Patent Publication Number: US-2022228551-A1

Title: Rotor blade and wind turbine

Description:
BACKGROUND 
     Technical Field 
     The invention relates to a rotor blade of a wind turbine that has a Gurney flap, to an associated wind turbine, and to an associated method. 
     Description of the Related Art 
     Wind turbines are fundamentally known. Modern wind turbines generally concern so-called horizontal-axis wind turbines, in the case of which the rotor axis is arranged substantially horizontally and the rotor blades sweep through a substantially vertical rotor area. Aside from a rotor arranged at a nacelle, wind turbines generally comprise a tower on which the nacelle with the rotor is arranged so as to be rotatable about a substantially vertically oriented axis. The rotor generally comprises one, two or more rotor blades of equal length. The rotor blades are slender components, which are commonly produced from fiber-reinforced plastic. 
     The shaping of the outer contour of the rotor blades is realized in the region close to the hub generally almost exclusively on the basis of requirements for design and structure. In a region adjoining the rotor blade flange, a rotor blade generally has a circular-cylindrical geometry. The circular-cylindrical geometry is generally necessary for realizing pitch-controlled rotor blades. Pitch-controlled rotor blades are mounted rotatably at the hub or another position in order for said rotor blades to be, at any rate partially, moved rotatably about their longitudinal axis. For structural reasons, a direct transition to an aerodynamically optimum rotor blade profile from said circular-cylindrical geometry is generally not possible since excessively large geometrical gradients would result in the formation of load peaks in this blade region, which is high loaded by impact loads and pivoting loads. Due to the focus on requirements for structure and design in the rotor blade region close to the hub, the aerodynamic properties here are often not optimal. Although  the existing systems and methods offer various advantages, further improvements are desirable. 
     The German Patent and Trademark Office has searched the following prior art in the priority application relating to the present application: DE 20 2016 101 461 U1, US 2015/0 267 681 A1, WO 2016/055 076 A1. 
     BRIEF SUMMARY 
     Provided is a rotor blade for a wind turbine, an associated wind turbine and an associated method that reduce or eliminate one or more of the stated disadvantages. Provided are one or more techniques that improve the aerodynamic properties in the region of a rotor blade that is close to the hub. 
     According to a first aspect, provided is a rotor blade for a wind turbine, having a rotor blade length, having a rotor blade depth which extends over the rotor blade length, having a rotor blade thickness which extends over the rotor blade length, and having a thickness of a trailing edge of the rotor blade, which thickness extends over the rotor blade length, said rotor blade comprising a Gurney flap, which has a height which extends over the rotor blade length, wherein the height of the Gurney flap is dimensioned according to the thickness of the trailing edge in such a way that a ratio of the height of the Gurney flap and the thickness of the trailing edge is between greater than 0% and 25%, in particular between 5% and 25%. 
     One or more embodiments are based on the realization that Gurney flaps having a height limited in this way result in a surprisingly positive flow influence. It is furthermore a realization that the ratio of the height of the Gurney flap and the thickness of the trailing edge can be used particularly advantageously for aerodynamic optimization of a rotor blade, in particular of a region of a rotor blade that is close to the hub. The inventors have discovered in particular that the most effective increase in lift possible in the region close to the hub depends in a particular way on the ratio of the height of the Gurney flap and the thickness of the trailing edge.  
     The rotor blade extends with a rotor blade length, a rotor blade depth and a rotor blade thickness. The rotor blade length is defined in particular as the distance between a rotor blade flange and a rotor blade tip between which the rotor blade extends. The rotor blade depth is oriented in particular substantially orthogonally to the rotor blade length. During operation, the rotor blade depth is oriented substantially parallel to the flow-on direction of the rotor blade. The rotor blade depth preferably extends between a leading edge and the trailing edge of the rotor blade. 
     The rotor blade extends in the direction of the rotor blade thickness orthogonally to the rotor blade length and to the rotor blade depth. At substantially every position along the rotor blade length, the rotor blade depth and the rotor blade thickness span an aerodynamic profile, which can also be understood as profile section. 
     In a region of the rotor blade that is close to the hub, said rotor blade may have a flat back profile, which is defined in more detail in the following text. A flat back profile is distinguished in particular in that it has no closed trailing edge profile with a runout. The flat back profile differs from the substantially drop-shaped form and has a substantially polygonal trailing edge geometry. 
     The direction of the thickness of the trailing edge is oriented substantially parallel to the rotor blade thickness and orthogonally to the rotor blade length and orthogonally to the rotor blade depth. A thickness of the trailing edge of the rotor blade may however also be defined for profiles which have rounded corners or else which have closed profiles, in particular in the form of a drop. 
     In particular the distance between the profile pressure-side contour and the profile suction-side contour at the root point of the Gurney flap orthogonal to the profile chord of the rotor blade is understood as the thickness of the trailing edge. General structural conditions, for example the buildup sequence or demoldability, can make it necessary to design the trailing edge of the rotor blade with a rounded shape both on the pressure side and on the suction side. In this case, it is advantageous to position the Gurney flap in a region before the actual flat profile trailing edge. The aforementioned profile pressure-side contour and profile suction-side contour preferably end before the rounding. Thus, the  rounding is in particular not part of the profile pressure-side contour and of the profile suction-side contour, so that the thickness of the trailing edge is determined before the actual flat trailing edge. 
     The rotor blade furthermore has the Gurney flap. A Gurney flap is in particular an edge projecting from the rotor blade surface. A Gurney flap generally has the effect that the flow is influenced at the trailing edge of the rotor blade, preferably in such a way that there is a resulting pressure rise in front of the Gurney flap and a resulting pressure drop behind the Gurney flap. The Gurney flap brings about an increase in circulation, wherein the flow-off angle, defined as the angle between the profile chord and the flow-off direction, and the diversion angle, defined as the angle between the flow-on direction and the flow-off direction, are increased. The separation region is generally shifted behind the Gurney flap. Consequently, the lift on the rotor blade and the drag are increased. 
     The Gurney flap projects from a rotor blade surface. The height of the Gurney flap is to be understood as meaning in particular a projection height. Furthermore, the height of the Gurney flap is defined in particular as the distance between a root point and a tip point of the Gurney flap. The root point of the Gurney flap is in particular the point at which the Gurney flap meets the rotor blade. The tip point of the Gurney flap is to be understood as meaning in particular the distal end of the Gurney flap. Said height can vary along the rotor blade length. The height of the Gurney flap is thus preferably not a constant value, but rather is variable as a function of the rotor blade length. 
     A ratio of the height of the Gurney flap and the thickness of the trailing edge can be determined for each profile section along the rotor blade length. The value of the ratio of the height of the Gurney flap and the thickness of the trailing edge is the result of a division of the height of the Gurney flap by the thickness of the trailing edge. Said ratio of the height of the Gurney flap and the thickness of the trailing edge is between greater than 0% and 25%. In particular, it is preferable for the ratio of the height of the Gurney flap and the thickness of the trailing edge to be between 5% and 25%. 
     Furthermore, it may be preferable for the ratio of the height of the Gurney flap and the thickness of the trailing edge to be greater than 5%, greater than 7.5%, greater than  10%, or greater than 15%. A ratio of the height of the Gurney flap and the thickness of the trailing edge that is unusually high in this way results in surprisingly high lift of the rotor blade. Furthermore, it is advantageous for the ratio of the height of the Gurney flap and the thickness of the trailing edge to be greater than 5%, greater than 7.5%, greater than 10%, or greater than 15%, in a region close to the hub, in particular between 0% and 50%, preferably between 3% and 35%, of the relative blade length. 
     Furthermore, it is preferable for the ratio of the height of the Gurney flap and the thickness of the trailing edge to be between greater than 0% and 25% in a rotor blade region close to the hub. The rotor blade region close to the hub is distinguished preferably in that said region extends from greater than or equal to 0% to at most 35% of the relative blade length. The relative blade length is preferably defined in such a way that it ranges from 0% to 100%, wherein 0% characterizes an end facing the hub, for example the rotor blade flange, and 100% characterizes the blade tip. For a relative blade length of greater than 35%, in particular greater than 50%, the ratio of the height of the Gurney flap and the thickness of the trailing edge preferably approaches zero. In particular, this may be the case up to the blade tip. 
     An advantageous development of the rotor blade is distinguished in that, proceeding from a rotor blade flange or proceeding from a region adjoining the rotor blade flange, the ratio of the height of the Gurney flap and the thickness of the trailing edge increases to a maximum value. 
     The height of the Gurney flap is preferably dimensioned along the rotor blade length in such a way that the ratio of the height of the Gurney flap and the thickness of the trailing edge, toward the blade tip, firstly increases. The Gurney flap preferably extends from an inner end to an outer end, wherein the inner end faces the rotor blade flange and the outer end faces the rotor blade tip. Proceeding from the inner end, the ratio of the height of the Gurney flap to the thickness of the trailing edge increases to a maximum of this ratio. From this maximum, the ratio of the height of the Gurney flap to the thickness of the trailing edge preferably decreases again toward the outer end of the Gurney flap. The maximum of the ratio may also occur at the outer end of the Gurney flap.  
     The ratio may increase immediately from the rotor blade flange, or from a region adjoining the rotor blade flange. This means in particular that the Gurney flap does not begin directly at the rotor blade flange by necessity, but may also begin spaced apart from the rotor blade flange. This spacing may amount for example to 3% of the rotor blade length. 
     In a preferred embodiment variant of the rotor blade, it is provided that the maximum value of the ratio of the height of the Gurney flap and the thickness of the trailing edge is attained between 5% and 15%, preferably between 6% and 12%, of the relative blade length. 
     The maximum value of said ratio is preferably attained between 5% and 15% of the relative blade length. This means, for example in the case of a rotor blade with a rotor blade length of 80 meters, that the maximal value of the ratio of the height of the Gurney flap and the thickness of the trailing edge is attained between 4 meters away from and 12 meters away from the rotor blade flange. In particular, it is preferable for said maximum value of the ratio to be 4.8 meters to 8.8 meters away from the rotor blade flange. 
     According to a further preferred embodiment variant of the rotor blade, it is provided that the Gurney flap is arranged on the rotor blade between 3% and 35% of the relative blade length. 
     In this embodiment variant, the inner end of the Gurney flap is spaced apart from the rotor blade flange. The inner end of the Gurney flap may be arranged for example at 3% of the relative blade length. The inner end of the Gurney flap may however also be more than 3% of the relative blade length away from the rotor blade flange. The outer end of the Gurney flap may be arranged at 35% of the relative blade length. It is furthermore preferable for the outer end of the Gurney flap to be arranged at 50%, or less than 50%, in particular at less than 35%, of the relative blade length, for example at 30%. 
     The Gurney flap preferably extends with a Gurney flap length from the inner end to the outer end. The Gurney flap length is preferably between greater than 0% and 50%, more preferably between greater than 0% and 30%, furthermore preferably between 10%  and 25%, of the relative blade length. It is furthermore preferable for the Gurney flap length to be less than or equal to 50% of the relative blade length. 
     A further preferred development of the rotor blade is distinguished in that the Gurney flap is arranged on a pressure side of the rotor blade. 
     Rotor blades generally have a pressure side and a suction side. The pressure side is in particular that side of the rotor blade on which the flow speed is low in comparison with the suction side and the pressure is high. 
     In a further preferred embodiment variant of the rotor blade, it is provided that an angle between the height of the Gurney flap and a profile chord of the rotor blade, which angle is determined proceeding from a leading edge of the rotor blade, which is arranged opposite the trailing edge, is between 90° and 170°, in particular is at least 100°, preferably is at least 110°. 
     The profile chord is preferably defined in the profile section in such a way that it is the connecting line between the central point of the trailing edge and the leading edge. The central point of the trailing edge is preferably that point on the trailing edge which is at the same distance from the pressure side as from the suction side. The leading edge is understood as meaning in particular that point of the profile outer contour which is furthest away from the central point of the trailing edge. 
     In the case of an angle between the height of the Gurney flap and the profile chord of the rotor blade of 90°, said Gurney flap projects downward in the profile section, wherein the projection direction is oriented substantially parallel to the rotor blade thickness. If this angle has been selected to be greater than 90°, the Gurney flap extends away from the leading edge. 
     According to a further preferred development of the rotor blade, it is provided that, at a profile section of the rotor blade length, a ratio of the height of the Gurney flap and the rotor blade depth is greater than 1%, greater than 2% and/or greater than 5%. 
     In particular, it is preferable that, at an arbitrary position between 0% and 35% of the relative blade length, in particular between 3% and 35%, a ratio of the height of the Gurney flap and the rotor blade depth is greater than 1%, greater than 2% and/or greater  than 5%. Furthermore, it may be preferable that, at an arbitrary position between 0% and 35% of the relative blade length, in particular between 3% and 35%, a ratio of the height of the Gurney flap and the rotor blade depth is greater than 10% and/or greater than 15%. In particular, it is preferable that, at an arbitrary position between 0% and 35% of the relative blade length, in particular between 3% and 35%, a ratio of the height of the Gurney flap and the rotor blade depth is between 1% and 20%, preferably between 2% and 15%, in particular between 5% and 15%. 
     In a further preferred embodiment variant of the rotor blade, it is provided that the ratio of the height of the Gurney flap and the thickness of the trailing edge is between 4% and 25%, in particular between 10% and 21%, in a range between 0% and 5% of the relative blade length; and/or is between 4% and 25%, in particular between 12% and 22%, in a range between 5% and 10% of the relative blade length; and/or is between 0% and 25%, in particular between 13% and 20%, in a range between 10% and 15% of the relative blade length; and/or is between 0% and 23%, in particular between 10% and 18%, in a range between 15% and 20% of the relative blade length; and/or is between 0% and 20%, in particular between 0% and 15%, in a range between 20% and 25% of the relative blade length; and/or is between 0% and 15%, in particular between 0% and 10%, in a range between 25% and 30% of the relative blade length; and/or is between 0% and 10% in a range between 30% and 35% of the relative blade length. 
     Thus, between 0% and 5% of the relative blade length, the ratio of the height of the Gurney flap and the thickness of the trailing edge is preferably between 4% and 25%. The Gurney flap may, for example, have an inner end at 3% of the relative blade length, and have a height such that a ratio of the height of the Gurney flap and the thickness of the trailing edge is 10%. 
     The range specifications for the ratio of the height of the Gurney flap and the thickness of the trailing edge apply to each profile section in the specified range of the relative blade length. The specified values for the ratio of the height of the Gurney flap and the thickness of the trailing edge are particular individual values at a defined position in the specified range, that is to say preferably for a profile section.  
     The ratio of the height of the Gurney flap and the thickness of the trailing edge is preferably determined by means of a quadratic function. In particular, it is preferable for the ratio of the height of the Gurney flap and the thickness of the trailing edge to lie in a design region, wherein the design region is defined by an upper limit-value curve f max  and by a lower limit-value curve f min . Preferably, the limit-value curves are determined by a function of the following form: 
     
       
         
           
             
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                 a 
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                       z 
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                 b 
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               c 
             
           
         
       
     
     In this relationship, r/R represents the relative radius position, this being the relative radius position with the rotor blade hub taken into consideration. Accordingly, the total rotor radius, with the rotor blade hub and the rotor blade length taken into consideration, is the reference length. The variable r is for example the distance to the considered position, in meters, from the axis of rotation of the rotor, and R is the sum of rotor blade length and distance from the blade flange to the axis of rotation. The relationship z/Z represents the relative blade length in the above-stated relationship. The variable z is for example the distance to the considered position, in meters, from the blade flange, and Z is the rotor blade length. 
     For the upper limit-value curve f max , it is preferable for the value a to have a minimum value of −5 or −4 or −3 or −2.5. It is furthermore preferable that, for the upper limit-value curve f max , the value a has a maximum value of −2.5 or −2 or −1 or 0. For the upper limit-value curve f max , it is preferable for the value b to have a minimum value of −1 or 0 or 0.3. It is furthermore preferable that, for the upper limit-value curve f max , the value b has a maximum value of 0.2 or 0.3 or 0.5 or 1. For the upper limit-value curve f max , it is preferable for the value c to have a minimum value of 0 or 0.1 or 0.2 or 0.3. It is furthermore preferable that, for the upper limit-value curve f max , the value c has a maximum value of 0.2 or 0.3 or 0.4 or 1. 
     For the lower limit-value curve f min , it is preferable for the value a to have a minimum value of −10 or −8 or −5 or −3. It is furthermore preferable that, for the lower limit-value curve f min , the value a has a maximum value of −5 or −4 or −3 or 0. For the  lower limit-value curve f min , it is preferable for the value b to have a minimum value of 0 or 0.3 or 0.5. It is furthermore preferable that, for the lower limit-value curve f min , the value b has a maximum value of 0.3 or 0.5 or 0.6 or 1. For the lower limit-value curve f min , it is preferable for the value c to have a minimum value of 0 or 0.01 or 0.02 or 0.03 or 0.04 or 0.05. It is furthermore preferable that, for the lower limit-value curve f min , the value c has a maximum value of 0.02 or 0.03 or 0.04 or 0.1. 
     According to a further preferred development of the rotor blade, it is provided that the rotor blade has a flat back profile with or without rounded edge regions, and the thickness of the trailing edge is defined as the distance between a profile contour of the pressure side and a profile contour of the suction side orthogonal to the profile chord. 
     In the case of rounded edge regions, the roundings are preferably not part of the profile contour of the pressure side and/or of the suction side. In the case of a flat back profile with rounded edge regions, the Gurney flap is preferably arranged at the transition of the profile contour of the pressure side toward the rounded edge region. Furthermore, the Gurney flap may, from this position, also be arranged toward the profile contour of the pressure side, or else may be arranged in the rounded edge region. 
     Furthermore, it is preferable for the rotor blade to have a closed profile, and for the thickness of the trailing edge to be defined as the distance between a profile contour of the pressure side and a profile contour of the suction side orthogonal to the profile chord at that point of the rotor blade depth at which a free pressure-side flow prevails during operation, wherein preferably said point is defined by the root point of the Gurney flap. 
     A closed profile is to be understood as meaning the customary geometry, substantially drop-shaped in principle, of a rotor blade cross section. In particular a closed profile is understood as meaning a profile which has a trailing edge with a runout, and consequently no significant thickness of the trailing edge can be established directly at the trailing edge. 
     In the case of the closed profile, the thickness of the trailing edge is understood as meaning the distance between the profile contour of the pressure side and the profile contour of the suction side orthogonal to the profile chord at that point of the rotor blade  depth at which a free pressure-side flow prevails during operation. The free pressure-side flow prevails, in the relevant operating range, in particular up to the point at which pressure-side flow separation occurs and at which a Gurney flap is preferably arranged. A relevant operating range is defined for example by the part-load range with optimum tip speed ratio up to the attainment of the rated power. In the case of a closed profile, the thickness of the trailing edge may be understood as meaning the distance between the profile contour of the pressure side and the profile contour of the suction side orthogonal to the profile chord at that point of the rotor blade depth at which the Gurney flap is arranged. 
     In the case of profile sections with large relative thicknesses, pressure-side separation takes place at the point where it is no longer ensured that the rotor blade is flowed around completely on the pressure side. 
     According to a further preferred development, the rotor blade comprises a transition region, wherein the transition region consists of a section with a flat back profile and a section with a circular-cylindrical profile, wherein the section with the circular-cylindrical profile faces the rotor blade flange, and wherein the Gurney flap is arranged in the transition region. 
     The transition region preferably comprises the rotor blade flange and/or a blade connector. The circular-cylindrical profile of a rotor blade is distinguished in particular in that it has a lift coefficient of substantially zero. Consequently, it is preferable for the lift coefficient-improving Gurney flap to be arranged in particular in this region. Here, in particular by way of the aforementioned ratio of the height of the Gurney flap and the thickness of the trailing edge, optimization can be made possible. The thickness of the trailing edge in the case of a circular-cylindrical profile is understood as meaning in particular the diameter of the circular cylinder. 
     A further preferred embodiment variant of the rotor blade provides that the Gurney flap extends in a direction of the height from a root point to a tip point, and/or the Gurney flap has, orthogonal to the direction of the height, a thickness which is oriented substantially parallel to the profile chord of the rotor blade, and/or the Gurney flap extends in a longitudinal direction orthogonally to the direction of the height and orthogonally to  the thickness, and an areal extent is preferably formed by the extent in the longitudinal direction and the height. 
     The thickness of the Gurney flap is to be understood as meaning in particular the material thickness of the material of the Gurney flap. The longitudinal direction of the Gurney flap may be straight, bent or curved. 
     In a further preferred embodiment variant of the rotor blade, it is provided that said rotor blade comprises a blade adapter and/or a blade extension, wherein the blade adapter and/or the blade extension have/has the rotor blade flange. 
     According to a further aspect, provided is a wind turbine comprising a rotor blade according to one of the embodiment variants discussed above. 
     According to a further aspect, provided is a wind farm having at least two wind turbines according to the preceding aspect. 
     According to a further aspect, provided is a method for designing a rotor blade of a wind turbine, having a rotor blade length, having a rotor blade depth which extends over the rotor blade length, having a rotor blade thickness which extends over the rotor blade length, and having a thickness of a trailing edge of the rotor blade, which thickness extends over the rotor blade length, said rotor blade comprising a Gurney flap, which has a height which extends over the rotor blade length, wherein the height of the Gurney flap is dimensioned according to the thickness of the trailing edge in such a way that a ratio of the height of the Gurney flap and the thickness of the trailing edge is between greater than 0% and 25%, in particular between 5% and 25%. 
     The method and its possible developments have features or method steps that make them particularly suitable for being used for the rotor blade according to the first aspect and its developments. For further advantages, embodiment variants and embodiment details of the further aspects and their possible developments, reference is also made to the description given above concerning the corresponding features and developments of the rotor blade.  
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Preferred exemplary embodiments will be discussed by way of example on the basis of the appended figures. In the figures: 
         FIG. 1  shows a schematic three-dimensional view of a wind turbine; 
         FIG. 2  shows a schematic three-dimensional view of an embodiment of a rotor blade; 
         FIGS. 3-5  show plan views of the rotor blade shown in  FIG. 2 ; 
         FIG. 6  shows a schematic two-dimensional view of a further embodiment of a rotor blade with a flat back profile; 
         FIG. 7  shows a schematic two-dimensional view of a rotor blade with a flat back profile with rounded corners; 
         FIG. 8  shows a schematic two-dimensional view of a rotor blade with a closed profile; and 
         FIG. 9  shows a schematic view of a design region for the ratio of the height of the Gurney flap and the thickness of the trailing edge. 
     
    
    
     In the figures, identical or substantially functionally identical or similar elements are denoted by the same reference signs. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic three-dimensional view of a wind turbine  100 . The wind turbine  100  has a tower  102  and a nacelle  104  on the tower  102 . An aerodynamic rotor  106  having three rotor blades  108 , which each have a rotor blade length R, and having a spinner  110  is provided on the nacelle  104 . During the operation of the wind turbine  100 , the aerodynamic rotor  106  is set in rotational motion by the wind and thereby also rotates an electrodynamic rotor or runner of a generator, which is coupled directly or indirectly to the aerodynamic rotor  106 . The electric generator is arranged in the nacelle  104  and generates electrical energy. 
     The pitch angles of the rotor blades  108  can be varied by way of pitch motors at the rotor blade roots of the respective rotor blades  108 . The rotor blades  108  have a thickness  of a trailing edge, which thickness extends over the rotor blade length R. The rotor blades  108  furthermore have Gurney flaps (not visible here), which have a height which extends over the rotor blade length R. The height of the Gurney flaps is dimensioned according to the thickness of the trailing edges, specifically in such a way that a ratio of the height of the Gurney flaps and the thickness of the trailing edge is between greater than 0% and 25%, in particular between 5% and 25%. 
       FIGS. 2 to 5  show schematic three-dimensional views of a further embodiment of a rotor blade  200 . The rotor blade  200  extends in a longitudinal direction L from a rotor blade flange  204  to a rotor blade tip (not shown). The rotor blade extends with a rotor blade depth T orthogonally to the longitudinal direction L and with a rotor blade thickness D orthogonally to the rotor blade length L and orthogonally to the rotor blade depth T. 
     In a region adjoining the rotor blade flange  204 , the rotor blade  200  has a circular-cylindrical profile  210 . On a side of the circular-cylindrical profile  210  that faces away from the rotor blade flange  204 , the rotor blade  200  has a flat back profile  212 . In a transition region consisting of a section with the circular-cylindrical profile  210  and of a section with the flat back profile  212 , the rotor blade  200  has a Gurney flap  214 . The Gurney flap  214  is arranged on the pressure side  206  of the rotor blade  200 . In particular, the Gurney flap  214  is arranged in a manner adjoining a trailing edge  202  of the rotor blade  200 . 
       FIG. 6  shows a schematic two-dimensional view of a further embodiment of a rotor blade  300  with a flat back profile. The rotor blade  300  extends in a rotor blade depth T from a leading edge  302  to a trailing edge  304 . The rotor blade  300  is described geometrically inter alia by a profile chord  322 . The profile chord  322  is defined as the connecting line between the central point of the trailing edge  304  and that point  324  of the leading edge  302  which is furthest away from the central point of the trailing edge  304 . 
     The profile has a rounded geometry at the leading edge  302 . The profile has a straight surface at the trailing edge  304 . The trailing edge may alternatively also have two or more surfaces which are arranged at an angle to one another. In particular, the trailing edge may have two surfaces which include an angle with one another, wherein one of said  surfaces includes an angle with the pressure side and the other surface includes an angle with the suction side. On the pressure side and on the suction side, said angles each form in particular a sharp end edge at which the attached flow departs from the profile. The trailing edge may also be of arched form. Trailing edges formed in this way are advantageously demoldable. Such a profile is referred to as flat back profile since the trailing edge  304  is substantially flat. In the case of a trailing edge  304  formed in this way, the thickness  310  of the trailing edge  304  can be determined directly by the spacing between the suction side  306  and the pressure side  308 . 
     The Gurney flap  312  is arranged at the transition from the pressure side  308  to the trailing edge  304 . The Gurney flap  312  extends from a root point  314  to a tip point  316 . The root point  314  of the Gurney flap  312  is arranged on the rotor blade  300 . The tip point  316  of the Gurney flap  312  is to be understood as meaning a distal end of the Gurney flap  312 , and thus faces away from the rotor blade  300 . 
     The Gurney flap  312  has a height  318 . The height  318  is defined as the distance between the root point  314  and the tip point  316 . The arrangement of the Gurney flap  312  is furthermore determined by an angle  320 . The angle  320  between the height  318 , or the direction of the height  318 , of the Gurney flap  312  and the profile chord  322 , which angle is determined proceeding from the leading edge  302 , may in particular be between 90° and 170°. In the present exemplary embodiment, the angle  320  is approximately 100°. 
     The rotor blade  300 ′ shown in  FIG. 7  differs from the rotor blade shown in  FIG. 6  by the profile geometry at the trailing edge  304 ′. The profile geometry is distinguished by rounded corners. The thickness  310  of the trailing edge  304 ′ is understood as meaning the distance between the profile contour of the pressure side  308  and the profile contour of the suction side  306  orthogonal to the profile chord  322  at that point of the rotor blade depth T at which a free pressure-side flow prevails during operation. 
       FIG. 8  shows a rotor blade  300 ″ with a closed trailing edge  304 ″. The thickness  310  of the trailing edge  304 ″ is to be understood as meaning the distance between the profile contour of the pressure side  308  and the profile contour of the suction side  306   orthogonal to the profile chord  322  at that point of the rotor blade depth T at which a free pressure-side flow prevails during operation. 
       FIG. 9  shows a schematic view of a design region for the ratio of the height  318  of the Gurney flap  312  and the thickness  310  of a trailing edge  304 ,  304 ′,  304 ″. The relative blade length is plotted on the abscissa in a value range of 0% to 35%. The ratio of the height  318  of the Gurney flap  312  and the thickness  310  of the trailing edge  304 ,  304 ′,  304 ″ is plotted on the ordinate in a value range between 0% and 30%. 
     A first design region  400 , which is defined by a first upper limit-value line  402  and a first lower limit-value line  404 , is illustrated in the diagram. The upper limit-value line  402  of the first design region  400  is preferably characterized by the following relationship: 
     
       
         
           
             
               
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                 ⁢ 
                 1 
               
             
           
         
       
     
     The lower limit-value line  404  of the first design region  400  is preferably characterized by the following relationship: 
     
       
         
           
             
               
                 f 
                 min 
               
               ⁡ 
               
                 ( 
                 
                   r 
                   R 
                 
                 ) 
               
             
             = 
             
               
                 
                   - 
                   
                     5 
                     . 
                     1 
                   
                 
                 ⁢ 
                 4 
                 ⁢ 
                 8 
                 * 
                 
                   
                     ( 
                     
                       z 
                       Z 
                     
                     ) 
                   
                   2 
                 
               
               + 
               
                 
                   0 
                   . 
                   5 
                 
                 ⁢ 
                 3 
                 ⁢ 
                 8 
                 ⁢ 
                 9 
                 * 
                 
                   ( 
                   
                     z 
                     Z 
                   
                   ) 
                 
               
               + 
               
                 
                   0 
                   . 
                   0 
                 
                 ⁢ 
                 3 
                 ⁢ 
                 5 
                 ⁢ 
                 6 
               
             
           
         
       
     
     In these relationships, r/R represents the relative radius position, this being the relative radius position with the rotor blade hub taken into consideration. Accordingly, the total rotor radius, with the rotor blade hub and the rotor blade length taken into consideration, is the reference length. The variable r is for example the distance to the considered position, in meters, from the axis of rotation of the rotor, and R is the sum of rotor blade length and distance from the blade flange to the axis of rotation. The ratio z/Z represents the relative blade length in the above-stated relationship. The variable z is for example the distance to the considered position, in meters, from the blade flange, and Z is the rotor blade length. 
     The second design region  410  is arranged within the first design region  400 . The second design region is defined by the second upper limit-value line  412  and the second lower limit-value line  414 .  
     REFERENCE SIGNS 
       100  Wind turbine 
       102  Tower 
       104  Nacelle 
       106  Rotor 
       108  Rotor blades 
       110  Spinner 
       200  Rotor blade 
       202  Trailing edge 
       204  Rotor blade flange 
       206  Pressure side 
       210  Circular-cylindrical profile 
       212  Flat back profile 
       214  Gurney flap 
       300 ,  300 ′,  300 ″ Rotor blade 
       302  Leading edge 
       304  Trailing edge 
       306  Suction side 
       308  Pressure side 
       310  Thickness of the trailing edge 
       312  Gurney flap 
       314  Root point of the Gurney flap 
       316  Tip point of the Gurney flap 
       318  Height of the Gurney flap 
       320  Angle 
       322  Profile chord 
       324  Leading-edge point 
       400  First design region 
       402  First upper limit-value line  
       404  First lower limit-value line 
       410  Second design region 
       412  Second upper limit-value line 
       414  Second lower limit-value line 
     D Rotor blade thickness 
     L Rotor blade length 
     T Rotor blade depth