AIRFOIL TRAILING EDGE APPARATUS FOR NOISE REDUCTION
A ridge (28) mounted or defined along a trailing edge (22) of an airfoil (20) for noise reduction. Sides (32, 34) of the ridge converge from respective suction and pressure sides (46, 48) of the trailing edge to a peak (30) of the ridge pointing aft. The sides of the ridge may be concave. The ridge may be hollow (42) or have a core (43) of a sound-absorbing material. The sides of the ridge may be perforated (44A, 44B) for pressure equalization across the ridge. The ridge may be covered with bristles (56) or be defined by the tips (58) of bristles (56A, 56B) of varying length. The peak of the ridge and/or at least one corner (64, 66) of the ridge and/or of the trailing edge (46, 48) may be serrated.
The invention relates to noise reduction devices on airfoils, and particularly to devices for noise reduction on wind turbine blades having thick trailing edges.
BACKGROUND OF THE INVENTIONAirfoils with thick (or blunt) trailing edges may be used on wind turbine blades to improve aerodynamic and structural performance. However, thick trailing edges are subjected to larger aerodynamic pressure gradients across the trailing edge than are thin trailing edges. This leads to the generation of von-Karman vortex streets downstream of the airfoil, resulting in undesirable acoustic noise.
The invention is explained in the following description in view of the drawings that show:
Each side 32, 34 of the ridge 28 may span between a respective corner 46, 48 of a squared trailing edge 22 and the peak 30 of the ridge 28. Herein “corner” in this context means a corner as seen in a cross sectional view. Each side 32, 34 may form an inward angle A, B of at least 20 degrees relative to an extension of the respective suction and pressure sides 36, 38 of the airfoil, or especially more than 30 degrees. The sides may be concave as shown. Such a ridge reduces the size of the stationary vortices 42 compared to that which would be produced by a flat splitter plate by providing a compact, smoothly contoured nest for each stationary vortex 42. This results in a smoother and more compact slipstream that produces less drag and noise. This noise reduction device 27A may be made of a material compatible with the airfoil material, such as carbon fiber/polymer composite or fiberglass/polymer composite, but are not limited to such materials. Alternately, the device 27A may be made of a more flexible material than the airfoil such as nylon, polyester or rubber that may deform to alleviate loads.
If the trailing edge is blunt without sharp corners 46, 48, the ridge 28 may be defined as comprising two walls 32, 34 converging to a peak 30 from a respective suction side and pressure side of the trailing edge 22, for example at inward angles A, B of at least 20 or 30 degrees.
The ridge 28 of the invention may be fabricated separately from the airfoil 20, and attached to it by adhesive or fasteners such as screws. With separate fabrication the ridge can use materials different from the airfoil that are specialized for sound reduction, such as flexible materials and sound absorbing core materials. The invention allows for site-specific trailing edge attachments. For example, high turbulence sites can use soft-passive trailing edge attachments. Alternatively, the ridge 28 can be cast along with the blade as long as it can withstand the temperatures experienced during casting.
Another benefit is that wind turbine blades with thick trailing edges can be transported without damage to the trailing edge because the ridge can be attached to the blade on site. Alternately, in some embodiments the ridge may be formed integrally with the airfoil, for example by grinding the trailing edge into a ridge geometry as described and shown herein. If the ridge is formed integral, the trailing edge as discussed herein is defined as an imaginary plane extending between the suction side and the pressure side between the corners 46, 48.
Benefits of thick trailing edges on wind turbine blades may include:
For some manufacturing methods, thicker trailing edges may result in better resistance to buckling than thin trailing edges. This may become increasingly relevant for swept blades.
A thick trailing edge allows blade designers to tailor the blade torsional stiffness. Reducing torsional stiffness allows a more twistable blade without sacrificing the flap stiffness needed to control tip deflections and avoid tower strikes.
A thick trailing edge allows greater freedom to add or remove edgewise stiffness to prevent dynamic structural/aerodynamic instabilities such as unstable whirling modes. Thickness is an alternative to large amounts of pre-deflection flap, which presents transportation and manufacturing issues.
Thin trailing edges are prone to damage by tight straps or other objects during transportation. Damaged trailing edges are likely to be repaired in a way that creates additional aerodynamic noise due to grinding roughness or inaccurate shape.
Benign extreme loads. Thick trailing edge sections exhibit lower flap loads under extreme conditions.
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.
Claims
1. An airfoil trailing edge apparatus for noise reduction, comprising:
- a ridge along a trailing edge of an airfoil;
- wherein the ridge comprises first and second sides converging from a respective suction side and pressure side of the trailing edge to an aft-pointing peak of the ridge.
2. The airfoil trailing edge apparatus of claim 1 wherein the first side of the ridge spans between the peak and a suction side corner of the trailing edge, and the second side of the ridge spans between the peak and a pressure side corner of the trailing edge.
3. The airfoil trailing edge apparatus of claim 1 wherein the sides of the ridge are concave.
4. The airfoil trailing edge apparatus of claim 3 wherein the peak of the ridge is serrated.
5. The airfoil trailing edge apparatus of claim 1 wherein the ridge is hollow.
6. The airfoil trailing edge apparatus of claim 5 wherein at least one side of the ridge is perforated.
7. The airfoil trailing edge apparatus of claim 1 wherein the ridge is perforated with a plurality of holes of at least two different diameters.
8. The airfoil trailing edge apparatus of claim 1 wherein the ridge comprises a core of a sound absorbing material.
9. The airfoil trailing edge apparatus of claim 1 wherein the ridge comprises a core of a breathable sound absorbing material, and both sides of the ridge are perforated with holes that provide pressure equalization between the two sides of the ridge through the breathable core.
10. The airfoil trailing edge apparatus of claim 1 wherein at least one of the sides of the ridge comprises a spoiler in the form of a secondary ridge.
11. The airfoil trailing edge apparatus of claim 10 wherein spoilers on the first and second sides of the ridge each comprise an outer surface aligned with a nearer one of a pressure side or suction side of the airfoil or aligned with a mean camber line or a chord line of the airfoil.
12. The airfoil trailing edge apparatus of claim 1, further comprising a plurality of bristles attached to at least one side of the ridge.
13. The airfoil trailing edge apparatus of claim 1, further comprising first and second pluralities of bristles of varying length that provide bristle tips arranged to define respectively the first and second sides of the ridge.
14. The airfoil trailing edge apparatus of claim 13, further comprising a splitter plate between the first and second pluralities of bristles, wherein the splitter plate is aligned with a mean camber line or a chord line of the airfoil, and a distal end of the splitter plate defines the peak of the ridge.
15. The airfoil trailing edge apparatus of claim 1, wherein the trailing edge further comprises a suction side corner, a pressure side corner, and a plurality of serrations along at least one of the corners of the trailing edge, the serrations crossing a respective attached corner of the ridge.
16. The airfoil trailing edge apparatus of claim 1, wherein the ridge comprises a suction side corner and a pressure side corner, and a plurality of serrations along at least one of the corners of the ridge.
17. An airfoil trailing edge noise reduction apparatus, comprising:
- a ridge along a trailing edge of an airfoil;
- wherein the ridge comprises first and second sides converging from a respective suction side and pressure side of the trailing edge at respective inward angles of at least 20 degrees relative to respective extensions of the pressure side or suction side of the airfoil, wherein the sides converge to an aft pointing peak.
18. The airfoil trailing edge of claim 17, wherein the ridge is separately formed and is attached to the airfoil at the trailing edge.
19. The airfoil trailing edge of claim 18, wherein the ridge is formed integral to the airfoil.
20. An airfoil trailing edge shaped for noise reduction, comprising:
- a suction side corner and a pressure side corner of the trailing edge; and
- a plurality of serrations along at least one of the corners of the trailing edge.
Type: Application
Filed: May 23, 2013
Publication Date: Feb 19, 2015
Inventors: Kristian R. DIXON (Boulder, CO), Alonso O. ZAMORA RODRIGUEZ (Boulder, CO), Michael J. ASHEIM (Golden, CO), Drew EISENBERG (Boulder, CO), Manjinder J. SINGH (Broomfield, CO), Arni T. STEINGRIMSSON (Erie, CO), Henrik Fredslund HANSEN (Aalborg), Peter J. RIMMINGTON (Superior, CO), Justin L. MULLINGS (Golden, CO)
Application Number: 13/900,756
International Classification: F01D 25/04 (20060101); F01D 5/14 (20060101);