Wind turbine and wind turbine rotor blade with add-on

Abstract

The invention relates to a wind turbine rotor blade with an aerodynamic structure to reduce noise. The wind turbine rotor blade is formed by an assembly of a rotor blade and an aerodynamic add-on structure. The aerodynamic add-on structure has a plurality of through-holes for reducing noise. The add-on is provided at the rotor blade trailing-edge section of the rotor blade. In a cross-section of the assembly across the rotor blade pressure-side and rotor blade suction-side surfaces and / or the add-on pressure-side and add-on suction-side surfaces, which cross-section preferably is generally perpendicular to a radial direction of the rotor blade, the assembly has a first thickness at the add-on leading edge, the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, and a ratio of the first thickness to the second thickness is larger than 1.5, and preferably between 3 and 8.

Description

WIND TURBINE AND WIND TURBINE ROTOR BLADE WITH ADD-ONBACKGROUND OF THE INVENTION1. Field of the Invention

[0001] The present invention relates generally to a wind turbine rotor blade, to an assembly of rotor blade and aerodynamic add-on structure for noise and / or drag reduction, to a wind turbine having rotor blades and methods for reducing noise for rotor blades. The invention specifically relates to reducing noise while having low drag at a trailing-edge portion of a wind turbine rotor blade.2. Description of the Related Art

[0002] A wind turbine is a device that is designed to capture the energy of the wind and convert it into a form that can be used to generate electricity. The wind turbine includes a rotor that is mounted on a tower. The wind turbine includes a hub and one or more rotor blades that are mounted on the hub and are configured to generate lift when the wind blows over them. The lift generated by the rotor blades causes the rotor to rotate about an axis of the hub, and this rotational energy is then transmitted to a generator via a drivetrain. The generator converts the rotational energy into electrical energy, which is then transmitted to a power grid or stored in a battery system for later use. The wind turbine may also include various sensors and control systems that are used to optimize its performance and ensure that it is operating safely and efficiently.

[0003] A wind turbine rotor blade has a leading edge, a trailing edge, and opposed suction side (upper) and pressure side (lower) side surfaces each beginning at the leading edge and terminating at the trailing edge. The one or more wind turbine rotor blades mounted on the hub extend from the hub in radial direction to a tip of the wind turbine rotor blade.

[0004] The wind turbine rotor blades create noise when they are operating. The rotor blades are moving through the air at a high speed, and this movement can create turbulence and vortices that produce noise. Noise reduction is particularly important for on-shore wind turbines because they are built on land. The acoustic power of the noise produced by a trailing-edge of a wind turbine blade scales exponentially to free-stream flow speed. It is hence important to have a proper noise reduction technology for installing large wind turbines with reasonable noise level at a location close to homes.

[0005] Due to the potential health hazard and its societal impact, governing bodies are establishing regulations and guidelines on noise requirements for wind turbines. For example, in 2018, the World Health Organization has issued guidelines for wind turbine noise, recommending the day-evening-night-weighted Sound Pressure Level (SPL) to be below 45 dB. The level of noise hence determines the region where the wind turbines can be installed, or correspondingly the manner in which the wind turbine can be operated in order to comply with noise requirements in the surroundings.

[0006] T urbulent boundary-layer trail i ng-edge noise (TBL-TE noise) is a dominant noise source of a wind turbine and is caused by the interaction between the turbulent boundary layer of air that flows from the blade's leading-edge to its trailing-edge, which is the back edge of the blade where the air flows off of it. One way in which TBL-TE noise can be generated is through the formation of vortices at the trailing-edge of the blade. These vortices can create a fluctuating pressure field that radiates noise into the surrounding air.

[0007] There are several factors that can influence the level of TBL-TE noise that is generated by a wind turbine rotor blade. These include the shape and size of the blade, the speed at which the blade is moving through the air, material of the blade, surface structure and property of the trailing-edge, and the properties of the air flow over the blade.

[0008] A promising approach for reducing TBL-TE noise is using through-holes near the trailing-edges. The through-holes can be formed integrally in a wind turbine rotor blade or can be attached as an add-on to a trailing-edge of a wind turbine rotor blade. This through-hole approach uses a series of small through-holes, openings, or through-holes in or near a trailing-edge portion of a blade. The through-holes allow air pressure communication between the suction-side and the pressure-side surfaces. As a result, the pressure differences between the suction-side and pressure-side merge more smoothly at the trailing-edge compared to a blade without trailing-edge through-holes, which can help to reduce the level of turbulence and noise generated by the blade as it moves through the air. One advantage of trailing-edge add-ons with through-holes is that they are relatively simple and low-cost to implement. They can be easily added to existing wind turbine blades, and they do not require any additional power or control systems to operate.

[0009] The Dutch patent NL2025831 B1 discloses a wind turbine rotor blade provided with a trailing-edge, wherein the rotor blade trailing-edge is provided with an aerodynamic add-on structure with through-holes. The add-on with through-holes is provided with through-holes such that the through-holes have an open area at the suction-side that is larger than an open area at the pressure-side. However, trailing-edges with through-holes can increase the drag on the blade, which can result in reducing the efficiency of the turbines.

[0010] Therefore, it is desired to have a wind turbine rotor blade having through-holes, which can reduce noise and keep the drag produced by the aerodynamic structure low.

[0011] The background section of this document is provided to place embodiments of the present invention in technological and operational context to assist those skilled in the art understanding the scope and utility of the present invention. Unless explicitly identified as such, no statement herein is admitted being prior art merely by its inclusion in the Background section.BRIEF SUMMARY OF THE INVENTION

[0012] The present invention addresses these problems by providing a wind turbine rotor blade according to claim 1. Other aspects of the invention relate to a method and wind turbine according to the independent claims. Further aspects are disclosed in the clauses.

[0013] According to another aspect of the invention a wind turbine rotor blade with aerodynamic add-on structure is provided comprising an assembly of rotor blade and add-on.Preferably the rotor blade has a rotor blade leading-edge. Preferably, the rotor blade has a rotor blade leading-edge section that has the rotor blade leading-edge. Preferably, the rotor blade has a rotor blade trailing-edge. Preferably, the rotor blade has a rotor blade trailing-edge section that has the rotor blade trailing-edge. Preferably, the rotor blade has a rotor blade pressure-side surface and a rotor blade suction-side surface. Preferably, the rotor blade pressure-side and suction-side surfaces extend between the rotor blade leading-edge and the rotor blade trailing- edge. Still according to an aspect, the aerodynamic add-on structure has a plurality of through- holes for reducing noise, the add-on provided at the rotor blade trailing-edge section of the rotor blade. Preferably the add-on has an add-on leading-edge. Preferably, an add-on leading-edge section has the add-on leading-edge. Preferably, the add-on has an add-on trailing-edge.Preferably an add-on trailing-edge section has the add-on trailing-edge. Preferably, the add on has add-on pressure-side and suction-side surfaces.

[0014] Preferably any of the leading- or trailing edges are the most upstream or downstream features of the rotor blade and / or add-on with reference to the air flow.

[0015] Any of the ratios disclosed herein can be combined with the features of the assembly of rotor blade and add-on. Embodiments of the assembly can be combined with any single one of the ratios disclosed in claim 1 , 2, 3 or 4. Also, any embodiment can be a combination of any of the two ratios disclosed herein.

[0016] In an embodiment, a thickness of rotor blade or add-on or assembly is defined as the shortest distance between the pressure-side surface and suction-side surface at a given point. The thickness will often be the distance in a plane defined by the radial direction of the assembly / rotor blade / add-on. The thickness can vary across the rotor blade / add-on / assembly. In embodiments, the thickness can be defined as the distance between pressure- and suction side perpendicular to the chord line or the camber line.

[0017] Any embodiment disclosed herein, in particular any of the ratios disclosed herein, can be combined with a first thickness of the assembly at the add-on leading edge, wherein the first thickness is at least 2.5mm, more preferably at least 3mm, most preferably at least 3.5mm, and / or the first thickness is at most 20mm, more preferably at most 10mm, most preferably at most 5mm.

[0018] Any embodiment disclosed herein, in particular any of the ratios disclosed herein, can be combined with a second thickness of the add-on at a through-hole positioned closest to the add-on leading edge, wherein the second thickness is at most 2,5mm, preferably at most 2.2mm, most preferably at most 2.05mm.

[0019] Also provided according to an aspect is a method for manufacturing a wind turbine rotor blade with aerodynamic add-on structure according to claim 17 or according to any of the method steps in any combination disclosed herein.

[0020] According to yet another aspect a wind turbine is provided having a mast, nacelle and having one or more wind turbine rotor blades according to any of the aspects disclosed herein.

[0021] Any of the aspects and embodiments herein, in particular the ratios disclosed herein, can be combined with disclosed thicknesses and lengths.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, showing several embodiments of the invention. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the spirit and scope of the invention to those skilled in the art. The reference is made to the appended figures.

[0023] FIG.1 illustrates a schematic view of a rotor blade.

[0024] FIG.2 illustrates a schematic view of a rotor blade with a plurality of aerodynamic add-on structures that have pressure communication channels for noise reduction while having a low drag.

[0025] FIG.3 illustrates a perspective cross-sectional view of a wind turbine rotor blade with an aerodynamic add-on structure.

[0026] FIGs. 4A-4B illustrate a cross-sectional view of the wind turbine rotor blade, wherein the aerodynamic add-on structure comprises a plurality of pressure communication channels.

[0027] FIGs. 5A-5E illustrate a cross-sectional view of embodiments of the wind turbine rotor blade with an aerodynamic add-on structure, wherein the aerodynamic structure comprises a plurality of pressure communication channels.

[0028] FIG. 6 illustrates an example of the result of a noise measurement showcasing the noise-reduction benefit of the present invention over the state of the art.DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0029] The following is a description of certain embodiments of the invention, given by way of example only and with reference to the drawings. Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts.

[0030] For simplicity and illustrative purposes, the present invention is described by referring mainly to exemplary embodiments thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be readily apparent to one of ordinary skill in the art that the present invention may be practiced without limitation to these specific details. In this description, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

[0031] Making first reference to FIG. 1 , this figure provides a view on a rotor blade 10 which has a leading-edge 110 and a trailing-edge 120. The leading-edge 110 is the most upstream part of the rotor blade 10. The leading-edge 110 faces the airflow upon rotation. The leading-edge 110 will cut through the air upon rotation. Upon rotation, the leading edge-will split the air in air at the pressure-side and the suction-side. The top view of FIG.1 shows the suction-side surface 130.Upon rotation, the air flows on both sides of the blade are reunited at the trailing-edge 120, where the pressure-side and suction-side surfaces meet.

[0032] The leading-edge 110 is part of a leading-edge section 11 . The leading-edge faces into from the airflow as the rotor blade 10 rotates. At the leading-edge section 11 , the thickness of the rotor blade increases away from the leading-edge 110. The trailing-edge 120 is part of a trailing-edge section 12. At the trailing-edge section 12, the thickness of the rotor blade decreases towards the trailing-edge 110.

[0033] The rotor blade has a span 16. Upon mounting the rotor blade 10 to a wind turbine, via a rotor hub, the rotor blade 10 spans from the rotor hub end 41 to the tip 42 along the radial direction 159. Generally, the radial direction 159 is generally perpendicular to the airflow direction of air flowing along the pressure-side and suction side surfaces.

[0034] In general, the trailing-edge 120 is sharper than the leading-edge 110. The trailing- edge section 12 is normally more flat and thinner, hence thinner than the leading-edge section 11. The trailing-edge section 12 is located at the downstream end of the airflow. The trailing-edge 12 and the leading-edge section 11 extend in a direction significantly perpendicular to the direction of the airflow 161.

[0035] Leading-edge 110 and trailing-edge 120 are curved lines extending from the rotor hub radially outward to the tip 42. In embodiments, the rotor blade length can be the, preferably shortest, distance between leading-edge 110 and trailing-edge 120 along the airflow direction 161 , which in the depiction of FIG.2 are generally horizontal distances. As one can see the blade starts with a rotor blade length near the hub, that rotor blade length increases towards typically at most a third of the total blade span length in the radial direction 159. The largest rotor blade length is indicated by distance 163. From that point on, outwards towards the tip 42, the rotor blade length decreases. In FIG.1 , the span 16 shows the line with middle points of the rotor blade lengths in the spanwise direction.

[0036] Span 16 is a reference line, which lies in the midpoints between the rotor blade leading-edge 110 and the rotor blade trailing-edge 120. Note that span 16 is not necessarily a straight line. Still, a spanwise direction 160 can be indicated as the general direction along the span 16, therefore extending from connecting end to the tip 42 of the rotor blade. Generally, the spanwise direction 160 is about perpendicular to the airflow direction of air flowing along the pressure-side and suction side surfaces.

[0037] In embodiments, the leading-edge section 11 and the trailing-edge section 12 can be defined with respect to the span line 16. The portion of the rotor blade 10 located between the leading-edge 110 and the span line 16 is the leading-edge section 11. Similarly, the portion of the rotor blade 10 located between the rotor blade trailing-edge 120 and the span line 16 is the rotor blade trailing-edge section 12.

[0038] FIG.2 shows a schematic point of rotation 51 of the rotor blade 10. Rotor blade 10 is provided with an aerodynamic add-on structure 20. The aerodynamic add-on structure 20 is connected to the trailing-edge 120 and / or the trailing-edge section 12 of the rotor blade 10. In the radial or spanwise direction 160 of the rotor blade, the add-on 20 is comprised of several add-onelements 20 positioned adjacent one and other along the trailing-edge 120 of the rotor blade. As shown the width, extending in the spanwise direction 160, of the add-on elements 20 can vary element by element. Also, the length in the airflow direction 161 of the add-on elements 20 can vary.

[0039] The add-on 20 connects to the rotor blade trailing-edge 120 / trailing-edge section 12 via the add-on leading-edge (section) 201 (202).

[0040] The add-on 20 is provided to improve properties of the rotating rotor blade 10. The improvement can be any combination of reducing drag and / or reducing noise. The add-on 20 can provide several features to improve the properties of the rotation rotor blade 10. Although the below will mostly refer to the use of through-holes, other features of the add-on can similarly improve properties of the rotating rotor blade.

[0041] FIG. 3 shows a cross-sectional view of the rotor blade 100 and aerodynamic add-on structure across the assembly pressure-side surfaces 130,207 and across the assembly suctionside surfaces 140,206, which cross-section preferably is generally perpendicular to a radial 159 or spanwise 160 direction of the rotor blade 100.

[0042] FIG. 3 shows a detailed perspective view of a part of the wind turbine rotor blade 100 formed by the rotor blade 10 and add-on 20. In the following, the wind turbine rotor blade 100 including rotor blade 10 with add-on 20 provided at the trailing-edge 120 of the rotor blade, will be referred to as assembly 100.

[0043] The rotor blade 10 has a rotor blade suction-side surface 140 and a rotor blade pressure-side surface 130. The surfaces 130,140 start at rotor blade leading-edge 110 and extend from the rotor blade lead-edge section 11 to the rotor blade trailing-edge section 12, meeting meet at the rotor blade trailing-edge 120. The rotor blade trailing edge 120 is relatively pointy. Pressure-side air flows 131 are shown by arrows. Suction-side air flows 141 are also shown by arrows.

[0044] FIG.3 shows that add-on suction-side surface 206 is flush with the rotor-blade suction-side surface 140. In preferred embodiments, the add-on pressure-side surface 207 is flush with rotor blade pressure-side surface 130, although schematic FIG.3 shows a blunt add-on leading edge 201. It is noted that a smooth add-on leading-edge 201 of increasing thickness, allowing a flush and smooth transition between rotor blade pressure-side surface and add-on pressure-side surface is preferable, as will be illustrated with reference to FIGs 5A-5G.

[0045] FIG.3 shows the span line 16 extending between rotor blade leading-edge 11 and the rotor blade trailing-edge 12. As FIG.3 shows a cross-section across the rotor blade 10 and add-on 20, the span line 16 can be represented by a surface generally half-way between leadingedge 110 and trailing edge 120 of the rotor blade. The spanwise direction 160 is also indicated.

[0046] This cross-section of the assembly across the rotor blade suction-side and rotor blade pressure-side surfaces 140,130 and / or the add-on suction-side and add-on pressure-side surfaces 206,207, and generally perpendicular to the radial direction 159 of the rotor blade allows indicating and measuring a thickness of rotor blade 10 and / or add-on 20 or the assembly 100. Athickness of the assembly 100 at any point thereof can be defined as the shortest distance between a point on the suction-side surfaces 140,206 and pressure-side surfaces 130,207.

[0047] The thickness at a location of the assembly can be defined as the shortest distance between outer surface of the rotor blade assembly at that location. In preferred embodiments, the thickness can be determined by a distance (straight line crossing the location) between: a- the assembly pressure-surface, such as the rotor blade pressure-side surface or the add-on pressure-side surface and b- the assembly suction-side surface, such as the rotor blade suction-side surface or the add-on suction-side surface,

[0048] Thickness 168 is indicated in FIG.3 as an example thickness across the pressure / suction-side surfaces and perpendicular to the radial direction 159. Accordingly, the thickness will be a length along a straight line oriented generally vertical in the view of FIG.3 However, for curved blades, the thickness can be a straight line at a small angle with respect to the vertical in FIG 3.

[0049] FIG.3 also shows an example of a camber line. The rotor blade camber line 15 is generally drawn halfway between rotor blade suction-side surface 140 and rotor blade pressureside surface 130. As the rotor blade 10 is curved in the airflow direction 161 , the camber line 15 is curved. The camber line 15 connects the half-way points between pressure-side and suction side. The camber line 15 can help to identify the shortest distance / thickness of the blade, as the shortest distance will cross the middle point, and the shortest distance will thus be perpendicular to the camber line 15.

[0050] FIG.3 shows the rotor blade chord line 17, which generally is a straight line connecting the rotor blade leading-edge section 10, close to the leading-edge 110, with the rotor blade trailing-edge section 20, close to the trailing-edge 110. In embodiments, the camber line 15 and chord line 17 generally share the same starting and end points near the leading and trailing edges.

[0051] In the FIG. 3 embodiment, the distance 178 of the chord line 17 of the rotor blade to the rotor blade suction-side surface 140, preferably defined sufficiently orthogonal to the chord line of the rotor blade, is larger than the distance 179 of the rotor blade chord line 17 to the pressure-side surface, preferably defined sufficiently orthogonal to the rotor blade chord line.

[0052] A chordwise cross-section can be defined in the plane sufficiently parallel to the chord line and sufficiently orthogonal to the pressure-side surface and suction-side surface. FIG.3 shows such a chordwise cross-section. In such a cross-section, such as the chordwise crosssection or a cross-section along the radial direction 159, the ratios of the invention are applied.

[0053] The wind turbine rotor blade / assembly 100 provides an aerodynamic add-on structure 20 connected to the rotor blade trailing-edge section 12 rotor blade trailing-edge 120. An add-on leading edge 201 with add-on leading-edge section 202 connects the aerodynamic addon structure 20 to the rotor blade trailing-edge section 12.

[0054] In embodiments, the aerodynamic add-on structure 20 has acoustic communication channels formed by a plurality of through-holes 21. The through-holes are provided in the add-on between add-on leading edge 201 and add-on trailing edge 22. In the add-on trailing-edge section 220 through-holes 21 are provided. The add-on 20 is air flow permeable, as will be further explained hereinafter. The through-holes 21 connect opposite sides of the add-on 20. The through-holes connect the add-on suction-side surface 206 with the add-on pressure-side surface 207.

[0055] FIG. 3 shows a repeating pattern of through-holes in the add-on. The density of the number of through-holes in the add-on 20 is shown to be constant. In other embodiments, the density of through-holes increases towards the add-on trailing-edge 22. Also, the size of the inlet (the surface area of the through-hole at the suction-side surface 206) can be varied. In embodiments, the size of the inlet decreases closer to the add-on trailing-edge 22.

[0056] The through-holes 7, that is the cavity in the add-on, can have a shape selected from the group comprising conical shape, pyramidal shape, honeycomb-line shape, and hourglass shape. The through-holes have a surface that connects continuously and flush with the respect add-on pressure-side and suction-side surfaces. A smooth transition into the through-hole is preferred. The add-on can be a moulded add-on. The add-on can be formed from a thermoplastic resin.

[0057] FIG.3 shows the add-on with extensions 90,91. The extensions extend the add-on in the airflow direction 161. The one or more extensions extend from the add-on trailing section. As a result, the trailing edge is further away from the leading-edge section and / or leading-edge. The shortest distance between the add-on leading edge and the add-on trailing edge that ends at the end of the extension will be longer at the extension 90, 91 and shorter at the valley 92.

[0058] The extensions 90, 91 can be shaped to have a repeating pattern in the rotor blade spanwise dimension. Examples include the following patterns: (a) straight, (b) zig-zag, (c) sinusoidal, (d) concave half-sinusoidal, (e) convex half-sinusoidal.

[0059] Also, for the add-on 20, a span line can be defined extending in the spanwise direction 160. The add-on also has a camber line. As add-on 20 has a more exotic shape / crosssection near the leading edge 201 as shown in FIG.3, the add-on camber line 15 can generally be drawn halfway between add-on suction-side surface 140 and add-on pressure-side surface 130 starting in the add-on leading-edge section 202 downstream of the rotor blade trailing edge 120 to the add-on trailing-edge 22. Also for the add-on, an add-on chord line can be defined, which generally is a straight line connecting the add-on leading-edge section 202, close to the trailing- edge 120, with the add-on trailing-edge section 220, close to the trailing-edge 22. In embodiments, the camber line 15 and chord line 17 generally share the same starting and end points near the leading and trailing edges. In embodiments, the start and end points of the camber line and chord line can be moved downstream and upstream from the respective leading and trailing edges in function of exotic shapes of the leading and trailing edges.

[0060] The transition of rotor blade 10 to add-on 20 in the embodiment of FIG.3 shows an overlapping part formed by a leading-edge section 202 of the add-on 20 extending off the rotorblade pressure-side surface 130.

[0061] Also for the add-on, the thickness 20 at any point thereof can be defined as the shortest distance between a point on the add-on suction-side surface 206 and add-on pressureside surface 207, which shortest line will preferably be generally perpendicular to the radial direction 159. An add-on thickness 189 is indicated in FIG.3 ahead of the first through-hole 21 , which is the through-hole that is positioned closest to the leading edge 201 of the add-on. The thickness of the assembly near the add-on leading edge 201 and rotor blade the trailing edge 120 can be a thickness across a part of the add-on 20 and a part of the rotor blade 10.

[0062] A first thickness 260 at the add-on leading-edge 201 , which is the thickness of the assembly at the add-on leading edge 201 is defined as the shortest distance between the respective pressure-side and suction-side surfaces of the assembly. In the embodiment of FIG.3 the first thickness 260 is the shortest distance between the rotor blade suction-side surface 140 and the add-on pressure-side surface 207 near the add-on leading-edge 201.

[0063] A second thickness 261 is the thickness of the add-on 20 at the leading through-hole 218 positioned closest to the add-on leading-edge 201. In the embodiment of FIG.3, the second thickness 261 is the shortest distance between the add-on suction-side surface 206 and the addon pressure-side surface 207 at the leading through-hole 218. In embodiments, the thickness is taken through the centre of the through-hole.

[0064] A third thickness 262 is the thickness of the add-on trailing through-hole 228 positioned closest to the add-on trailing edge 22. In the embodiment of FIG.3, the third thickness 262 is the shortest distance between the add-on suction-side surface 206 and the add-on pressure-side surface 207 at the trailing through-hole 228.

[0065] The first, second, and third thickness are preferably thicknesses of the assembly / rotor blade / add-on taken in the same / one cross-sectional plane, such as the cross-section shown in FIG.3. The first-third thicknesses 260-262 in the same plane allow to identify aerodynamic properties of the aerodynamic add-on structure 20 for rotor blade 10.

[0066] The following table shows embodiments of the thickness:0067] Similar to defining thicknesses for the assembly / rotor blade / add-on, lengths and distances can be defined for the assembly / rotor blade / add-on. The (flat) plane of the crosssection of the assembly as shown in FIG 3, which is a flat plane across the rotor blade suctionside and rotor blade pressure-side surfaces 140,130 and / or the add-on suction-side and add-on pressure-side surfaces 206,207, and generally perpendicular to the radial direction 159 of the rotor blade, allows indicating and measuring a lengths of assembly 100, rotor blade 10 and / or add-on 20. A length or distance of the assembly 100 in the cross-section perpendicular to the radial direction 159 can be defined as the shortest distance between two points. In embodiments, the point, such as a through-hole, has a size. The distance can be to any of the points forming the finite size of the through-hole. Preferably however, the distance to a through-hole is measured from a centreline of the through-hole and / or the connecting line through the centers of the inlet / outlet of the through-hole.

[0068] The first through-hole distance 381 is defined as the shortest distance between addon leading edge 201 and the through-hole 261 positioned closest to the add-on leading edge 201. In the section shown in FIG 3, through-hole 218 is positioned closest to the leading edge 201 of the add-on. In this embodiment, a row of through-holes in the radial direction 159 is formed in the add-on, which together form the row of through-holes that are closest to the add-on leading edge 201 . In other embodiments, the through-hole closest to the leading edge can be a through-hole located downstream according to air flow direction 161 from the through-hole 218.

[0069] The second through-hole distance 380 is defined as the shortest distance between add-on leading edge 201 and the through-hole 228 positioned closest to the add-on trailing edge 201 . In the section shown in FIG 3, through-hole 228 is positioned closest to the trailing edge 22 of the add-on. However, in other sections, through-hole 229 can be the closest to the add-on trailing edge 22.

[0070] The third through-hole distance 382 is defined as the shortest distance between a through-hole 261 positioned closest to the add-on leading edge 201 and a through-hole 228 positioned closest to the add-on trailing edge 22.

[0071] In embodiments, the second through-hole length 380 is equal to the first through- hole distance 381 and the third through-hole distance 382.

[0072] Although the section of FIG.3 shows the through-holes 218,228 in a single section, it is possible that the first and last through-hole are not in the same plane perpendicular to theradial direction 159. Then through-holes closest to that plane are to be defined as the closest to the leading and trailing-edges of the add-on. Similarly, thickness of leading and trailing through- holes can be defined closest to a plane similar to the section of FIG.3.

[0073] In the FIG.3 embodiment, several, in casu four, through-holes are positioned along the shortest line connecting the add-on trailing edge 220 at the illustrated section to the add-on leading edge at the illustrated section of FIG. 3.

[0074] The add-on length 390 can defined as the shortest distance between the add-on leading edge 201 and the add-on trailing edge 22. In some embodiments, the add-on length 390 and the second through-hole distance 380 are close to interchangeable. In embodiments, the add-on length 390 is equal to the second through-hole distance 380 plus the length of the extension.

[0075] As a result of the extensions 90,91 , the add-on length 390 varies dependent on the location of the plane of the cross-section generally perpendicular to the radial direction 159.

[0076] The add-on 20 can have two or more extensions 90,91 , which form the add-on trailing edge. Extension 90 has one through-hole, that is positioned more downstream than the add-on length at a position or valley 92 between the extensions 90,91.

[0077] The following table shows embodiments of the distances:

[0078] Embodiments 7-12 can be examples in which the add-on length is measured in a section that includes the extension.

[0079] Any of the embodiments 1-18 for the thickness can be combined with the 1-23 examples for the distances. Specifically, the combinations of thickness embodiment 1 + distances embodiment 1 can be made. Specifically, the combinations of thickness embodiment 14 + distances embodiment 17 can be made. Specifically, the combinations of thickness embodiment 5 + distances embodiment 3 can be made. Specifically, the combinations of thickness embodiment 13 + distances embodiment 19 can be made. Specifically, the combinations of thickness embodiment 4 + distances embodiment 15 can be made. Specifically, the combinations of thickness embodiment 18 + distances embodiment 23 can be made. Specifically, the combinations of thickness embodiment 9 + distances embodiment 7 can be made. Specifically, the combinations of thickness embodiment 15 + distances embodiment 16 can be made. Specifically, the combinations of thickness embodiment 2 + distances embodiment 22 can be made. Specifically, the combinations of thickness embodiment 17 + distances embodiment 18 can be made. Specifically, the combinations of thickness embodiment 6 + distances embodiment 10 can be made. Specifically, the combinations of thickness embodiment 16 + distances embodiment 21 can be made. Specifically, the combinations of thickness embodiment 11 + distances embodiment 14 can be made. Specifically, the combinations of thickness embodiment 8 + distances embodiment 20 can be made. Specifically, the combinations of thickness embodiment 12 + distances embodiment 9 can be made. Specifically, the combinations of thickness embodiment 7 + distances embodiment 6 can be made. Specifically, the combinations of thickness embodiment 10 + distances embodiment 23 can be made. Specifically, the combinations of thickness embodiment 3 + distances embodiment 19 can be made. Specifically, the combinations of thickness embodiment 18 + distances embodiment 11 can be made. Specifically, the combinations of thickness embodiment 13 + distances embodiment 5 can be made. Specifically, the combinations of thickness embodiment 16 + distances embodiment 15 can be made.

[0080] FIG. 3, and as discussed below FIG.4 and FIGs 5A-5E, illustrate a wind turbine rotor blade with an aerodynamic add-on structure to reduce noise. The wind turbine rotor blade is formed by an assembly 100 of a rotor blade 10 and an aerodynamic add-on structure 20. The rotor blade can have a rotor blade leading-edge section 11 having a rotor blade leading-edge 110. The rotor blade 10 has a rotor blade trailing-edge section 12 having a rotor blade trailing- edge 120. The rotor blade 10 has a rotor blade pressure-side surface 130 and a rotor blade suction-side surface 140, which extend between the rotor blade leading-edge section and the rotor blade trailing-edge section. The aerodynamic add-on structure has a plurality of through- holes 21 , 218, 228 for reducing noise. The add-on 20 is provided at the rotor blade trailing-edge section 12 of the rotor blade. The add-on has an add-on leading-edge section 202 with an add-onleading-edge 201. The add-on also has an add-on trailing-edge section 220 with an add-on trailing-edge 22. An add-on suction-side surface 206 and an add-on pressure-side surface 207 extends between the add-on leading-edge section to the add-on trailing-edge section.

[0081] The inventors have found that the properties for noise reduction and / or drag reduction for rotor blades using add-ons can be improved by adapting predetermined ratios for thicknesses and lengths into the assembly.

[0082] As indicated above, the thicknesses and lengths, and thus the ratios according to the invention, can be defined in a cross-section of the assembly across the rotor blade pressure-side and rotor blade suction-side surfaces. Similarly, the parameters can be defined in the cross section across the add-on pressure-side and add-on suction-side surfaces. The cross-sections can be cross-sections that in a plane generally perpendicular to a radial or spanwise direction of the rotor blade.

[0083] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly wherein the ratio of the first thickness 260 to the second thickness 261 is larger than 1 .4, preferably larger than 1.5, and more preferably between 2 and 5, because the noise-reducing characteristics of the leading through- hole 218 positioned nearest to the add-on leading-edge 201 is significantly improved, thereby significantly improving the noise-reducing characteristics of all subsequent through-holes in a direction from the add-on leading-edge 201 to the add-on trailing-edge 22, thereby significantly reducing the noise of the airflows 131 , 141 over the rotor blade 10 as shown in FIG. 6.

[0084] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the following ratio:{first thickness 260 — the second thickness 261} first through — hole distance 381 is between 0.04 and 0.3, preferably between 0.05 and 0.2, and more preferably between 0.1 and 0.15, because these ratio values define a slope that strengthens the noise-reduction mechanism of the leading through-hole 218 positioned nearest to the add-on leading-edge 201 , thereby aiding the noise-reducing mechanism of all subsequent through-holes in a direction from the addon leading-edge 201 to the add-on trailing-edge 22, thereby significantly reducing the noise of the airflows 131 ,141 over the rotor blade 10.

[0085] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the ratio of the first through-hole distance 381 to the second through-hole distance 380 is more than 0.05, preferably more than 0.1, more preferably more than 0.2, and / or less than 0.5, preferably less than 0.4.These ratios obtain good results in combination with first and second thickness of more than 3mm and less than 2.1 mm, respectively.

[0086] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the following ratio:first thickness 260 — second thickness 261 second thickness 261 — third thickness 262 is at least 0.6, preferably at least 0.8, more preferably at least 1 and most preferably at least 2, and / or less than 12, preferably less than 10, and preferably between 2-10, more preferably between 3-8, as these ratios guide the airflows 131 ,141 as to significantly improve the distribution of noise-reduction properties of the through-holes starting from the leading through-hole 218 positioned nearest to the add-on leading-edge 201 and ending at the trailing through-hole 228 positioned nearest to the add-on trailing-edge 22, thereby significantly reducing the noise of the airflows 131 , 141 over the rotor blade 10.

[0087] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the ratio of the first thickness 260 to the third thickness 262 is between larger than 1 .4, preferably larger than 1.5, more preferably larger than 2 and / or less than 10, more preferably less than 8, and more preferably less than 4.5, and most preferably between 3-7, as these ratios strongly balance the proportion of noise-reduction properties along the through-holes starting from the leading through-hole 218 positioned nearest to the add-on leading-edge 201 and ending at the trailing through-hole 228 positioned nearest to the add-on trailing-edge 22, thereby significantly reducing the noise of the airflows 131 , 141 over the rotor blade 10.

[0088] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the following ratio: first thickness 260 — third thickness 262 second through — hole distance 380 is more than 0.02, preferably more than and 0.025, and / or less than 0.3, preferably less than 0.25, more preferably less than 0.2 or less than 0.1, and most preferably between 0.02 and 0.25, and even more preferably between 0.03 and 0.06, because these ratio values define a slope that maximizes the noise-reduction potential of the trailing through-hole 228 positioned nearest to the add-on trailing-edge 22, thereby significantly reducing the noise of the airflows 131 ,141 over the rotor blade 10.

[0089] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the ratio of the first through-hole distance 381 to the second through-hole distance 380 is more than 0.05, preferably more than 0.1, more preferably more than 0.2, and / or less than 0.5, preferably less than 0.4, as this helps in preparing the airflows 131 , 141 for significantly better receptivity to the noisereducing working mechanism of the plurality of through-holes, thereby significantly reducing the noise of the airflows 131 ,141 over the rotor blade 10.

[0090] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the ratio of the secondthickness 261 to the third thickness 262 is between 1 and 3, preferably between 1.5-2.9, more preferably between 1.56- 2.1 , as this significantly helps in the targeting of specific noisegeneration mechanisms in the airflows 131 ,141 by the plurality of through-holes starting from the leading through-hole 218 positioned nearest to the add-on leading-edge 201 and ending at the trailing through-hole 228 positioned nearest to the add-on trailing-edge 22, thereby significantly reducing the noise of the airflows 131 , 141 over the rotor blade 10.

[0091] The inventors discovered that the noise and / or the drag of the airflows 131 ,141 over the rotor blade 10 can be reduced by a design of the assembly, wherein the following ratio: second thickness 261 — third thickness 262 third through — hole distance 382 is between 0.005 and 0.15, preferably between 0.007 and 0.1 , more preferably between 0.01- 0.03, since this significantly improves the low-friction guidance of airflows 131 ,141 over the region of the add-on 20 which contains the plurality of through-holes 21 , 218, 228 as to achieve a noisereducing add-on 20 with low drag.

[0092] For any of the ratios, the section of the rotor blade with add-on can be a plane having local minimum add-on length, that is, a section across a valley 92.

[0093] Any of the ratios can be combined with a minimum difference between the first thickness and second thickness of 2mm. Any of the ratios can be combined with a maximum second thickness of 2.2mm. Any of the ratios can be combined with a minimum third thickness of 0.5mm. Any of the ratios can be combined with a maximum difference between the second thickness and third thickness of 1.0mm. Any of the ratios can be combined with a minimum first through-hole distance of 10mm. Any of the ratios can be combined with a maximum second through-hole distance of 200mm. Any of the ratios can be combined with a minimum third through-hole distance of 30mm.

[0094] FIGs. 4A and 4B illustrate further examples of embodiments according to one or more of the aspects of the invention.

[0095] Both FIGs. 4A and 4B show an assembly 100 of rotor blade 10 and add-on 20 is shown. The rotor blade trailing-edge 120 abuts at an interface the add-on leading-edge 201. A fixation, such as glue or a mechanical connection, is present to connect the rotor blade and addon.

[0096] At the interface, the first thickness 260 is more than double the size of the second thickness 261 at the first through-hole 218. Between leading-edge section of the add-on 202 continuously reduces in thickness 202 from first thickness 260 to second thickness 261 at the first pore 261. The first through-hole distance 381 is at least 12mm to allow the reduction of the thickness in the add-on leading-edge section 202 to reduce, without disturbing the air flows 131 ,141 on the suction-side and pressure-side of the add-on.

[0097] Downstream of the front through-hole 218, more through-holes are formed, including a last through-hole 228 at a second through-hole distance 380. A last through-hole 228 is closestto the trai I i ng-edge of the add-on 20. In the illustrated embodiment of FIGs.4A and 4B, the third through-hole distance 382 > first through-hole distance 380.

[0098] However, in FIGs.4A and 4B the thickness of the add-on 20 decreases only in a limited amount between first through-hole 218 and last through-hole 228. In fact, the decrease in thickness is less than 50%, preferably less than 40%, more preferably less than 25%. Any add- on / assembly can have such a limited decrease in thickness.

[0099] In FIGs.4A,4B, a cross-section of the assembly 100 is provided through a valley 92 between extensions 90,91. The cross-section shows an add-on without extensions. In embodiments, the add-on is without extensions.

[0100] FIGs. 4A and 4B also show the rotor blade chord line 17 and the add-on chord line 23. Rotor blade chord line 17, shown extended downstream of the trailing edge 120, connects a rotor blade leading-edge section halfway point to a rotor blade trailing-edge section halfway point. The halfway points are points about halfway between the rotor blade pressure-side surface and rotor blade suction-side surface at the rotor blade trailing-edge section, preferably close to (less than 10%, preferably less than 5% of the total length) rotor blade trailing edge 120. The add-on chord line 23 connects an add-on leading-edge section halfway point with an add-on trailing-edge section halfway point. The halfway points are points about halfway between the add-on pressureside surface and add-on suction-side surface of the add-on trailing-edge section, preferably close to the respective leading / trailing edges

[0101] In FIG. 4a, the add-on chord line 23 is tilted with respect to the rotor blade chord line 17 by an angle 410 towards the pressure-side surface. This angle is about 9 degrees. In embodiments, the angle 410 can be between 0-12, preferably between 0-10 degrees..

[0102] In FIG. 4b, the add-on chord line 23 is tilted with respect to the rotor blade chord line 17 by an angle 410 towards the suction-side surface. The angle 410 with respect to the rotor blade chord line 17 towards the pressure is about -4 degrees. In embodiments, the angle 410 can be between -10 and 0, preferably between -6 and 0 degrees.

[0103] In embodiments, the angle 410 (with respect to the rotor blade chord line 17 towards the pressure-side surface) is between -10 to 20 degrees, preferably between -5 to 15 degrees, and most preferably between 1-10 degrees. -10 degrees is an angle of 10 degrees tilted towards the suction-side surface.

[0104] In the embodiments of FIGs. 4A and 4B, the rotor blade chord line 17 and the addon chord line 23 meet at the rotor blade trailing-edge 120.

[0105] As can be seen in the add-on leading-edge section 202, the distance between the add-on pressure-side surface and chord line 23 decreases from leading edge to trailing edge faster than the distance between suction-side surface and chord line 23. In the add-on leadingedge section 202, the shortest distance between the add-on chord line 23 and the suction-side surface is shorter than a shortest distance between the add-on chord line and the pressure-side surface.

[0106] In other embodiments, the shortest distance between the add-on chord line 23 and the pressure-side surface is larger than a shortest distance between the add-on chord line and the suction-side surface.

[0107] FIGs 5A-5E show similar cross-sections as the cross-section of FIG.4 with similar reference numerals for similar parts.

[0108] First, second, and third thickness 260-262 are indicated in line with the provided definitions. First, second, and third through-hole distances 380-382 are indicated in line with the provided definitions.

[0109] FIG. 5A shows a thin add-on leading-edge section 2020. The larger part of the interface 40 is at the rotor blade trailing edge 120.

[0110] FIG. 5B shows a thin add-on leading-edge section 2021 and adhesive 25 for connecting that thin add-on leading-edge section 2021 to a part of the rotor blade trailing-edge section 1200. The larger part of the interface 40 is at the rotor blade trailing edge 120.

[0111] FIG. 5C shows an add-on leading-edge section 2022, which is formed as a flange that is received in a cavity on rotor blade pressure side surface of the rotor blade trailing-edge section 1201. The larger part of the interface 40 is at the rotor blade trailing edge 120. The interface 40 is acute with respect to the thickness, providing a larger surface area for allowing connection of the add-on 20 to the rotor blade 10.

[0112] FIG. 5D shows a cavity on the pressure-side of both the rotor blade trailing-edge section 1202 and the add-on leading-edge section 2023. Another trailing edge attachment, such as a plate 24, is received in the cavities and aides in connecting the add-on 20 to the rotor blade 10. Here the interface 40 and add-on leading-edge are generally located at the same position.

[0113] FIG. 5E shows a rotor blade trailing-edge section 1203 connected via adhesive 24 to the leading-edge section 2024 of the add-on 20. The add-on leading-edge is provided upstream from the adhesive that forms the interface 40.

[0114] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. For example, different combinations of thickness and distances may be combined based on the disclosed thicknesses and distances.

[0115] Further embodiments are provided in the following clauses. The clauses can, individually or in combination, be combined with any of the embodiments or clauses or claims disclosed herein.[Clause 1 .] A wind turbine rotor blade with an aerodynamic structure to reduce noise, formed by an assembly of a rotor blade and an aerodynamic add-on structure, wherein the rotor blade comprises: a rotor blade leading-edge section having a rotor blade leading-edge, a rotor blade trailing-edge section having a rotor blade trailing-edge, anda rotor blade pressure-side surface and a rotor blade suction-side surface, the rotor blade pressure-side surface and rotor blade suction-side surface extending between the rotor blade leading-edge section and the rotor blade trailing-edge section, wherein the aerodynamic add-on structure has a plurality of through-holes for reducing noise, the add-on provided at the rotor blade trailing-edge section of the rotor blade, wherein the add-on has: an add-on leading-edge section with an add-on leading-edge, an add-on trailing-edge section with an add-on trailing-edge, and an add-on pressure-side surface and an add-on suction-side surface, which surfaces extend from the add-on leading-edge section to the add-on trailing-edge section.[Clause 2.] Wind turbine according to any of the clauses or any of the embodiments disclosed herein or any of the claims. wherein in a cross-section of the assembly across the rotor blade pressure-side and rotor blade suction-side surfaces and / or the add-on pressure-side and add-on suction-side surfaces, which cross-section preferably is generally perpendicular to a radial direction of the rotor blade: the assembly has a first thickness at the add-on leading edge, the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, and a ratio of the first thickness to the second thickness is larger than 1 .4.[Clause 3.] Wind turbine according to clause 2, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of the first thickness to the second thickness is larger than 1.5.[Clause 4.] Wind turbine according to clause 2, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of the first thickness to the second thickness is between 2 and 5.[Clause 5.] Wind turbine according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein in a cross-section of the assembly across the rotor blade pressure-side and rotor blade suction-side surfaces and / or the add-on pressure-side and add-on suction-side surfaces, which cross-section preferably is generally perpendicular to a radial direction of the rotor blade: the assembly has a first thickness at the add-on leading edge, the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, the assembly has a first through-hole distance defined as the shortest distance between the add-on leading edge and a through-hole positioned closest to the add-on leading edge, and wherein a ratio of {the first thickness minus the second thickness} to the first through-hole distance is between 0.04 and 0.3.[Clause 6.] Wind turbine according to clause 5, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to the first through-hole distance is between 0.05 and 0.2.[Clause 7.] Wind turbine according to clause 5, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to the first through-hole distance is between 0.1 and 0.15.[Clause 8.] Wind turbine according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein in the cross-section of the assembly across the rotor blade pressure-side and suction-side surfaces, preferably in a plan perpendicular to the radial direction of the rotor blade, the assembly has a first thickness at the add-on leading edge, the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, and a ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is at least 0.6.[Clause 9.] Wind turbine according to clause 8, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is at least 0.8.[Clause 10.] Wind turbine according to clause 8, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is at least 1.[Clause 11.] Wind turbine according to clause 8, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is at least 2.[Clause 12.] Wind turbine according to any of the clauses 8-11 , according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is less than 12.[Clause 13.] Wind turbine according to any of the clauses 8-11 , according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is less than 10.[Clause 14.] Wind turbine according to clause 8, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is between 2-10.[Clause 15.] Wind turbine according to clause 8, according to any of the clauses or any of the embodiments disclosed herein or any of the claims, wherein the ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is between 3-8.[Clause 16.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, characterized in that in the cross-section of theassembly across the pressure-side and suction-side surfaces, preferably in a plane perpendicular to the radial direction of the rotor blade: the assembly has a first thickness at the add-on leading edge, the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, and a ratio of the first thickness to the third thickness is larger than 1 .4.[Clause 17.] Wind turbine rotor blade according to clause 16, wherein the ratio of the first thickness to the third thickness is larger than 1 .5.[Clause 18.] Wind turbine rotor blade according to clause 16, wherein the ratio of the first thickness to the third thickness is larger than 2.[Clause 19.] Wind turbine rotor blade according to clause 16, wherein to the ratio of the first thickness to the third thickness is less than 10.[Clause 20.] Wind turbine rotor blade according to clause 16, wherein to the ratio of the first thickness to the third thickness is less than 8.[Clause 21.] Wind turbine rotor blade according to clause 16, wherein the ratio of the first thickness to the third thickness is less than 4.5.[Clause 22.] Wind turbine rotor blade according to clause 16, wherein the ratio of the first thickness to the third thickness is between 3-7.[Clause 23.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a first thickness at the add-on leading edge, the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, the assembly has a second through-hole distance defined as the shortest distance between the add-on leading edge and the through-hole positioned closest to the add-on trailing edge, and wherein a ratio of {the first thickness minus the third thickness} to the second through-hole distance is more than 0.02.[Clause 24.] Wind turbine rotor blade according to clause 23, wherein the ratio of {the first thickness minus the third thickness} to the second through-hole distance is more than 0.025.[Clause 25.] Wind turbine rotor blade according to clause 23 or 24, wherein the ratio of {the first thickness minus the third thickness} to the second through-hole distance is less than 0.3.[Clause 26.] Wind turbine rotor blade according to clause 23 or 24, wherein the ratio of {the first thickness minus the third thickness} to the second through-hole distance is less than 0.25.[Clause 27.] Wind turbine rotor blade according to clause 23 or 24, wherein the ratio of {the first thickness minus the third thickness} to the second through-hole distance is less than 0.2.[Clause 28.] Wind turbine rotor blade according to clause 23 or 24, wherein the ratio of {the first thickness minus the third thickness} to the second through-hole distance is less than 0.1 .[Clause 29.] Wind turbine rotor blade according to clause 23, wherein the ratio of {the first thickness minus the third thickness} to the second through-hole distance is between 0.02 and 0.25.[Clause 30.] Wind turbine rotor blade according to clause 23, wherein the ratio of {the first thickness minus the third thickness} to the second through-hole distance is between 0.03 and 0.06.[Clause 31.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a first through-hole distance defined as the shortest distance between add-on leading edge and a through-hole positioned closest to the add-on leading edge, the assembly has a second through-hole distance defined as the shortest distance between the add-on leading edge and the through-hole positioned closest to the add-on trailing edge, and wherein a ratio of the first through-hole distance to the second through-hole distance is more than 0.05.[Clause 32.] Wind turbine rotor blade according to clause 31 , a ratio of the first through-hole distance to the second through-hole distance is more than 0.1.[Clause 33.] Wind turbine rotor blade according to clause 31 , a ratio of the first through-hole distance to the second through-hole distance is more than 0.2.[Clause 34.] Wind turbine rotor blade according to clause 31 or 32 or 33, a ratio of the first through-hole distance to the second through-hole distance is less than 0.5.[Clause 35.] Wind turbine rotor blade according to clause 31 or 32 or 33, a ratio of the first through-hole distance to the second through-hole distance is less than 0.4.[Clause 36.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, and wherein a ratio of the second thickness to the third thickness is between 1 and 3.[Clause 37.] Wind turbine rotor blade according to clause 36, wherein the ratio of the second thickness to the third thickness is between 1.5-2.9.[Clause 38.] Wind turbine rotor blade according to clause 36, wherein the ratio of the second thickness to the third thickness is between 1.56- 2.1 ,[Clause 39.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, the assembly has a third through-hole distance defined as the shortest distance between the through-hole positioned closest to add-on leading edge and the through-hole positioned closest to the add-on trailing edge, andwherein a ratio of {the second thickness minus the third thickness} to the third through-hole distance is between 0.005 and 0.15.[Clause 40.] Wind turbine rotor blade according to clause 39, wherein the ratio of {the second thickness minus the third thickness} to the third through-hole distance is between is 0.007 and 0.1.[Clause 41.] Wind turbine rotor blade according to clause 39, wherein the ratio of {the second thickness minus the third thickness} to the third through-hole distance is between 0.01-0.03. [Clause 42.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a thickness at a location of the assembly is defined as the shortest distance between outer surfaces of the assembly at that location, preferably a shortest distance, between a- the outer assembly (rotor blade or add-on) pressure-side surface and b- the outer assembly (rotor blade or add-on) suction-side surface.[Clause 43.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the cross-section of the assembly across the rotor blade / add-on pressure-side and rotor blade / add-on suction-side surfaces includes the shortest distance between the rotor blade / add-on pressure-side surface and rotor blade / add-on suction-side surfaces, preferably in a plane perpendicular to the radial direction of the rotor blade. [Clause 44.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the cross-section of the assembly across the rotor blade / add-on pressure-side and rotor blade / add-on suction-side surfaces includes a shortest distance between the add-on leading edge and a through-hole positioned closest to the add-on trailing edge.[Clause 45.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the add-on extends in a radial dimension along the rotor blade, wherein any of the ratios are present in a middle part of the radial dimension of the add-on, or are present at at least two, three or more positions along the radial dimension of the add-on. [Clause 46.] Wind turbine rotor blade according to clause 45, according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the two, three, or more positions are separated by at least 10% of the radial dimension of the add-on.[Clause 47.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a thickness at a location of the assembly is defined as the shortest distance between outer surfaces of the assembly at that location, preferably a shortest distance, between a- the outer assembly (rotor blade or add-on) pressure-side surface and b- the outer assembly (rotor blade or add-on) suction-side surface,wherein the assembly has a first thickness at the add-on leading edge, wherein the first thickness is at least 2.5mm.[Clause 48.] Wind turbine rotor blade according to clause 47, wherein the first thickness is at least 3mm.[Clause 49.] Wind turbine rotor blade according to clause 47, wherein the first thickness is at least 3.5mm[Clause 50.] Wind turbine rotor blade according to clause 47 or 48 or 49, wherein the first thickness is at most 20mm.[Clause 51 .] Wind turbine rotor blade according to clause 47 or 48 or 49, wherein the first thickness is at most 10mm.[Clause 52.] Wind turbine rotor blade according to clause 47 or 48 or 49, wherein the first thickness is at most 5mm,[Clause 53.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the add-on has a second thickness at a through-hole positioned closest to the add-on leading edge, wherein the second thickness is at most 2.5mm.[Clause 54.] Wind turbine rotor blade according to clause 53, wherein the second thickness is at most 2.2mm.[Clause 55.] Wind turbine rotor blade according to clause 53, wherein the second thickness is at most 2.05mm.[Clause 56.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, wherein the third thickness is at least 0.05mm.[Clause 57.] Wind turbine rotor blade according to clause 56, wherein the third thickness is at least 0.1 mm.[Clause 58.] Wind turbine rotor blade according to clause 56, wherein the third thickness is at least 0.5mm.[Clause 59.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a first through-hole distance defined as the shortest distance between the add-on leading edge and a through-hole positioned closest to the add-on leading edge, wherein the first through-hole distance is at least 5mm.[Clause 60.] Wind turbine rotor blade according to clause 59, wherein the first through-hole distance is at least 8mm.[Clause 61 .] Wind turbine rotor blade according to clause 59, wherein the first through-hole distance is at least 10mm.[Clause 62.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a second through-hole distance defined as the shortest distance between the add-on leading edge and a through-holepositioned closest to the add-on trailing edge, wherein the second through-hole distance is at least 10mm.[Clause 63.] Wind turbine rotor blade according to clause 62, wherein the second through-hole distance is at least 15mm.[Clause 64.] Wind turbine rotor blade according to clause 62, wherein the second through-hole distance is at least 20mm.[Clause 65.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the assembly has a third through-hole distance defined as the shortest distance between a through-hole positioned closest to the addon leading edge and a through-hole positioned closest to the add-on trailing edge, wherein the third through-hole distance is at least 4mm.[Clause 66.] Wind turbine rotor blade according to clause 65, wherein the third through-hole distance is at least 6mm.[Clause 67.] Wind turbine rotor blade according to clause 65, wherein the third through-hole distance is at least 10mm.[Clause 68.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the add-on has an add-on length defined as the shortest distance between the add-on leading edge and the add-on trailing edge, wherein the add-on length is at least 11 mm.[Clause 69.] Wind turbine rotor blade according to clause 68, wherein the add-on length is at least 15mm.[Clause 70.] Wind turbine rotor blade according to clause 68, wherein the add-on length is at least 20mm.[Clause 71.] Wind turbine rotor blade according to clause 68, wherein the add-on length is at least 25mm.[Clause 72.] Wind turbine rotor blade according to clause 68 or 69 or 70 or 71 , wherein the addon length is at most 400mm.[Clause 73.] Wind turbine rotor blade according to clause 68 or 69 or 70 or 71 , wherein the addon length is at most 300mm.[Clause 74.] Wind turbine rotor blade according to clause 68 or 69 or 70 or 71 , wherein the addon length is at most 200mm.[Clause 75.] Wind turbine rotor blade according to clause 68 or 69 or 70 or 71 , wherein the addon length is at most 150mm.[Clause 76.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the aerodynamic add-on structure has at least one or more extensions forming the add-on trailing edge.[Clause 77.] Wind turbine rotor blade according to clause 76, wherein the one or more extensions extend from the add-on trailing section.[Clause 78.] Wind turbine rotor blade according to clause 76 or 77, wherein the aerodynamic addon structure has a total add-on length defined as the shortest distance between add-on leadingedge and the add-on trailing edge in a cross-section through the add-on pressure-side surface and suction-side surface, which cross-section is a plane perpendicular to the radial direction of the rotor blade, wherein the add-on total length varies along a spanwise dimension of the add-on. [Clause 79.] Wind turbine rotor blade according to clause 76 or 77 or 78, wherein the add-on trailing edge comprises a repeating pattern of extensions in the rotor blade spanwise dimension. [Clause 80.] Wind turbine rotor blade according to clause 79, wherein the repeating pattern of extensions in the rotor blade spanwise dimension includes at least one of the following patterns:(a) straight,- (b) zig-zag,(c) sinusoidal,(d) concave half-sinusoidal, and / or(e) convex half-sinusoidal.[Clause 81.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the add-on comprises one, two, or more through-holes along a shortest line connecting the add-on trailing edge to the add-on leading edge.[Clause 82.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the add-on has two or more extensions forming the add-on trailing edge, and the two or more extensions have one, two, or more through- holes along a shortest line connecting the add-on trailing edge formed by the extension to the add-on leading edge.[Clause 83.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a rotor blade leading-edge section halfway point is a point about halfway between rotor blade pressure-side surface and rotor blade suction-side surface at the rotor blade leading-edge section.[Clause 84.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a rotor blade trailing-edge section halfway point is a point about halfway between the rotor blade pressure-side surface and rotor blade suction-side surface at the rotor blade trailing-edge section.[Clause 85.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein an add-on leading-edge section halfway point is a point about halfway between the add-on pressure-side surface and add-on suction-side surface at the add-on leading-edge section.[Clause 86.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein an add-on trailing-edge section halfway point is a point about halfway between the add-on pressure-side surface and add-on suction-side surface of the add-on trailing-edge section.[Clause 87.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a rotor blade chord line is defined by ashortest straight line connecting the rotor blade leading-edge section halfway point and the rotor blade trailing-edge section halfway point.[Clause 88.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein an add-on chord line is defined by a shortest straight line connecting the add-on leading-edge section halfway point and the add-on trailing-edge section halfway point.[Clause 89.] Wind turbine rotor blade according to clause 88, according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the add-on chord line is tilted with respect to the rotor blade chord line.[Clause 90.] Wind turbine rotor blade according to clause 89, wherein the add-on chord line is tilted towards the pressure-side surface of the wind turbine rotor blade.[Clause 91.] Wind turbine rotor blade according to clause 89, wherein the add-on chord line is tilted with respect to the rotor blade chord line towards the pressure-side surface of the wind turbine rotor blade with an angle between -10 to 20 degrees.[Clause 92.] Wind turbine rotor blade according to clause 89, wherein the add-on chord line is tilted between -5 to 15 degrees.[Clause 93.] Wind turbine rotor blade according to clause 89, wherein the add-on chord line is tilted between 1-10 degrees.[Clause 94.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein an add-on leading-edge section halfway point is a point about halfway between the add-on pressure-side surface and add-on suction-side surface at the add-on leading-edge section, wherein an add-on trailing-edge section halfway point is a point about halfway between the add-on pressure-side surface and add-on suction-side surface of the add-on trailing-edge section, and wherein an add-on chord line is defined by a shortest straight line connecting the add-on leading-edge section halfway point and the add-on trailing-edge section halfway point, wherein a shortest distance between the add-on chord line and the pressure-side surface is larger than a shortest distance between the add-on chord line and the suction-side surface. [Clause 95.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a density of the plurality of through-holes increases in the direction from the add-on leading-edge to the add-on trailing-edge.[Clause 96.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a cross-sectional area of at least one through-hole, preferably measured in a plane of the add-on pressure- or suction-side surface, has a diameter between 0.5 - 2.0 mm.[Clause 97.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the cross-sectional area of the through- hole on the suction-side surface is different, larger or smaller, than the cross-sectional area of the through-hole on the pressure-side surface.[Clause 98.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the aerodynamic add-on structure is connected to the rotor blade using any or any combination of the following means:(i) a lap joint at the wind turbine rotor blade pressure-side surface,(ii) a lap joint at the wind turbine rotor blade suction-side surface,(iii) a butt joint at the wind turbine rotor blade trailing-edge,(iv) a tongue and groove joint at the wind turbine rotor blade trailing-edge,(v) a dovetail joint at the wind turbine rotor blade trailing-edge,(vi) a mortise joint at the wind turbine rotor blade trailing-edge,(vii) a socket at the wind turbine rotor blade suction-side surface,(viii) a socket at the wind turbine rotor blade pressure-side surface,(ix) a hinge,(x) a flexible connecting member, or(xi) flexible connecting sheet elements.[Clause 99.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the aerodynamic add-on structure is applied to the rotor blade as an individual component from a set of aerodynamic structures, wherein the set comprises between 1 and 100 aerodynamic structures.[Clause 100.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a spanwise width of the aerodynamic add-on structure is between 100-500 mm,[Clause 101.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein a spanwise width of the aerodynamic add-on structure is between 200-400 mm,[Clause 102.] Wind turbine rotor blade according to any of the clauses, according to any of the embodiments or according to any of the claims, wherein the aerodynamic add-on structure has a color that is different from the rotor blade.[Clause 103.] A method of producing the wind turbine rotor blade according to any of the previous clauses.[Clause 104.] A wind turbine comprising: a wind turbine tower having a top and a bottom; a nacelle arranged at the top of the wind turbine tower; a rotor hub rotatably mounted to the nacelle; one or more wind turbine blades according to or comprising any of the clauses or according to any of the embodiments disclosed herein or according to any of the claims, mounted to the rotor hub, wherein the wind turbine blades define a rotor plane; a shaft coupled to the rotor hub; and a generator coupled to the shaft, wherein the generator is configured to transform a torque of the shaft into electrical power.

[0116] Further modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

Claims

1. -29-CLAIMS1. A wind turbine rotor blade with an aerodynamic structure to reduce noise, formed by an assembly of a rotor blade and an aerodynamic add-on structure, wherein the rotor blade comprises: a rotor blade leading-edge section having a rotor blade leading-edge, a rotor blade trailing-edge section having a rotor blade trailing-edge, and a rotor blade pressure-side surface and a rotor blade suction-side surface, the rotor blade pressure-side surface and rotor blade suction-side surface extending between the rotor blade leading-edge section and the rotor blade trailing-edge section, wherein the aerodynamic add-on structure has a plurality of through-holes for reducing noise, the add-on provided at the rotor blade trailing-edge section of the rotor blade, wherein the add-on has: an add-on leading-edge section with an add-on leading-edge, an add-on trailing-edge section with an add-on trailing-edge, and an add-on pressure-side surface and an add-on suction-side surface, which surfaces extend from the add-on leading-edge section to the add-on trailing-edge section, characterized in that, in a cross-section of the assembly across the rotor blade pressure-side and rotor blade suction-side surfaces and / or the add-on pressure-side and add-on suction-side surfaces, which cross-section preferably is generally perpendicular to a radial direction of the rotor blade: the assembly has a first thickness at the add-on leading edge, the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, and a ratio of the first thickness to the second thickness is larger than 1.5, and the assembly has a first thickness at the add-on leading edge, the assembly has a second thickness at a through-hole positioned closest to the add-on leading edge, the assembly has a first through-hole distance defined as the shortest distance between the add-on leading edge and a through-hole positioned closest to the add-on leading edge, and a ratio of {the first thickness minus the second thickness} to the first through-hole distance is between 0.04 and 0.3.

2. Wind turbine rotor blade according to claim 1 , the ratio of the first thickness to the second thickness is larger than 2, and more preferably between 3 and 83. Wind turbine rotor blade according to claim 1 or 2, the ratio of {the first thickness minus the second thickness} to the first through-hole distance is between 0.05 and 0.25, preferably between 0.08 and 0.2, and more preferably between 0.1 and 0.15-30-4. Wind turbine rotor blade according to any of the previous claims, characterized in that, in the cross-section of the assembly across the rotor blade pressure-side and suction-side surfaces, preferably in a plane perpendicular to the radial direction of the rotor blade, the assembly has a first thickness at the add-on leading edge, the assembly has a second thickness at a through- hole positioned closest to the add-on leading edge, the assembly has a third thickness at a through- hole positioned closest to the add-on trailing edge, and a ratio of {the first thickness minus the second thickness} to {the second thickness minus the third thickness} is at least 1 , preferably at least 3, more preferably at least 5, and / or less than 20, preferably less than 15, and more preferably less than 10.

5. Wind turbine rotor blade according to the preamble of claim 1 or according to claim 1, characterized in that, in the cross-section of the assembly across the pressure-side and suction-side surfaces, preferably in a plane perpendicular to the radial direction of the rotor blade, the assembly has a first thickness at the add-on leading edge, the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, the assembly has a second through-hole distance defined as the shortest distance between the add-on leading edge and the through-hole positioned closest to the add-on trailing edge, and a ratio of {the first thickness minus the third thickness} to the second through-hole distance is between 0.02 and 0.25, preferably between 0.03 and 0.15, and more preferably between 0.04 and 0.1.

6. Wind turbine rotor blade according to any of the previous claims, wherein a thickness at a location of the assembly is defined as the shortest distance between outer surfaces of the assembly at that location, preferably a shortest distance, between a- the outer assembly (rotor blade or add-on) pressure-side surface and b- the outer assembly (rotor blade or add-on) suction-side surface, wherein preferably the cross-section of the assembly across the rotor blade / add-on pressure-side and rotor blade / add-on suction-side surfaces includes the shortest distance between the rotor blade / add-on pressure-side surface and rotor blade / add-on suction-side surfaces, preferably in a plane perpendicular to the radial direction of the rotor blade, wherein preferably the cross-section of the assembly across the rotor blade / add-on pressure-side and rotor blade / add-on suction-side surfaces includes a shortest distance between the add-on leading edge and a through-hole positioned closest to the add-on trailing edge, wherein preferably the add-on extends in a radial dimension along the rotor blade, wherein any of the ratios are present in a middle part of the radial dimension of the add-on, or are present at at least two, three or more positions along the radial dimension of the add-on, preferably wherein the two, three, or more positions are separated by at least 10% of the radial dimension of the addon.

7. Wind turbine rotor blade according to any of the previous claims, wherein a thickness at a location of the assembly is defined as the shortest distance between outer surfaces of the assembly at that location, preferably a shortest distance, between a- the outer assembly (rotor blade or add-on) pressure-side surface and b- the outer assembly (rotor blade or add-on) suction-side surface, wherein the assembly has a first thickness at the add-on leading edge, wherein the first thickness is at least 2.5mm, more preferably at least 3mm, most preferably at least 3.5mm, and / or the first thickness is at most 20mm, more preferably at most 10mm, most preferably at most 5mm, and / or wherein the add-on has a second thickness at a through-hole positioned closest to the add-on leading edge, wherein the second thickness is at most 2.5mm, preferably at most 2.2mm, most preferably at most 2.05mm, and / or wherein the assembly has a third thickness at a through-hole positioned closest to the add-on trailing edge, wherein the third thickness is at least 0.05mm, more preferably at least 0.1 mm, most preferably at least 0.5mm.

8. Wind turbine rotor blade according to any of the previous claims, wherein preferably in the cross-section of the assembly across the rotor blade pressure-side and rotor blade suction-side surfaces and / or the add-on pressure-side and add-on suction-side surfaces, which cross-section is a plane perpendicular to the radial direction of the rotor blade, wherein the assembly has a first through-hole distance defined as the shortest distance between the add-on leading edge and a through-hole positioned closest to the add-on leading edge, wherein the first through-hole distance is at least 5mm, more preferably at least 8mm, most preferably at least 10mm, and / or wherein the assembly has a second through-hole distance defined as the shortest distance between the add-on leading edge and a through-hole positioned closest to the add-on trailing edge, wherein the second through-hole distance is at least 10mm, more preferably at least 15mm, most preferably at least 20mm, and / or wherein the assembly has a third through-hole distance defined as the shortest distance between a through-hole positioned closest to the add-on leading edge and a through-hole positioned closest to the add-on trailing edge, wherein the third through-hole distance is at least 4mm, more preferably at least 6mm, most preferably at least 10mm, and / or wherein the add-on has an add-on length defined as the shortest distance between the addon leading edge and the add-on trailing edge, wherein the add-on length is at least 11 mm, more preferably at least 15mm, most preferably at least 20mm and even more preferably at least 25mmand / or wherein the add-on length is at most 400mm, more preferably at most 300mm, most preferably at most 200mm and even more preferably at most 150mm.

9. Wind turbine rotor blade according to any of the preceding claims, wherein the aerodynamic add-on structure has at least one or more extensions forming the add-on trailing edge, wherein preferably the one or more extensions extend from the add-on trailing section, wherein preferably the aerodynamic add-on structure has a total add-on length defined as the shortest distance between add-on leading edge and the add-on trailing edge in a cross-section through the add-on pressure-side surface and suction-side surface, which cross-section is a plane perpendicular to the radial direction of the rotor blade, wherein the add-on total length varies along a spanwise dimension of the add-on, wherein preferably the add-on trailing edge comprises a repeating pattern of extensions in the rotor blade spanwise dimension, including more preferably at least one of the following patterns: (a) straight, (b) zig-zag, (c) sinusoidal, (d) concave half-sinusoidal, (e) convex half-sinusoidal.

10. Wind turbine rotor blade according to any of the preceding claims, wherein the add-on comprises one, two, or more through-holes along a shortest line connecting the add-on trailing edge to the add-on leading edge, wherein preferably the add-on has two or more extensions forming the add-on trailing edge, and the two or more extensions have one, two, or more through-holes along a shortest line connecting the add-on trailing edge formed by the extension to the add-on leading edge.

11. Wind turbine rotor blade according to any of the preceding claims, wherein a rotor blade leading-edge section halfway point is a point about halfway between rotor blade pressure-side surface and rotor blade suction-side surface at the rotor blade leading-edge section, a rotor blade trailing-edge section halfway point is a point about halfway between the rotor blade pressure-side surface and rotor blade suction-side surface at the rotor blade trailing-edge section, an add-on leading-edge section halfway point is a point about halfway between the addon pressure-side surface and add-on suction-side surface at the add-on leading-edge section, an add-on trailing-edge section halfway point is a point about halfway between the addon pressure-side surface and add-on suction-side surface of the add-on trailing-edge section, wherein a rotor blade chord line is defined by a shortest straight line connecting the rotor blade leading-edge section halfway point and the rotor blade trailing-edge section halfway point, wherein an add-on chord line is defined by a shortest straight line connecting the add-on leading-edge section halfway point and the add-on trailing-edge section halfway point, wherein the add-on chord line is tilted with respect to the rotor blade chord line, preferably towards the pressure-side surface of the wind turbine rotor blade,-33- wherein preferably the add-on chord line is tilted with respect to the rotor blade chord line towards the pressure-side surface of the wind turbine rotor blade with an angle between -10 to 20 degrees, preferably between -5 to 15 degrees, and most preferably between 1-10 degrees.

12. Wind turbine rotor blade according to any of the preceding claims, wherein an add-on leading-edge section halfway point is a point about halfway between the addon pressure-side surface and add-on suction-side surface at the add-on leading-edge section, an add-on trailing-edge section halfway point is a point about halfway between the addon pressure-side surface and add-on suction-side surface of the add-on trailing-edge section, wherein an add-on chord line is defined by a shortest straight line connecting the add-on leading-edge section halfway point and the add-on trailing-edge section halfway point, wherein a shortest distance between the add-on chord line and the pressure-side surface is larger than a shortest distance between the add-on chord line and the suction-side surface.

13. Wind turbine rotor blade according to any of the preceding claims, wherein a density of the plurality of through-holes increases in the direction from the add-on leading-edge to the add-on trailing-edge.

14. Wind turbine rotor blade according to any of the preceding claims, wherein a cross-sectional area of at least one through-hole, preferably measured in a plane of the add-on pressure- or suction-side surface, has a diameter between 0.5 - 2.0 mm.

15. Wind turbine rotor blade according to any of the preceding claims, wherein the cross-sectional area of the through-hole on the suction-side surface is different, larger or smaller, than the cross-sectional area of the through-hole on the pressure-side surface.

16. Wind turbine rotor blade according to any of the preceding claims, wherein the aerodynamic add-on structure is connected to the rotor blade using any or any combination of the following means: (i) a lap joint at the wind turbine rotor blade pressure-side surface, (ii) a lap joint at the wind turbine rotor blade suction-side surface, (iii) a butt joint at the wind turbine rotor blade trailing-edge, (iv) a tongue and groove joint at the wind turbine rotor blade trailing-edge, (v) a dovetail joint at the wind turbine rotor blade trailing-edge, (vi) a mortise joint at the wind turbine rotor blade trailing-edge, (vii) a socket at the wind turbine rotor blade suction-side surface, (viii) a socket at the wind turbine rotor blade pressure-side surface, (ix) a hinge, (x) a flexible connecting member, or (xi) flexible connecting sheet elements, wherein preferably the aerodynamic add-on structure is applied to the rotor blade as an individual component from a set of aerodynamic structures, wherein the set comprises between 1 and 100 aerodynamic structures, wherein preferably a spanwise width of the aerodynamic add-on structure is between 100- 500 mm, preferably between 200-400 mm,-34- wherein, even more preferably, the aerodynamic add-on structure has a color that is different from the rotor blade.

17. A method of producing the wind turbine rotor blade according to any of the preceding claims.

18. A wind turbine comprising: a wind turbine tower having a top and a bottom; a nacelle arranged at the top of the wind turbine tower; a rotor hub rotatably mounted to the nacelle; one or more wind turbine blades according to any of the claims 1-16 mounted to the rotor hub, wherein the wind turbine blades define a rotor plane; a shaft coupled to the rotor hub; and a generator coupled to the shaft, wherein the generator is configured to transform a torque of the shaft into electrical power.

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