Abstract:
A wind turbine includes a tower supporting a drive train with a rotor, at least one blade extending radially from the rotor; and a plurality of substantially flat flaps extending substantially perpendicular from a suction surface of the blade and along different chord lines near a tip of the blade for capturing and directing tip vortices toward a trailing edge of the blade.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The subject matter described here generally relates to fluid reaction surfaces with specific blade structures, and, more particularly, to wind turbines having vortex breakers near the tip of the blades. 
         [0003]    2. Related Art 
         [0004]    A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by the machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is converted to electricity, then the machine may also be referred to as a wind generator or wind power plant. 
         [0005]    Wind turbines use an airfoil in the form of a blade to generate lift and capture momentum from moving air that is them imparted to a rotor. The blade is typically secured to a rotor at its root end, and extends radially to free, tip end. The front, or leading edge, of the blade connects the forward-most points of the blade that first contact the air. The rear, or trailing edge, of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A chord line connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of a chord line is simply referred to simply as the chord. 
         [0006]    The outboard ends of the blade are called tips and the distance from the tip to the root, at the opposite end of the blade, is called the span. Since many blades change their chord over the span from root to tip, the chord length is referred to as the root chord, near the root, and the tip chord, near the tip of the blade. The resulting shape of the blade, when viewed perpendicular to the direction of flow, is called the planform. Since the thickness of a blade will typically vary across the planform, the term thickness is typically used to describe the maximum distance between the low pressure suction surface and the high pressure surface on the opposite side of the blade. As with other airfoils, wind turbine blades are sometimes provided with flat, usually thin, plates attached at one edge, and referred to as flaps. 
         [0007]    Wind turbines are typically categorized according to the vertical or horizontal axis about which the turbine rotates. Horizontal configurations are most common in modern wind turbine machines and one so-called horizontal-axis wind generator is schematically illustrated in  FIG. 1 , copied from U.S. Pat. No. 7,144,216. This particular configuration for a wind turbine  1  includes a tower  2  supporting a drive train  4  with a rotor  6  that is covered by a protective enclosure referred to as a nacelle. Blades  8  are arranged at one end of the rotor  6  outside the nacelle for driving a gearbox  10  and electrical generator  12  at the other end of the drive train  4  inside the nacelle. 
         [0008]    The “upwind” configuration shown in  FIG. 1 , where the rotor  6  faces into the wind, helps to reduce noise by eliminating the wind shadow of the tower  2  that can other result in an impulsive thumping sound. Other aspects of wind turbine noise have also been addressed by providing quieter gearboxes, soundproofed nacelles, and streamlined nacelles and towers. More efficient blade designs that convert less of the wind&#39;s energy into aerodynamic noise have also been sought. 
         [0009]    For example, U.S. Patent Publication No. 20060216153 discloses a rotor blade for a wind power plant with a tip that is curved or angled at it end region in the direction of the pressure side of the blade. In order further to reduce the levels of sound emission, the blade is curved or angled in its edge region in the direction of the trailing edge of the rotor blade in the plane of the rotor blade. An English-language abstract of World Intellectual Property Organization Publication No. 2006059472 also discloses a propeller wherein the tip parts of the propeller blades of a horizontal-shaft windmill are tilted in the front direction of the propeller blades. However, these and other conventional approaches to blade tip configuration do not adequately address the problems of aerodynamic efficiency and wing tip noise because they can actually increase blade drag which correspondingly reduces turbine power. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    These and other drawbacks associated with such conventional approaches are addressed here in by providing, in various embodiments, a blade for a wind turbine including a plurality of substantially flat flaps extending from a suction surface of the blade and along different chord lines near a tip of the blade. The subject matter disclosed here also relates to a wind turbine, including a tower supporting a drive train with a rotor, at least one blade extending radially from the rotor, and a plurality of substantially flat flaps extending substantially perpendicular from a suction surface of the blade and along different chord lines near a tip of the blade. Also provided is a noise reduction system for a wind turbine blade, including a plurality of substantially flat flaps for breaking vortices near a tip of the wing, where each flap extending substantially perpendicular from a base for securing to a suction surface of the blade with an edge of each flap extending along a different chord line near a tip of the blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Various aspects of this technology will now be described with reference to the following figures (“FIGs.”) which are not necessarily drawn to scale, but use the same reference numerals to designate corresponding parts throughout each of the several views. 
           [0012]      FIG. 1  is a schematic illustration of a conventional wind turbine. 
           [0013]      FIG. 2  is an orthographic view of a wind turbine blade tip vortex breaker system. 
           [0014]      FIG. 3  is an end view of the wind turbine blade tip vortex breaker system illustrated in  FIG. 2 . 
           [0015]      FIG. 4  is an orthographic view of another wind turbine blade tip vortex breaker system. 
           [0016]      FIG. 5  is an end view of the wind turbine blade tip vortex breaker system illustrated in  FIG. 4 . 
           [0017]      FIGS. 6-12  are end views of other wind turbine blade tip vortex breaker systems. 
           [0018]      FIG. 13  is a comparative plot of calculated sound pressure level versus log of frequency for wind turbine blade tip vortex breaker system with and without the vortex breaker system illustrated in  FIG. 2 . 
           [0019]      FIG. 14  is a comparative plot of calculated relative minimum pressure at vortex center versus relative chord position for the wind turbine blade vortex breaking system illustrated in  FIG. 2 . 
           [0020]      FIG. 15  is an enlarged orthographic view of a single flap for use with the wind turbine blade tip vortex breaker system of  FIG. 2 . 
           [0021]      FIG. 16  is an enlarged orthographic view of a pair of flaps. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 2  is an orthographic view of an enlarged tip portion of a wind turbine blade including one example of a tip vortex breaker system  20 .  FIG. 3  is an end view of the wind turbine blade vortex breaker system  20  illustrated in  FIG. 2 . In  FIGS. 2 and 3 , the tip vortex breaker system  20  is illustrated in connection with the blade  8  shown in  FIG. 1 . However, any other wind turbine blade may also be used. For example, the vortex breaker system  20  may be used with blades having other planforms, cambers, thicknesses, aspect ratios, and/or tip geometries in addition to the ones illustrated in the figures here. 
         [0023]    In  FIGS. 2 and 3 , the dashed line represents a tip chord line  22  that extends from the leading edge  24  to the trailing edge  26  of the turbine blade  8 . Although the length of this chord line  22 , or chord, is illustrated in these figures as being constant for the span of the blade  8  from the root (not show) to the tip  28 , the chord may vary along the span of the blade. The maximum thickness  30  of the blade  8  at the tip chord line  22  shown here is illustrated by another dashed line which extends from the upper, low pressure, or suction surface  32  to the lower, high pressure surface  34 . The maximum thickness  30  may also vary along the span of the blade  8 . 
         [0024]    The blade  8  is provided with two or more flaps  40  and  42  extending from the suction surface  32  near the tip of the blade. However, any number of additional flaps may also be provided inboard and/or outboard of the illustrated flaps  40  and  42 . The flaps  40  and  42  are spaced-apart from each other by a distance that will allow air flow between the flaps. For example, the spacing between the flaps  40  and  42  may be up to twice the maximum thickness  30  of the blade  8 , or between once or twice the maximum thickness  30 . The maximum thickness at any chord line near the tip may be approximated may be approximated by the maximum thickness at the tip chord line, and/or at the outermost chord line which is still substantially perpendicular to the leading and trailing edges of the blade. 
         [0025]    The flaps  40  and  42  are arranged at or near the tip  28  of the blade  8  so that, during normal operation, it is expected that some or all of the vortex flow emanating from the high pressure zone near the, high pressure surface  34  and moving around the tip  28  toward the low pressure zone near the suction surface  32  will be captured in the space between the flaps  40  and  42  (and/or others, not shown). The captured vortices will then be flushed over the trailing edge  26  between the flaps so as to minimize aerodynamic noise. Since this vortex flow is typically most severe near the trailing edge  26  of the tip  34 , the trailing edge of the flaps  40  and  42  may be arranged at or near the trailing edge of the blade  8 . However since noisy vortex flows can also occur in front of, and/or behind, the trailing edge  26  of the blade  8 , one or more of the flaps  40  and  42  may be displaced in front of or behind the trailing edge  26  as discussed below with regard to  FIGS. 4 ,  5 ,  11 , and  12 . The number of flaps  40  and  42  may be chosen so as to catch most or all the vortex flow between the flaps and is typically expected to include about two to ten, or, more specifically, two to five flaps. 
         [0026]    Each of the inboard flap  40  and the outboard flap  42  is illustrated here as being fixed to the blade  8  along one edge, with each of those fixed edges being aligned with a different chord line in the direction of flow across the blade  8 . However, one or both of the flaps  40  and  42  may be moveably positioned relative to the suction surface  32  or other part of the blade  8 . For example, one or both of the flaps  40  and  42  may be retractable into the blade  8 , hinged at their connected edges, and/or rotatable on an axis which is perpendicular to the suction surface  32  in order to better align the flaps with airflow across the blade in order to minimize drag. The positioning of such moveable flaps may also be automatically controlled based upon various environmental conditions, such as wind speed, and/or operating set points, such as rotor speed. 
         [0027]    The embodiments illustrated here also show the flaps  40  and  42  extending substantially perpendicular from the suction surface  32  and substantially parallel to each other. However, one or both of the flaps may slant, curve, or be otherwise directed to the inboard and/or outboard direction of the blade  8 . For example, the inboard flap  40  may be curved in order to generally correspond to the radius of the expected vortex flow while the outboard flap  42  slants toward the tip  34  in order to provide a wider top opening between the blades in which to receive the vortex flow. 
         [0028]    In  FIGS. 2 and 3 , the flaps  40  and  42  are substantially the same in size, shape, and position along different chord lines relative to the trailing edge  26  of the blade  8 . Arranging the flaps  40  and  42  along chord lines, which generally correspond to the direction of flow across the blade  8 , minimizes the apparent cross-section of the flaps  40  and  42  and corresponding drag on the blade  8 . The flaps  40  are also substantially flat and as thin as possible in order to further minimize drag while still maintaining their structural rigidity. In some circumstances, rotating or thickening the flaps  40  and  42  so as to unintentionally or knowingly increase their apparent cross-section to the flow across the blade  8  may nonetheless result in acceptable levels of drag. 
         [0029]    The leading edges of the flaps  40  and  42  shown in  FIGS. 2 and 3  are shorter than the trailing edges of the blades in order to further minimize the size of the leading edge cross-section and thus reduce drag. For example, the height of leading edge of the flaps  40  and  42  may be up to the maximum thickness  30  of the blade  8 , or, more specifically, between one-half and one times the maximum thickness  30  on the corresponding chord line for the flap or tip chord for the blade  8 . The height of the trailing edge of the flaps  40  and  42  may be up to four times the maximum thickness  30  of the blade  8 , or, more specifically, between two and four times the maximum thickness  30  on the corresponding chord line for the flap, the chord line at the maximum overall thickness of the blade, or the tip chord for the blade  8 . However, other leading edge and trailing edge sizes and shapes may also be used. For example, with a blade  8  having a fifty meter span length from root to tip, the flaps  40  and  42  may be up to about up to 0.1 meters tall or about 0.2% of the span length. For shorter blades  8 , the flaps  40  and  42  may be up to about 0.5% of the span length for a flap height range for various blade sizes of around 0.2% to 0.5% of span. 
         [0030]    In  FIGS. 2 and 3 , the length of the flaps  40  and/or  42  along the corresponding chord line may be between 0.1 and 0.7 times, or, more specifically, between 0.2 and 0.6 times the length of the chord line on which the flap is aligned. However, other flap lengths may also be used. The chord length on which the flap is aligned can be approximated as the tip chord length for the blade  8 . For blade tips that are not squared off (unlike those illustrated here) and/or other blade configurations, the tip chord may be approximated as the length of the outermost chord line which is still substantially perpendicular to the leading and trailing edges of the blade. 
         [0031]    With regard to the chord position of the flaps  40  and/or  42 , the flaps may extend up to 0.2 times the corresponding chord length beyond trailing edge, or more specifically, up to 0.1 times the corresponding chord length beyond the trailing edge. The flaps may start at least 0.15 times the corresponding chord length downstream of the leading edge, or more specifically, up to 0.3 times the corresponding chord length downstream of the leading edge. However, other chord positions may also be used. The chord length on which the flap is aligned can be approximated as the tip chord length for the blade  8 . For blade tips that are not squared off (unlike those illustrated here) and/or other blade configurations, the tip chord may be approximated as the length of the outermost chord line which is still substantially perpendicular to the leading and trailing edges of the blade. 
         [0032]    With regard to the span position of the flaps  40  and  42 , the flaps may be located inboard of the tip within four times the corresponding chord length on which the flap is aligned, or, more specifically, inboard of the tip within two times the corresponding chord length. However, other span positions may also be used. The chord length on which the flap is aligned can be approximated as the tip chord length for the blade  8 . For blade tips that are not squared off (unlike those illustrated here) and/or other blade configurations, the tip chord may be approximated as the length of the outermost chord line which is still substantially perpendicular to the leading and trailing edges of the blade. 
         [0033]    With regard to shape, the top edge  44  of the flaps  40  and  42  in  FIGS. 2 and 3  is substantially parallel to the chord line  22 . However, other flap shapes may also be used, including the shapes discussed in more detail below. For example,  FIGS. 4 and 5  illustrate another embodiment of the wind turbine blade vortex breaking system  20  where the inboard flap  40  and outboard flap  42  have different sizes, shapes, and positions along their corresponding chord lines. In  FIGS. 4 and 5 , the inboard flap  40  is taller than the outboard flap  42 , and displaced from the trailing edge  26  of the blade  8 . In addition, the leading edge of the inboard flap  40  is substantially the same height from the suction surface  32  along the entire length of the flap  40 . Consequently, the shape of the top edge  44  of the inboard flap  40  generally corresponds to the slope of the suction surface near the trailing edge  26 . However, in other embodiments, the leading edge of one or both of the flaps  40  and  42  may be taller than the trailing edge of the corresponding flap. 
         [0034]      FIG. 6-12  illustrates various other embodiments of wind turbine blade vortex breaking systems  20  showing the flap  40  and/or  42  which, for simplicity, are referred to here as flaps  40 . In  FIG. 6 , the flap  40  has a leading edge which is substantially shorter than a trailing edge of the flap so that the top edge  44  of the flap slopes toward the suction surface  32 .  FIG. 7  illustrates another embodiment of the wind turbine blade vortex breaking system  20  where the leading edge of the flap  40  has been further reduced so as create a three-sided flap  40  with a straight to edge  44 . In  FIG. 8 , another three-sided flap  40  has been provided with a convex top edge  44  while the three-sided flap  40  shown in  FIG. 9  has been provided with a concave top edge  44 . In  FIG. 10 , the top edge  44  of the flap  40  has been arranged substantially parallel to the suction surface  32 . In  FIG. 11 , the trailing edge of the flap  40  is arranged behind, and the bottom of the trailing edge is arranged below, the trailing edge of the blade  8 . In  FIG. 12 , the trailing edge of the flap  40  is arranged in front of the trailing edge of the blade  8 . 
         [0035]    Contour plots (not shown here) from CFXpost software were used to compare calculated vorticity and pressure for a wind turbine blade vortex breaking system  20  similar to the one illustrated in  FIGS. 2 and 3 . For the base case without flaps  40  and  42 , the highest vorticity and lowest pressure in the center of the vertex occurred near the trailing edge of the blade  8 . Similar plots for the wind turbine blade vortex breaking system  20  with flaps  40  and  42  showed significantly lower vorticity and higher pressure in the center of these resulting vortices with the flaps  40  and  42 . Such higher pressure is generally desirable for reducing fluid rotation, while lower vorticity tends to suggest lower noise and improved aerodynamic efficiency of the blade  8 . 
         [0036]      FIGS. 13 and 14  illustrate the results of another computational fluid dynamics simulation for a wind turbine blade vortex breaking system  20  similar to the one illustrated in  FIGS. 2 and 3 . In these FIGs., the plot line  50  represents the blade vortex breaking system  20  (with flaps) while the plot line  60  represents the blade vortex breaking system  20  without the flaps  40  and  42 . 
         [0037]      FIG. 13  is a comparative plot of calculated sound pressure level in decibels versus log of frequency for wind turbine blade tips with and without the vortex breaker system illustrated in  FIG. 2 . The noise calculation used a correlation given by “Airfoil Tip Vortex Formation Noise,” by Thomas F. Brooks and Michael A. Marcolini, in American Institute of Aeronautics and Astronautics Volume 24, Number 2. In this correlation, a parameter called “wetted length” is calculated at a plane ( 1/10th of chord) after the trailing edge by inspection turbulent kinetic energy for a contour value of 0.05. The wetted length is then used with a maximum MACH number to calculate a sound pressure level at the tip SPL(tip) for various frequencies. The plot illustrates a 6-8 decibel noise reduction for most frequencies. 
         [0038]    A sound pressure level difference of 3 dB is roughly half the corresponding power level difference and thus commonly used as a point of reference in sound measurement. In practical terms, sound that is radiated from a point source drops in level at approximately 6 dB per doubling of distance. Therefore, if you start at 50 feet from the source and move to 100 feet from the source you will have a 6 dB drop in level. Similarly, if you move from 500 feet to 1000 feet, you will have a 6 dB drop in level. The wind turbine blade vortex breaking system  20  illustrated in  FIGS. 2 and 3  is therefore expected to provide at least similar reductions in noise level. 
         [0039]      FIG. 14  is a plot of calculated relative minimum pressure at vortex center in Pascals versus relative chord position for wind turbine blade tips with and without the vortex breaker system illustrated in  FIG. 2 . The circled portion of the chart in  FIG. 14  illustrates an approximate 3500 to 4000 Pascal pressure difference over the baseline configuration without flaps near the trailing edge  26  of the blade  8  and up to 30% of the chord beyond the trailing edge. 
         [0040]      FIGS. 15 and 16  show enlarged orthographic views of a flaps  40  and/or  42  for use with the wind turbine blade tip vortex breaker system  20  in any of the previous FIGs.  FIG. 15  illustrates a single flap configuration while  FIG. 16  is an enlarged orthographic view of a pair of flaps. Each of these T-shaped configurations includes a base member  70  for securing to the suction surface  32  of the blade  8  by an suitable means including adhesive or fastening. For example, each of the base members  70  may be provided with fastener holes  72  for receiving bolts, pins, or other fasteners that secure the base member to the blade  8 . The base member may also be curved to the general shape of the suction surface  32  as illustrated in  FIG. 15 . One or more arm members  80  then extends from the base member  70  for capturing and guiding vortex airflow as discussed above. 
         [0041]    The previously described embodiments offer have many advantages. For example,  FIGS. 13 and 14  show that the wind turbine blade vortex breaking system  20  illustrated in  FIGS. 2 and 3  is expected to provide significantly improved noise reduction. It also is expected that other configurations of the wind turbine blade vortex breaking system  20 , including those illustrated in  FIGS. 4-12  and  15 - 16  would provide at least some level of beneficial noise reduction. 
         [0042]    It should be emphasized that the embodiments described above, and particularly any “preferred” embodiments, are merely examples of various implementations that have been set forth here to provide a clear understanding of various aspects of this technology. One of ordinary skill will be able to alter many of these embodiments without substantially departing from scope of protection defined solely by the proper construction of the following claims.