Patent Abstract:
A rotor blade for a wind turbine includes a surface having a plurality of aerodynamics feature elements formed therein. The elements for influencing an airflow at the surface during operation of the wind turbine and arrayed in a two dimensional pattern.

Full Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to rotor blades for a wind turbine, and more specifically to the surface of a rotor blade for a wind turbine. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Rotor blades are primary elements of wind turbines for the conversion of wind energy into electrical energy. The working principle of the rotor blades resembles that of airplane wings. A cross-section of a typical blade, during operation thereof, enables air to flow along both sides of the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. 
     In addition, an attached-flow region has a mainly laminar flow along an outer surface area of the blade. In contrast, a detached-flow region in the wake of flow separation has a more turbulent flow. Flow separation depends on a number of factors, such as incoming air flow characteristics (e.g. Reynolds number, wind speed, in-flow atmospheric turbulence) and characteristics of the blade (e.g. airfoil sections, blade chord and thickness, twist distribution, pitch angle, etc). 
     The lift force is predominantly created in the attached-flow region, whereas the detached-flow region leads to an increase in drag force, mainly due to a pressure difference between the upstream attached-flow region and the downstream detached-flow region. 
     The force component used to produce electrical power is a portion of the lift force acting as torque on the rotor main shaft. Hence, in order to increase the energy conversion efficiency during normal operation of the wind turbine, it is desired to maximize the lift force. On the other hand, it is generally desired to minimize the drag force. To this purpose, it is advantageous to increase the attached-flow region and to reduce the detached-flow region by having the flow separation near a trailing edge of the blade, i.e. in a downstream region of the blade. Also, it is generally desired to have a stable flow separation, e.g. in order to increase the working stability or to decrease noise generation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are cross-sectional views through a standard wind turbine blade; 
         FIGS. 3 and 4  are cross-sectional views through a wind turbine blade having a dimple skin; 
         FIG. 5  is a top view of a blade having different surface sections, e.g. with differently sized dimples; 
         FIGS. 6 through 13  are enlarged views of dimple skins having different dimple size and depth; and 
         FIGS. 14 through 31  are top views of examples of aerodynamic feature elements immersed inwardly into the blade surface or protruding outwardly from the blade surface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a cross sectional view of a typical blade  100  including a suction side  102  and a higher pressure side  104 . As illustrated by lines  106 , air flows along both sides  102 ,  104  of blade  100 . A pressure difference develops between sides  102 ,  104 , such that side  102 , which experiences a lower pressure, is a suction side, and side  104 , which experiences a higher pressure, is a pressure side. Consequently, a lift force, directed from pressure side  104  towards suction side  102 , acts on blade  100 . 
     Also shown in  FIG. 1  is a flow separation between a region of attached air flow  108 , and a region of detached air flow  110 . Attached-flow region  108  has a mainly laminar flow along an outer surface area of blade  100 . In contrast, detached-flow region  110  in the wake of the flow separation has a more turbulent flow. Flow separation depends on a number of factors, such as incoming air flow characteristics (e.g. Reynolds number, wind speed, turbulence) and characteristics of the blade (e.g. blade thickness, pitch angle, etc). 
       FIG. 2  is a cross sectional view of a blade  120  in which similar reference numbers indicate the same features as described in  FIG. 1 . Blade  120  includes a smaller pitch angle than blade  100  (shown in  FIG. 1 ). Consequently, the region of flow separation in  FIG. 2  is further downstream, i.e. closer to the blade&#39;s trailing edge, compared to the flow separation in  FIG. 1 . 
     Referring to  FIGS. 3 and 4 , embodiments of the present invention are shown, which include dimples as aerodynamic feature elements on a surface of a blade. Dimples are also known in golf balls, where they are commonly used to improve the aerodynamic properties of the golf balls as a bluff body. 
     Specifically,  FIG. 3  illustrates a cross section of a blade  150  including a surface  152  having aerodynamic feature elements  154  on both a pressure side  156  and a suction side  158  of blade  150 . In the illustrated embodiment, surface  152  includes aerodynamic feature elements  154  on the entire blade, i.e. from a leading edge  160  to a trailing edge  162  on both sides of blade  150 . 
       FIG. 4  illustrates a cross section of a blade  170  in which similar reference numbers indicate the same features as described in  FIG. 3 . As illustrated in  FIG. 4 , aerodynamic feature elements  154  are provided only on a trailing edge portion  172  of blade  170 , i.e. between spar caps and a downstream trailing edge  162  of blade  170 . In other embodiments, aerodynamic feature elements  154  may further be provided only on a leading edge portion  160  of blade  170 , i.e. between the spar caps and an upstream leading edge (not shown). The latter arrangement may be useful for a thick or cylindrical section near or at the blade root. Other arrangements of aerodynamic feature elements on the blade surface are also possible, depending on the blade geometry and the desired blade characteristics. For example, the extension of the surface having aerodynamic feature elements depends, in one embodiment, on the radial position on the blade. 
     As is shown in  FIGS. 3 and 4 , aerodynamic feature elements  154  are be provided integrally with a skin sheet. Hereby, the aerodynamic feature elements are defined as a height profile of the surface of the skin sheet. In the embodiments shown in  FIGS. 3 and 4 , surface  152  defines a smooth surface area, into which aerodynamic feature elements  154  are immersed, i.e. from which aerodynamic feature elements are extending in an inwardly direction. 
     Aerodynamic feature elements  154  in the embodiment shown in  FIGS. 3 and 4  all have the same shape, size, and arrangement. However, in other embodiments given the different airfoil section size, local air flow speed and Reynolds number, it may be desirable to provide aerodynamic feature elements that vary in shape, size, arrangement, and/or orientation, depending on the position on blades  150 ,  170 . For example, aerodynamic feature elements  154  can be very large (having a length between 0.3 m and 10 m and a width and depth each between 0.3 cm and 5 cm) at the root section and very small at the tip region (having a length, width and depth each between 0.3 mm and 5 mm). 
     The above variation aerodynamic feature elements  154  can be continuous or stepwise. Further, the variation can be in a radial, in a circumferential, or in some other direction of the blades  150 ,  170 . Further, aerodynamic feature elements  154  can be different on pressure side  156  and on suction side  158  of blades  150 ,  170 . 
     An example for a stepwise variation of aerodynamic feature elements  154  is illustrated in  FIG. 5 . As shown in  FIG. 5 , a blade  180  has several aerodynamic feature element surfaces A to F and A′ to F′, whereby each of the surfaces may include aerodynamic feature elements  154  of a particular shape, size, arrangement and/or orientation. Thus, a stepwise variation of aerodynamic feature elements  154  is achieved. 
     In this way, a difference in air flow velocities and other air flow conditions between the respective blade sections can be accounted for. Further, aerodynamic feature elements  154  may serve different purposes in the respective sections. For example, surfaces A to C and A′ to C′ may mainly promote flow transition stability in a region of comparably low blade velocity. On the other hand, surfaces D to F and D′ to F′ may mainly serve to extend flow transition as far downstream as possible in a region of comparably high blade velocity, in order to reduce drag. In  FIG. 5 , aerodynamic feature elements surfaces D to F and D′ to F′ are provided along the outmost 50% of the blade span in order to extend flow transition as far as possible to the trailing edge in order to reduce drag. Further, the use of areas having differently shaped or sized aerodynamic feature elements  154  may serve to trigger a progressive flow transition, especially at high pitch angle. 
     Aerodynamic feature elements  154  on the different aerodynamic feature elements surfaces of  FIG. 5  may differ in various respects. As a first example, the size of the aerodynamic feature elements may be varied. For example, surface A may include large, deep aerodynamic feature elements, whereas surfaces B to F may include aerodynamic feature elements of increasing extension and depth. Further, surfaces A′ to F′, which are located near a leading edge portion of blade  180 , may be smaller than corresponding surfaces A to F on a trailing edge portion of blade  180  to be adapted to the generally more laminar air flow near the leading edge. 
     For example, aerodynamic feature elements  154  on surface A may be smaller in each direction by a respective factor of one half to one tenth than those on surface F, whereas aerodynamic feature elements  154  on surfaces B to E have intermediate sizes. For example, the elements on surface F may have a maximum extension along the surface of 1 to 10 cm and a maximum depth of 0.1-1 cm, whereas surface A may have elements having a maximum extension along the surface of 1 to 10 mm and a maximum depth of 0.1-1 mm. Thereby, the skin F comprising aerodynamic feature elements  154  may be relatively thin (e.g. having an outer layer about 1 mm thick), and skin A may be thicker (e.g. having an outer layer about 4 mm thick). Alternatively, both dimple skins A and F may be of the same thickness. In both cases, the bottom surface of e.g. a dimple skin may have the shape of the bottom of the dimples, or it may be smooth. 
     In order to have a continuous cross-over between a surface area having comparably large elements and a surface area having comparably small elements, it is also possible to have elements of different sizes on one surface. 
     As a second example, the shape of aerodynamic feature elements may be varied. Examples for a variation in shape are shown in  FIGS. 6-13 . As described herein, any embodiment of  FIGS. 6-13 , may correspond to any aerodynamic feature elements surface of  FIG. 5 . As shown in  FIGS. 6-13 , each shape is adapted to a particular air flow characteristic. For example, elongated structures in  FIGS. 8 ,  9 ,  12  and  13  are adapted to a preferred overall air flow direction, whereas circular shapes, such as shown in  FIGS. 5 ,  6 ,  7 ,  10  and  11  do not have a preferred air flow direction. Further, the immersed structures of  FIGS. 6-9  tend to induce micro-turbulent flow within the immersed cavities, whereas the protruding elements of  FIGS. 10-13  tend to induce micro-turbulent flow in the wake of the elements. Therefore, it can be advantageous to form the latter elements in an asymmetric pattern that distinguishes between a generally upstream and a generally downstream region of each element (not shown). 
     As a third example, the aerodynamic feature elements on each of the surfaces A to F and A′ to F′ of  FIG. 5  may be the same. The advantage of this structure is that the blade skin needs not be made from a single-piece skin sheet, but can also be made from a plurality of skin sheet sections. Hereby, the skin sheet sections can have the form of tiles and can be applied in a tile-like fashion on the blade or on part of the blade. The number of tiles is not limited to 2×6 tiles as shown in  FIG. 5 , nor is the tiling limited to a quadratic tiling, but the skilled person will recognize that there are various ways of tiling a blade surface or part of a blade surface using aerodynamic feature element surfaces. 
     In other embodiments (not shown), different aerodynamic feature element surfaces may be used on the suction side and on the pressure side of the blade. Hereby, the term “different” can e.g. signify that the shape, size, arrangement, or orientation of the aerodynamic feature elements may be different. Further, more or less than 2×6 tiles or aerodynamic feature element surfaces may be provided. Further, the aerodynamic feature element characteristics may be varied in any direction within one surface. In a further embodiment, there are variations in the arrangement and/or the orientation of the aerodynamic feature elements. 
     It is typically desired to influence the flow separation behaviour. To this end, a region of potential flow separation should, if possible, be covered with aerodynamic feature elements. It can further be desired to improve the aerodynamic and noise performance at the root region of the blade, which is usually characterized by having a thick airfoil and a low local flow velocity. To this end, it may be advantageous to provide large aerodynamic feature elements near the root region, such as to energize sooner a stable turbulent boundary layer. 
     In the tip region, on the other hand, which is characterized by thin airfoils and a high local flow velocity, the priorities may be different. For example, here it may be desired to effectively restrict frictional drag but still stabilize the flow separation and other flow behaviour. This may lead to an improved aerodynamic and noise performance over a large operating domain (e.g. pitch, rotor speed). Therefore, the size of the aerodynamic feature elements should not be too large, such as to limit the frictional drag due to induced turbulences. Analogously, the use in other parts of the blade should be made dependent on a number of further factors, such as the relative importance of the frictional drag. 
     The aerodynamic feature elements surface is typically a polymeric skin sheet. In one embodiment, it comprises a hard polymer compound. In another embodiment, a thermoplastic ioniomeric resin is used as a polymer compound, such as, for example, “Surlyn”, produced by Dupont (see U.S. Pat. No. 4,884,814), or “Escor” and “lotek”, produced by Exxon (see U.S. Pat. No. 4,911,451). In the exemplary embodiment, the surface is manufactured from pre-moulded material and may have a pattern curved shell, which is typically similar to a composite sandwich. Its outer layer thickness is typically about 1-4 mm for a normal blade length. For large blades, i.e. a blade span of more than 50 m, the skin thickness is scaled accordingly by a scale factor. In most cases, the blade span divided by 50 m is used as scale factor. 
     The aerodynamic feature elements may be arranged in a variety of two-dimensional and three-dimensional patterns. Exemplary patterns include hexagonal, rectangular, quadratic, body-centred quadratic, and other regular patterns. Further, it is possible to arrange the dimples in a random irregular pattern. The patterns can be cyclic or acyclic. The random arrangement is isotropic in the sense that no direction is preferred. 
     Although dimples were mainly used as examples in the above description, other aerodynamic feature elements can be used in a similar manner. These other elements can be defined by their height profile in the aerodynamic feature element surface. A number of such elements are illustrated in  FIGS. 14-31 . Hereby, it is possible to distinguish elements protruding outwardly from the surface and elements immersed inwardly into the surface. The terms “protruding” and “immersed” are used with respect to a smooth surface area defined by the surface between the aerodynamic feature elements. 
       FIGS. 14-31  display examples of aerodynamic feature elements. The aerodynamic feature elements are either immersed inwardly into the surface or protruding outward from the surface.  FIGS. 14 ,  17 , and  20  illustrate the slope of each element,  FIGS. 15 ,  18 , and  21  illustrate the elements immersed inwardly and  FIGS. 16 ,  19 , and  22  illustrate the elements protruding outwardly.  FIGS. 23-31  illustrates elongated slots. As is shown in  FIGS. 23-25 , the slots have either rounded or sharp edges and can be symmetric or asymmetric. In addition, they have various shapes and cross-sections. The embodiments illustrated in  FIGS. 26-31 , include slots that have a straight, a curved, multiple curvatures, or a zig-zag form (similar to a magnified groove of a vinyl record). The slots, or grooves, are to be open-ended or closed-ended. In addition, the slots are straight faceted edge slots and curved rounded edge slots. 
       FIGS. 16 ,  19 , and  22 , illustrate a section of an outward spherical shape, polygonal shapes, and rounded polygonal shapes.  FIGS. 23-31  illustrate ribs that correspond to the above description of the slots, the difference being that the ribs are outwardly protruding from the surface. 
     Other embodiments of intrusions that are suitable as aerodynamic feature elements include pores, inverted cones, and grooves. The grooves include, for example, a U-shaped or a V-shaped vertical cross-section. Further examples for protrusions are shark teeth, pyramids, cones, hemispherical sections, fins and ribs. The ribs include, in one example, a vertical cross-section shape as an inverse U or an inverse V. 
     Further, the aerodynamic feature elements may be asymmetrically deformed or otherwise anisotropic and thus may have a designated orientation, e.g. an upstream side and a downstream side. Examples for aerodynamic feature elements having a designated orientation include shark teeth and wave-type elements (i.e. asymmetrically deformed ribs). For example, the wave-type elements are arranged having a long side along a direction of air flow or orthogonal to a direction of air flow. 
     If arranged in a suitable way, the use of anisotropic elements can be a way of adapting the aerodynamic feature elements to an anticipated air flow direction. Further, it may have an effect of directing the air flow along the blade surface. This can have further advantageous effects on the overall air flow. For example, large-scale laminar flow along a defined direction on the blade surface may be promoted. This can lead to a decrease of noise production. 
     Further, elements having sharp or rounded edges can be used. Further, other elements are within the scope of the present invention, e.g. elements comprising both protruding and immersed portions. Generally, the aerodynamic feature elements can be characterized, among others, in terms of positive or negative cavity, cavity curvature, cavity facets, sharp or rounded edges, random or cyclic pattern layouts, and isotropic or anisotropic shape. 
     The aerodynamic feature elements shown in  FIGS. 6-31  have a similar effect as the dimples described above, namely to influence the air flow near the boundary layer at the blade surface. Therefore, aerodynamic feature elements formed to correspond to the above description, but in which the dimples are replaced by other aerodynamic feature elements such as the ones shown in  FIGS. 6-31 . 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.

Technology Classification (CPC): 5