Structures and methods for a flexible winglet of a blade

Abstract

A structure and method for a passive flexible winglet that can be attached onto the tip of an elongate body is proposed The flexible winglet is flexible insomuch as it obtains a desired angle of attack relative to the incoming flow vector and aerodynamic load it experiences. The winglet contains a winglet body having an internal geometry such as a honeycomb, Voronoi, lattice or laminated composite sheets which enables passive bending and twisting in response to aerodynamic loads, and an internal cavity such that it can be placed over the tip of a wind turbine blade. The winglet body is arced and conforms to an arc of circumference of a circle traversed by the winglet during movement of the elongate body about a rotational axis

Description

STRUCTURES AND METHODS FOR A FLEXIBLE WINGLET OF A BLADE CROSS-REFERENCE

[0001] This Application is a non-provisional of, and claims all benefits, including priority, to U.S. Provisional Application No. 63 / 764,570, filed on February 27th, 2025, titled “STRUCTURES AND METHODS FOR A FLEXIBLE WINGLET OF A BLADE”, which is incorporated by reference herein.FIELD

[0002] Embodiments of the present disclosure relate to the field of aerodynamic structures adapted to traverse fluid environments, and more specifically, embodiments relate to devices, systems and methods for improved winglets used in fluid power systems.INTRODUCTION

[0003] Aerodynamic structures, such as airfoils, are designed to traverse a fluid medium, such as air or water, and act as either a power producing device or as a power output device. When operating as a power producing device, the aerodynamic structure is typically coupled to a generator which coverts the kinetic energy extracted from the fluid flow into electric power which can be stored and transported for consumption. When operating as a power output device, the airfoil is controlled by a power source which imparts rotary motion on the airfoil in order to “push” or “pull” a fluid in order to generate thrust.

[0004] With the increasing adoption of renewable energy sources to supplement and replace less environmentally friendly sources of energy, wind turbines have seen growing adoption as a power producing device. As blades traverse through a fluid environment, particularly at higher linear velocities, the blade may generate aeroacoustic noise and turbulence which can reduce efficiency at both the immediate turbine and downstream turbines. A primary source of inefficiency along a blade is the drag resulting from scattered vortices at the tip of the blade. For example, aeroacoustic noise emitted from the region of the wingtip of a rotor blade is generally called tip vortex noise. Tip vortex noise is an indicator that a scattered vortex is being created due to the configuration of the rotor blade at the wingtip,which decreases the efficiency of the blade by creating undue drag. Therefore, structures and methods for improved blade efficiency through the reduction of tip vortex noise is desirable.SUMMARY

[0005] As an object traverses a fluid medium, the interaction between the surface of the object and the fluid flow causes noise as a result of the interference of the natural flow vector of the fluid flow.

[0006] The proposed embodiments relate to a flexible winglet that can be attached onto the tip of a wind turbine blade, the winglet being flexible insomuch as it obtains a desired angle of attack relative to the incoming flow vector and aerodynamic load it experiences. The winglet contains a proposed internal geometry and physical configuration (e.g., honeycomb, Voronoi, lattice, laminated composite layers) which enables passive bending and twisting in response to aerodynamic loads, and an internal cavity in the base of the flexible winglet such that it can be placed over the tip of a wind turbine blade and have its external surface (e.g., the surface in contact with the fluid flow) mate flush to the external surface of a blade. In some embodiments, safety critical features, such as the lightning receptor of a moisture venting plug, can be embedded within the structure. The physical configuration can include specific materials selection, which may be modified depending on the type of manufacturing process (e.g., additive, subtractive). The winglet further has a contour that extends above a center line of the wind turbine blade towards a pressure side, and then arcs back towards a trailing edge of the wind turbine blade, the tip of the winglet terminating at a point behind the trailing edge of the wind turbine blade. In this way, the most external radial geometry of the winglet conforms to the radial arc of rotation of the host blade.

[0007] The wake generated by turbine blades may reduce flow velocities of the working fluid (i.e. air) and this effect can be felt as far as 100km away in certain conditions. The proposed aerodynamic structure and method may reduce the production of horseshoe vortices at the wingtip and reduce undesirable loads on the blade, including axial thrust and unsteady vibrations. The reduction of horseshoe vortices may further reduce the creation of unwanted aero-acoustic noise. The resulting technical improvement in performance generates a beneficial downstream wake recovery due to the controlled vortex shedding achieved throughthe proposed flexible winglet’s geometry and mode of action. This may result in improved turbine performance on the host turbine and improvements to wind turbines downstream in the wind farm as a whole.

[0008] The proposed internal geometry of the flexible winglet leads to a passive flexibility to obtain desirable local angles of attack relative to the local fluid flow in order to produce beneficial aerodynamic and acoustic effects. The passive flexibility incorporated into the aerodynamic structure of the winglet may be based on the aerodynamic load the winglet is expected to experience, the desired local angles of attack the winglet geometry should obtain relative to the angles of attack of the turbine blade, and the local twist, pre-bend and airfoils of the host blade. A series of refinement steps including an aero-structural analysis may be completed based on a selected material and a series of defined coordinate geometries of the winglet, associated with a range of aerodynamic loads, to obtain internal geometric structures for one or more layers of the flexible winglet. $

[0009] The design of the flexible winglet can be performed pre-manufacture using data available from previous projects, obtained from sensors on-site or expected results. The design of the flexible winglet can also be optimized based on results and technical data stemming from the in-operation performance of a previous iteration of the flexible winglet, or results from lab-based experiments.

[0010] The material for the flexible winglet may include composites such as fiber glass, carbon fiber, polyurethanes (PU) or Thermoplastics (TPU), but may also include a combination of materials that lend themselves to moulding, additive or reductive manufacturing processes, automized processes and hand-based processes. The flexible winglet may take on complex internal geometries that move, bend, twist or otherwise act to conform the winglet body into the beneficial geometric state and local angle of attack given the external aerodynamic load it is experiencing.

[0011] The flexible winglet can be manufactured as a standalone unit, or retrofit onto a turbine assembly, or in another variation, be built into a proposed rotor blade prior to installation of the blade. Proposed are also manufacturing approaches and processes, as well as tooling for making same. Manufacturing approaches for the proposed configurationsinclude additive (e.g., 3-D printing) or subtractive (e.g., cutting) manufacturing, moulding, infusion, and lay-up.

[0012] Embodiments of the flexible winglet may include a structure adapted to traverse a fluid environment, the structure comprising an elongate body having a root, a wingtip, a leading edge and a trailing edge, and a flexible winglet associated with the wingtip and having a body extending from the elongate body to a termination point that is rearward of the trailing edge, the body extends from the wingtip and curves upwards while sweeping backwards with respect to a direction of rotation of the elongate body. The flexible winglet includes a base section of the body which is integral with the elongate body and having a central cavity which receives the wingtip of the elongate body, a mid section of the body that is adjacent to the base section, and a winglet tip section of the body that is adjacent to the mid section and extends to the termination point of the winglet body, wherein, responsive to an aerodynamic load, at least one of bending and torsion occurs along the mid section of the flexible winglet to conform to a prescribed local angle of attack based on at least a local air flow vector.

[0013] In some embodiments, the mid section contains one or more cross-sectional layers, and each layer contains an internal geometric grid structure having anisotropic bending and torsional responses to the local air flow vector, and each of the internal geometric grid structures contains a plurality of cells and a plurality of orifices, and each cell of the plurality of cells forms a bounding box for an orifice of the plurality of orifices.

[0014] In some embodiments, the anisotropic bending and torsional response of the mid section is passively controlled through a moment of inertia of each of the cells of the plurality of cells within the internal geometric structure and a cross-sectional surface area of the plurality of orifices.

[0015] In some embodiments, a central aperture having a variable cross-sectional surface area extends axially within at least one of the mid section and winglet tip section to provide localized rigidity to the winglet body.

[0016] In some embodiments, the winglet tip section and the base section each have an internal structure that is substantially stiff and resistant to bending and torsion under aerodynamic loads.

[0017] In some embodiments, the plurality of orifices are about 40-80% of the cross-sectional surface area of the mid section of the flexible winglet. In some embodiments, the mid section comprises 25%-75% of the winglet body on a volume basis. In some embodiments, the mid section comprises 40%-60% of the winglet body on a volume basis.

[0018] In some embodiments, torsion of the flexible winglet occurs along an axial length of the winglet body extending parallel to the elongate body, and bending of the flexible winglet occurs along an axis of rotation of the elongate body.

[0019] In some embodiments, at least one of the base section, mid section and winglet tip section are encapsulated by a layer of laminated composite sheets having bi-axial or tri-axial fibers configured to provide directional rigidity.

[0020] In some embodiments, the at least one of bending and torsion of the flexible winglet conforms to the local angle of attack within a range of about -3 to 10 degrees relative to the local air flow vector.

[0021] In some embodiments, the winglet body is arced and conforms to an arc of circumference of a circle traversed by the winglet during movement of the elongate body about a rotational axis.

[0022] In some embodiments, the internal geometric structure of the flexible winglet is manufactured from a layer by layer aero-structural analysis of a pre-determined material composition, the aero-structural analysis based at least on a range of aerodynamic loads and a corresponding range of local angle of attacks for the flexible winglet.

[0023] In some embodiments, the plurality of cells within each of the layers form one of an a-symmetric lattice or Voronoi pattern, or a symmetric repeating honeycomb pattern. In some embodiments, the elongate body is an airfoil.

[0024] In some embodiments, the trailing edge of the winglet body may contain a serrated element that has additional flexibility, and extends from the winglet trailing edge, so as to impart a passive-adaptive flap component to the winglet body that aids to further reduce aerodynamic drag, loading and noise production.

[0025] In another embodiment, a method of manufacture is proposed for a flexible winglet adapted to traverse a fluid environment, the flexible winglet is configured for coupling to an elongate body containing a root, a wingtip, a leading edge and a trailing edge, the method comprises sensing a range of aerodynamic loads expected at the wingtip of the elongate body and associating the range of aerodynamic loads with corresponding angles of attack for the flexible winglet, modeling and tuning a plurality of layers of the flexible winglet, each layer of the plurality of layers containing an internal geometry based on a bending and torsional response to a pressure gradient generated by the range of aerodynamic loads, outputting a model of an internal geometric structure for the flexible winglet, wherein the model is a digital representation of each of the layers of the plurality of layers as a finite element, forming, based on the model of the internal geometric structure, each of the layers of the plurality of layers using additive or subtractive manufacturing. The plurality of layers include a base section of the flexible winglet having a central cavity for receiving the wingtip of the elongate body, a mid section of the flexible winglet that is adjacent to the base section, and configured for at least one of bending and torsion responsive to the aerodynamic loads to conform to a prescribed angle of attack based on at least a local air flow vector, and a winglet tip section of the flexible winglet that is adjacent to the mid section and extends to a termination point of the flexible winglet.

[0026] In some embodiments, the additive or subtractive manufacturing processes comprise 3D printing, moulding, laser cutting, water jet cutting, hand-layup, infusion, spray coating, or a combination.

[0027] In some embodiments, the one or more layers of the mid section contain an internal geometric grid structure having anisotropic bending and torsional responses to the local air flow vector, and wherein each of the internal geometric grid structures contains a plurality of cells and a plurality of orifices, and each cell of the plurality of cells forms a bounding box for an orifice of the plurality of orifices.

[0028] In some embodiments, the anisotropic bending and torsional response of the mid section is passively controlled through a moment of inertia of each of the cells of the plurality of cells within the internal geometric structure and a cross-sectional surface area of the plurality of orifices.

[0029] In some embodiments, a central aperture having a variable cross-sectional surface area extends axially within at least one of the mid section and winglet tip section to provide localized rigidity to the winglet body.

[0030] In some embodiments, the winglet tip section and the base section each have an internal structure that is substantially stiff and resistant to bending and torsion under aerodynamic loads.

[0031] In some embodiments, the plurality of orifices are about 40-80% of the cross-sectional surface area of the mid section of the flexible winglet. In some embodiments, the mid section comprises 25%-75% of the winglet body on a volume basis.

[0032] In some embodiments, torsion of the flexible winglet occurs along an axial length of the winglet body extending parallel to the elongate body, and bending of the flexible winglet occurs along an axis of rotation of the elongate body.

[0033] In some embodiments, at least one of the base section, mid section and winglet tip section are encapsulated by a layer of laminated composite sheets having bi-axial or tri-axial fibers configured to provide directional rigidity.

[0034] In some embodiments, the at least one of bending and torsion of the flexible winglet conforms to the local angle of attack within a range of about -3 to 10 degrees relative to the local air flow vector.

[0035] In some embodiments, the internal geometric structure of the flexible winglet is manufactured from a layer by layer aero-structural analysis of a pre-determined material composition, the aero-structural analysis based at least on a range of aerodynamic loads and a corresponding range of local angle of attacks for the flexible winglet.

[0036] In some embodiments, the plurality of cells within each of the layers form one of an a-symmetric lattice or Voronoi pattern, ora symmetric repeating honeycomb pattern. In some embodiments, the elongate body is an airfoil.

[0037] The improvements proposed herein improve the adoption of certain technologies such as, but not limited to, wind turbines and tidal turbines, and adoption of these green renewable technologies can help mitigate environmental impacts and renewable energies help conserve the natural environment and natural resources.DESCRIPTION OF THE FIGURES

[0038] In the figures, embodiments are illustrated by way of example. It is to be expressly understood that the description and figures are only for the purpose of illustration and as an aid to understanding.

[0039] Embodiments will now be described, by way of example only, with reference to the attached figures, wherein in the figures:

[0040] FIG. 1 is a side elevation view of a horizontal axis wind turbine according to the prior art.

[0041] FIG. 2 is a front perspective view of a blade in the horizontal axis wind turbine of FIG. 1 , which is also prior art.

[0042] FIG. 3 is a front view of a wingtip section of a blade including a flexible winglet, according to some embodiments.

[0043] FIG.4A is a front view of a wingtip section of a blade including a flexible winglet with bending axes shown, according to some embodiments.

[0044] FIG. 4B is a front view of a flexible winglet with cross-sectional layers shown for a base (A1-A1), mid (A2-A2) and winglet tip (A3-A3) section, according to some embodiments.

[0045] FIG.5 is a front view of a flexible winglet with example cross-sectional layers shown for a mid section (B-B), according to some embodiments.

[0046] FIG. 6 is a method diagram showing a method of tuning the flexible winglet to achieve the desired aerodynamic performance in response to an aerodynamic load, according to some embodiments.

[0047] FIGs. 7A, 7B, 7C and 7D are experimental results for power capture (FIGs. 7A-7B) and wake recovery (FIGs. 7C-7D) which compare a baseline turbine blade and a turbine blade with the proposed flexible winglet, according to some embodiments.DETAILED DESCRIPTION

[0048] The proposed embodiments relate to a flexible winglet that can be attached onto the tip of a wind turbine blade, the winglet being flexible insomuch as it obtains a desired angle of attack relative to the incoming flow vector and aerodynamic load it experiences. The flexible winglet can be a passive configuration in which the internal geometry and physical configuration conform to the incoming flow vector based on the pressure gradient generated at the tip of the turbine blade.

[0049] The winglet contains an internal geometry which incorporates structures which facilitate passive bending and twisting in response to aerodynamic loads experienced by the winglet. The internal geometry may include geometric orifices, lattices and laminated composite sheets of fibers, which provide desirable flexibility while still maintaining the strength to withstand the aerodynamic loads experienced at the tip of a blade. In operation, a resultant fluid flow vector, which is the relative sum of a radial component coming from the rotation of the rotor, the centripetal force and the incident wind on the turbine blade, will have a flow path that bends slightly away from the arc of rotation of the wind turbine. As a result, the resultant fluid flow vector generates a resulting pressure gradient along the surface of the flexible winglet which provides a torque to the flexible winglet which results in passive bending and twisting of the flexible winglet to achieve relative equilibrium with the resulting pressure gradient and maintain alignment to the circumferential arc of the rotation of the turbine blade, such that the beneficial energy capture by the winglet is maintained without causing a large vortex at the tip of the turbine blade.

[0050] The flexible winglet also contains an internal cavity (e.g., orifice or aperture) such that the flexible winglet can be placed over the tip of a wind turbine blade and have the external surface of the flexible winglet mate flush to the external surface of the wind turbine blade.

[0051] Various configurations of winglets have been used to improve the efficiencies of wind turbines as a whole by limiting the vortices that may be created upon rotation of the rotor blades. These winglets can either deviate into the oncoming wind incident with the rotor blade or to deviate away from the oncoming wind in the direction of the tower, without going forward or rearward of the leading or trailing edges of the rest of the rotor blade. This is a technical deficiency and technically undesirable. Such configurations have addressed only the handling of wind incident on the front of the rotor blades that causes the rotor blades to rotate, and have not considered improvements in how the rotor blades might operate in respect of the air that is encountered at high-speed by the rotor blades during their high-speed traversal of their rotational path. In particular, the wind incident on the front of the rotor blades may reasonably be moving at only up to about thirty (30) kilometres per hour (kph), whereas the linear speed of the wingtip region as it traverses its rotational path may reasonably reach up to three hundred and twenty (320) kph for a very rapidly-rotating rotor blade. The higher-speed in this respect can be responsible for the bulk of the noise, load, wake and inefficiencies at the wingtip.

[0052] Therefore, the flexible winglet may have an external geometry (i.e., surface of the winglet interacting with the fluid flow) that extends above a center line of the wind turbine blade towards a pressure side, and then arcs back towards a trailing edge of the wind turbine blade, the tip of the winglet terminating at a point behind the trailing edge of the wind turbine blade. In this way, the most external radial geometry of the winglet conforms to the radial arc of rotation of the host wind turbine blade.

[0053] As the proposed winglet is coupled at the tip of a turbine blade (i.e., distal to the radial center of the turbine), the winglet will experience a change in effective geometry (i.e., local angle of attack of the winglet) as the angle of attack of the turbine blade is adjusted in a pitch-operated turbine system. For example, as pitchable turbine blades have become standard for modern wind turbines, the effective geometry of the winglet is subject to the pitch angle of the turbine blade to which it is attached. Therefore, a rigid winglet may experienceundesirable or reduced performance characteristics (i.e., reduced noise and vortex attenuation) as the angle of attack of the turbine blade is adjusted due to the rigid winglet being incapable of adjusting to the change in aerodynamic loading and fluid angle. As such, there is only one specific angle of attack of a turbine blade that a rigid winglet is optimal at, with all other angles of attack being at sub-optimal efficiencies, following a substantially parabolic efficiency curve.

[0054] By incorporating passive bending and twist-coupling into the winglet, the flexible winglet may respond to varying aerodynamic loads experienced by the winglet, resulting from changes in the turbine blade’s angle of attack and incident fluid flow, by maintaining a desirable local angle of attack with the oncoming local fluid flow to contribute to the formation of laminar flow that may increase aerodynamic efficiency, reduce the intensity of vortex shedding and corresponding reductions in wake, loads and noise. Therefore, as the aerodynamic loads vary during operation of the turbine, the flexible winglet may bend and / or twist to conform to the natural flow vector and the circumferential arc of rotation of the rotor given the angle of attack of the turbine blade and the associated operational flow conditions.

[0055] The winglet body may also incorporate a passive-adaptive trailing edge serration which may adjust to changes in flow conditions and loads in real-time. In this way, it may act as a dampener or reduce the impact of severe turbulence. The trailing edge serrations may also further aid the winglet body in reducing drag and imparting a beneficial wake characteristic to the flow. In so doing, the acoustic signature may be reduced, especially in the low-frequency bands below 1000 Hz.

[0056] For example, the fluid flow vector may provide a torque to the flexible winglet which results in passive bending and twisting of the flexible winglet such that the local angle of attack of the flexible winglet is adjusted to maintain the beneficial energy capture by the tip portion of the winglet, without causing a large vortex at the tip of the blade.

[0057] However, when incorporating passive bending and twist-coupling into the winglet, there is a risk that, as a result of aerodynamic loading on the winglet, the winglet will overbend or over-twist, resulting in a reduction in the attenuation of noise, undesirable loading and vortices. Further, over-bending and over-twisting may damage the structure of the winglet,leading to potential material fracture, fatigue, and plastic deformation. Therefore, while it is desirable for the winglet to bend and twist as a result of loading, the bending / twisting profile must be within a desired range, and it may be desirable to have structural protections in place which brace or limit the bending / twisting of the winglet past a certain point of deformation and maintain the degree of bend and twist-coupling within a threshold amplitude.

[0058] FIG. 1 shows a side elevation view of a horizontal axis wind turbine 100 according to the prior art.

[0059] Wind turbine 102 includes a tower 104 supported by and extending from a surface S, such as a ground surface. Supported by towers 104, in turn, is a nacelle 106 extending horizontally. A hub with a spinner 108 is rotatably mounted at a front end of nacelle 106 and is rotatable with respect to nacelle 106 about a rotation axis R. Spinner 108 receives and supports multiple rotor blades 110 that each extend outwardly from spinner 108. Rotor blades 400 catch incident W flowing towards the wind turbine 10 and are caused to rotate. Due to their being supported by spinner 108, rotor blades 110 when rotating cause spinner 108 to rotate about rotation axis R thereby to cause rotational motion that can be converted in a well-known manner into usable electrical or mechanical power. In this sense, rotor blades 110 are each structures adapted to traverse a fluid environment, where the fluid in this embodiment is ambient air. Nacelle 106 may be rotatably mounted to tower 100 such that nacelle 106 can rotate about a substantially vertical axis (not shown) with respect to tower 100, thereby to enable rotor blades 110 to adaptively face the direction from which incident wind Wi is approaching wind turbine 10. A nose cone 112 of generally a uniform paraboloidal shape is shown mounted to a front end of spinner 108 to deflect incident wind Wi away from spinner 108.

[0060] FIG. 2 shows a front perspective view of a blade 110 in the horizontal axis wind turbine 102 of FIG. 1. Rotor blade 110 includes an elongate body that extends from a root 202 through a main section 204 to terminate at a wingtip 206. Root 202 extends from nacelle 106 when attracted thereto or integrated therewith, whereas wingtip 206 is the portion of the elongate body that is distal to nacelle 106. The elongate body has a leading edge 208 and a trailing edge 210, where leading edge 208 leads trailing edge 210 when rotor blade 110 is in motion rotating with nacelle 106 about rotation axis R in the direction D. A suction side 212 ofthe elongate body is shown in FIG. 2, and a pressure side 214, shown in dotted lines, is opposite the elongate body from suction side 212.

[0061] FIG. 3 shows a front view of a wingtip section 300 of a blade 200 including a flexible winglet 302, according to some embodiments. The flexible winglet 302 may reduce the production of horseshoe vortices at the wingtip 206 and reduce undesirable loads on the blade 110, including unsteady axial thrust and unsteady vibrations. The resulting improvement in performance generates a beneficial downstream wake recovery due to the controlled vortex shedding achieved through the proposed flexible winglet 302.

[0062] In one embodiment, blade 110 includes a flexible winglet 302 that is coupled to the wingtip 206. In this embodiment, flexible winglet 302 is integral with the wingtip 206. Flexible winglet 302 has a planar winglet body 304, which may be substantially normal to, and extending from the suction 212 or pressure side 214 of wingtip 206 of the blade 110 to a termination point, or tip 306 that is rearward of the base of the flexible winglet 302 during movement in direction D about rotation axis R, thereby to present a thin edge to the air it moves through during rotation. As shown, termination point 306 is rearward of trailing edge 210 by a distance x. The distance x as shown in FIG. 3 does not necessarily have to be the same across all embodiments.

[0063] Flexible winglet 302 has a leading edge 310 slicing at high speed into wind Whs during rotation, a trailing edge 312, and a wingtip 306. A flexible winglet 302 may be integral with wingtip 206 and extends smoothly from wingtip 206 by gently twisting so as to curve upwards while sweeping backwards with respect to the direction of rotation D so as to extend substantially normal to the suction side 308. The rigid winglet 302 continues to twist a total of about 180 degrees (to reveal the pressure side 314) and sweep backwards to a termination point, or a tip, 306, which is rearward of the trailing edge 212 by a distance x. The termination point 306 points away from the wind WhS. Winglet body 304 is substantially planar and is generally parallel to the tangent T of the circle traversed by the flexible winglet 302 during movement of a turbine. The twisting configuration of the flexible winglet 302 may contribute to the formation of laminar flow that may reduce the intensity of vortex shedding and corresponding reduction in noise.

[0064] Winglet 302 is configured to have a termination point 306 that is rearward of trailing edge 312 of rotor blade 110 in order to allow vortex shedding at this region to be gradual and less abrasive when compared to prior art designs. For example, instead of “ripping” the air, the configuration shown in FIG. 3 enables air to more efficiently run along the winglet body 304 such that less resistance is encountered. The proposed configuration may reduce turbulence and noise emissions at the region of wingtip 206 as a result of this reduction in resistance to the oncoming fluid flow.

[0065] In some embodiments, flexible winglet 302 is integral with the wingtip 206. In some embodiments, flexible winglet 302 may have a base that is substantially normal to and, extending from, pressure side 214 of the elongate body.

[0066] In a further embodiment, flexible winglet 302 may be retro-fitted onto a wingtip 206 of a blade 110 through an adhesive, mechanical fastener or chemical bonding.

[0067] In some embodiments, winglet body 304 may be curved or arced, such that it generally conforms to a radial arc of circumference of a circle traversed in the direction D by the winglet 302 during movement of the blade.

[0068] In some embodiments, termination point 306 may converge to a point which faces away from the wind Whs. Winglet body 304 is substantially planar and is generally parallel to the tangent T of the circle traversed by the wingtip during movement of a turbine.

[0069] In some embodiments, a connection mechanism coupled between the wingtip 206 and the flexible winglet 302 may be a rigid joint which holds the flexible winglet 302 in a preset fixed position relative to the wingtip 206.

[0070] FIGs. 4A, 4B and 5 show a front view of a flexible winglet 302 with bending axes shown (FIG. 4A) and cross-sectional layers (FIG. 4B and FIG. 5) shown, according to some embodiments. The flexible winglet 302 is configured to bend to achieve a desired angle of attack relative to the local fluid flow vector. The flexible winglet 302 contains a base section 402 which is configured to couple flexible winglet 302 to the wingtip 206 of blade 110. Base section 402 may have an aperture which defines a central cavity 408 configured to receive wingtip 206 of blade 110. For example, base section 402 may be substantially hollow suchthat base section 402 may be inserted onto wingtip 206 (see FIG. 4B). In this example, a portion of wingtip 206 resides inside base section 402 and may be fastened through mechanical or chemical adhesives. The exterior surface of base section 402 may be substantially parallel to the exterior surface of wingtip 206. Therefore, as seen in FIG. 4B, base section 402, and therefore flexible winglet 302, is integral with wingtip 206 such that the transition from blade 110 to flexible winglet 302 is smooth, and reduces turbulence inducing edges and rough / uneven surfaces.

[0071] Flexible winglet 302 further contains a mid section 404 which is positioned radially outwards from, and adjacent to, base section 402 relative to the central hub of the host turbine. Mid section 404 may be a transitional section between base section 402 and a winglet tip section 406 of flexible winglet 302. Mid section 404 may initially be substantially planar, and extend radially outward from base section 402 (i.e., extending away from wingtip 206). In some embodiments, the portion of mid section 404 proximal to base section 402 may be substantially parallel to base section 402. As mid section 404 extends radially outwards away from base section 402, mid section 404 may gradually curve rearward of base section 402 relative to the movement in direction D.

[0072] Flexible winglet 302 contains a winglet tip section 406 which comprises the portion of flexible winglet 302 which is furthest from blade 110. Winglet tip section 406 extends between mid section 404 and a termination point (i.e., a tip) of flexible winglet 302, and is adjacent to mid section 404. Winglet tip section 406 may extend rearward of base section 402, relative to the movement of direction D, such that the tip of winglet 302 is a distance x (shown in FIG. 3) from the trailing edge of base section 402. Winglet tip section 406 is the portion of flexible winglet 302 which is primarily responsible for the aerodynamic effect imparted on the working fluid. The curvature of winglet tip section 406 may present a thin leading edge to the air it moves through during rotation in order to allow vortex shedding at this region to be gradual and less abrupt. For example, instead of ‘ripping’ the air, the configuration shown in FIG. 3 better enables air encountered during rotation to run from the base section 402 to the winglet tip section 406 smoothly and with reduced resistance, imparting a natural gradual spiraling movement to the air, thereby reducing sudden pressure equalization, turbulence formation and noise emissions at the winglet tip section 406. In some embodiments, the radialgeometry of the winglet tip section 406 conforms to the radial arc of rotation of the host wind turbine blade 110.

[0073] As effective operation of flexible winglet 302 for noise and turbulence attenuation is based on the ability of flexible winglet 302 to channel the air encountered during rotation to run from base section 402 to winglet tip section 406 smoothly and with reduced resistance, the angle of attack and effective geometry of the flexible winglet 302 will impact the vortex shedding performance of the flexible winglet 302. This is because as the angle of attack of the blade 110 is altered to achieve the desired blade polars, the flexible winglet 302 may need to be adjusted along a different vector. However, as the flexible winglet 302 is incapable of manual adjustment, and mechanically assisted adjustment may require expensive and bulky sensors and actuators which may increase weight, drag and engineering complexity, the flexible winglet 302 is configured to passively adjust the local angle of attack and effective geometry based on the pressure differential around the winglet structure. In so doing, flexible winglet 302 will passively re-position itself to an effective geometry which is in relative equilibrium with its external pressure. Therefore, as the aerodynamic loads vary during operation of the turbine, the flexible winglet 302 may bend and / ortwistto conform to the natural flow vector and the circumferential arc of rotation of the rotor given the angle of attack of the blade 110 and the associated operational flow conditions.

[0074] The passive-adaptive principle of operation of flexible winglet 302 may also allow for peak load / thrust shaving, which usually occurs in times of high turbulence when pitching the blade 110 for load reduction is not possible given the level of change of the fluid flow speed. For example, a fixed and rigid winglet in this case would increase loads by failing to passively adjust and therefore reduce unwanted loads on the blade 110.

[0075] The inability of a rigid winglet to conform to the natural flow vector at the wingtip 206 of blade 110 is exacerbated when the rigid winglet is implemented in advanced systems which can adjust / modify the angle of attack of an airfoil or blade in response to changes in the fluid flow conditions and operational demands. As the angle of attack of the airfoil or blade is adjusted, the natural flow vector at the wingtip 206 of blade 110 will experience a corresponding adjustment which makes it difficult to design a rigid projection with a preset curvature or offset which will be viable across the broad range of operating conditions whichmay be expected. Therefore, under certain operating conditions when the rigid winglet does not align with the aerodynamic load and local median air pressure vector, a rigid winglet will rip through the air and as a result create undesired noise and loads.

[0076] The turbulence created by rigid winglets can result in downstream disturbances being produced which may reduce the strategic opportunities, such as the efficiency and performance, of a turbine farm. These inefficiencies are due to the impact that the wake generated by a rigid winglet may have on downstream turbines in a turbine farm. For example, upstream wake may reduce flow velocities of the working fluid (i.e. air) and this effect can be felt as far as 100km away in certain conditions.

[0077] The flexible winglet 302 is designed to bend during operation to conform to the local median air pressure vector arc which is generated by the combination of airflow incident to the leading edge of the blade 110 and the linear speed of the wingtip 206 as the blade 110 traverses its rotational path. This results in a resultant fluid flow vector which is the relative sum of these forces. The predominant fluid vector in this case is the radial component coming from the rotation of the rotor. When the radial fluid vector is combined with the centripetal force and incident wind on the blade 110, the resultant fluid vector is bent slightly away from the arc of rotation. As the geometry of the winglet tip section 406 conforms to the circumferential arc of the blade’s 110 rotation, the fluid flow vector may provide a torque to the flexible winglet 302 which results in passive bending and twisting of the flexible winglet 302 such that the beneficial energy capture by the winglet tip section 406 is maintained, without causing a large vortex at the tip of blade 110.

[0078] The local median air pressure vector arc is dependent on the flow conditions of the blade 110, and therefore may change as operational (i.e., blade angel of attack) and external conditions fluctuate. For example, the local median air pressure vector arc may be impacted by changes in the angle of attack and speed of the blade 110 during operation. The local median air pressure vector may also be impacted by changes, both sudden and gradual, in free stream airflow direction and velocity. Therefore, the flexible winglet 302 may be designed to react and experience elastic deformation, across a broad range of local median air pressure vector arcs. In some embodiments, the flexible winglet 302 is designed to bend in response to a pressure gradient imparted by a local median air pressure vector of about 1 to 50 MPa. Ifthe flexible winglet 302 is not conforming to the air flow vector, it may create turbulence which is a source of noise and reduced aerodynamic performance.

[0079] Since the flexible winglet 302 may perform optimally when it conforms to the local median air pressure vector arc, there may be certain considerations which must be taken into account to ensure that the flexible winglet 302 does not over-bend or over-twist as a result of the forces imparted by the pressure gradient at the wingtip 206 of the blade 108. Over-bending and over-twisting of the flexible winglet 302 may result in the flexible winglet 302 no longer conforming to the local median air pressure vector arc and instead interfering with the natural flow of the fluid which may result in undesirable turbulence. Further, over-bending and overtwisting may result in the flexible winglet 302 experiencing increased material stress, plastic deformation and fatigue impacting component lifetime and performance, and potential material failure at high aerodynamic loads.

[0080] Therefore, an underlying objective when designing the flexible winglet 302 may be to have an internal structure which allows the flexible winglet 302 to conform to the local median air pressure vector arc within a range of expected operating conditions of the turbine 100. For example, as the aerodynamic loads and angle of attack of the blade 110 vary, the flexible winglet 302 may be configured to maintain a local angle of attack (i.e., relative to the local flow vector) of between about -3 to 10 degrees. If the flexible winglets 302 are too stiff, then they will not bend enough to conform to the pressure vector arc when acted upon by the force vectors coming off of the wingtip 206. If the flexible winglets 302 are too pliable and / or do not contain braces or limits which restrict over-bending and over-twisting, then they may either bend past the point of conforming to the pressure vector arc or experience material fracture or plastic deformation.

[0081] In some embodiments, the flexible winglet 302 is configured to bend and I or twist to achieve a local angle of attack varying between about -3 to 10 degrees relative to the local air flow vector, based on a number of factors including the aerodynamic load imposed on the flexible winglet 302 and the angle of attack of the blade 110. In orderto conform to the desired angle of attack, the flexible winglet 302 may bend along axial plane A-A and I or twist along axial plane B-B. In some embodiments, the body of flexible winglet 302 is configured to primarily twist along the portion of the mid section 404 which is normal to the wingtip 206, andprimarily bend along the portion of the mid section 404 that is swept backwards. In some embodiments, flexible winglet 302 is configured to primarily twist and bend at mid section 404, while the winglet tip section 406 and base section 402 remain substantially rigid. In some embodiments, mid section 404 is configured for large amplitudes of twisting and bending, winglet tip section 406 is configured for smaller amplitudes of twisting and bending, and base section 402 remains substantially rigid.

[0082] As discussed above, in some embodiments, the flexible winglet 302 will, in response to aerodynamic loads, achieve a desired angle of attack relative to the local fluid vectorthrough a combination of bending and torsion of the flexible winglet 302. The flexible winglet 302 may be manufactured, through a selection of material and geometric properties, to bend along the axial length of the body (i.e., along axis A-A), and to twist primarily along the axis of rotation of the turbine (i.e., along axis B-B).

[0083] To achieve bending and twisting to conform to the desired angle of attack, the flexible winglet 302 can have advanced internal geometry which is tuned to achieve the desired bending and twist-coupling performance when under aerodynamic load. Another important mechanical consideration which impacts the bending and twist-coupling performance of the flexible winglet 302 is the material selection. Through combining different internal geometric structures along with a desired material selection, such as polymers, carbon fibers, glass fibers, resins, PUs, TPUs etc., the performance of the flexible winglet 302 can be fine-tuned to achieve the desired aerodynamic performance (through bending and twist-coupling) in response to the expected aerodynamic loads which will be experienced. For example, it may be desirable to select an initial material selection and then optimize the internal geometry of the flexible winglet 302 based on the expected aerodynamic loads of the turbine blades 110.

[0084] The passive flexibility within the body of the flexible winglet 302 should respond to the aerodynamic loads experienced at the tip 206 of the blade 110 by bending and / or twisting to achieve a desired local angle of attack with the local fluid flow. The bending and twisting performance of the flexible winglet 302 may be generated by determining a maximum and minimum aerodynamic load which is expected to be experienced by the flexible winglet and associating those upper and lower thresholds to a maximum and minimum bending and twisting performance. For example, the flexible winglet 302 should be designed to beginbending and / or twisting at a prescribed minimum aerodynamic load experienced by the wingtip 206, but should be able to resist over bending and / or over twisting (which may lead to undesirable vibrations) at a maximum aerodynamic load experienced at the wingtip 206.

[0085] In some embodiments, flexible winglet 302 may have a pre-bend such that under low or minimal aerodynamic loads where the flexible winglet 302 does not bend or twist, the local angle of attack of flexible winglet 302 is preset to a desirable degree for low aerodynamic loads. As the aerodynamic loads on flexible winglet 302 increase, bending and / or twisting may occur within flexible winglet 302 to adapt the effective geometry of flexible winglet 302 to maintain desirable fluid flow and vortex shedding.

[0086] As shown in aerodynamic structure 400B in FIG. 4B, the flexible winglet 302 contains a base section 402, a mid section 404 and a winglet tip section 406. Each of the sections 402, 404 and 406 may each be composed of a one or more layers of a plurality of layers 502A-502n which have an internal geometry configured for a desired bending and twisting profile. By way of example, in FIG. 4B, the base section 402 is shown with a single layer 502A, the mid section 404 is shown with two layers 502B and 502C, and the winglet tip section 406 is shown with a single layer 502n. Further, in FIG.5, example internal geometries 500 and compositions for mid section 404 are shown.

[0087] The plurality of layers 502A-502n are sandwiched between adjacent layers such that each layer forms a cross-sectional portion of the flexible winglet 302. In other terms, each layer of the plurality of layers 502A-502n form a finite element within the structure of the flexible winglet 302. The plurality of layers 502A-502n each contain an internal structure which can be one or more of a solid material, composite layers and geometric orifices.

[0088] When manufacturing the flexible winglet 302, each layer of the plurality of layers 502A-502n may be manufactured individually and then bonded together into a single unitary structure. Alternatively, using additive manufacturing techniques such as 3D printing, each layer 502A-502n may be printed as a single unitary structure beginning from the base section 402 and proceeding to the mid section 404 and finally the winglet tip section 406. In some embodiments, after manufacturing the internal geometry of the flexible winglet 302, one or more layers of laminated composite sheets 504, having bi-axial ortri-axial fibers for example,can be placed along the exterior surface of the flexible winglet 302 to enclose the flexible winglet 302 in laminated sheets 504. Incorporating laminated sheets 504 on the exterior surface of the internal geometry may increase the strength of the flexible winglet 302, especially when high aerodynamic loads are expected. In some embodiments, the laminated sheets 504 may be aligned layer by layer to assist with the desired bending and twisting response to varying aerodynamic loads. By adjusting the alignment of the fibers of the layers of laminated composite sheets 504, the flexible winglet 302 can be tuned to obtain improved bending performance at certain locations along the length of the body of the flexible winglet 302 (such as along the back swept region of mid portion 404) and to achieve improved torsion performance along the length of the flexible winglet 302 (such as near the portion of the mid section 404 proximal to the base section 402).

[0089] When layers of laminated composite sheets 504 are used for the structure of the flexible winglet 302, the material and mechanical properties that may impact the strength, bending and torsion performance of the flexible winglet 302 include at least the material composition of the composite sheets 504, the alignment of the fibers within the composite sheets 504, the alignment of the fibers between two adjacent composite sheets and the number of layers within the composite sheets 504.

[0090] Further, in embodiments with and without the laminated sheets 504, a shell 506 (shown in FIG. 4B) may be placed on the exterior surface of flexible winglet 302 to provide a smooth surface for fluid flow. For example, the shell may be a smooth and thin layer of material or coating which encapsulates the internal geometry, and in some embodiments, layers of laminated sheets 504, of the flexible winglet 302. The shell 506 may be configured to reduce surface friction on the fluid flow, and may have an elasticity which enables the shell 506 to bend and twist based on the response of the internal geometry to the aerodynamic loads on the flexible winglet 302.

[0091] As mentioned above, the flexible winglet 302 contains an internal geometric structure which contains a plurality of complex geometric shapes that define a plurality of orifices 508. In some embodiments, the complex geometric shapes are concentrated within the mid section 404 of the flexible winglet. As can be seen, in layers 502B and 502C, the internal geometric structure creates a mesh of geometric shapes which extend an axial lengthof the flexible winglet 302. For example, in FIG. 4B, layer 502B may extend 40% of the axial length x of midsection 404, and layer 502C may extend 60% of the axial length x of mid section 404.

[0092] The geometric shapes may be 2D or 3D configurations which provide directional strength and anisotropic bending and twisting performance.

[0093] As an example, but without limiting the scope of geometric shapes and configurations which are possible forthe proposed flexible winglet 302, a honeycomb structure is shown in FIG. 4B comprising a grid of repeating multi-faced geometric shapes defining a plurality of orifices 508. Additive manufacturing techniques can be used, such as 3D printing for example, to manufacture the honeycomb structure of the flexible winglet 302. By utilizing additive manufacturing techniques, the honeycomb structure can be manufactured in layers (i.e., slices of the winglet beginning at the base), and each layer of the plurality of layers 502A-502n may have a similar or unique internal geometry which provides varying structural and mechanical properties which determine the bending and twisting performance of the flexible winglet 302.

[0094] The internal geometry creates a plurality of orifices 508 (i.e., micro-cavities) within the internal structure of the flexible winglet 302, which enable the internal geometry to bend and twist in a controlled manner in response to an external force.

[0095] As each orifice 508 within the grid of layers 502B, 502C is defined by a corresponding surface area bounded on all sides by the walls of each honeycomb shape, the cross-sectional shape and moment of inertia of each honeycomb structure within the grid will impact the response of layer 502B, 502C within the plurality of layers 502A-502n to an aerodynamic load. Therefore, each orifice 508 is defined by a cell within the grid of orifices 508, where each cell defines the boundary walls of an orifice 508.

[0096] For example, as the moment of inertia represents a structures ability to resist bending, the bending and torsional response of the flexible winglet 302 to an external aerodynamic load can be controlled through the shape and size of each cell defining an orifice 508 within the plurality of layers 502A-502n. Further, each layer within the plurality of layers502A-502n can be configured with a unique bending and torsional response to varying aerodynamic loads, such that bending and twisting can be localized at desired locations. For example, bending can be localized within layers 502B, 502C within mid section 404, while base section 402 and winglet tip section 406 may be substantially stiff and experience minimal to no bending.

[0097] The layer(s) which compose the base section 402 may be configured with an internal structure which has a high stiffness and resistance to bending and twisting (i.e., minimal to no orifices 508 to increase the high moment of inertia and structural stiffness), in order to avoid any bending or twisting occurring at the base section 402. This may be desirable in order to ensure a robust and secure coupling is maintained between the wingtip 206 of blade 110 and the flexible winglet 302, and to ensure a smooth transition between the wingtip 206 and base section 402 of the flexible winglet 302 to avoid generating turbulence at this transition point. As shown in FIG. 4B, the internal structure of base section 402 is shown in cross-sectional layer 502A. Layer 502A is substantially solid, and contains minimal to no orifices 508 which encourage bending and torsion in response to aerodynamic loads. Therefore, layer 502A may be a rigid structure and may, as shown in FIG. 4B, be encapsulated by laminated sheets 504 to increase the stiffness and directional strength of base section 402. A central cavity 408 extends along some or all of the axial length z of base section 402, and is configured to receive and be fastened to the wingtip 206 of blade 110.

[0098] The winglet tip section 406 may be substantially rigid along its axial length y, such that it experiences minimal bending or torsion as a result of aerodynamic loads. In some embodiments, a portion of winglet tip section 406 proximal to the mid section 404 may be configured for small amplitudes of bending and twisting (i.e., compared to the larger amplitude of bending and twisting occurring along the mid section 404). For example, about 1-25% of the axial length y of winglet tip section 406 proximal to the mid section 404 may have orifice 508 which permit bending and torsion, while the remaining axial length y of winglet tip section 406 is substantially rigid. A technical problem with flexible winglets 302 is that there is a risk of vibrations occurring within the winglet structure (e.g., as a result of turbulence in the working fluid) which have an adverse effect on both the aerodynamic performance and structural integrity of the winglet 302. For example, vibration of the flexible winglet 302 may result in theflexible winglet 302 no longer conforming to the local median air pressure vector arc and instead interfering with the natural flow of the working fluid and generating undesirable turbulence.

[0099] Therefore, the flexible winglet 302 should resist uncontrolled vibrations during a nonstalled fluid flow through dampening and stiffness along the flexible winglet 302. As the winglet tip section 406 is the most radially outward portion of the flexible winglet 302, this portion of the flexible winglet 302 may experience the highest-pressure gradient. Further, since the winglet tip section 406 contains the smallest cross-sectional surface area, it may be susceptible to over-bending and vibrations as a result of the high-pressure gradient.

[0100] Lastly, as the winglet tip section 406 is the most radially outward portion of the flexible winglet 302, it will experience a force vector by the local median air pressure vector across a longer moment arm and therefore a larger internal force vector will be imparted onto the winglet tip section 406.

[0101] Therefore, the winglet tip section 406 may be configured to be substantially stiff and resist bending and torsion, especially proximal to the most radially outward portion of the winglet tip section (i.e., the termination point of the flexible winglet 302). As shown in FIG.4B, an example internal structure of the winglet tip section 406 is shown as cross-sectional layer 502n, and the internal structure is substantially solid with no orifices 508 or openings. The internal structure of layer 502n may have a high stiffness, which minimizes bending and torsion. Alternatively, the internal structure of layer 502n may permit small amplitudes of bending and torsion, but has a higher stiffness than layers 502B and 502C of mid section 404 which are the primary locations for bending and torsion. According to this embodiment, layers of laminated sheets 504 are also included on layer 502n, which may provide additional stiffness and directional strength.

[0102] The layers which compose the mid section 404 may be configured as the primary section for bending and twisting in response to varying aerodynamic loads. The mid section 404 may be 25-75% of the entire flexible winglet 302 on a volume basis, while the base section 402 and winglet tip section 406 may each be 15-40% of the entire flexible winglet 302 on a volume basis. In some embodiments, the bending and twisting occurs primarily at the midsection 404 so as to ensure that the winglet tip section 406 of the winglet 302 retains its effective geometry relative to the airflow. Allowing bending ortwisting at the winglet tip section 406 of the winglet 302 may further cause vibrations, and may significantly reduce the power capture of the turbine system.

[0103] In the embodiment shown in FIG. 4B, the base section 402 and the winglet tip section 406 may be substantially rigid and formed of a substantially solid material having no or minimal orifices 508. The mid section 404 contains layers 502B and 502C of the plurality of layers 502A-502n which each have an internal geometry containing a plurality of orifices 508 formed by a grid of geometric cells. In FIG. 4B, the two layers 502B, 502C of mid section 404 both contain the same internal geometry, with the only difference being that layer 502C is encapsulated by laminated sheets 504, while layer 502B is not. However, it would be understood that FIG. 4B is a non-limiting example, and mid section 404 may contain one or more distinct cross-sectional layers, which contain the same or distinct internal geometries.

[0104] For example, mid section 404 may contain distinct layers 502B, 502C, 502D which each extend 33% of the axial length x of mid section 404. Each of the layers 502B, 502C, 502D may have the same or different internal geometries (such as the examples shown in FIG. 5). Even when two or more of the layers 502B, 502C, 502D have the same internal geometries, the surface area of the orifices 508 may increase or decrease based on the amplitude of bending and torsion desired along the axial length x of mid section 404.

[0105] The grid of geometric cells within the internal structure of mid section 404 provides an anisotropic bending and torsional property to the mid section 404 such that bending along the axis A-A and torsional movement along axis B-B (shown in FIG. 4A) may be directionally controlled based on aerodynamic loading resulting from incident fluid flow.

[0106] The geometric cells which create the grid within the internal structure of mid section 404 each define an orifice 508 which extends the axial length of its associated layer within the plurality of layers 502A-502n. For example, if layer 502B extends 45% of axial length x of mid section 404, the orifices 508 within layer 502B extend the same distance.

[0107] In FIG.5, a cross-section B-B is shown of mid section 404, and multiple non-limiting examples of the internal structure 500 contemplated for the flexible winglet 302 are shown. Cross sections 512A, 512B and 512C each show a geometric grid which contain a plurality of orifices 508 extending along the axial length x of mid section 404. As mentioned, the geometric grid which makes up the internal structure of mid section 404 provides the bending and torsional response to the pressure gradient generated by the incident fluid flow vector.

[0108] As mentioned above, one or more layers of geometric structures can be combined within mid section 404, such that specific bending and torsional responses are localized along the axial length x of mid section 404. For example, it may be desirable to prioritize twisting of the mid section 404 proximal to the base section 402 and to prioritize bending of mid section 404 proximal to the winglet tip section 406. This may be desirable when excessive vibrations are a concern, since excessive bending proximal to the base section 402 may result in larger amplitude vibrations at the winglet tip section 406 (due to the amplitude of vibration being proportional to the axial distance). Therefore, by localizing the bending response at the portion of mid section 404 that is proximal to the wingtip section 406, the amplitude of vibration that could be experienced by wingtip section 406 will be reduced, and the natural frequency of the flexible winglet 302 will be increased.

[0109] In some embodiments, to achieve the desired bending and torsional response within mid section 404, the plurality of orifices 508 comprise about 40-80% of the cross-sectional surface area of the mid section 404 of the flexible winglet 302. In a further embodiment, the plurality of orifices 508 comprise about 50-60% of the cross-sectional surface area of the mid section 404 of the flexible winglet 302.

[0110] Achieving a directional bending and torsional response using the internal geometric structures within cross-sections 512A-512C may be based on at least the shape of each cell within the geometric grid and the size of the plurality of orifices 508. For example, by increasing the surface area of the orifices 508, the amplitude of bending or torsion may increase. The size of the orifices 508 may also act as braces against over-bending. As the mid section 404 bends, the internal structure will compress causing the surface area of the orifices 508 to be reduced. As the internal structure is compressed, the walls which define the orifices 508 willbegin to engage with one another and act as a brace against further bending (i.e., by restricting further compression within the internal geometry).

[0111] For example, for bending or torsion along a specific plane, there will exist a series of cross-sectional orifices 508 within the layers 512A-512C that align, the size of said orifices 508 being the bounding-box within which the structure can bend at that given point. These aligned orifices 508 may differ in size along an axial plane of the flexible winglet, and be structurally aligned with orifices 508 along a cross-sectional plane, such that when one orifice 508 has fully collapsed and the opposing walls of orifice’s 508 that are parallel to the load are touching (i.e., opposing walls of a cell are compressed against each other), continued load on the structure will compress adjacent orifices 508 in the perpendicular plane to generate a torsional response that is bounded by the cross-sectional area and geometric shape of the orifices 508. It is the three-dimensional layout of these orifices 508 that determine the overall three-dimensional response of the winglet 302 to the aerodynamic load. The size of the orifices 508 may be tailored to the expected blade 110 and / or wind conditions, such that the winglet 302 is protected from over-bending or over-twisting, while ensuring the performance and desired torque capture by the winglet 302 is translated to the blade 110.

[0112] Further, as a structures cross-sectional shape and moment of inertia significantly impact its response to bending and torsional loads, the geometric cells within the grid can be configured with directional strength that resists bending and torsion along certain planes, but enables it along others.

[0113] In some embodiments, such as that seen in cross-section 512B, an aperture 510 may extend along some or all of the axial length x of the mid section 404 and axial length y of winglet tip section 406. Aperture 510 may be aligned with the central axis of the winglet 302, and be encapsulated by the grid of geometric cells defining the plurality of orifices 508. In some embodiments, aperture 510 may be defined by an internal layer of laminated sheets 502 which provide additional localized rigidity to the flexible winglet 302. Therefore, aperture 510 may provide additional rigidity and further act as a safety margin on the bounding box of flexibility for the winglet 302 (i.e., aperture 510 with a larger cross-sectional surface area will enable increase bending / torsional amplitude). For example, the cross-sectional surface area of the aperture 510 may vary along the axial length of the mid section 404 and winglet tipsection 406, such that larger bending or torsion responses can be localized along axial lengths of the mid section 404 having an aperture 510 with a wider cross-sectional area, while portions of the winglet tip section 406, for example, having an aperture 510 with a smaller cross-sectional area will be stiffer and therefore experience smaller or minimal bending or torsion.

[0114] In some embodiments, the aerodynamic loads experienced by the flexible winglet 302 may reach a maximum of 50-75 MPa. In some embodiments, the range of desired local angles of attack for the flexible winglet 302 may be any one of -3, -2, -1 , 0, 1 , 2, 3, 4, 5, 6, 7, 8, 9 and 10 degrees relative to the local air flow vector.

[0115] FIG.6 is a method diagram showing a method 600 for tuning the flexible winglet 302 to achieve the desired aerodynamic performance in response to an aerodynamic load, according to some embodiments. To achieve the desired aerodynamic performance of the flexible winglet 302, the flexible winglet 302 should passively align with a desired local angle of attack.

[0116] The range of aerodynamic loads which the flexible winglet 302 will experience should be identified and then associated with a range of desired local angles of attack the flexible winglet 302 should obtain in response (602). When identifying the range of desired local angles of attack, the broader turbine structure 100, and its response to aerodynamic loads, should be taken into account in order to have an accurate bending and twisting response within the flexible winglet 302. For example, the desired local angles of attack of the flexible winglet 302 should be determined relative to the angles of attack the blade 110 will obtain at certain aerodynamic loads, the local pre-set twist of the blade 110, the pre-bend of the blade 110 and resulting bend in response to aerodynamic loads, and the aerodynamic geometry of the blade 110. The aerodynamic geometry of the blade 110 includes relevant aerofoil design considerations such as camber, thickness, chord length, and other relevant airfoil design parameters. In some embodiments, the aerodynamic loads are determined through sensors on operational turbines, sensors placed on site at a turbine location, experimental results, and historical data from turbine operations.

[0117] Utilizing the identified aerodynamic loads and corresponding range of desired local angle of attacks of the flexible winglet 302, an aero-structural analysis is performed (604)which assesses how various geometric and material parameters of the flexible winglet 302 respond to the identified aerodynamic loads. The results of the aero-structural analysis are compared to the desired local angles of attack identified earlier, and a series of refinement steps (606) are performed within a confines of a material selection to optimize the internal geometric structure within the flexible winglet 302.

[0118] The refinement of the internal geometric structure builds up the flexible winglet 302 layer by layer based on a desired maximal twisting and bending which should be achieved within a load set. By having a selected material composition of the flexible winglet 302, the internal geometric structure can be tuned based on the parameters identified earlier. In some embodiments, the layer-by-layer sensitivity analysis (608) begins at the base of the flexible winglet 302 and builds up the flexible winglet 302 based on the desired response characteristics. As would be understood, the internal structure of the flexible winglet 302 can vary along its axial length, including by using a combination of internal geometric structures and composite laminated layers.

[0119] Based on the layer-by-layer model generated by the aero-structural analysis (604), refinements (606) and sensitivity analysis (608), a final modeling of the external structural composition and internal geometry for the winglet 302 may be generated which can be output as a digital representation for an additive or subtractive manufacturing process. As mentioned previously, the internal geometry of the flexible winglet 302 may be separated into distinct layers which each contain unique cross-sectional structures. For example, base section 402 may have a rigid internal structure and an internal cavity for receiving the tip of blade 110. Mid section 404 may have one or more layers of geometric grids containing cells which define a plurality of orifices. In some embodiments, mid section 404 may have a central aperture which extends along some or all of the axial length of mid section 404. Winglet tip section 406 may have a substantially rigid internal structure which contains minimal to no orifices. In some embodiments, the portion of winglet tip section 406 proximal to the mid section 404 may have geometric grids containing cells which define a plurality of orifices, while the portion of winglet tip section 406 proximal to the termination point of flexible winglet 302 may be substantially rigid and resist bending and torsion.

[0120] When manufacturing the flexible winglet 302, each layer of the plurality of layers may be manufactured individually and then bonded together into a single unitary structure. Alternatively, using additive manufacturing techniques such as 3D printing, each layer may be printed successively to form as a single unitary structure beginning from the base section 402 and proceeding to the mid section 404 and finally the winglet tip section 406.

[0121] FIG. 7A-7D shows experimental results of a comparison of a baseline turbine blade against a proposed turbine blade containing a flexible winglet 302. The results show improvements in power capture (FIGs. 7A-7B) and wake recovery (FIGs. 7C-7D) achieved through implementation of the flexible winglet. In FIGs. 7A-7D, the baseline geometry used for the comparison was a turbine having a radius of 89.6 m. A detailed computational investigation utilizing Detached Eddy Simulation (DES) turbulence models was performed to obtain the results in FIGs. 7A-7D.

[0122] In FIGs. 7A and 7B, the averaged unsteady experimental results suggest that the addition of the proposed winglet increases the thrust by 1% (FIG. 7A) and the power by 4.5% (FIG. 7B). The average power (peak to mean) is also more consistent by 1 / 3.

[0123] In FIGs. 7C and 7D, the velocity recovery and turbulence intensity were considered behind the turbine. FIG. 7C shows a 0.1 m / s improvement in velocity recovery at a distance of 5Xthe rotor diameter behind the turbine. A 0.1 m / s increase in velocity is equivalent to a power increase of 2.5% behind the turbine. FIG. 7D shows a 1% decrease in turbulence intensity at a distance of 5X the rotor diameter behind the turbine.

[0124] The flexible winglet may be manufactured in tandem with a turbine blade and be fitted onto the turbine blade prior to installation of the blade onto a turbine. The flexible winglet may also be retro-fitted for installation onto a turbine blade which is already installed on an operational turbine.

[0125] In some embodiments, the flexible winglet, configured to traverse a fluid environment, contains a base section extending from the tip of the turbine blade. The base section contains an internal cavity which is configured to receive the tip of the turbine blade, and is fastened through either mechanical or chemical bonding. The base section of theflexible winglet extends substantially parallel to the turbine blade, and acts as the transition between the turbine blade and a mid section of the flexible winglet. The mid section of the flexible winglet is adjacent to the base section, and initially extends substantially parallel to the base section. A distal portion of the mid section (i.e., distal to the base section), may have a curvature which sweeps backwards in the direction of the rotation of the turbine blade. The mid section acts as a transition between the base section and a winglet tip section of the flexible winglet. The winglet tip section is adjacent to the mid section and curves upwards while sweeping backwards with respect to a direction of rotation of the turbine blade.

[0126] The mid section of the flexible winglet is composed of one or more cross-sectional layers which are configured as the primary portions of the flexible winglet which provide passive-adaptive bending and twist response. Each layer of the one or more layers of the mid section has an axial length, and contain an internal geometric structure having a bending and torsional response profile based on the operating conditions and natural fluid flow vector at the tip of the turbine blade. The internal geometric structure of the layer(s) within the mid section contains a grid of cells where each cell defines the walls of an orifice. As each cell within the grid contains a specific moment of inertia and directional strength, the surface area and geometric shape of each cell will impact the aggregate bending and torsional response profile of the mid section of the flexible winglet. In some embodiments, a central aperture may extend along some or all of the mid section.

[0127] The winglet tip section may be substantially rigid along its axial length, and resist bending ortorsion as a result of incoming fluid flow. In other embodiments, a portion of winglet tip section proximal to the mid section may have one or more layers of internal geometric structures having a bending and torsional response profile based on the operating conditions and natural fluid flow vector at the tip of the turbine blade. In this embodiment, the portion of winglet tip section proximal to the termination point of the flexible winglet may be substantially rigid to mitigate harmful vibrations. In some embodiments, a central aperture may extend along some or all of the winglet tip section.

[0128] The term “connected” or "coupled to" may include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements).

[0129] Although the embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification.

[0130] Processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the embodiments are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

[0131] As can be understood, the examples described above and illustrated are intended to be exemplary only.

Claims

1. WHAT IS CLAIMED IS:1 . A structure adapted to traverse a fluid environment, the structure comprising:an elongate body having a root, a wingtip, a leading edge and a trailing edge;a flexible winglet associated with the wingtip and having a body extending from the elongate body to a termination point that is rearward of the trailing edge, the body extends from the wingtip and curves upwards while sweeping backwards with respect to a direction of rotation of the elongate body;a base section of the body which is integral with the elongate body and having a central cavity which receives the wingtip of the elongate body;a mid section of the body that is adjacent to the base section; anda winglet tip section of the body that is adjacent to the mid section and extends to the termination point of the winglet body;wherein, responsive to an aerodynamic load, at least one of bending and torsion occurs along the mid section of the flexible winglet to conform to a prescribed local angle of attack based on at least a local air flow vector.

2. The structure of claim 1 , wherein the mid section contains one or more cross-sectional layers, and each layer contains an internal geometric grid structure having anisotropic bending and torsional responses to the local airflow vector; andwherein each of the internal geometric grid structures contains a plurality of cells and a plurality of orifices, and each cell of the plurality of cells forms a bounding box for an orifice of the plurality of orifices.

3. The structure of claim 2, wherein the anisotropic bending and torsional response of the mid section is passively controlled through a moment of inertia of each ofthe cells of the plurality of cells within the internal geometric structure and a cross-sectional surface area ofthe plurality of orifices.

4. The structure of claim 2, wherein a central aperture having a variable cross-sectional surface area extends axially within at least one of the mid section and winglet tip section to provide localized rigidity to the winglet body.

5. The structure of claim 2, wherein the winglet tip section and the base section each have an internal structure that is substantially stiff and resistant to bending and torsion under aerodynamic loads.

6. The structure of claim 3, wherein the plurality of orifices are about 40-80% of the cross- sectional surface area of the mid section of the flexible winglet.

7. The structure of claim 2, wherein the mid section comprises 25%-75% of the winglet body on a volume basis.

8. The structure of claim 3, wherein the mid section comprises 40%-60% of the winglet body on a volume basis.

9. The structure of claim 1 , wherein torsion of the flexible winglet occurs along an axial length of the winglet body extending parallel to the elongate body, and bending of the flexible winglet occurs along an axis of rotation of the elongate body.

10. The structure of claim 1 , wherein at least one of the base section, mid section and winglet tip section are encapsulated by a layer of laminated composite sheets having bi-axial or tri-axial fibers configured to provide directional rigidity.

11. The structure of claim 1 , wherein a trailing edge of the winglet body incorporates a flexible trailing edge serration.

12. The structure of claim 1 , wherein the at least one of bending and torsion of the flexible winglet conforms to the local angle of attack within a range of about -3 to 10 degrees relative to the local air flow vector.

13. The structure of claim 1 , wherein the winglet body is arced and conforms to an arc of circumference of a circle traversed by the winglet during movement of the elongate body about a rotational axis.

14. The structure of claim 5, wherein the internal geometric structure of the flexible winglet is manufactured from a layer by layer aero-structural analysis of a pre-determined material composition, the aero-structural analysis based at least on a range ofaerodynamic loads and a corresponding range of local angle of attacks for the flexible winglet.

15. The structure of claim 2, wherein the plurality of cells within each of the layers form one of an a-symmetric lattice or Voronoi pattern, or a symmetric repeating honeycomb pattern.

16. The structure of claim 1 , wherein the elongate body is an airfoil.

17. A method of manufacture of a flexible winglet adapted to traverse a fluid environment, the flexible winglet is configured for coupling to an elongate body containing a root, a wingtip, a leading edge and a trailing edge, the method comprising:sensing a range of aerodynamic loads expected at the wingtip of the elongate body and associating the range of aerodynamic loads with corresponding angles of attack for the flexible winglet;modeling and tuning a plurality of layers of the flexible winglet, each layer of the plurality of layers containing an internal geometry based on a bending and torsional response to a pressure gradient generated by the range of aerodynamic loads;outputting a model of an internal geometric structure for the flexible winglet, wherein the model is a digital representation of each of the layers of the plurality of layers as a finite element;forming, based on the model of the internal geometric structure, each of the layers of the plurality of layers using additive or subtractive manufacturing, wherein the plurality of layers include:a base section of the flexible winglet having a central cavity for receiving the wingtip of the elongate body;a mid section of the flexible winglet that is adjacent to the base section, and configured for at least one of bending and torsion responsive to the aerodynamic loads to conform to a prescribed angle of attack based on at least a local air flow vector; anda winglet tip section of the flexible winglet that is adjacent to the mid section and extends to a termination point of the flexible winglet.

18. The method of claim 17, wherein the additive or subtractive manufacturing processes comprise 3D printing, moulding, laser cutting, water jet cutting, hand-layup, infusion, spray coating, or a combination.

19. The method of claim 17, wherein the one or more layers of the mid section contain an internal geometric grid structure having anisotropic bending and torsional responses to the local air flow vector; andwherein each of the internal geometric grid structures contains a plurality of cells and a plurality of orifices, and each cell of the plurality of cells forms a bounding box for an orifice of the plurality of orifices.

20. The method of claim 19, wherein the anisotropic bending and torsional response of the mid section is passively controlled through a moment of inertia of each ofthe cells of the plurality of cells within the internal geometric structure and a cross-sectional surface area ofthe plurality of orifices.

21. The method of claim 19, wherein a central aperture having a variable cross-sectional surface area extends axially within at least one of the mid section and winglet tip section to provide localized rigidity to the winglet body.

22. The method of claim 19, wherein the winglet tip section and the base section each have an internal structure that is substantially stiff and resistant to bending and torsion under aerodynamic loads.

23. The method of claim 20, wherein the plurality of orifices are about 40-80% ofthe cross- sectional surface area ofthe mid section ofthe flexible winglet.

24. The method of claim 19, wherein the mid section comprises 25%-75% of the winglet body on a volume basis.

25. The method of claim 17, wherein torsion of the flexible winglet occurs along an axial length of the winglet body extending parallel to the elongate body, and bending of the flexible winglet occurs along an axis of rotation ofthe elongate body.

26. The method of claim 17, wherein at least one of the base section, mid section and winglet tip section are encapsulated by a layer of laminated composite sheets having bi-axial or tri-axial fibers configured to provide directional rigidity.

27. The method of claim 17, wherein the at least one of bending and torsion of the flexible winglet conforms to the local angle of attack within a range of about -3 to 10 degrees relative to the local air flow vector.

28. The method of claim 22, wherein the internal geometric structure of the flexible winglet is manufactured from a layer by layer aero-structural analysis of a pre-determined material composition, the aero-structural analysis based at least on a range of aerodynamic loads and a corresponding range of local angle of attacks for the flexible winglet.

29. The method of claim 19, wherein the plurality of cells within each of the layers form one of an a-symmetric lattice or Voronoi pattern, or a symmetric repeating honeycomb pattern.

30. The method of claim 17, wherein the elongate body is an airfoil.

31. An article of manufacture according to the method of claims 17-30.

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