Patent Publication Number: US-2015064003-A1

Title: Airflow modifying element for suppressing airflow noise

Description:
FIELD OF THE INVENTION  
     The present invention relates generally to the field of wind turbines, and more particularly to a rotor blade assembly for a wind turbine having airflow modifying elements for suppressing airflow noise. 
     BACKGROUND OF THE INVENTION  
     Wind turbine rotor blades are the primary elements of wind turbines for converting wind energy into electrical energy. The working principle of the blades resembles that of an airplane wing. The blades have the cross-sectional profile of an airfoil such that, during operation, air flows over 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. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity. 
     Some rotor blades may include one or more drain holes used for draining water that may become trapped within the rotor blade during operation. The drain holes are typically located in body shell of the rotor blade, in either the pressure or suction sides, as well as side edges near the blade tip. Such drain holes, however, may cause noise or whistling in surrounding areas due to the interaction of a moving mass (e.g. air inside the drain hole) with a shear layer at the opening of the drain hole. More specifically, the moving mass may cause shear layer instabilities that, in return, amplify the movement of the mass. 
     It is known in the art to change the aerodynamic characteristics of wind turbine blades by adding dimples, protrusions, or other airflow modifying elements on the surface of the blade. These structures are often referred to as “vortex generators” or “vortex elements” and serve to create local regions of turbulent airflow over the surface of the blade. Conventional vortex generators are typically sheet metal and defined as “fins” or shaped structures on the suction side of the turbine blade. 
     As such, the industry would benefit from a rotor blade design that reduced drain-hole noise and/or whistling. More specifically, the industry would benefit from a rotor blade assembly having airflow modifying element that reduces drain-hole noise and/or whistling. 
     BRIEF DESCRIPTION OF THE INVENTION  
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In accordance with aspects of the invention, a rotor blade assembly is provided having at least one rotor blade including a body shell extending between a blade root and a blade tip. The body shell has a pressure side surface and a suction side surface. The rotor blade includes at least one drain hole having a diameter. The drain hole is configured on the body shell of the rotor blade. At least one airflow modifying element is configured on the body shell a predetermined distance from the drain hole such that the airflow modifying element reduces airflow noise caused by the drain hole. In addition, the predetermined distance is substantially equal to the diameter of the drain hole. 
     In a further aspect, another embodiment of a rotor blade assembly for a wind turbine is disclosed. The rotor blade assembly includes a rotor blade having a suction side surface and a pressure side surface. Further, the rotor blade assembly includes a drain hole configured on the pressure side surface. At least one airflow modifying element is configured on the pressure side surface. The airflow modifying element is located a predetermined distance from the drain hole such that the airflow modifying element reduces airflow noise caused by the drain hole. In addition, the airflow modifying element extends a perpendicular distance from the surface of the blade to define a maximum height, the maximum height being a function of a boundary layer thickness. 
     In still another aspect, a method for reducing airflow noise caused by a drain hole of a wind turbine is disclosed. The method includes measuring an airflow noise near a wind turbine using a sensor; providing the airflow noise to a controller; and, actuating an airflow modifying element, by the controller, when the airflow noise exceeds a predetermined threshold. The airflow modifying element is located a predetermined distance from the drain hole. Further, the predetermined distance is equal to or less than the diameter of the drain hole. Moreover, the airflow modifying element reduces the airflow noise caused by the drain hole when the airflow modifying element is in an actuated position. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING  
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  is a perspective view of a conventional wind turbine; 
         FIG. 2  is a perspective view of an embodiment of a rotor blade assembly in accordance with aspects of the invention; 
         FIG. 3  is side view of a rotor blade rotor blade assembly in accordance with aspects of the invention; 
         FIG. 4  is an enlarged view of the rotor blade assembly of  FIG. 3 ; 
         FIG. 5  is an enlarged view of another embodiment of a rotor blade assembly in accordance with aspects of the invention; and, 
         FIG. 6  is another enlarged view of an alternative embodiment of a rotor blade assembly in accordance with aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention include such modifications and variations as come within the scope of the appended claims and their equivalents. 
     The present invention is described herein as it may relate to a component of a wind turbine blade. It should be appreciated, however, that the unique airflow modifying element configuration in accordance with principles of the invention is not limited to use on wind turbine blades, but is applicable to any type of airfoil or flow surface that would benefit from the airflow modifying elements. Examples of such surfaces include airplane wings, boat hulls, sails, and so forth. 
     Generally, the present invention relates to a rotor blade assembly and method for reducing drain hole whistling. The rotor blade assembly includes a rotor blade including a body shell that extends from a blade root to a blade tip. The body shell includes a pressure side surface and suction side surface. At least one drain hole is configured on the body shell, on either or both of the suction or pressure side or a tip edge surface. An airflow modifying element is configured on the same surface as the at least one drain hole. As such, the airflow modifying element is designed to have a specific shape and location so as to reduce airflow noise caused by the drain hole, such as whistling. More specifically, the airflow modifying element enhances mixing of higher energetic flows in an outer boundary layer with lower energetic flows in an inner boundary layer and forms two counter-rotating vortices which are oriented in the flow direction. Such vortices flow over the drain hole opening and stabilize the shear layer at the opening of the drain hole. Accordingly, the resonating interaction of the mass within the drain hole (e.g. water within the blade) with the shear layer at the opening of the drain hole is thus suppressed and whistling is reduced or avoided. 
     Referring now to the drawings,  FIG. 1  illustrates a wind turbine  10  of conventional construction. The wind turbine  10  includes a tower  12  with a nacelle  14  mounted thereon. A plurality of turbine blades  16  are mounted to a rotor hub  18 , which is in turn connected to a main flange that turns a main rotor shaft. The wind turbine power generation and control components are typically housed within the nacelle  14 . For example, as shown, a controller  40  is provided within the nacelle  14  to control various wind turbine components. The view of  FIG. 1  is provided for illustrative purposes only to place the present invention in an exemplary field of use. It should be appreciated that the invention is not limited to any particular type of wind turbine configuration. 
       FIG. 2  depicts a rotor blade assembly  100  incorporating aspects of the invention. The rotor blade assembly  100  includes rotor blade  16  having a suction side surface  20 , a pressure side surface  22 , a leading edge  24 , and a trailing edge  26 . Further, the rotor blade  16  is defined by a body shell  28  extending from a blade root  30  to a blade tip  32 . At least one airflow modifying element  102  in accordance with aspects of the invention described in greater detail below is formed on either of the pressure or suction surfaces  22 ,  24 . In the embodiment illustrated in  FIG. 2 , the airflow modifying element  102  is depicted on the pressure side surface  22  for illustrative purposes only. It should be appreciated that the airflow modifying element  102  could also be provided on the suction side surface  20 . More specifically, the airflow modifying element  102  may be placed at any location on either or both of the blade&#39;s flow surfaces  20 ,  22  wherein it is desired to reduce noise and/or whistling caused by a drain hole  104  configured within the body shell  28 . 
     As mentioned, one or more drain holes  104  are typically provided in the body shell  28 . Such drain holes  104  are important to wind turbine operation so as to provide draining of from within the body shell  28 . Further, if a lightning strike occurs, water within the body shell  28  may heat up instantly into steam requiring substantially more volume. As such, the drain holes may also provide pressure-relief to the blade  16  such that the blade  16  is not damaged during operation. 
     Still referring to  FIG. 2 , the drain holes  104  are generally located at an outboard location, such as near the blade tip  32 . Alternatively, the drain holes  104  may be provided at any location along a span  42  or chord  44  of the body shell  28  in either the pressure or suction sides  20 ,  22  or a tip edge surface. As such, the drain holes  104  may provide draining capabilities and pressure relief anywhere along the body shell  28 . Typically, the drain holes  104  remain open; however, in some embodiments, it may be beneficial to provide removable covers (not shown). The cover may keep debris, water, or other contaminants from entering the body shell  28  during transportation, installation, and maintenance. Further, the cover may be in communication with the controller  40  such that it may be opened or closed during operation of the wind turbine  10 . 
     Referring now to  FIG. 3 , the airflow modifying element  102  and drain hole  104  are illustrated on the pressure side surface  22  at a chord  44  length of between about 50% to about 75% as measured from the leading edge  24  of the rotor blade  16 . More specifically, in one embodiment, the airflow modifying element  102  and drain hole  104  may be located at about 60% chord length as measured from the leading edge  24  of the rotor blade  16 . Alternatively, the airflow modifying element  102  and drain hole  104  may be located less than 50% or more than 75% of the chord length as measured from the leading edge  24  of the rotor blade  16 . 
     Additionally, the airflow modifying element  102  and drain hole  104  configuration may be disposed closer to the blade tip  32 , as compared to the blade tip  32 , as shown in  FIG. 3 , or may be closer to the blade tip  32  as compared to the blade root  30 . It should be understood that the invention is not limited to any particular placement of the airflow modifying element  102  and drain hole  104  and the configuration may be located at any location on any of the flow surfaces of the rotor blade  16 . 
     In regards to the location of the airflow modifying element  102  relative to the drain hole  104 , the device is typically be placed upstream of the drain hole  104 , which is theoretically perpendicular to the pitch axis (the axis that extends between the blade root and the blade tip). Thus, the airflow modifying element  102  is typically the same distance to blade root  30  and tip  32  as the drain hole  104 . Practically, however, streamlines are slightly bended towards the blade tip  32  in the region of the drain hole  104 . In such an embodiment, the airflow modifying element  102  should be placed closer to the blade root  30  than the drain hole  104 , which may be referred to herein as the blade-root side. Alternatively, the airflow modifying element  102  may be placed closer to the blade tip  32  than the drain hole  104 , or the blade-tip side. 
     Referring to  FIGS. 5 and 6 , the drain hole  104  may have a diameter D. Further, the airflow modifying element  102  may be located a predetermined distance d from the drain hole  104 . In one embodiment, the predetermined distance d may be approximately equal to the diameter D of the drain hole  104 . In further embodiments, the predetermined distance d may be approximately half or less than half of the diameter D of the drain hole  104 . In still further embodiments, the predetermined distance d may be equal to more than the diameter D of the drain hole  104 . In one embodiment, the predetermined distance d may be approximately 10 millimeters (mm). 
     As particularly shown in  FIG. 4 , the airflow modifying element  102  may also extend a perpendicular distance from a surface (e.g. from the pressure side surface  22 ) of the blade  16  to define a maximum height H. Further, the maximum height H may be a function of a local boundary layer thickness. For example, the maximum height H may extend approximately 150% of the local boundary layer thickness. Further, the boundary layer thickness may depend on flow speed, angles of attack, and various other parameters. As used herein, the term “boundary layer thickness” is defined as an aerodynamically-determined distance from a surface of the body shell  28  to an undisturbed flow field above the surface of the body shell  28 . Further, the term “local boundary layer thickness” is defined as the boundary layer thickness at or near the location of the drain hole and the airflow modifying element. Accordingly, the maximum height H may be dependent upon the location of the drain hole on the suction side, the pressure side, or at the side edge of the blade tip  32 . More specifically, in one embodiment, the height H may be approximately 5 mm. In still further embodiments, the height H may be greater than 5 mm or less than 5 mm. 
     The airflow modifying element  100  may also define a width W and a length L as shown in  FIGS. 5 and 6 . The width W and length L are optimized so as to create appropriate vortices  52  over the drain hole  104 , thereby reducing drain hole whistling. For example, in one embodiment, the width W may be approximately 10 mm and the length L may be approximately 15 mm, respectively. In further embodiments, the width may be greater than or less than 10 mm. Similarly, the length L may be greater than or less than 15 mm. 
     It should be understood that the airflow modifying elements  102 ,  202  described herein may have different shape configurations within the scope and spirit of the invention. For example, as shown in  FIG. 5 , the airflow modifying element  102  contains a base  106  having two sides  108 ,  110 . The base  106  may be attached to the blade  16  using any suitable adhesive, such as tape or glue. Alternatively, the base  106  may be attached to an actuator  60  as will be described in further detail later. Further, the base  106  may have any suitable shape. For example, as shown, the base  106  may have a substantially trapezoidal shape wherein the sides  108 ,  110  define a skew angle θ with the flow direction  50 . It should be understood that the skew angle θ may be any suitable angle ranging from 0 degrees to less than 90 degrees, more preferably about 40 degrees, more preferably about 30 degrees, still more preferably about 20 degrees. In further embodiments, the base may have a rectangular, square, triangular, circular, or similar shape. 
     The sides  108 ,  110  may also define any suitable shape having respective top edges  112 ,  114 . Further, the respective top edges  112 ,  114  may have corresponding slopes that increase from a minimum height to a maximum height as the airflow modifying element  100  approaches the drain hole  104 . For example, the minimum height may be approximately equal to the one half of the maximum height. Alternatively, the sloping edges  112 ,  114  may decrease from a maximum height to a minimum height as the airflow modifying element  100  approaches the drain hole  104 . Further, the slopes of the edges  112 ,  114  may be different or may correspond with one another. In further embodiments, the sides  108 ,  110  may have flat, pointed, or arcuate edges. 
     In another embodiment, as shown in  FIG. 6 , the airflow modifying element  202  may have a base  206  and a top surface  208 . The base  206  may be attached to the blade  16  using any suitable adhesive, such as tape or glue. Further, the base  206  may include feet (not shown) to assist in attaching the base  206  to one of the rotor blade surfaces  20 ,  22 . In further embodiments, the base  206  may be attached to the actuator  60  as will be described in further detail later. As such, a slit of groove may be cut within the body shell  28  such that the base  206  may slide within the slit and the top surface  204  may lay substantially flush with the pressure side  22  when in a recessed position. Moreover, the base  206  may extend in generally the same direction as the flow direction  50 . 
     The top surface  208  may define any suitable shape having respective edges  210 ,  212 . For example, as shown, the top surface  208  may define a generally trapezoidal shape. As such, the respective edges  210 ,  212  may taper outwardly at a skew angle θ as the airflow modifying element  200  approaches the drain hole  104  ( FIG. 6 ). Alternatively, the respective edges  210 ,  212  may taper inwardly at a skew angle θ as the edges  210 ,  212  approach the drain hole  104 . In still further embodiments, the top surface  208  may have a rectangular, square, triangular, circular, or similar shape. As such, the respective edges may be parallel to one another, may diverge with one other, or may have an arcuate shape. Further, the slopes of the edges  210 ,  212  may be different or may correspond with one another. 
     In still further embodiments, the airflow modifying elements  102 ,  202  may be any suitable shape known in the art. For example, the airflow modifying elements  102 ,  202  may be shaped like conventional vortex generators, including fin or wedge-type shapes. The descriptions of the shapes of the airflow modifying elements  102 ,  202  described herein are not meant to be limiting and are provided for illustrative purposes only. 
     The relationship of the dimensions of the airflow modifying element  102  as described herein (i.e. predetermined distance d, drain-hole diameter D, height H, width W, length L, and skew angle θ) and the location of airflow modifying element with respect to the drain hole both contribute to the reduction in drain-hole whistling and/or noise in surrounding areas. More specifically, as illustrated in  FIGS. 5 and 6 , the incoming air stream  50  is modified by the airflow modifying elements  102 ,  202  such that the airstream after the airflow modifying elements  102 ,  202  forms two counter-rotating vortices  52  that flow over the drain hole  104 . Further, the airflow modifying elements  102 ,  202  enhance mixing of higher energetic flows in the outer boundary layer with lower energetic flows in the inner boundary layer and form the counter-rotating vortices  52  which are oriented in the flow direction. As such, the vortices  52  above the drain hole  104  stabilize the shear layer at the opening of the drain hole  104 . Accordingly, the resonating interaction of the mass within the drain hole (e.g. water within the blade) with the shear layer at the opening of the drain hole is thus suppressed and whistling is reduced or avoided. 
     As mentioned and referring back to  FIG. 3 , each airflow modifying element  102  may be coupled to an actuator  60  disposed within the rotor blade  16 . In general, the actuator  60  may be configured to displace the airflow modifying element  102  between a recessed position (i.e. within the blade shell) to an actuated position (i.e. above the blade shell). Accordingly, it should be appreciated that the actuator  60  may generally comprise any suitable device capable of moving the airflow modifying element  102  relative to the shell  28 . For example, in several embodiments, the actuator  60  may comprise a linear displacement device configured to linearly displace the airflow modifying element  102  between the actuated and recessed positions. In the context of the present subject matter, the term “linearly displace” refers to the displacement of a surface feature along a straight line. Thus, in one embodiment, the actuator  60  may comprise a hydraulic, pneumatic or any other suitable type of cylinder configured to linearly displace a piston rod  62 . Thus, as shown, the airflow modifying element  102  may be attached to the piston rod  62  such that, as the piston rod  62  is actuated, the airflow modifying element  102  is linearly displaced relative to the shell  28 . In other embodiments, the actuator  60  may comprise any other suitable linear displacement device, such as a rack and pinion, a worm gear driven device, a cam actuated device, an electro-magnetic solenoid or motor, other electro-magnetically actuated devices, a scotch yoke mechanism and/or any other suitable device. Alternatively, the airflow modifying elements  102 ,  200  may be affixed to the body shell  28  such that they remain in place and are not actuated between a recessed and an actuated position. 
     The airflow modifying elements  102 ,  202  and associated drain hole  104  may also be in communication with the controller  40  housed within the nacelle  14  ( FIG. 1 ). More specifically, the controller  40  may be supplied with control signals in response to the respective wind or other conditions experienced by the individual blade  16  (i.e. increased water or pressure within the blade  16 ) as detected by any manner of sensor provided in or around the blade  16 . As such, the one or more sensors may supply a signal to the controller  40  for near-instantaneous control of the airflow modifying elements  102 ,  200  and/or drain hole(s)  36  associated with each of the respective blades  16 . For example, the controller  40  may send one or more signals to the actuator  60  that may move the airflow modifying elements  102 ,  202  from the recessed to the actuated position. Further, the drain hole  104  may include a cover that moves between an open position and a closed position. As such, the controller  40  may send signals to the drain hole  104  so as to move the cover between the open and closed positions. Accordingly, in one embodiment, the airflow modifying element  102  may move from the recessed position to the actuated position so as to reduce noise associated with an open drain hole  104 . 
     In another embodiment, a method for reducing airflow noise caused by one or more drain holes of a wind turbine is disclosed. The method may include measuring an airflow noise near the wind turbine using one or more sensors. The sensors may be configured to detect a decibel value caused by the drain hole. The airflow noise (e.g. the decibel value) may then be provided to the controller  40 . As such, the controller  40  may be configured to actuate one or more airflow modifying element when the airflow noise (or decibel value) exceeds a predetermined threshold. Further, actuating the airflow modifying element may be completed as a function of the decibel value. In addition, the airflow modifying element may be located a predetermined distance from the drain hole. Moreover, the predetermined distance may be equal to or less than the diameter of the drain hole. Accordingly, the method as described herein reduces the airflow noise caused by the drain hole when the airflow modifying element is in an actuated position. 
     In another embodiment, the method may also include actuating the airflow modifying element to a maximum height. As mentioned, the maximum height is typically a perpendicular distance from a surface of the blade. Further, the maximum height may be equal to or less than half of the diameter of the drain hole. 
     In still further embodiments, the blade  16  may incorporate the airflow modifying elements  102 ,  202  and drain-hole configuration  36  described herein with conventional aerodynamic vortex generators  34 . For example, as depicted in  FIG. 2 , the airflow modifying elements  102 ,  200  may be provided at a defined region of the blade  16  near the drain hole  104 , while the conventional vortex generators  34  may be provided at a different region of the blade  16 . In a particular embodiment, the airflow modifying elements  102 ,  202  may be configured on the pressure side  22  at the blade tip  32  of the rotor blade  16  (as shown in  FIG. 2 ), while conventional wedge or fin-type vortex generators  34  may be provided on the suction side of the blade  16 , or both the pressure and suction sides  20 ,  22 . In an alternate embodiment, the airflow modifying elements  102 ,  202  may be located closer to the blade tip  32  than the blade root  30 . 
     It should also be understood that the present invention encompasses any configuration of the wind turbine  10  ( FIG. 1 ) that includes one or more rotor blade assemblies  100  incorporating at least one of the unique airflow modifying elements  102 ,  202  and drain holes  104  as described herein. 
     While the present subject matter has been described in detail with respect to specific exemplary embodiments and methods thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily produce alterations to, variations of, and equivalents to such embodiments. As mentioned, it should also be appreciated that the invention is applicable to any type of flow surface, and is not limited to a wind turbine blade. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.