FIELD OF THE INVENTION
The invention relates to vortex generators on wind turbine blades, and particularly to such vortex generators shaped for noise reduction.
BACKGROUND OF THE INVENTION
Vortex generators are known to be used to induce vortical flow structures that improve the performance of a wind turbine blade by entraining momentum from the free stream relative flow into the boundary layer, and consequently preventing or delaying flow separation on the wind turbine blade during operation.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in the following description in view of the drawings that show:
FIG. 1 is a perspective view of a prior art wind turbine blade with vortex generators.
FIG. 2 is a perspective view of a prior art vortex generator.
FIG. 3 is a top view of a pair of diverging prior art vortex generators.
FIG. 4 is a perspective view of an embodiment of the invention.
FIG. 5 is a prior art transverse sectional view taken along line 5-5 of FIG. 3.
FIG. 6 is a transverse sectional view of an embodiment of the invention taken along line 6-6 of FIG. 4.
FIG. 7 is a transverse sectional view of another embodiment of the invention.
FIG. 8 is a transverse sectional view of another embodiment of the invention.
FIG. 9 is a transverse sectional view of another embodiment of the invention.
FIG. 10 is a perspective view of another embodiment of the invention.
FIG. 11 is a perspective view of another embodiment of the invention.
FIG. 12 is a perspective view of another embodiment of the invention.
FIG. 13 is a suction side view of another embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have recognized that current vortex generator (VG) designs create separated flow regions around the VG that do not contribute to the generation of the beneficial vortex, but increase aerodynamic noise and drag. The inventors have also found that aerodynamic noise is a limiting factor in the design and optimization of wind turbines due to strict regulations around the world. Thus, the present invention was developed to improve wind turbine efficiency, and at the same time, to reduce aerodynamic noise to meet regulations and to minimize site objections.
FIG. 1 shows a prior art wind turbine blade 20 with a suction side aerodynamic surface 22 on which is mounted a row of pairs of diverging vortex generators 26, 28. A free stream airflow 24 relative to the turbine blade generates counter-rotating vortices 27, 29. These vortices entrain free stream energy into the boundary layer, which delays or prevents flow separation from the aerodynamic surface 22.
FIG. 2 shows a prior art vortex generator (VG) 26, which is a small airfoil extending from the larger aerodynamic surface 22 of the wind turbine blade 20. It has a pressure side 30 (hidden), suction side 32, leading edge 34, trailing edge 36, a root portion 38 attached to the larger aerodynamic surface, and a distal portion or tip 40. Such foils are commonly triangular or delta-wing-shaped plates as shown, and have a high leading edge sweep angle Λ, such as 50-80 degrees.
FIG. 3 shows a top view of two diverging vortex generators 26, 28, separated by a distance called a pitch P. Each VG is a foil with a length L and an angle of incidence φ relative to the free stream flow 24. A high incidence angle φ, such as 10-40 degrees, creates a high pressure difference between the pressure and suction sides of the VG. The combination of high incidence angle φ and high sweep angle Λ (FIG. 2) promotes leakage of high pressure flow from the pressure side 30 to the suction side 32. As the local flow 40 wraps around the VG leading edge 34, it forms a shear layer that rolls into a vortical flow structure 29 called a leading edge vortex. The exemplary incidence angle φ shown in the drawing is 15 degrees.
Noise is generated by angular edges and corners on the VG, and by separation areas or gaps between the suction side of the VG and the vortex 29 that promote noisy waves and eddies. High VG incidence angles φ, such as 10-40 degrees, are beneficial for maintaining the vortex 29 over a range of relative wind speeds. However, separation areas form where the vortex 29 diverges from the highly angled VG toward the free steam 24. In addition, VG trailing edge (TE) waves such as von Karman streets can create noise.
The inventors have recognized that noise can be reduced by providing a vortex nest along the suction side of the VG that eliminates or reduces flow separation areas between the vortex and the VG. The nest is a structure that fits partially around the vortex so that the generally cylindrical or conical shape of the vortex does not abut flat surfaces or inside corners, but instead nests in a mating concave surface that fills the areas that otherwise would be prone to separation.
FIG. 4 shows a VG 26A in an embodiment that provides a progressive fillet 42 in a root portion of the VG, between the suction side 32A of the VG and the larger aerodynamic surface 22. This fillet has a radius that increases with distance from the front 44 of the VG, providing a concave nest 43A that matches or closely approximates the generally conical shape of the vortex. This embodiment also provides rounded corners, tapered span-wise thickness (FIG. 6), and tapered longitudinal thickness to reduce aerodynamic noise by eliminating or reducing flow separations. Herein “span-wise” means generally normal to the larger aerodynamic surface 22. A smooth and thin leading edge reduces momentum losses in developing the vortex. An exemplary manufacturing method is injection molding. A second VG 28A in this embodiment is shown, forming a divergent pair 26A, 28A.
FIG. 5 is a transverse sectional view of the prior art VG 26 of FIG. 3. Areas A and B are separated flow regions that generate noise. Angular edges or apexes 46 and 48 also generate noise and drag along the leading and trailing edges without contributing to the beneficial vortex 27. A vortex centerline 50 is indicated for reference.
FIG. 6 is a transverse sectional view of a VG embodiment 26A of the invention, with a pressure side 30A, suction side 32A, leading edge 34A, and local airflow 40. This embodiment has two improvements over the prior art. Firstly, the progressive fillet 42 eliminates gap A of FIG. 5 by a progressively increasing radius that matches the progressively increasing vortex 27, providing a vortex nest 43. For example, the fillet radius may be designed to be substantially or approximately centered on the vortex centerline 50. Secondly, the leading edge 34A has only one apex in transverse sectional views along at least most of the leading edge. This minimizes disruptions to the flow 40 caused by multiple apexes 46, 48 as in FIG. 5 that generate noise.
FIG. 7 is a transverse sectional view of a VG embodiment 26B of the invention. This embodiment reduces gap B compared to FIG. 6 by means of net curvature or asymmetry leaning from the pressure side 30B toward the suction side 32B of the VG as viewed in transverse sections along at least most of the leading edge 34B.
FIG. 8 is a transverse sectional view of a VG embodiment 26C of the invention with a pressure side 30C, suction side 32C, distal portion 40C, and local airflow 40. This embodiment eliminates gap B by the distal portion 40C curling over the suction side 32C, thus forming a suction side concavity or vortex nest 43 as seen in transverse section. The curl of this embodiment may be continuous from the progressive fillet 42 to the distal portion 40C as shown, or it may be formed as a main VG airfoil portion and a dihedral tip as in FIG. 9. In either case, a dihedral angle r may be defined between a plane P1 normal to the larger aerodynamic surface 22 and a plane P2 of the distal portion 40C, 40D of the VG. If the curl is continuous as in FIG. 8, the second plane P2 is defined tangent to the distal portion of the VG as shown. An optimum range for this dihedral angle is 20-70 degrees.
FIG. 9 is a transverse sectional view of a VG embodiment 26D of the invention with a pressure side 30D, suction side 32D, distal portion 40D, and local airflow 40. This embodiment eliminates gap B by a tip portion 40D of the VG that curls or leans over the suction side 32C with a dihedral angle r, thus forming a suction side concavity or vortex nest 43 as seen in transverse section.
A high leading edge sweep angle Λ (FIG. 2), such as 50-80 degrees, induces the vortex 27 to form by mid-span of the VG. The distal portion or tip 40 of such VG does not contribute to the vortex, and only increases drag and noise due to flow separation. However, the embodiments of FIGS. 8 and 9 promote flow rollup around the leading edge by providing a geometric path for the roll to follow. Thus the distal portion 40C, 40D of the VGs of FIGS. 8 and 9 contributes to the vortex 27 and contains and directs it, making it smaller and more intense. Reducing the vortex size increases its wave frequencies, which attenuate more rapidly in the air, thus effectively reducing the distance of threshold noise.
FIG. 10 is a perspective view of a VG embodiment 26E of the invention with a suction side 32E, leading edge 34E, trailing edge 36E, distal portion 40E, and free stream flow 24. A trailing edge (TE) fillet 52E may be blended with the trailing edge 36E to increase the effective root chord of the VG, which increases the VG Reynolds number and the strength of the induced leading edge vortex. A ridge 54E may be formed by the trailing edge 36E and the fillet 52E having a single apex in chord-wise or span-wise sectional views thereof. This ridge may be aligned with the leading edge 34E or alternately it may be aligned with the suction side 32E as indicated at 54E, thus extending the vortex 43 nest aft. The ridge for example may be shaped as a continuation of the leading edge shape 34A or 34B of FIGS. 6 and 7.
FIG. 11 is a perspective view of an embodiment of the invention showing a pair of diverging vortex generators 26F, 28F separated by a distance P between the front ends 44 of their leading edges 34. A progressive fillet 42F on the suction side 32F is illustrated by dashed contour lines. Progressive fillets 42F on the suction sides 32F of two VGs 26F, 28F are illustrated by dashed contour lines. These fillets converge toward each other from front to back along the larger aerodynamic surface 22. This convergence 56 accelerates the flow between the VGs by the Venturi Effect and Bernoulli Principle, which increases the pressure difference across each VG, providing a stronger leading edge vortex. The pressure-side fillet 58 shown with dashed contour lines on the pressure side 30 of the VG 28F may or may not be progressive, and it may be smaller than the fillet 42F on the suction side since it does not need to converge with a corresponding fillet of an adjacent VG. A ridge 54F may be formed by the trailing edge 36F and the TE fillet 52F. This ridge may extend aft from the trailing edge 36F in alignment with the suction side 32F. It may extend a contour of the suction side 32F, thus providing a smooth vortex nest 43F. This nest may have a generally conical shape with an axis that diverges from the incidence angle of the VG toward the free stream 24. As a further enhancement, the nest 43F may have an axis of concavity 50F (meaning a curve drawn along a centerline or focus of the fillet 42F) that curves toward the free stream direction 24 by at least 5 degrees. This curvature causes the vortex nest to guide or follow the vortex as it curves toward the free stream 24 away from the incidence angle of the VG (FIG. 3). This allows a smoother direction change and reduces gaps and noise.
FIG. 12 is a perspective view of an embodiment of the invention showing a pair of diverging vortex generators 26G, 28G separated by a distance P between front ends 44 of their leading edges 34B. As in FIG. 11, a progressive fillet 42G on the suction side 32G is illustrated by dashed contour lines. A ridge 54G may be formed by the trailing edge 36E and the TE fillet 52E having a single apex in chord-wise or span-wise sectional views. This ridge may extend aft from the trailing edge 36G in alignment with the suction side 32G. It may extend a contour of the suction side 32G, thus providing a smooth vortex nest 43G. This nest may have a generally conical shape with an axis that diverges from the incidence angle of the VG toward the free stream 24. As a further enhancement, the nest 43G may have an axis of concavity 50G that curves toward the free stream direction 24 by at least 5 degrees. Each leading edge 34B may have net curvature or asymmetry leaning from the pressure side 30G toward the suction side 32G of the VG as viewed in transverse sections along at least most of the leading edge 34B as shown for example in FIG. 7.
FIG. 13 is a suction side view of a VG embodiment 26H with serrations 60 along the trailing edge 36H. The serrations may continue along the trailing edge fillet ridge 54G, if any. Although the main flow pattern in a vortex generator is the leading edge vortex 27 (previously shown), there is also flow over the trailing edge 36H. Since the flow over the suction side 32G is accelerated by the leading edge vortex, there can be a strong a pressure difference at the trailing edge 36H in some designs. This can create a coherent pressure wave that is a source of aerodynamic noise. Serrations such as chevron-shaped cuts add a variety of smaller wave structures that disrupt the coherence of the trailing edge pressure wave, and reduce noise. This effect also reduces disruption of the leading edge vortex 27 by trailing edge pressure waves such as von Karman streets.
The embodiments described and shown herein can be use separately or combined. For example, the serrations of FIG. 13 may be added to the VGs 26G, 28G of FIG. 12. As another example, the leading edge 34B of FIG. 7 was combined with the progressive fillet 42F of FIG. 11 in the embodiment of FIG. 12.
The invention provides a vortex nest that reduces noisy pockets of flow separation. It reduces separation between the vortex and flat surfaces and inside corners. In some embodiments it also reduces separation by reducing the number of abrupt outside edges. It reduces momentum losses due to flow separation and friction by specifically contouring portions of the VG that do not contribute to generating the vortex but only increase drag and noise. It also optimizes the generation of the beneficial leading edge vortex by reducing the mixing of trailing separated flow with the vortex.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
Patent Application
20150010407
