Vortex generator arrangement for an airfoil

Abstract

A particular arrangement of vortex generators for an airfoil is described. The vortex generators are provided in pairs, preferably on a wind turbine blade, wherein by arranging the vortex generators according to specified characteristics, a surprising improvement in blade performance is provided over the prior art systems.

Claims

1. A wind turbine blade having an airfoil profile, the wind turbine blade comprising an arrangement of vortex generators, said airfoil profile having a leading edge and a trailing edge, said vortex generators provided as an array of pairs of vortex generators, said vortex generators comprising substantially triangular vortex generator vanes projecting from a surface of said airfoil profile of said wind turbine blade, each of said pairs comprising a first vortex generator and a second vortex generator, wherein said first and second vortex generators each comprise: a first leading edge end provided towards said leading edge; a second trailing edge end provided towards said trailing edge; a base extending between said first leading edge end and said second trailing edge end adjacent the surface of the airfoil profile; and a tip distal end of each said substantially triangular vortex generator vane provided near said second trailing edge end, wherein said first and second vortex generators are skewed relative to a main flow direction of the airfoil profile, wherein the first leading edge ends of the first and second vortex generators form a narrow, relatively convergent end of a pair, and that the second trailing edge ends of the first and second vortex generators form a wider, divergent end of the pair, and wherein l is a length of the base of the first and second vortex generators; s is a distance between the respective second trailing edge ends forming the wider, divergent end of the first and second vortex generators in the pair; h is a height from said base to a surface of said distal end of the first or second vortex generator in the pair; z is a distance between nominal center lines defined between the first and second vortex generators of adjacent pairs in the array; and β is an angle of skew of the first and second vortex generators relative to the main flow direction of the airfoil profile, wherein: l/h is between 1-5, s/h is between 6-15; z/h is between 7-20; and β is between 6-12 degrees, whereby s/h is selected to reduce drag and increase lift.

2. The wind turbine blade of claim 1, wherein said vortex generators comprise right angle triangle vortex generator vanes, wherein a hypotenuse of said right angle triangle vortex generator vanes extends from the base at said first end to the distal end at said second end.

3. The wind turbine blade of claim 1, wherein l/h is 2.

4. The wind turbine blade of claim 1, wherein s/h is between 6-10.

5. The wind turbine blade of claim 4, wherein s/h is 7.

6. The wind turbine blade of claim 1, wherein z/h is between 8-15.

7. The wind turbine blade of claim 6, wherein z/h is 10.

8. The wind turbine blade of claim 1, wherein β is between 9-12 degrees.

9. The wind turbine blade of claim 8, wherein β is 12 degrees.

10. The wind turbine blade of claim 1, wherein the angle β is measured from the respective first ends towards the second ends.

11. The wind turbine blade of claim 1, wherein said wind turbine blade has a length greater than 30 metres.

12. A wind turbine having at least one wind turbine blade as claimed in claim 1.

13. The wind turbine blade of claim 1, wherein s/h is between 7-15.

14. The wind turbine blade of claim 13, wherein z/h is between 8-20.

15. The wind turbine blade of claim 1, wherein z/h is between 8-20.

16. The wind turbine blade of claim 1, wherein l/h is between 2-5.

17. The wind turbine blade of claim 1, wherein z/h is between 7-10.

18. A wind turbine blade having an airfoil profile, the wind turbine blade comprising an arrangement of vortex generators, said airfoil profile having a leading edge and a trailing edge, said vortex generators provided as an array of pairs of vortex generators, said vortex generators comprising substantially triangular vortex generator vanes projecting from a surface of said airfoil profile of said wind turbine blade, each of said pairs comprising a first vortex generator and a second vortex generator, wherein said first and second vortex generators each comprise: a first leading edge end provided towards said leading edge; a second trailing edge end provided towards said trailing edge; a base extending between said first leading edge end and said second trailing edge end adjacent the surface of the airfoil profile; and a tip distal end of each said substantially triangular vortex generator vane provided near said second trailing edge end, wherein said first and second vortex generators are skewed relative to a main flow direction of the airfoil profile, wherein the first leading edge ends of the first and second vortex generators form a narrow, relatively convergent end of a pair, and that the second trailing edge ends of the first and second vortex generators form a wider, divergent end of the pair, and wherein l is a length of the base of the first and second vortex generators; s is a distance between the respective second trailing edge ends forming the wider, divergent end of the first and second vortex generators in the pair; h is a height from said base to a surface of said distal end of the first or second vortex generator in the pair; z is a distance between nominal center lines defined between the first and second vortex generators of adjacent pairs in the array; and β is an angle of skew of the first and second vortex generators relative to the main flow direction of the airfoil profile, wherein: l/h is between 1-5, s/h is between 7-15; z/h is between 7-20; and β is between 6-16 degrees, whereby s/h is selected to reduce drag and increase lift.

19. A wind turbine blade having an airfoil profile, the wind turbine blade comprising an arrangement of vortex generators, said airfoil profile having a leading edge and a trailing edge, said vortex generators provided as an array of pairs of vortex generators, said vortex generators comprising substantially triangular vortex generator vanes projecting from a surface of said airfoil profile of said wind turbine blade, each of said pairs comprising a first vortex generator and a second vortex generator, wherein said first and second vortex generators each comprise: a first leading edge end provided towards said leading edge; a second trailing edge end provided towards said trailing edge; a base extending between said first leading edge end and said second trailing edge end adjacent the surface of the airfoil profile; and a tip distal end of each said substantially triangular vortex generator vane provided near said trailing edge second end, wherein said first and second vortex generators are skewed relative to a main flow direction of the airfoil profile, wherein the first leading edge ends of the first and second vortex generators form a narrow, relatively convergent end of a pair, and that the second trailing edge ends of the first and second vortex generators form a wider, divergent end of the pair, and wherein l is a length of the base of the first and second vortex generators; s is a distance between the respective second trailing edge ends forming the wider, divergent end of the first and second vortex generators in the pair; h is a height from said base to a surface of said distal end of the first or second vortex generator in the pair; z is a distance between nominal center lines defined between the first and second vortex generators of adjacent pairs in the array; and β is an angle of skew of the first and second vortex generators relative to the main flow direction of the airfoil profile, wherein: l/h is between 1-5, s/h is between 6-15; z/h is between 8-20; and β is between 6-16 degrees, whereby s/h is selected to reduce drag and increase lift.

Description

DESCRIPTION OF THE INVENTION

(1) An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a wind turbine;

(3) FIG. 2 shows a schematic view of a wind turbine blade according to the invention;

(4) FIG. 3 shows a schematic view of an airfoil profile of the blade of FIG. 2;

(5) FIG. 4 shows an enlarged view of an arrangement of vortex generators according to the invention;

(6) FIG. 5 shows a plot of the lift coefficient against the angle of attack from experimental results of two embodiments of the invention in comparison with prior art systems;

(7) FIG. 6 shows a plot of the drag coefficient against the angle of attack from experimental results of two embodiments of the invention in comparison with prior art systems; and

(8) FIG. 7 shows a plot of the (lift coefficient/drag coefficient) against the angle of attack from experimental results of two embodiments of the invention in comparison with prior art systems and

(9) FIG. 8 shows a plot of the lift coefficient against the drag coefficient from experimental results of two embodiments of the invention in comparison with prior art systems.

(10) FIG. 1 illustrates a conventional modern upwind wind turbine 2 according to the so-called “Danish concept” with a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub 8 and three blades 10 extending radially from the hub 8, each having a blade root 16 nearest the hub and a blade tip 14 furthest from the hub 8. The rotor has a radius denoted R.

(11) FIG. 2 shows a schematic view of a first embodiment of a wind turbine blade 10 according to an embodiment of the invention. The wind turbine blade 10 has the shape of a conventional wind turbine blade and comprises a root region 30 closest to the hub, a profiled or an airfoil region 34 furthest away from the hub and a transition region 32 between the root region 30 and the airfoil region 34. The blade 10 comprises a leading edge 18 facing the direction of rotation of the blade 10, when the blade is mounted on the hub, and a trailing edge 20 facing the opposite direction of the leading edge 18.

(12) The airfoil region 34 (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region 30 due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade 10 to the hub. The diameter (or the chord) of the root region 30 is typically constant along the entire root area 30. The transition region 32 has a transitional profile 42 gradually changing from the circular or elliptical shape 40 of the root region 30 to the airfoil profile 50 of the airfoil region 34. The chord length of the transition region 32 typically increases substantially linearly with increasing distance r from the hub.

(13) The airfoil region 34 has an airfoil profile 50 with a chord extending between the leading edge 18 and the trailing edge 20 of the blade 10. The width of the chord decreases with increasing distance r from the hub.

(14) It should be noted that the chords of different sections of the blade normally do not lie in a common plane, since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub.

(15) FIG. 3 shows a schematic view of an airfoil profile 50 of a typical blade of a wind turbine depicted with the various parameters, which are typically used to define the geometrical shape of an airfoil. The airfoil profile 50 has a pressure side 52 and a suction side 54, which during use—i.e. during rotation of the rotor—normally face towards the windward (or upwind) side and the leeward (or downwind) side, respectively. The airfoil 50 has a chord 60 with a chord length c extending between a leading edge 56 and a trailing edge 58 of the blade. The airfoil 50 has a thickness t, which is defined as the distance between the pressure side 52 and the suction side 54. The thickness t of the airfoil varies along the chord 60. The deviation from a symmetrical profile is given by a camber line 62, which is a median line through the airfoil profile 50. The median line can be found by drawing inscribed circles from the leading edge 56 to the trailing edge 58. The median line follows the centres of these inscribed circles and the deviation or distance from the chord 60 is called the camber f. The asymmetry can also be defined by use of parameters called the upper camber and lower camber, which are defined as the distances from the chord 60 and the suction side 54 and pressure side 52, respectively.

(16) Airfoil profiles are often characterised by the following parameters: the chord length c, the maximum camber f, the position df of the maximum camber f, the maximum airfoil thickness t, which is the largest diameter of the inscribed circles along the median camber line 62, the position dt of the maximum thickness t, and a nose radius (not shown). These parameters are typically defined as ratios to the chord length c.

(17) Preferably, the wind turbine blades are longer than 30 metres between said root and tip ends, preferably longer than 40 metres.

(18) An array of vortex generators 100 are provided on the suction side 54 of the airfoil 50, towards the leading edge 56. The vortex generators 100 act to induce turbulent vortices in the incident airflow over the airfoil 50, which prevent flow separation.

(19) With reference to FIG. 4, the vortex generators (or VGs) 100 are provided as pairs of triangular VG vanes 100 for use on an airfoil, e.g. a wind turbine blade. The pairs of VGs are arranged on the suction side of the airfoil, extending along at least a portion of the airfoil in the longitudinal direction, i.e. along the direction of radius R. Each pair of VGs comprises a first vortex generator 102 and a second vortex generator 104 provided adjacent one another. The individual VGs 102,104 have a first end 102a,104a arranged towards a leading edge 56 of the airfoil and a second end 102b,104b arranged towards a trailing edge 58 of the airfoil.

(20) The VGs 102, 104 have a right-angled triangle profile, a first leg of said triangular profile forming the base 105 of the VG attached to the surface of the airfoil and a second leg of said triangular profile projecting from the base at the said second end 102b, 104b of the VG, having height h. The hypotenuse of said profile extends from the first end 102a, 104a of the VG at the base (i.e., point 103 of VG 102 and point 109 of VG 104) to the distal point, or tip, of the second end 102b, 104b of the VG (i.e., to point 107 of VG 102 and point 111 of VG 104). However, it will be understood that other VG constructions may be used, and the invention is not limited to a right-angled triangle profile.

(21) The VGs 102,104 of each pair 100 are skewed from the direction of the incident flow (indicated by arrows A) on the airfoil by an angle β, measured from the respective first ends towards the second ends, such that the first ends of the adjacent VGs form a narrow, relatively convergent end of the VG pair, and that the second ends of the adjacent VGs form a wider, divergent end of the VG pair.

(22) Through variation of the characteristic dimensions of the VGs, a surprising improvement in aerodynamic performance was achieved. In particular, with regard to the dimensions of: the inter-vane distance s between the second ends 102b,104b of VGs in a VG pair; the length l of the base of the triangular VG vane 100; the height h of the tip end of the VG vanes, provided at the second end 102b,104b of the VGs; the inter-pair distance z measured between nominal centre lines extending between the VGs of adjacent VG pairs; and the offset angle β—measured as the angle at which a VG vane extends with respect to the direction of inflow at the vortex generators.

(23) In comparison to the state of the art described in Godard, while having an l/h ratio of between 4-15, by increasing the ratio of s/h to between 4-15, increasing the ratio of z/h to between 7-20, and reducing 13 to between 6-16 degrees, a surprising improvement in aerodynamic performance was discovered, resulting in an improved configuration of vortex generator pairs on an airfoil.

(24) While individually any one of the above described adjustments to the vortex generator arrangement would result in an increase in drag and a negative impact on aerodynamic performance, the combination of these feature adjustments presents an improvement over the prior art, which is not thought or suggested in the state of the art.

(25) The following table illustrates the improvement in performance obtained from two embodiments according to the proposed arrangement, in comparison to two prior art constructions, according to the accepted state of the art configuration (taken from Godard et al.). The results are illustrated for the starting condition wherein l/h=2, with the results of the proposed arrangement denoted “LM Wind Power”. In the table the maximum values of the polars and other significant values are given.

(26) TABLE-US-00001 TABLE 1 Experimental results Optimum VC parameter β z/h s/h c.sub.L,max AOA.sub.Stall ${\left(\frac{c_L}{c_D}\right)}_{\max }$ AOA.sub.Des c.sub.L,Design c.sub.D,Design configuration [*] [−] [−] [−] [*] [−] [*] [−] [−] LM Wind 9 10 7 1.81 13.9 117.6 6.1 1.12 0.0095 Power 12 10 7 1.81 13.4 118.1 7.1 1.24 0.0105 Godard 18 5 2 1.66 12.4 86.1 7.1 1.22 0.0142 18 7.5 3 1.71 12.7 94.9 7.1 1.23 0.0130

(27) It can be seen that the proposed arrangement of vortex generator pairs, having a relatively smaller angle β, larger z/h and s/h ratios, results in an increase in C.sub.Lmax, the maximum lift coefficient for the airfoil, over the Godard system. Additionally, the (C.sub.L/C.sub.D) ratio is increased relative to Godard, and the airfoil will only enter stall at a higher angle of attack (AOA).

(28) With reference to FIGS. 5-8, a series of illustrative plots are provided showing experimental results involving the embodiments of the invention described in the above table in comparison to the state of the art proposed in Godard. The individual plots are denoted VG_A_S_Z, wherein A is the β angle used, S is the s/h ratio, and Z is the z/h ratio. Accordingly, the plots denoted VG_A12_S7.0_Z10 and VG_A9_S7_Z10 are indicative of a vortex generator configuration according to the invention, and the plots VG_A18_S2.0_Z5 and VG_A18_S3.0_Z7.5 are indicative of prior art vortex generator configurations, with reference to the above Table 1.

(29) The experimental investigations of the effect of the vortex generator configuration on the maximum lift coefficient, C.sub.l,max and on maximum glide ratio, (C.sub.l/C.sub.d).sub.max, have been carried out in the LM Wind Power wind tunnel on a DU 91-W2-250 profile, which is a wind turbine dedicated airfoil developed at Delft University of Technology [W. A. Timmer & R. P. J. O. M. van Rooij; Summary of the Delft University Wind Turbine Dedicated Airfoils; ASME Journal of Solar Energy 125 (2003) 488-496]. The Reynolds number for the presented results is 3 million.

(30) By way of comparison to the experimental results, the performance of the airfoil used without any vortex generators is also shown in FIGS. 5-8, by the plot denoted ‘Clean’.

(31) In FIG. 5, a plot is shown of the lift coefficient C.sub.l against angle of attack AOA. Here it can be seen that the prior art systems (indicated in the plots denoted by the X and by the triangle) experience a reduction in the C.sub.l provided by the airfoil at an earlier AOA in comparison to the embodiments of the invention, i.e. the system of the invention will surprisingly enter stall at a later angle of attack than the prior art systems.

(32) In FIG. 6, a plot is shown of the drag coefficient C.sub.d against angle of attack AOA. Here it can be seen that the prior art systems (indicated in the plots denoted by the X and by the triangle) experience a greater overall C.sub.d across all AOAs in comparison to the embodiments of the invention, and accordingly the proposed invention provides an improvement in the drag performance of the airfoil.

(33) In FIG. 7, a plot is shown of (C.sub.l/C.sub.d) against angle of attack AOA, while in FIG. 8 C.sub.l is plotted against the C.sub.d for the tested systems. It is clear that the embodiments of the system of the invention provide a significant and surprising improvement in the lift-to-drag ratio over the prior art systems, resulting in an improvement in overall airfoil performance.

(34) Further testing has indicated to the inventors that the performance advantages indicated in the above table and the accompanying figures extend across the proposed range of characteristic values.

(35) It will be understood that the VGs may have any suitable structure and cross-section. The VGs are substantially planar, or at least a portion of the VGs may be tapered, e.g. a tapered second end 102b,104b, or a tapered tip end.

(36) Such a system provides an improved performance when installed on a wind turbine blade, resulting in greater energy production for such a wind turbine blade over the prior art systems. Initial calculations have shown that blades having such an arrangement of vortex generators will experience an increase in AEP (Annual Energy Production) of between approximately 0.3%-1% over the lifetime of the blade.

(37) The arrangement comprises at least 2 VG pairs. The arrangement of VG pairs may be provided along a straight line, a curved line, and/or a multi-part line in the longitudinal direction of the blade. The VG pairs may be arranged substantially equidistantly.

(38) It will be understood that the individual VG pairs may be provided having a separate or a common base. Furthermore, the individual vanes of the VG pairs may also be provided having a separate or a common base for the VG pair. In addition, it will be understood that the invention also covers the VG arrangement wherein a first vane of a first VG pair and a second vane of an adjacent second VG pair may be provided on a common base, wherein the VG pairs of the invention are defined by adjacent vanes of adjacent VG elements or modules.

(39) The herein described embodiments present an array of vortex generator pairs for use on an airfoil, preferably a wind turbine blade. The arrangement of vortex generators in a manner different to that thought by the prior art presents a surprising improvement in aerodynamic performance over the prior art systems.

(40) The invention is not limited to the embodiment described herein, and may be modified or adapted without departing from the scope of the present invention.