness. In example, but not limited to, the local height may be about one-half of the local boundary layer thickness. This allows the airflow modifying elements to control the direction of the airflow and thereby modify the turbulence in the boundary layer. This configuration also saves material and reduces the loadings on the airflow modifying ele- ments. This configuration further reduces the drag of the airflow modifying elements compared to the vane pair of WO 2016/001420 A1 .

It is known to the skilled person that the local boundary layer thickness distribution at the trailing edge varies in the spanwise direction. The local boundary layer thickness can be easily extracted using any simulation techniques. For example, the local boundary layer thickness may be determined using a panel method, such as XFOIL ^® or PROFILE ^® . For example, the boundary layer thickness may be determined when the wind turbine blade is operated at a rated rotational speed (rpm), preferably without said noise reducing device.

According to a first special embodiment, the at least one airflow modifying element comprises at least two airflow modifying elements arranged at a distance from each other, the distance is equal to or less than one-third of the local boundary layer thickness.

A number of airflow modifying elements may be arranged on each noise reducing element in a predetermined pattern in a longitudinal and/or transverse direction. The longitudinal direction of the noise reducing device corresponds to the spanwise direction of the wind turbine blade when installed. The transverse direction of the noise reducing device corresponds to the chordwise direction of the wind turbine blade when installed. In example, one airflow modifying element may be arranged on one or both side surfaces of the noise reducing element. This divides the airflow into two modified airflows. In example, at least two airflow elements may be arranged on one or both side surfaces of the noise reducing element. Said at least two airflow elements may be equally spaced, or the spacing may vary between individual airflow elements. The number of airflow modifying elements may be selected dependent of the local boundary layer thickness and/or the local width of the serrations. This divides the airflow into a plurality of modified airflows which exits along peripheral edge of the noise reducing element. This also allows the number of individual airflow modifying elements to be adapted to the dimensions of each respective noise reducing element.

In one example, a first airflow modifying element may be positioned at a distance from at least a second airflow element, wherein said distance may be measured in the longitudinal direction. The wind tunnel tests have further shown that the spacing, i.e. dis- tance, between the individual airflow modifying elements have a significant influence on the performance of the modified airflows. Therefore, it has surprisingly been found that the effect of the airflow modifying element can be further improved by lowering the spacing compared the vane pair of WO 2016/001420 A1 . In example, the distance may be equal to or less than one-third of the local boundary layer thickness. In example, but not limited to, the distance may be between one-fourth and one-tenth of the local boundary layer thickness. In example, but not limited to, the distance may be about one-sixth of the local boundary layer thickness. This provides the greatest effect and thus allowing for the best control of the modified airflows.

The noise reducing device can thus be manufactured with a standardised spacing between the individual airflow modifying elements, which allows for a simple and fast manufacturing process. A kit or set of noise reducing devices may be provided, wherein each noise reducing device has a different spacing between its airflow modifying el- ements. A particular noise reducing device can thus be selected for installation at a selected position along the trailing edge of a particular wind turbine blade.

According to a second special embodiment, the at least one airflow modifying element comprises at least two airflow modifying elements arranged at a distance from each other, the distance is determined as function of characteristics of a trailing edge noise or of said local boundary layer. The distance may alternatively or additionally be determined as function of the trailing edge noise characteristics and/or of the boundary layer characteristics at the trailing edge. These characteristics may be defined by one or more measurable or quantifiable parameters descriptive of the aerodynamic or acoustic characteristics.

In example, the acoustic characteristics may comprise at least a main frequency or frequency range having the highest noise level, i.e. amplitude value or averaged amplitude value. The acoustic characteristics may comprise one or more further relevant parameters, e.g. other frequencies and/or amplitude values, which can be used to deter- mine the distance. The distance may thus be determined based on the main frequency or frequency range, e.g. in combination with the other relevant parameters. This allows the arrangement of airflow modifying elements to be adapted to the trailing edge noise characteristics. The abovementioned frequencies, amplitude values and other parameters may be determined using any type of model descriptive of the trailing edge noise, e.g. an aeroa- coustic model. In example, but not limited to, the aeroacoustic model may include the BPM-model, the TNO-Blake model or computational aerodynamics (e.g. Ffowcs- Williams and Hawkings equation). Other known techniques, such as measurements or simulations, may also be used to determine or estimate the respective parameters of the trailing edge noise.

In example, the aerodynamic characteristics may comprise at least a velocity, e.g. convection velocity, of the turbulent airflow at one or more heights. The aerodynamic char- acteristics may comprise one or more further relevant parameters, e.g. temperatures, pressures and/or air density, which can be used to determine the distance. The distance may thus be determined based on the velocity, e.g. in combination with the other relevant parameters. The spacing between the individual airflow modifying elements can thus be adapted according to the aerodynamic characteristics of the local bounda- ry layer.

The abovementioned velocities and other parameters may be determined using any type of model of the boundary layer characteristics, e.g. an aerodynamic model. Alternatively, a combined model descriptive of both the aerodynamic and acoustic charac- teristics may be used. In example, but not limited to, the aerodynamic model may include panel methods (e.g. XFOIL ^® ), analytic methods (e.g. Corcos model), or computa- tional fluid dynamic methods (e.g. RANS, LES or a hybrid thereof). Other known techniques, e.g. measurements or simulations, may also be used to determine or estimate the respective parameters of the local boundary layer. The noise reducing device can thus be customised for a particular site, a particular aerodynamic blade profile, and/or a particular location of the wind turbine blade. The efficiency of the noise reducing device can thereby be tuned according to the actual aerodynamic or acoustic characteristics. According to one embodiment, the at least one airflow modifying element comprises at least one outermost airflow modifying element and at least one innermost airflow modifying element, wherein a first length of the at least one innermost airflow modifying element is greater than a second length of the at least one outermost airflow modifying element.

A local length of each airflow modifying element is measured from a local first end facing the first end of the noise reducing device to a local second end facing the second end of the noise reducing device. When installed, the local first end face towards the leading edge of the wind turbine blade and the local second end face in the opposite direction, i.e. away from the trailing edge.

The number of airflow modifying elements may each have the same local length or a local length which varies gradually along a local width of the noise reducing element. The noise reducing element further has a local length measured between the second end, e.g. a tip, and a proximal end of the noise reducing device. The total length of the noise reducing device is measured from the first end to the second end.

An innermost airflow modifying element defines a first length while an outermost airflow modifying element defines a second length. Any number of intermediate airflow modify- ing elements arranged between the innermost and outermost airflow modifying elements, and may have a local length which is equal to or greater the second length. Alternatively, the intermediate airflow modifying elements may have a local length which is smaller than the second length. The wind tunnel tests have also shown that the local length, e.g. the first and second lengths, of the individual airflow modifying elements has a minimal influence on the performance of the modified airflows. Therefore, it has surprisingly been found that the local length of the airflow modifying element can be changed, e.g. lowered, without ad- versely affecting the effect of the airflow modifying element compared to the noise reducing device of WO 2016/001420 A1 . This may result in less drag and increased structural strength and also a simpler and faster manufacturing process.

According to a third special embodiment, said local first end is arranged at the first end or at the proximal end, and said local second end is arranged at a first or second peripheral edge of the at least one noise reducing element.

The airflow modifying element may extend from the peripheral edge of the noise reducing element to the proximal end or the first end. The local first end may alternatively be positioned at a distance from the proximal end or the first end. The local second end may alternatively be positioned at a distance from the peripheral edge. The airflow modifying element may thus have a substantially straight profile. The airflow modifying element may thus extend partly or fully along the side surface without projecting beyond the peripheral edge. This guides and thus controls the modified airflow along the length of airflow modifying elements.

Additionally or alternatively, the airflow modifying element may project beyond the peripheral edge and from a freely projecting portion located on the pressure or suction side of the noise reducing element. This freely projecting portion may optionally have a sub-portion extending into the opposite side of the noise reducing element. The airflow modifying element may thus have a substantially L-shaped profile. This guides and thus controls the modified airflow beyond the peripheral edge to a point located in the wake, thereby extending the effect of airflow modifying elements. This allows an airflow modifying element located on one side surface to partly guide and controls the modified airflow on the opposite side surface.

The abovementioned profiles of the outermost airflow modifying elements may thus have a shorter local length than the pair of vanes in WO 2016/001420 A1 . Further, the abovementioned profiles of the innermost airflow modifying elements may thus have a larger local length than the pair of vanes in WO 2016/001420 A1 . According to a fourth special embodiment, the at least one noise reducing element has a first side surface, a second side surface and a first or second peripheral edge, where said at least one airflow modifying element extends over said first or second peripheral edge so that said local first end is arranged at the first end or proximal end of said first side surface and said local second end is arranged at the first end or proximal end of said second side surface.

In yet another example, the airflow modifying element may extend around the peripheral edge and thus extend partly or fully along the opposite side surface. The airflow modifying element may thus have a substantially U-shaped profile. This allows the airflow modifying element guides and controls the modified airflow on both side surfaces.

The sub-length of the freely projection portion in the abovementioned examples may substantially correspond to the local boundary layer thickness.

According to one embodiment, said local height of the at least one airflow modifying element is substantially constant between the local first and second ends, or said local height of the at least one airflow modifying element tapered from the local first or second end towards the other local end.

The airflow modifying element may have a substantially uniform cross-sectional profile between the local first and second ends, thus having a substantially constant height along its length. This allows for a simple and fast manufacturing process, e.g. using thermoforming or injection moulding.

The airflow modifying element may alternatively have a tapered cross-sectional profile which tapers from the local first end towards the local second end, or vice versa. The tapered profile may instead taper from at least one intermediate point towards the local first end and/or the local second end. In example, but not limited to, the airflow modify- ing element may have a rectangular, a convex, a trapezoid, a semi-circular or a semi- elliptical profile. This allows the cross-sectional profile of the airflow modifying element to be adapted to the aerodynamic blade profile and/or the boundary layer profile. This reduces drag and allows the noise reducing elements to reduce noise more effectively. The airflow modifying element may further have a substantially uniform or tapered cross-sectional profile in the longitudinal direction. The tapered profile may taper from the side surface of the noise reducing element towards the free tip of the airflow modifying element, or vice versa. In example, but not limited to, the airflow modifying element may have a rectangular, a convex, a trapezoid, a semi-circular or a semi-elliptical cross-sectional profile. This further allows the cross-sectional profile of the airflow modifying element to be adapted to the aerodynamic blade profile and/or the boundary layer profile. The cross-sectional profile may also be selected to ensure a strong attachment to the side surface and/or to reduce the flexibility of the airflow modifying element. According to one embodiment, said top of the at least one airflow modifying element at the local first end and/or the local second end had a rounded edge in a plane defined by the at least one airflow modifying element.

The respective corners of the local first and/or second ends may comprise a rounded or elliptical edge. The corner(s) may alternatively comprise as a straight edge which positioned in an inclined angle relative to the longitudinal direction or the side surface of the noise reducing element. This reduces the drag force and allows for a more optimal airflow around the first and second ends. According to one embodiment, the at least one airflow modifying element has a substantially straight or curved profile extending between the local first end and the local second end, wherein said straight or curved profile is arranged in a predetermined angle relative to the proximal end. The airflow modifying elements in the transverse direction may have a substantially straight profile, wherein the local first and second ends may be substantially aligned in the transverse direction. This straight profile may be aligned with the main flow direction, e.g. placed perpendicularly relative to the proximal end. Alternatively, this straight profile may be positioned in an inclined angle relative to the proximal end. The airflow modifying elements may also have a curved profile, wherein the local second end is offset relative to the first end, or vice versa. The local first or second end may curve towards or away from the centreline of the noise reducing element. This allows the modified airflow to be guided towards or away from the centreline dependent on the orientation of the airflow modifying elements. This configuration contributes to substantially aligning the modified airflow with the main flow direction when the noise reducing elements are misaligned and thereby reducing the negative affect on the passing airflow. According to one embodiment, a first airflow modifying element located on the first side surface is aligned with or offset in a longitudinal direction relative to a second airflow modifying element located on the second side surface. A first number of first airflow modifying elements may be arranged on a first side surface, e.g. the pressure side, of the noise reducing element. A second number of second airflow modifying elements may be arranged on a second side surface, e.g. the suction side, of the noise reducing element. The first and second numbers may be the same or differ from each other. The numbers of first and second airflow modifying ele- ments may be selected dependent of the desired spacing, the local width of the serrations and/or the frequencies of the trailing edge noise as mentioned earlier.

The first and second airflow modifying elements may be aligned so the local modified airflows on the pressure and suction sides exit at the same location seen in the longitu- dinal or spanwise direction. This reduces the amount of turbulence generated by the airflow modifying elements themselves.

Alternatively, the first and second airflow modifying elements may be offset relative to each other so the local modified airflows on the pressure and suction sides exit at dif- ferent locations seen in the longitudinal or spanwise direction. This allows the respective local airflows on the pressure and suction sides to mix when exiting the noise reducing device. This may further reduce the coherent patterns in the existing airflows and further reduce the scattering efficiency of the trailing edge. According to one embodiment, said at least one noise reducing element is serrations and/or said at least one airflow modifying element is vanes.

The noise reducing elements may be, but not limited to, shaped as serrations. The serrations may have a substantially triangular, elliptical or semi-circular profile. This changes the original trailing edge profile from a substantially straight edge to an alternating edge. This reduces the scattering efficiency and the generated trailing edge noise of the original trailing edge.

The noise reducing elements may be aligned with a base part to form a substantially straight noise reducing device. Alternatively, the noise reducing elements may be bend towards the pressure or suction side and thus placed in an inclined angle relative to the base part. Said angle may be between 1 degree and 45 degrees, e.g. between 5 degree and 30 degrees.

The airflow modifying elements may be, but not limited to, shaped as vanes. Here, the term "vane" is defined as any elongated elements or structure having a substantially slender cross-sectional profile seen in the transverse direction. The airflow modifying elements, e.g. vanes, may be formed by a single continuous element or structure or a plurality of sub-elements or structures which together forms the vane. The airflow modifying elements may be manufactured separately from the noise reducing elements. The airflow modifying elements may thus be attached to the noise reducing element, e.g. using adhesive, fasteners like bolts or screws, a press fit, a mechanical coupling or another suitable attachment technique. This allows the use of different manufacturing processes and allows for replacement of the airflow modifying elements.

The airflow modifying elements may also be integrated into the noise reducing elements, e.g. using injection moulding, thermoforming or another suitable manufacturing technique. This adds structural strength to the noise reducing device and reduces the flexibility of the noise reducing elements.

The noise reducing elements and/or the airflow modifying elements may be made of a flexible material, such as thermoplastics, composite materials, polymer or other suitable materials or composites. The noise reducing elements and/or the airflow modifying elements may be made of a rigid material or composite, such as fibre reinforced mate- rials or composites or metals. This reduces the fluttering of the serrations/vanes during operations.

An object of the invention is additionally achieved by a wind turbine blade, the wind turbine blade extends in a spanwise direction from a blade root to a tip end and in a chordwise direction from a leading edge to a trailing edge, the wind turbine blade comprises an aerodynamic profile having a first side surface and a second side surface, characterised in that at least one noise reducing device as described above is installed on the first or second side surface relative to the trailing edge or at the trailing edge, e.g. on a trailing edge surface. This provides a wind turbine blade with improved trailing edge noise reduction compared to the use of other conventional noise reducing devices, such as WO 201 16/001420 A1 . This also allows for the design of longer and faster-rotating wind turbine blades. This may, in turn, result in an increase in the annual energy production (AEP) by about 1 %.

The noise reducing device described above is capable of reducing the trailing edge noise by about 9 dB to 10 dB compared to a wind turbine blade without any noise reducing devices.

An array of noise reducing devices may be arranged in the spanwise direction along a part of the trailing edge. The array may comprise two or more of noise reducing devices, e.g. all or some of these noise reducing devices may be configured as described above. In example, the array may be located between the spanwise position of maxi- mum chord length and the tip end. In example, but not limited to, the array may be located in the outer half of the wind turbine blade.

The number of airflow modifying elements and/or the local height of said airflow modifying elements located on each noise reducing element may be the same or differ. The total number of airflow modifying elements and/or the individual local heights thereof may thus be optimised within the same noise reducing device.

Alternatively or additionally, the number of airflow modifying elements and/or the local height of said airflow modifying elements of each noise reducing device within the abovementioned array may be the same or differ. The configuration of each noise reducing device may thus be optimised within the same array as described above.

The trailing edge is a substantially sharp trailing edge or a blunt trailing edge having a trailing edge surface. The proximal end of said noise reducing device is aligned with or retracted relative to the sharp trailing edge or the trailing edge surface. The noise reducing elements thereby partly or fully project outwards from the trailing edge of the wind turbine blade.

Description of drawings

The invention is explained in detail below with reference to embodiments shown in the drawings, in which Fig. 1 shows a wind turbine,

Fig. 2 shows an exemplary embodiment of the wind turbine blade,

Fig. 3 shows a perspective view of a first exemplary embodiment of a noise reducing device according to the invention,

Fig. 4 shows another perspective view of the noise reducing device of fig. 3,

Fig. 5 shows the noise reducing device of fig. 3 seen from the second end,

Fig. 6 shows a second exemplary embodiment of the second noise reducing device seen from the second side surface,

Fig. 7 shows the second noise reducing elements of fig. 6 seen from the first side surface,

Fig. 8 shows a cross-sectional view of a third exemplary embodiment of the noise reducing device,

Fig. 9 shows a cross-sectional view of a fourth exemplary embodiment of the noise reducing device,

Fig. 10 shows a cross-sectional view of the wind turbine blade with the noise reducing device installed at the trailing edge.

Figs. 1 1 a-c show three different embodiments of the local second end, and

Figs. 12a-b shows two different embodiments of the airflow modifying elements. List of references

1 . Wind turbine

2. Wind turbine tower

3. Nacelle

4. Hub

5. Wind turbine blades

6. Pitch bearing

7. Blade root

8. Tip end

9. Leading edge

10. Trailing edge

1 1 . Blade shell

12. Pressure side

13. Suction side

14. Blade root portion

15. Aerodynamic blade portion

16. Transition portion 17. Length of wind turbine blade

18. Chord length of wind turbine blade

19. Noise reducing device

20. Second side surface

21 . First end of noise reducing element

22. Second end of noise reducing element

23. Base part

24. Proximal end

25. Noise reducing elements

26. Airflow modifying elements

27. Innermost airflow modifying element

28. Outermost airflow modifying element

29. First side surface

30. First peripheral edge

31 . Second peripheral edge

32. First end of airflow modifying element

33. Second end of airflow modifying element

34. Installation surface

35. Airflow modifying elements

36. Centreline

Lia-b First local lengths of innermost first and second airflow modifying elements l_2a-b Second local lengths of outermost first and second airflow modifying elements ha b Local heights of first and second airflow modifying elements

tb Local boundary layer thickness

wia-b Distances between adjacent first and second airflow modifying elements w _2a -b Distances between outermost first and second modifying elements

w ₃ Local width of noise reducing element

The listed reference numbers are shown in abovementioned drawings where no all ref- erence numbers are shown on the same figure for illustrative purposes. The same part or position seen in the drawings will be numbered with the same reference number in different figures.

Detailed description of the drawings

Fig. 1 shows a modern wind turbine 1 comprising a wind turbine tower 2, a nacelle 3 arranged on top of the wind turbine tower 2, and a rotor defining a rotor plane. The na- celle 3 is connected to the wind turbine tower 2, e.g. via a yaw bearing unit. The rotor comprises a hub 4 and a number of wind turbine blades 5. Here three wind turbine blades are shown, but the rotor may comprise more or fewer wind turbine blades 5. The hub 4 is connected to a drive train, e.g. a generator, located in the wind turbine 1 via a rotation shaft.

The hub 4 comprises a mounting interface for each wind turbine blade 5. A pitch bearing unit 6 is optionally connected to this mounting interface and further to a blade root of the wind turbine blade 5.

Fig. 2 shows a schematic view of the wind turbine blade 5 which extends in a longitudinal direction from a blade root 7 to a tip end 8. The wind turbine blade 5 further extends in a chordwise direction from a leading edge 9 to a trailing edge 10. The wind turbine blade 5 comprises a blade shell 1 1 having two opposite facing side surfaces defining a pressure side 12 and a suction side 13 respectively. The blade shell 1 1 further defines a blade root portion 14, an aerodynamic blade portion 15, and a transition portion 16 between the blade root portion 14 and the aerodynamic blade portion 15.

The blade root portion 14 has a substantially circular or elliptical cross-section (indicat- ed by dashed lines). The blade root portion 14 together with a load carrying structure, e.g. a main laminate combined with a shear web or a box beam, are configured to add structural strength to the wind turbine blade 5 and transfer the dynamic loads to the hub 4. The load carrying structure extends between the pressure side 1 2 and the suction side 13 and further in the longitudinal direction.

The blade aerodynamic blade portion 15 has an aerodynamically shaped cross-section (indicated by dashed lines) designed to generate lift. The cross-sectional profile of the blade shell 1 1 gradually transforms from the circular or elliptical profile into the aerodynamic profile in the transition area 16.

The wind turbine blade 5 has a longitudinal length 17 of at least 35 metres, preferably at least 50 metres. The wind turbine blade 5 further has a chord length 18 as function of the length 17, wherein the maximum chord length is found between the blade aerodynamic blade portion 15 and the transition area 16.

Figs. 3-4 show a first exemplary embodiment of a noise reducing device 19 configured to be installed on the wind turbine blade 5. The noise reducing device 19 has a first side surface (see fig. 4), a second side surface (see fig. 4), a first end 21 and a second end 22. The noise reducing device 19 comprises a base part 23 extending from the first end 21 to a proximal end 24 and a number of noise reducing elements 25 extending from the proximal end 24 towards the second end 22. Here the noise reducing elements 25 are shaped as serrations for lowering the scattering efficiency of the trailing edge 10. The noise reducing device 19 further comprises a number of airflow modifying elements 26 extending from the respective side surfaces of the noise reducing elements 25. An innermost airflow modifying element 27 is located closest to a centreline (see fig. 12a-b) of the noise reducing element 25. An outermost airflow modifying element 28 is located farthest to a centreline (see fig. 12a-b) of the noise reducing element 25. Here, the airflow modifying elements 26 are shaped as vanes for controlling the direction of the existing airflow and substantially aligning it with a main flow direction of the wind turbine blade 5.

A first number of first airflow modifying elements project from the first side surface 29 of the noise reducing element 25, as illustrated in fig. 4. A second number of second airflow modifying elements project from the second side surface 20 of the noise reducing element 25, as illustrated in fig. 3. Here, ten airflow modifying elements are on either side of the serrations. Fig. 5 shows the noise reducing device 19 seen from the second end 22. Each noise reducing element 25 has a first peripheral edge 30 and a second peripheral edge 31 . The first and second peripheral edges 30, 31 of adjacent noise reducing elements 25 form a gap between the tips of said adjacent noise reducing elements 25, as illustrated in fig. 5.

The airflow modifying elements 26 have a local length measured from a local first end 32 to a local second end 33. The first airflow modifying elements 26a arranged on the first side surface 29 are aligned in a longitudinal direction with the second airflow modifying elements 26b arranged on the second side surface 20. The local first and second ends 32, 33 are further aligned in a transverse direction and thus the airflow modifying element 26 has a straight profile. The local modified airflows on the pressure and sue- tion sides of the wind turbine blade 5 thus exist the noise reducing device 19 at the same location.

Fig. 6 shows a second exemplary embodiment of the second noise reducing device 19' seen from the second side surface 20. Here, the local first end 32b of the second airflow modifying elements 26b are arranged at the first end 21 . The local second end 33b of the second airflow modifying elements 26b are arranged at the first or second peripheral edge 30, 31 . The noise reducing element 25 has a local width, w ₃ , measured in the longitudinal direction and a local length in the longitudinal direction measured from the proximal 24 to its tip, e.g. the second end 22.

As illustrated in fig. 6, the local length of the second airflow modifying elements 26b varies along the local width w ₃ . The innermost 27b of the second airflow modifying elements 26b has a first length, Lib, and the outermost 28b of the second airflow modifying elements 26b has a second length, L _2b . The local lengths of remaining second airflow modifying elements 26b gradually changes between the first and second lengths Lib, L ₂ b, as illustrated in fig. 6.

The individual second airflow modifying elements 26b on the respective noise reducing element 25 are spaced apart by a local distance wi _b measured between the centrelines or opposite facing side surfaces of two adjacent second airflow modifying elements 26b. The outermost second airflow modifying elements 28b on two adjacent noise re- ducing elements 25 are further spaced apart by another local distance w ₂ b. The local distances wi _b , w ₂ b can be the same or differ.

Fig. 7 shows the second noise reducing elements 19' seen from the first side surface 29. Here, the local first end 32a of the first airflow modifying elements 26a are arranged at the proximal end 24. The local second end 33a of the first airflow modifying elements 26a are arranged at the first or second peripheral edge 30, 31 .

As illustrated in fig. 7, the local length of the second airflow modifying elements 26a varies along the local width w ₃ . The innermost 27a of the first airflow modifying ele- ments 26a has a first length, Li _a , and the outermost 28a of the first airflow modifying elements 26a has a second length, L _2a . The local lengths of remaining first airflow mod- ifying elements 26a gradually changes between the first and second lengths l_i _a , L _2a , as illustrated in fig. 7.

The individual first airflow modifying elements 26a on the respective noise reducing el- ement 25 are spaced apart by a local distance wi _a measured between the centrelines or opposite facing side surfaces of two adjacent second airflow modifying elements 26a. The outermost first airflow modifying elements 28a on two adjacent noise reducing elements 25 are further spaced apart by another local distance w _2a . The local distances wia, w _2a can be the same or differ.

The base part 23 has an installation surface configured to contact a matching contact surface on a side surface or a trailing edge surface of the wind turbine blade 5. The base part 23 and thus the noise reducing device 19 can be suitably attached, e.g. using an adhesive, to the wind turbine blade 5.

Fig. 8 shows a cross-sectional view of a third exemplary embodiment of the noise reducing device 19". Here, the first airflow modifying elements 26a have a local height, h _a , measured from the first side surface 29 to its free tip facing away from the noise reducing element 25. The second airflow modifying elements 26b have further a local height, h _b , measured from the second side surface 20 to its free tip facing away from the noise reducing element 25. The local heights h _a , h _b is substantially constant along the local lengths of the first and second airflow modifying elements 26a, 26b.

The noise reducing elements 25 are arranged in an inclined angle relative to the base part 23, as illustrated in fig. 8. By bending the noise reducing elements 25 can be substantially aligned with the main flow direction at the trailing edge 10 of the wind turbine blade 5.

The local second ends 33a, 33b of the first and second airflow modifying elements 26a, 26b are arranged at the peripheral edge 30, 31 of the noise reducing element 25. The local second ends 33a, 33b has a rounded profile in the plane define by the airflow modifying element 26.

The local first end 32b of the second airflow modifying element 26b is arranged at or near the proximal end 24 of the noise reducing device 19". The first airflow modifying element 26a extends past the proximal end 24 so that the local first end 32a of the first airflow modifying element 26a is arranged at the first end of the noise reducing device 19". The first airflow modifying elements 26a thus have a greater local length that the second airflow modifying elements 26b, as illustrated in figs. 6 and 7. The local first ends 32a, 32b has a greater rounded profile compared to the profile of the local second ends 33a, 33b.

Fig. 9 shows a cross-sectional view of a fourth exemplary embodiment of the second noise reducing device 19". Here, the local heights h _a , hb of both the first and second airflow modifying elements 26a, 26b taper from the local second ends 33a, 33b to the local first ends 32a, 32b.

Fig. 10 shows a cross-sectional view of the trailing edge area of the wind turbine blade 5. The trailing edge 10 has a trailing edge surface facing the noise reducing elements 25, as illustrated in fig. 10. The noise reducing device 19 is installed, e.g. attached, to one side surface of the wind turbine blade, as illustrated in fig. 10. The noise reducing device 19 is alternatively installed on the trailing edge surface.

The local airflow (indicated by arrows) passing along the side surface forms a local boundary layer (located between dashed line and side surface) having a local bounda- ry layer thickness, tb. The local airflow is guided along the airflow modifying elements 26 and exits at the second end 22, e.g. at the local second ends 33a, 33b. The thickness of this local boundary layer is typically defines as the distance from the side surface to the point at which the velocity of the turbulent airflow is 99% of the free stream velocity, U∞.

At least one of the local heights h _a , h _b , the local lengths l_i _a , Lib and the local distances wia, wib is suitably determined as function of the local boundary layer thickness tb. In example, the local heights h _a , h _b are equal to or less than two-thirds of the local boundary layer thickness tb. In example, the local distances wi _a , wi _b are equal to or less than one-thirds of the local boundary layer thickness tb.

Figs. 1 1 a-c show three different embodiments of the local second end 33a, 33b. The local second end 33a, 33b can extend over the peripheral edge 30, 31 , as illustrated in fig. 1 1 a. The first and second airflow modifying elements 26a, 26b can thus be com- bined to form a substantially U-shaped profile. The local second end 33a, 33b can form a free end portion projecting outwards from the peripheral edge 30, 31 and further have a free sub-portion which projects towards the opposite side surface, as illustrated in fig. 1 1 b. The airflow modifying elements 26 can thus influence the local airflow located on that side surface and partly influence the local airflow located on the opposite side surface.

The local second end 33a, 33b can form a free end portion without any sub-portions projecting towards the opposite side surface, as illustrated in fig. 1 1 c. The airflow modifying elements 26 thus influences only the local airflow located on that side surface.

Figs. 12a-b shows two different embodiments of the airflow modifying elements 35. The airflow modifying elements 35, e.g. the first and/or second airflow modifying elements, can have a straight profile which is positioned in an incline angle, as illustrated in fig. 12a. The inclined angle is measured either relative to the centreline 36 of the noise re- ducing element 25 or to the proximal end 24. Here, the local second ends 33a, 33b are angled away from the centreline 36, but they may instead be angled towards the centreline 36. This reduces the loss of noise reduction when the noise reducing elements 25 are not aligned with the main flow direction. The airflow modifying elements 35, e.g. the first and/or second airflow modifying elements, can also have a curved profile. Here, the local second ends 33a, 33b are curving away from the centreline 36, but they may instead be curving towards the centreline 36. This also reduces the loss of noise reduction when the noise reducing elements 25 are not aligned with the main flow direction.

The abovementioned embodiments may be combined in any combinations without deviating from the present invention.