Anti-stall device for an aerofoil

Abstract

An aerofoil (1) is provided with a boundary layer energisation device (100) comprising at least one element (10) having a crosspiece (40) extending between a pair of struts (20) in spaced relation to the aerofoil surface (7) to define a closed boundary surrounding a flowpath (11). The flowpath (11) is open in the airflow direction Da and separated by the closed boundary from an outwardly open flow region (101) exterior to the device element (10). The device (100) may include a set of such elements (10) spaced apart in series along the length dimension of the aerofoil (1) to generate a pair of counter-rotating vortices V that energise the air flowing through the gap (50) between each adjacent pair of elements (10).

Description

[0001] ANTI-STALL DEVICE FOR AN AEROFOIL

[0002] This invention relates to devices for energising the boundary layer of an aerofoil to prevent or delay stalling, and in particular (although not exclusively) to devices for use on the rotor blade of a wind turbine.

[0003] Stalling means the separation of the high speed airflow from the surface of the aerofoil in advance of the trailing edge, resulting in a loss of lift and increased drag.

[0004] The separation point tends to move closer to the leading edge when the angle of attack increases, and when the blade profile is distorted due to roughness at the leading edge.

[0005] Most modern commercial wind generators (i.e. wind turbines that generate electricity) are HAWTs with lift-based rotors - which is to say, the rotor blades are aerofoils which rotate about the rotor axis at a tip speed higher than the wind speed, typically with a tip speed ratio of around 8 to 10. The air flowing over the aerofoil generates lift (a force acting across the direction of the relative wind, i.e. the mean direction of the air impinging on the blade as it rotates), which drives the rotor in rotation.

[0006] The blade of a HAWT rotor extends radially outwardly away from the rotor axis along a blade length axis from the root of the blade at the rotor hub to the tip. As the radial distance from the hub increases, the tangential speed of the blade also increases and so the direction of the relative airflow moves progressively away from the direction of the free wind (which flows through the rotor along its axis of rotation) to oppose the rotational direction of the blade. For this reason, the blade is twisted along its length axis, with the leading edge turning progressively towards the direction of rotation with increasing radial distance from the hub, to maintain an optimal angle of attack a (the angle of the chord of the aerofoil relative to the airflow).

[0007] The aerofoil profile is shaped to generate a pressure differential between the downwind (lower pressure or "suction") side of the blade and the upwind (higher pressure or "pressure") side of the blade. The pressure differential across the blade appears as lift, which includes a thrust component, which acts along the direction of the axis of rotation, and a torque component which urges the blades in the direction of rotation and is converted into shaft power at the rotation axis. Thrust must be reacted by the hub and supporting structures and so increases capital cost. The angle of attack a is therefore selected to maximise torque relative to thrust.

[0008] The air flowing over the blades gives rise to reaction forces, referred to collectively as drag, which act on the blades in a direction opposed to the direction of rotation. Drag consumes some of the useful torque and thus reduces the output shaft power of the rotor.

[0009] As the blades rotate, the pressure differential across the upwind and downwind sides of each blade causes the air to flow around the blade from the higher pressure, upwind side to the lower pressure, downwind side. Most of the flow is along the direction of the chord, i.e. in the width direction of the blade, from its leading edge to its trailing edge, hence contributing to generate the pressure differential which produces the lift and hence the torque which drives the blade in rotation.

[0010] The commercial case for a wind turbine installation depends largely on its mass efficiency, which is to say, its average output shaft power relative to the mass moment of inertia of the rotor.

[0011] HAWT blades represent a compromise between aerodynamic and structural objectives. By extending the chord proximate the blade root it would be possible to form the entire blade with an optimal aerofoil profile, and so to obtain additional lift. However, this would result in a disproportionate reduction in mass efficiency and increase in thrust. A typical HAWT blade will therefore have a profile that is circular at the blade root where it connects to the rotor hub, gradually blending into an optimal aerofoil profile as it moves away from the root towards the tip.

[0012] As air flows over the aerofoil a boundary layer is formed, which clings to the aerofoil while the faster airstream flows over the boundary layer. The low Reynolds number and aerodynamically suboptimal profile in the inboard portion of a HAWT blade (i.e. the radially inward portion of the blade, extending from the root) produce a thickened boundary layer and a high propensity to stall.

[0013] It is known to use vortex generators to mitigate stalling on wind turbine blades and other aerofoils, particularly in the inboard portion of the blade. Vortex generators are typically small, triangular projections that are mounted on the aerofoil to mix faster flowing air into the boundary layer at the blade surface. It is also known to arrange a secondary aerofoil or slat in spaced relation to the inboard portion of a wind turbine blade to suppress stall. The slat may be mounted on spaced supports having an aerodynamic profile, for example, as taught by DE102008026474 Al or US9151270 B2.

[0014] US20140286787 Al teaches a guide vane mounted in spaced relation to the inboard portion of a wind turbine blade. The guide vane is supported by a set of spaced apart fences to form a box structure defining a series of flow ducts. Each flow duct may converge in the flow direction.

[0015] EP2940292 Al teaches a device for reducing the sensitivity of a wind turbine blade to leading edge roughness without increasing its maximum lift coefficient. The device includes a flow guiding plate that is inclined or tapered with respect to the blade surface. The flow guiding plate may be mounted between side walls to form a channel. The flow guiding plate may comprise first and second sub-plates separated by a gap which generates a pair of counter-rotating vortices downstream of the device. Alternatively, the flow guiding plate and the side walls may form a closed channel such that the airflow is accelerated along its passage along the closed channel. The flow guiding plates of several devices can be connected directly together to form a continuous set of flow guiding devices.

[0016] It is a general objective of the present invention to provide a device that provides better performance when applied to an aerofoil, particularly on the blade of a horizontal axis wind turbine (HAWT), to energise the boundary layer.

[0017] Accordingly, the present invention provides a boundary layer energisation device for use on an aerofoil.

[0018] The device includes at least one device element which is mounted or mountable in a use position, downstream of the leading edge of the aerofoil in the flow direction, to extend from an aerofoil surface of the pressure side or suction side of the aerofoil.

[0019] The device element includes, in the use position, a pair of struts extending from the aerofoil surface, and a crosspiece extending between the struts, in spaced relation to a bounded portion of the aerofoil surface, to define a flowpath extending in the flow direction and bounded by the bounded portion of the aerofoil surface and by respective, inwardly facing surfaces of the crosspiece and the struts. When considered in a second plane (P2) perpendicular to the aerofoil surface and perpendicular to the flow direction, the flowpath is undivided, and the device element and the bounded portion of the aerofoil surface together form a closed boundary separating the flowpath from an outwardly open flow region exterior to the device element, and the crosspiece and the struts have outwardly facing surfaces opposite said inwardly facing surfaces, and the outwardly facing surfaces of the crosspiece and both of the struts are exposed to the outwardly open flow region.

[0020] The device may include a plurality of said device elements, which are spaced apart in series relation along a portion of the length of the aerofoil so that the respective outwardly facing surfaces of respective struts of each adjacent pair of device elements are arranged in confronting relation to define a gap between said respective outwardly facing surfaces. The gap has a minimum width Wg perpendicular to the flow direction between the respective outwardly facing surfaces.

[0021] The outwardly open flow region is common to all of said device elements and bounded by a portion of the aerofoil surface in the gap.

[0022] Further features and advantages will be apparent from the following illustrative embodiments which will now be described, purely by way of example and with reference to the accompanying drawings, in which:

[0023] Fig. 1 shows the hub and one rotor blade of a HAWT (horizontal axis wind turbine) rotor, having a first device mounted on its surface with the device elements aligned along the length of the blade.

[0024] Fig. 1A shows part of another HAWT rotor blade having a variant device in which the device elements are staggered in the airflow direction along the length of the blade.

[0025] Fig. 2 is a section in plane Pl through the rotor blade of Fig. 1, looking towards the tip of the blade.

[0026] Fig. 3 is an enlarged section in the same plane Pl as Fig. 2, looking towards the hub of the rotor, showing the first device and a small part of the blade. Fig. 4 is a section corresponding to Fig. 3 but taken in the plane P4 (Fig. 1) in the gap between two of the device elements.

[0027] Fig. 5 is an enlarged view of part of the first device, looking towards the suction side of the blade as shown in Fig. 1, but with the view rotated 180° with respect to the flow direction.

[0028] Fig. 5A shows a variant device including a baseplate on which the device elements are mounted.

[0029] Figs. 6 and 7 show part of the first device looking along the blade surface, respectively in the flow direction (Fig. 6) and against the flow direction (Fig. 7).

[0030] Fig. 8 is a section through an adjacent pair of device elements in the plane P2 of Figs. 1 and 3.

[0031] Fig. 9 shows one device element in the same view as Fig. 5, with the crosspiece partially cut away to show one of the struts sectioned in the plane P3a of Fig. 3.

[0032] Fig. 10 shows part of the device element in the same view as Fig. 9, illustrating how the aerofoil profile of the strut is twisted along the height of the flowpath.

[0033] Figs. 11A - 11D show various modifications to the leading and trailing edges of the crosspiece.

[0034] Figs. 12a and 12b show a CFD simulation of the airflow at the downstream end of one element of the first device.

[0035] Figs. 13a and 13b illustrate how paired, counter-rotating vortices are generated at the downstream end of the first device.

[0036] Figs. 14a, 14b and 14c show a CFD simulation of the airflow around the aerofoil, respectively with no devices (Fig. 14a), with conventional vortex generators (Fig. 14b), and with the first device (Fig. 14c).

[0037] Fig. 15 shows the conventional vortex generators used in the CFD simulation of Fig. 14b. Figs. 16a, 16b and 16c present the results of wind tunnel tests comparing the aerodynamic efficiency of the first device with the same prior art arrangements.

[0038] Figs. 17A - 17D show various modifications in which fins are arranged on the outwardly facing surfaces of the struts.

[0039] Reference numerals and characters that appear in more than one of the figures indicate the same or corresponding elements in each of them.

[0040] Definitions

[0041] In this specification, an aerofoil profile means a profile that is configured as known in the art to define a pressure side and a suction side so as to generate a pressure differential between those two sides, thus producing lift, when arranged in an airflow having a flow direction Da perpendicular to its length axis, from its leading edge to its trailing edge.

[0042] In this specification, a chord means a straight line between the leading and trailing edges of the profile of any element where the profile extends in the flow direction Da, as defined in the plane of that profile, irrespective of whether it is an aerofoil profile or not. The leading and trailing edges are the foremost and rearmost points of the profile with respect to the airflow direction in the plane containing the chord. If the profile is an aerofoil then the leading edge may be defined as the point of minimum radius facing against the airflow direction.

[0043] The chord of an aerofoil is defined as a straight line between the leading and trailing edges of the profile in the plane of the profile, which is perpendicular to the length axis of the aerofoil. A flatback aerofoil is an aerofoil where the profile is truncated to define a flat surface at the trailing edge, in which case the trailing edge may be defined as a point in the middle of that flat surface.

[0044] The flow direction Da is taken to be perpendicular to the length axis XI of the aerofoil (Fig. 1), hence in the plane Pl of the profile (the plane of the drawing in Fig. 2). In the case of a wind turbine, the flow direction Da will vary with the speed of the rotor and the speed of the oncoming wind. When considered in the plane Pl, the flow direction Da may be defined nominally at the optimal angle of attack a of the aerofoil, i.e. the angle of attack a between the chord 6 of the aerofoil and the flow direction Da at which the aerofoil is designed to generate maximum lift and minimum drag, i.e. the optimal lift-to-drag ratio L / D. In this specification the terms "upwards" and "downwards" are used as a convenient shorthand to mean "away from the aerofoil surface" and "towards the aerofoil surface", respectively.

[0045] A bounded portion 7' of the aerofoil surface is defined as that portion of the aerofoil surface 7 inbetween the struts 20 of a device element 10, forming part of the boundary of the flowpath 11.

[0046] The first plane Pl is defined to extend centrally through the device element 10 in the flow direction Da, perpendicular to the bounded portion 7' of the aerofoil surface (further explained below.)

[0047] The second plane P2 is defined as perpendicular to the first plane Pl.

[0048] Generally herein, unless otherwise stated, it is assumed that the second plane P2 is in a nominal position, perpendicular to the bounded portion 7' of the aerofoil surface 7, and bisecting the chord of the crosspiece (and so lies approximately mid-way along the flowpath in the flow direction). This definition may be convenient when the leading and trailing edges of the struts are approximately perpendicular to the surface 7 of the aerofoil 1 when viewed in the length direction of the aerofoil, as illustrated for example in solid lines in Fig. 4.

[0049] In alternative embodiments, insofar as the leading or trailing edges of the struts may be angled so as to slope height-wise in the flow direction Da to a degree that would preclude a closed section in the second plane P2 if defined in that nominal position, then the second plane P2 may be defined to intersect the chord of the crosspiece anywhere along its length, preferably also being perpendicular (or as nearly perpendicular as possible) to the bounded portion 7' of the aerofoil surface 7. If the struts are strongly angled along the airflow direction then the second plane P2 may be inclined to the bounded portion 7' of the aerofoil surface.

[0050] Each third plane P3a, P3b, P3c, P3d is defined to pass through the flowpath 11, parallel with the bounded portion 7' of the aerofoil surface on which the device is mounted.

[0051] Of course, the bounded portion 7' of the aerofoil surface will normally have curvature at least in the airflow direction Da, and so the third plane P3a, P3b, P3c, P3d is defined as a flat plane, as near as possible parallel with the bounded portion 7' of the aerofoil surface in the region mid-way between the struts 20 and mid-way along the length of the chord 46 of the crosspiece 40.

[0052] The fourth plane P4 is a plane of symmetry extending centrally through the gap 50 between adjacent device elements 10 in the flow direction Da, perpendicular to the aerofoil surface 7" (and so parallel or nearly parallel with the first plane Pl.)

[0053] For ease of description, the confined, more energetic, higher velocity and lower pressure flow leaving the trailing end 13 of the flowpath will be referred to as F. The unconfined, relatively lower energy, higher pressure air in the outwardly open flow region 101 in the boundary layer of the aerofoil will be referred to as O.

[0054] The aerofoil

[0055] Referring to Figs. 1 and 2, a horizontal axis wind turbine rotor 110 includes three blades 111, one of which is shown in the drawings, mounted on a hub 112 for rotation about a nominally horizontal axis X2. Each blade defines an aerofoil 1 extending radially outwardly for a length LI along a length axis XI from a blade root 113 at the hub to a blade tip 114.

[0056] The aerofoil 1 defines a surface 7, a leading edge 2, a trailing edge 3, a suction side 4, a pressure side 5, and a chord 6 having a length Ca. The chord 6 of the aerofoil 1 is a straight line between the leading and trailing edges 2, 3 of the aerofoil 1 perpendicular to its length axis XI.

[0057] The profile of the blade 111 varies along the length of the blade and can be defined in any plane perpendicular to the length axis XI, but is shown by way of example in Fig. 2 in the plane Pl of Fig. 1.

[0058] The device

[0059] Referring also to Figs. 3 - 8, the aerofoil 1 includes a boundary layer energisation device 100 which is mounted or mountable in a use position as shown.

[0060] The device 100 includes at least one device element 10, and optionally (as shown) a plurality of said device elements 10. The device 100 or each device element 10 is mounted or mountable in a use position as shown, downstream of the leading edge 2 of the aerofoil 1 in the flow direction Da, to extend from the surface 7 of the pressure side 5 or suction side 4 of the aerofoil 1. In the illustrated example it is arranged on the suction side 4.

[0061] The device 100 (including one or more device elements 10) may be formed as an integral part of the aerofoil 1 or may be formed as a retrofit accessory, in which case it may or may not include the bounded portion 7' of the aerofoil surface. For example, if retrofitted, the device 100 may include a baseplate as shown in Fig. 5A so that, after fitting, the surface of the baseplate defines the bounded portion 7' of the aerofoil surface. Alternatively, the struts 20 could be mounted directly on the aerofoil surface 7. The device 100 may be fitted by adhesive bonding or mechanical fasteners or otherwise as known in the art.

[0062] When considered in its use position, the or each device element 10 includes a pair of struts 20 that extend from the aerofoil surface 7, and a crosspiece 40 that extends between the struts 20, in spaced relation to a bounded portion 7' of the aerofoil surface 7, to define a flowpath 11. The flowpath 11 extends in the flow direction Da and is bounded by the bounded portion 7' of the aerofoil surface and by respective, inwardly facing surfaces 48, 28 of the crosspiece 40 and the struts 20.

[0063] When considered in the first plane Pl, the crosspiece 40 defines a leading edge 42, a trailing edge 43, and a chord 46.

[0064] The crosspiece 40 may have an aerofoil profile, or could just be aerodynamically shaped (e.g. straight, flat, thin and smooth) to reduce drag without generating lift. Irrespective of the profile of the crosspiece, the chord 46 is defined as a straight line between the leading and trailing edges 42, 43 of the crosspiece 40, and has a length Cc. (If the leading or trailing end of the crosspiece is flat rather than radiused, then the respective leading or trailing edge 42, 43 can be taken to be the point in the middle of the flat edge.)

[0065] As shown in Fig. 3, the flowpath 11 has a height H in the first plane Pl between the inwardly facing surface 48 of the crosspiece 40 and the bounded portion 7' of the aerofoil surface.

[0066] The flowpath As best seen in Fig. 8, when considered in the second plane P2 (the plane of the figure) perpendicular to the first plane Pl, the flowpath 11 is undivided, and the device element 10 and the bounded portion 7' of the aerofoil surface together form a closed boundary separating the flowpath 11 from an outwardly open flow region 101 exterior to the device element 10.

[0067] By outwardly open flow region is meant that the flow region 101 (when considered in the second plane P2) is not enclosed within a boundary, but is in fluid communication (in this plane) with open space around the aerofoil 1.

[0068] In the second plane P2 it can also be seen that the crosspiece 40 and the struts 20 have outwardly facing surfaces 49, 29 opposite their inwardly facing surfaces 48, 28, and the outwardly facing surface 49 of the crosspiece 40, and the outwardly facing surfaces 29 of both of the struts 20, are exposed to (in fluid communication with) the outwardly open flow region 101.

[0069] As best seen in Fig. 3, the flowpath 11 extends in the flow direction Da from a leading end defining a front opening 12, formed by the leading edge 42 of the crosspiece and the leading edges 22 of the struts, to a trailing end defining a rear opening 13, formed by the trailing edge 43 of the crosspiece and the trailing edges 23 of the struts, so that the airflow enters the flowpath through the front opening 12 and exits through the rear opening 13.

[0070] The section area of the flowpath 11 perpendicular to the flow direction Da may be constant or may vary in the flow direction Da. A modest convergence in the flow direction Da will help to increase local velocity and so enhance performance. This can be provided by arranging for the height H of the flowpath 11 to reduce progressively over part or all of the length of the flowpath 11 in the flow direction Da.

[0071] However, too much convergence will cause the device element 10 to obstruct rather than channel the airflow, resulting in a net reduction in performance due to increased drag and exacerbating the stall.

[0072] For this reason, preferably, the height H in the first plane Pl between the inwardly facing surface 48 of the crosspiece 40 and the bounded portion 7' of the aerofoil surface, when considered along the flow direction Da between the leading and trailing edges 42, 43 of the crosspiece 40, is invariate or varies by not more than + / - 20% of the crosspiece chord length Cc. Preferably, for optimal performance, the length Cc of the crosspiece chord 46 in the first plane Pl is from 0.01 Ca to 0.20 Ca, when the length of the aerofoil chord Ca is defined in the first plane Pl. Relatively little additional lift is obtained by a chord length smaller than this range, while a longer chord adds relatively little further benefit in terms of aerodynamic efficiency.

[0073] Preferably, the average height H of the flowpath in the first plane Pl is from 0.02 Ca to 0.08 Ca, when the length of the aerofoil chord Ca is defined in the first plane Pl. This helps to optimise the improvement in aerodynamic efficiency (Cl / Cd) over a wide range of angle of attack. If the height H is too small then the flow entering the flowpath will have relatively little additional energy compared with the open flow region, and this little additional energy will be lost as it travels through the flowpath. If the height H is too large then the energetic airflow leaving the flowpath will have relatively little effect on the boundary layer.

[0074] As mentioned above, the confined, more energetic, higher velocity and lower pressure flow leaving the trailing end 13 of the flowpath will be referred to as F. The unconfined, relatively lower energy, higher pressure air in the outwardly open flow region 101 will be referred to as O.

[0075] Importantly, the flowpath 11 is undivided when considered in the second plane P2. In consequence, the pressure differential between F and O appears across each of the struts 20, and so each of the struts 20 will generate a vortex at its trailing edge 23 which helps to energise the boundary layer.

[0076] This offers a peformance advantage over prior art arrangements that provide an elongate leading edge slat or secondary aerofoil supported by multiple struts that are streamlined in the airflow direction Da, thus dividing the slot (the aperture formed below the slat or secondary aerofoil) into multiple contiguous flowpaths separated by the struts. In such prior art arrangements, since there is no pressure differential across each of the dividing struts, the dividing strut will add drag without contributing a vortex to help energise the boundary layer, so that the performance benefit of the slat is offset by the added drag of the struts.

[0077] The leading and / or trailing edges of the device element 10 at any or all of the crosspiece 40 and the struts 20 (and the radiused junctions 70 mentioned below, if present) may be radiused when considered in their thickness dimension, to reduce drag. For optimal aerodynamic efficiency (Cl / Cd) over a wide range of angle of attack, preferably the entire leading edge 42, 22 of the device element 10 is radiused in the thickness dimension of each part with a radius from 0.3% to 5% of the length Cc of the chord 46 of the crosspiece 40.

[0078] When the blade is close to stall, as long as the flowpath is not too convergent, the momentum of the airflow entering the slot is maintained as it travels through the flowpath (which, if somewhat convergent, will moderately accelerate the flow); whereas the momentum of the airflow in the boundary layer in the gap 50 is lost, because unconfined in the outwardly open flow region. The retained momentum of the confined airflow is then injected into the stagnant boundary layer as the more energetic, lower pressure airflow F exits the flowpath 11 at the trailing end 13 of the device element 10.

[0079] This can be achieved even if, for example, the inwardly facing surfaces 48, 28 of the crosspiece and the struts are all flat and straight (or parallel with the aerofoil surface 7) in the flow direction Da, and the flowpath 11 has a constant section in the flow direction Da.

[0080] Preferably however, the geometry of the device element is enhanced for greater effect.

[0081] By making the flowpath 11 somewhat convergent in the flow direction Da, a modest improvement is obtained.

[0082] A more significant improvement is obtained by providing the crosspiece 40 or the struts 20, and preferably both the crosspiece 40 and the struts 20, with an aerofoil profile. A more significant improvement is also obtained by providing the struts 20 with a twist.

[0083] Most preferably, for optimal effect, both the crosspiece 40 and the struts 20 are provided with an aerofoil profile, and the struts 20 are provided with a twist, since the aerodynamic effects of these combined features are synergistic.

[0084] Preferably therefore, as illustrated, when considered in the first plane Pl, the crosspiece 40 defines an aerofoil profile having a suction side 44 and a pressure side 45 extending between its leading and trailing edges 42, 43, wherein the chord 46 is a chord of the profile. The suction side 44 is configured to generate reduced pressure relative to the pressure side 45 in the airflow along the crosspiece 40.

[0085] Preferably, for optimal aerodynamic efficiency (Cl / Cd), the aerofoil profile of the crosspiece 40 may have a maximum thickness from 5% to 21% of the length Cc of its chord 46, wherein the maximum thickness is defined in the first plane Pl perpendicular to its chord 46. (For reference, the corresponding line 8 of maximum thickness of the aerofoil 1 is shown in Fig. 2.)

[0086] Preferably, for optimal aerodynamic efficiency (Cl / Cd), the aerofoil profile of the crosspiece 40 may have a camber from 2% negative camber to 8% positive camber. The camber is defined as the maximum deviation of the camber line of the profile (the locus of points mid-way between its pressure and suction side surfaces 44, 45) from its chord 46, expressed as a percentage of the length Cc of the chord 46 of the crosspiece. The camber is considered to be positive if the camber line lies between the chord 46 and the suction side 44 of its profile, and negative if the camber line lies between the chord 46 and the pressure side 45 of its profile.

[0087] Irrespective of which side of the aerofoil 1 the device 100 is mounted on, the pressure side 45 of the crosspiece profile forms the inwardly facing surface 48 of the crosspiece 40, i.e. the surface facing the bounded portion 7' of the aerofoil surface on which the device element 10 is mounted. The inwardly facing surface 48 controls the momentum of the flow through the flowpath 11, compressing the boundary layer as the flow exits the flowpath, and entraining more distant, high energy flow closer to the aerofoil surface. The outwardly facing suction side 44 of the crosspiece profile helps to avoid flow separation (stall) in the airflow over the suction side surface 49, which otherwise could be an additional source of drag.

[0088] When considered in the second plane (P2), the crosspiece 40 may blend into each of the struts at a radiused junction 70 (which can be considered part of the strut.) Hence, the crosspiece terminates at each end of the crosspiece at the radiused junction 70 with the respective strut, with both the inwardly and outwardly facing surfaces of the crosspiece and strut (including their aerofoil profiles, if present) being blended and radiused at the junction 70. This reduces drag. The inwardly facing surface 71 of the junction 70 is radiused with an internal bend radius r about a point in the flowpath 11. The value of r can be constant or may vary along the bend, and can be any value greater than 0%, up to 100% of the height H of the flowpath 11. A larger bend radius r results in lower drag as compared with a smaller radius. The twisted part of the strut 20 may terminate at the beginning of the bend radius r, and hence a larger bend radius r may locate the point of vortex generation closer to the aerofoil surface. The bend radius r may be selected to locate the vortices at the desired height, depending on where on the blade the device element 10 is mounted.

[0089] When considered in the second plane P2, a central portion 41 of the crosspiece 40 may extend in parallel with the bounded portion 7' of the aerofoil surface for a width Wcc (and so terminates at the radiused junctions 71, if present.) Preferably, the width Wcc of the central portion 41 of the crosspiece is from 50% to 300% of the length Cc of the crosspiece chord 46 (i.e. Wcc is from 0.5 Cc to 3 Cc.) This helps to optimise the improvement in aerodynamic efficiency (Cl / Cd) over a wide range of angle of attack a. For a more powerful and stable vortex generating effect, a width Wcc towards the lower end of the range may be selected to enhance the interaction between the counter-rotating vortices shed from the struts of the respective device element 10. Relatively little interaction is obtained above the upper end of the range.

[0090] Struts: profile and twist

[0091] The struts 20 of the or each device element 10 may be mirror-symmetric about the first plane Pl.

[0092] Alternatively or (preferably) in addition to the aerofoil profile of the crosspiece 40, each of the struts 20 may define an aerofoil profile when considered in a third plane P3a, P3b, P3c, P3d, parallel with the bounded portion 7' of the aerofoil surface on which the device 100 is mounted.

[0093] The third plane can be defined mid-way between the inwardly facing surface 48 of the crosspiece 40 and the bounded surface 7' of the aerofoil 1 (so in the position of P3b in Fig. 3), or at any other position along the height H of the flowpath 11.

[0094] When considered in the third plane, the profile of the strut 20 defines a leading edge 22, a trailing edge 23, and a chord 26 which is a straight line between the leading and trailing edges 22, 23.

[0095] If the profile of the strut 20 is an aerofoil profile then it further defines a suction side 24 and a pressure side 25, as shown. The suction side 24 of the aerofoil profile is configured to generate reduced pressure relative to the pressure side 25 in the airflow over the strut 20. This arrangement will now be described with reference to Figs. 3, 9 and 10.

[0096] Fig. 3 shows four mutually parallel third planes P3a, P3b, P3c, P3d, all parallel with the bounded portion 7' of the aerofoil surface and passing through the flowpath 11. The four planes are spaced apart in the dimension of the height H of the flowpath 11 from the uppermost plane P3a (which is to say, the plane closest to the to the crosspiece 40) to the lowest plane P3d (which is to say, the plane closest to the aerofoil surface 7 at the base of the strut 20.)

[0097] Fig. 10 shows the aerofoil profile of the strut 20 in each of the four third planes P3a, P3b, P3c, P3d, and for ease of reference, each profile is identified by its respective plane. Since the chords 26 of the profiles are very close together, they are projected out as lines 26' and indicated simply as a, b, c and d, indicating respectively the profile of the respective third plane P3a, P3b, P3c, P3d in which each one is defined.

[0098] Fig. 9 shows the profile and chord 26 only in the uppermost third plane P3a.

[0099] When considered in each of two, upper and lower third planes (for example, P3b and P3d, Fig. 3), wherein the upper third plane P3b is defined between the crosspiece 40 and the lower third plane P3d, each of the struts 20 defines a leading edge 22, a trailing edge 23, and a chord 26 which is a straight line between its leading and trailing edges 22, 23.

[0100] As shown in Fig. 10, a twist angle A2 is defined between the chord 26 of the respective strut 20 in the lower third plane P3d and the chord 26 of the same respective strut 20 in the upper third plane P3b when projected onto the lower third plane P3d.

[0101] The twist angle A2 is greater than 0° and not more than 10°.

[0102] It can be seen that the twist is progressive in the height H dimension of the flowpath 11 from each of the third planes to the next. The twist may extend from the base of the strut 20 through all or part of the height H of the flowpath 11. The proportion of the height H of the flowpath through which the twist extends will affect the strength and position of the generated vortex V and the aerodynamic effectiveness of the device element 10. When considered in the airflow direction Da and upwards away from the aerofoil surface, the strut 20 may twist either inwards towards the closed domain or outwards towards the open domain. Thus, the respective chords 26 of the strut 20 as defined in the respective third planes may be divergent or convergent in the flow direction Da, depending on the direction of the twist which will affect the direction of the vortex V shed from the strut 20.

[0103] In alternative embodiments, the strut 20 may be formed with a twist as shown, but without an aerofoil profile. In further alternative embodiments, the strut 20 may have an aerofoil profile but no twist. For optimal effect however, both these features may be combined as shown.

[0104] If the struts 20 have an aerofoil profile then the pressure side 25 of that profile can form either the inwardly facing surface 28 (as illustrated) or the outwardly facing surface 29 of the strut 20, depending on the desired rotation direction of the vortex V as further discussed below.

[0105] For optimal vortex generation and aerodynamic effectiveness, the aerofoil profile of the strut 20 in the lowest third plane P3d at the base of the strut may have a camber of not more than 6% negative camber to 6% positive camber. The camber is defined as the maximum deviation of the camber line of the profile (the locus of points mid-way between its pressure and suction side surfaces 24, 25) from its chord 26, expressed as a percentage of the length Cc of the chord 46 of the crosspiece 40. The camber is considered to be positive if the camber line lies between the chord 26 and the suction side 24 of its profile, and negative if the camber line lies between the chord 26 and the pressure side 25 of its profile.

[0106] For optimal vortex generation and aerodynamic effectiveness, the maximum thickness of the aerofoil profile of the strut 20 in the lowest third plane P3d at the base of the strut 20, perpendicular to its chord 26, may be within + / - 21% of the length Cc of the chord 46 of the crosspiece 40 as defined in the central plane Pl.

[0107] Referring now to Fig. 4, for optimal vortex generation and aerodynamic effectiveness, the length Cs of the chord 26 of the aerofoil profile of the strut 20 in the lowest third plane P3d at the base of the strut 20 may be from 50% to 200% of the length Cc of the chord 46 of the crosspiece 40 as defined in the central plane Pl. Fig. 4 illustrates the leading and trailing edges 22, 23 of the strut 20 in a generally perpendicular configuration where the length of the chords 26, 46 is approximately equal.

[0108] For optimal vortex generation and aerodynamic effectiveness, the length of the chord 26 of the strut 20 is preferably within the range from approximately 200% Cc in the alternative trailing edge position 23', to approximately 50% Cc in the alternative trailing edge position 23".

[0109] Fig. 4 also illustrates how the trailing edge 23 of the strut 20 can alternatively be angled to slope in the flow direction Da, either downwardly 23' from the trailing edge 43 of the crosspiece 40 to the aerofoil surface 7, or upwardly 23" from the aerofoil surface 7 to the trailing edge 43 of the crosspiece 40 so that the trailing edge 43 of the crosspiece 40 overhangs the leading end of the strut 20 when considered in the airflow direction Da. The trailing edge of the strut may also be curved to follow the expected boundary layer velocity profile, for example, as illustrated by alternative trailing edge positions 23'", 23"".

[0110] Device element dimensions, orientation, and location on the aerofoil surface

[0111] The device 100 or device element 10 can be located on the suction side 4 of the aerofoil 1, as exemplified by the illustrated embodiments. Alternatively, it can be located on the pressure side 5 of the aerofoil 1.

[0112] Preferably, when used on a wind turbine blade, the or each device element 10 is located on either the suction or pressure side of the blade in a region where the aerofoil maximum thickness 8 (i.e. the maximum dimension of the aerofoil 1 perpendicular to the aerofoil chord Ca in the first plane Pl) lies in the range from 75% Ca to 18% Ca. In a large blade this may be radially outwardly from a distance of about 2m from the blade root (roughly the position of boundary line 115 in Fig. 1.)

[0113] Preferably, when used on a wind turbine blade, the leading edge 42 of the crosspiece 40 of the or each device element 10 in the first plane Pl is located in a range from 9% to 40% of the aerofoil chord length Ca from the leading edge 2 of the aerofoil 1 (the distance DI in Fig. 2) - which is to say, when considered in the first plane Pl, between respective lines projected from those points perpendicular to the chord 6 of the aerofoil 1. This provides the best improvement in aerodynamic efficiency (Cl / Cd, coefficient of lift over coefficient of drag.) Referring to Fig. 2, the angle of attack Al of the crosspiece 40 is defined with respect to the orientation of the aerofoil by an angle Al in the first plane (Pl) between the chord 46 of the crosspiece 40 and the chord 6 of the aerofoil 1. (The chords are projected in the figure as lines 46' and 6' respectively, for ease of illustration.) Preferably, the angle Al lies in a range from a negative angle Al of -30° through a parallel orientation of 0° to a positive angle Al of +15°, wherein the angle Al is defined as negative if the chords 46, 6 are divergent in the airflow direction Da, as zero if the chords 46, 6 are parallel in the airflow direction Da, and as positive if the respective chords 46, 6 are convergent in the airflow direction Da. A value in the preferred range helps to optimise the improvement in aerodynamic efficiency (Cl / Cd) over a wide range of angle of attack a of the aerofoil 1.

[0114] Series arrangement

[0115] As illustrated, the device 100 may include a plurality of device elements 10, for example, at least three device elements 10.

[0116] Referring particularly to Figs. 5, 6 and 7, the device elements 10 are spaced apart in series relation along a portion of the length LI of the aerofoil 1 so that the respective outwardly facing surfaces 29 of respective struts 20 of each adjacent pair of device elements 10 are arranged in confronting relation to define a gap 50 between the respective pair of outwardly facing surfaces 29. The gap 50 has a minimum width Wg perpendicular to the flow direction Da. The minimum width Wg may be defined in the second plane P2, particularly if the respective adjacent struts 20 are mirror- symmetric about the plane of symmetry P4 as further discussed below.

[0117] The device elements 10 may be aligned along the plane P2 which may be common to all the device elements 10 in the row and may extend approximately centrally through all of them in series, e.g. bisecting the chord 46 of each crosspiece 40 as shown in Fig. 1.

[0118] Alternatively, the device elements 10 may be staggered or offset in the airflow direction along the length of the aerofoil, in which case the plane P2 may be defined individually for each device element or for each pair or group of device elements, as shown in Fig. 1A.

[0119] The outwardly open flow region 101 is common to all of the device elements 10 and bounded by (i.e. in fluid communication with) a portion 7" of the aerofoil surface 7 in the gap 50. Adjacent struts 20 of each adjacent pair of device elements 10 may be mirror-symmetric about a plane of symmetry P4 extending centrally through the gap 50 in the flow direction Da, perpendicular to the aerofoil surface 7" (and so parallel or nearly parallel with the first plane Pl.)

[0120] Where multiple device elements 10 are arranged in series to form a gap 50 between the confronting surfaces 29 of their adjacent struts 20, the vortices V generated at the trailing edges 23 of the struts 20 will entrain the relatively higher energy air further away from the aerofoil surface and bring it closer to the boundary layer close to the surface 7" of the aerofoil 1. Where the struts 20 are mirror-symmetric (or would be mirror-symmetric if aligned along the airflow direction), they will generate vortices V that rotate in opposite directions.

[0121] Ceteris paribus, the longer the chord length Cc of the crosspiece 40, hence the longer the flowpath 11 in the flow direction Da, the greater will be the pressure differential between F and O (because the energy of the confined airflow is maintained, while the energy of the stagnant airflow O in the boundary layer declines in the flow direction Da) - and so, the more powerful the vortex V.

[0122] The width Wg of the gap 50 may be from 0.25 CC to 2 Cc. In this range, it is found that the counter-rotating vortices V will develop a stable circulation that helps them to cling to the blade surface 7, so that they more effectively entrain the relatively higher energy air further away from the aerofoil and bring it closer to the aerofoil boundary layer.

[0123] Fig. 5A illustrates a variant device including a baseplate 102. As illustrated, a set of two, three, four or more device elements 10 may be mounted on a common baseplate 102 to form a single device, defining a gap 50 between each adjacent pair of device elements. Alternatively a single device element 10 could be mounted on an individual baseplate. The whole device 100 may be formed as a unitary body, e.g. by moulding or 3D printing. The baseplate 102 is mounted on the wind turbine blade or other element that defines the aerofoil so that the upper surface of the baseplate becomes part of the aerofoil surface 7 and defines the bounded portion 7' of the aerofoil surface within each device element 10. The baseplate may be retrofitted by mounting it directly on the original aerofoil surface so that, in use, it defines a new, slightly raised portion of the aerofoil surface. Alternatively, the baseplate may be inset into a shallow recess in the aerofoil so that the original surface profile of the aerofoil remains unchanged. The baseplate 102 may be attached to the blade by adhesive bonding or mechanical fasteners as known in the art.

[0124] Vortex generation

[0125] A conventional vortex generator creates a vortex about a free edge that projects into the airflow and extends in the airflow direction. Irrespective of whether the vane defining the free edge is arranged in parallel with the aerofoil surface or perpendicular to the aerofoil surface, a pressure differential across the vane will be relieved by a flow that includes a rotational component in a direction around the free edge away from the aerofoil surface (so for example upwards, if the aerofoil surface is facing upwards.) This away or upward component of the flow means that the vortex is formed partly above the free edge. The vortex is shed from the trailing corner of the free edge which is spaced above the blade surface, and so tends to move away from the aerofoil surface and so out of the boundary layer.

[0126] In contrast, the novel device element 10 generates vortices that are generated at the trailing edge 23 of each strut 20.

[0127] Figs. 12a and 12b show the pressure differential generated across the trailing edges 23 of the struts 20 in three-dimensional CFD (Computational Fluid Dynamics) simulations, looking at the trailing end 13 of the device element 10, so against the airflow direction Da. The contour or gradient shows the Lambda 2 criterion, which reflects the vorticity in the flow.

[0128] The vortex V is produced by the airflow downstream of the trailing edge 23 (so out of the plane of the drawing.) The struts 20 are mirror-symmetric about the central plane Pl, but the aerofoil profile and twist angle of the struts 20 is adjusted between the two simulations to change the direction of vorticity from clockwise to counter-clockwise.

[0129] The struts 20 generate uniform, steady and quantifiable vorticity into the flow as shown in the image.

[0130] Figs. 13a and 13b show how the mirror-symmetric struts 20 can generate paired, counter-rotating vortices V in the gap 50, again in either direction as desired (shown respectively in Figs. 13a and 13b) depending on the aerofoil profile and twist angle of the struts 20. The vortices are shed from the trailing edge of each strut 20. If the strut 20 is twisted, then the twist will introduce a transverse vector component into the airflow and so the vortex will tend to be seeded at the point where that transverse flow occurs, which may be a little above the aerofoil surface as illustrated. If the strut is not twisted, then a relatively weaker vortex will be shed close to the aerofoil surface where the difference in velocity within the boundary layer is greatest.

[0131] Since the airflow F is confined within the flowpath 11 between the struts 20, its speed is higher and so its pressure is lower than the less energetic air O in the boundary layer of the outwardly open flow region 101.

[0132] The vortex V is initiated at each strut 20 by the pressure differential between F (the confined, more energetic, higher velocity and lower pressure flow leaving the trailing end 13 of the flowpath 11) and O (the unconfined, relatively lower energy, higher pressure air in the boundary layer of the outwardly open flow region 101.)

[0133] At the trailing end 13 of the device element 10, this pressure differential causes an initial flow from the higher pressure side to the lower pressure side, which in turn causes a balancing flow in the opposite direction, thus initiating the vortex V.

[0134] The speed of the airflow is lowest, hence pressure is highest, at the aerofoil surface 7" in the boundary layer of the outwardly open flow region 101. Hence, the pressure differential between F and O will tend to be greater across the trailing end 23 of the strut 20 (in the length direction of the aerofoil surface) than across the trailing edge 43 of the crosspiece (in the direction away from the aerofoil surface). For this reason, a pair of vortices V are shed from the trailing edges 23 of the two struts 20 of the device element 10, typically near the baseplate or in the region where the strut is twisted, as discussed above.

[0135] Since each vortex V is generated by the pressure differential between O and F, wherein F is the flow leaving the flowpath 11 from below the crosspiece 40, it tends to be closer to the aerofoil surface 7 than would be the case with a vortex generated by the free edge of a conventional vortex generator. Since it is closer to the aerofoil surface 7, it remains in the boundary layer for longer (ceteris paribus) and so is relatively more effective in energising the boundary layer. In addition, the effect of the crosspiece is to compress the boundary layer, further pushing the vortices closer to the aerofoil surface. The vortices V mix the moving (higher energy) airflow F into the boundary layer, thus energising the boundary layer and preventing or delaying flow separation (stalling) by moving the flow separation (stall) point towards the trailing edge 3 of the aerofoil 1.

[0136] If the struts 20 and the crosspiece 40 are all configured as flat plates with no twist and aligned with the flow direction Da, then the vortex V will tend to be initiated with a direction of rotation that is downwards towards the aerofoil surface 7" on the outwardly facing side 29 of the strut 20, i.e. downwards towards the aerofoil surface 7" in the region immediately downstream of the gap 50 in a series arrangement of the device elements 10.

[0137] The strength and direction of the vortices V can be controlled by providing the struts 20 with an aerofoil profile and / or a twist, as discussed above.

[0138] The orientation, aerofoil profile and twist of the mirror-symmetric struts 20 may be configured to produce the counter-rotation of both vortices V in the region downstream of the gap 50 in either direction as desired, either upwards away from the aerofoil surface 7 or downwards towards the aerofoil surface 7. Upward rotation can be selected to help improve peak efficiency, while downward rotation can be selected to provide a slightly lower peak efficiency but a smoother stall behaviour.

[0139] In use, the novel device 100 reduces flow separation in the inboard part of a wind turbine blade 111 by reducing pressure drag. The device compresses the boundary layer and pushes high energy air F towards the blade surface 7. At the same time, the struts 20 shed small vortices V that entrain additional high energy flow near the blade surface 7. In combination, these effects guide the flow to better follow the shape of the blade 111 against adverse pressure gradients and thereby generate more power.

[0140] Apart from the effect of energising the boundary layer, the novel device 100 helps to entrain additional air mass into the flow over the aerofoil and so can be employed to increase lift.

[0141] The benefit is maximised by deploying the device at the optimal position on the airfoil 1. On a

[0142] HAWT blade this will generally be in the inboard portion of the blade 111, nearer the root 113 than the tip 114, where the novel device tends to improve overall aerodynamic efficiency (lift to drag ratio) by both reducing drag and increasing lift.

[0143] Figs. 14a, 14b and 14c show the results of three CFD (Computational Fluid Dynamics) simulations of the aerofoil 1 as seen in the first plane Pl, looking towards the tip 114 of the blade, so the same thick aerofoil profile in the same view as Fig. 2. Each figure shows the total energy of the airflow, presented as a gradient where the region E- has the lowest energy and the region E+ has the highest energy. The angle of attack a of the aerofoil 1 was high at 15° in each of the three simulations.

[0144] In Fig. 14a the aerofoil 1 is shown on its own without any device fitted.

[0145] In Fig. 14b the aerofoil 1 has a row of conventional, industry standard vortex generators 140, optimised for the aerofoil.

[0146] For reference, Fig. 15 shows the conventional vortex generators 140 of Fig. 14b in three different views. The conventional vortex generators 140 are configured as triangular plates having a height 1401 of about 0.5 to 1% of the length Ca of the chord 6 of the aerofoil, sloping upwardly from the surface 7 in the airflow direction Da and inclined alternately in opposite directions by an optimal angle in the length direction of the aerofoil 1. The vortex generators 140 were spaced apart in the length dimension of the aerofoil 1 by distances 1402 and 1403 in the range of about two to six times the height 1401.

[0147] In Fig. 14c the aerofoil 1 is shown with the same novel device 100 configured as a row of device elements 10 and fitted in the same position as shown in Fig. 2.

[0148] It can be seen that the novel device 100 (Fig. 14c) provides a substantially higher energy in the airflow close to the surface 7 of the aerofoil 1 when compared with the plain aerofoil 1 (Fig. 14a) and the same aerofoil 1 with conventional vortex generators 140 (Fig. 14b), resulting in improved aerodynamic performance.

[0149] Figs. 16a, 16b and 16c present the results of wind tunnel tests comparing three different result sets, all based on the same thick aerofoil profile as shown in Fig. 2. In each figure:

[0150] Result set (a) represents the aerofoil 1 on its own without any device fitted, the same as in Fig. 14a.

[0151] Result set (b) represents the aerofoil 1 fitted with conventional, industry standard vortex generators 140, the same as in Figs. 14b and 15.

[0152] Result set (c) represents the aerofoil 1 fitted with the novel device 100, the same as in Fig. 14c.

[0153] Fig. 16a shows the coefficient of lift Cl relative to the angle of attack a of the aerofoil 1.

[0154] Fig. 16b shows the coefficient of drag Cd relative to the angle of attack a of the aerofoil 1.

[0155] Fig. 16c shows the lift to drag ratio Cl / Cd relative to the angle of attack a of the aerofoil 1.

[0156] It can be seen that the aerofoil 1 fitted with the novel device 100 develops more lift than the aerofoil 1 either with or without conventional vortex generators 140, across the whole (wide) range of angle of attack a covered by the tests.

[0157] The novel device 100 also develops less drag than the conventional vortex generators 140, and in part of the range of angle of attack a even reduces the drag compared with the aerofoil 1 on its own.

[0158] In combination, this delivers a significant improvement in the lift to drag ratio Cl / Cd over the whole range of angle of attack a.

[0159] Modifications

[0160] Referring now to Figs. 11A - 11D, the leading and trailing edges 42, 43 of the crosspiece 40 need not be straight in the length dimension of the aerofoil 1, as shown in the other figures. Instead, each of the device elements may be modified with a leading or trailing edge formed, for example, as shown in a respective one of Figs. 11A - 11D.

[0161] Figs. 11A and 11B illustrate how the trailing edge 43 of the crosspiece 40 of each of the device elements 10 may extend in the airflow direction Da, from each of the struts 20, to terminate at a pointed extremity 60. On either side of the pointed extremity 60 the trailing edge 43 may be straight (Fig. 11B) or concave (Fig. 11A).

[0162] Fig. 11C illustrates how the trailing edge 43 of the crosspiece 40 of each of the device elements 10 may be serrated, defining a row of contiguous serrations 61.

[0163] Fig. 11D illustrates how the leading edge 42 of the crosspiece 40 of each of the device elements 10 may be scalloped, defining a row of contiguous convex portions 62, each extending against the flow direction Da to an rounded extremity 63.

[0164] In each example, the size and spacing of the projecting elements can be varied to optimise performance.

[0165] As best seen in Fig. 8, when considered in the second plane P2, an outwardly facing boundary 49, 29 of the or each device element 10, terminating at the aerofoil surface 7, may be substantially convex without re-entrant surfaces.

[0166] Alternatively, as exemplified by the further alternative embodiments of Figs. 17A - 17D, a respective fin 80 may be arranged to project outwardly from the outwardly facing surface 29 of each respective one of the struts 20, optionally from the radiused portion 70 where the strut blends 20 into the crosspiece 40, so defining re-entrant surfaces on the outwardly facing profile of the device element 10.

[0167] The fins 80 are spaced apart from the aerofoil surface 7 and may be arranged at a height (in the height dimension H of the device element 10) selected to seed additional vortices. The fins 80 may be shaped and optionally twisted in the outward direction away from the strut 20 so as to control or enhance the vorticity created by the struts 20. The projections 80 may have a leading edge that is straight or swept forwardly or (as shown in Fig. 17A) backwardly with respect to the flow direction Da.

[0168] Figs. 17B - 17D show the fins 80 as seen from the rear of the device element 10.

[0169] The fins 80 may extend in parallel with the aerofoil surface 7, as shown in Fig. 17B. Alternatively, the fins 80 may be twisted in the outward direction away from the strut 20, as illustrated in Fig. 17C.

[0170] Alternatively, the fins 80 may slope downwardly towards or, as illustrated in Fig. 17D, upwardly away from the aerofoil surface 7 as they extend away from the struts 20 along the length dimension of the aerofoil 1.

[0171] In summary, an aerofoil 1 is provided with a boundary layer energisation device 100 comprising at least one element 10 having a crosspiece 40 extending between a pair of struts 20 in spaced relation to the aerofoil surface 7 to define a closed boundary surrounding a flowpath 11. The flowpath 11 is open in the airflow direction Da and separated by the closed boundary from an outwardly open flow region 101 exterior to the device element 10. The device 100 may include a set of such elements 10 spaced apart in series along the length dimension of the aerofoil 1 to generate a pair of counter-rotating vortices V that energise the air flowing through the gap 50 between each adjacent pair of elements 10.

[0172] Although the novel device 100 is described herein with particular reference to its effect in energising the boundary layer, hence delaying or preventing stall at increased angles of attack a, it also serves to augment mass flow over the aerofoil 1 by entraining more air mass and so for this reason also enhances the aerodynamic efficiency (Cl / Cd, coefficient of lift over coefficient of drag) of the aerofoil 1 or wind turbine rotor 110.

[0173] A single device element 10 can be used on its own, but preferably then is augmented by an aerofoil profile(s) and / or twisted struts as further explained herein so as to have a more powerful energising effect.

[0174] The novel device 100 may be used to improve mass flow and suppress stalling, particularly on wind turbine rotor blades 111, especially HAWT (horizontal axis wind turbine) rotor blades, but also on other aerofoils.

[0175] Many further adaptations are possible within the scope of the claims. In the claims, reference numerals and characters are provided in parentheses, purely for ease of reference, and should not be construed as limiting features.

Claims

CLAIMS1. A boundary layer energisation device (100) for use on an aerofoil (1); the aerofoil (1) extending for a length (LI) along a length axis (XI) and defining a surface (7), a leading edge (2), a trailing edge (3), a suction side (4), a pressure side (5), and a chord (6) having a length Ca, the chord (6) of the aerofoil (1) being a straight line between the leading and trailing edges (2, 3) of the aerofoil (1) perpendicular to the length axis (XI); the aerofoil (1) being configured to generate lift in an airflow having a flow direction (Da) from the leading edge (2) to the trailing edge (3) and perpendicular to the length axis (XI); the device (100) including at least one device element (10); the device element (10) being mounted or mountable in a use position, downstream of the leading edge (2) of the aerofoil (1) in the flow direction (Da), to extend from the surface (7) of the pressure side (5) or suction side (4) of the aerofoil (1); the device element (10) including, in the use position: a pair of struts (20) extending from the aerofoil surface (7), and a crosspiece (40) extending between the struts (20), in spaced relation to a bounded portion (7') of the aerofoil surface (7), to define a flowpath (11) extending in the flow direction (Da) and bounded by the bounded portion (7') of the aerofoil surface and by respective, inwardly facing surfaces (48, 28) of the crosspiece (40) and the struts (20); wherein, when considered in a first plane (Pl) extending centrally through the device element (10) in the flow direction (Da), perpendicular to the bounded portion (7') of the aerofoil surface, the crosspiece (40) defines a leading edge (42), a trailing edge (43), and a chord (46) being a straight line between the leading and trailing edges (42, 43) of the crosspiece (40) and having a length Cc; wherein, when considered in a second plane (P2) perpendicular to the first plane (Pl): the flowpath (11) is undivided, and the device element (10) and the bounded portion (7') of the aerofoil surface together form a closed boundary separating the flowpath (11) from an outwardly open flow region (101) exterior to the device element (10), and the crosspiece (40) and the struts (20) have outwardly facing surfaces (49, 29) opposite said inwardly facing surfaces (48, 28), and the outwardly facing surface (49) of the crosspiece (40), and the outwardly facing surfaces (29) of both of the struts (20), are exposed to the outwardly open flow region (101).

2. A boundary layer energisation device (100) according to claim 1, wherein the device (100) includes a plurality of said device elements (10); and, in the use position: the device elements (10) are spaced apart in series relation along a portion of the length (LI) of the aerofoil (1) so that the respective outwardly facing surfaces (29) of respective struts (20) of each adjacent pair of device elements (10) are arranged in confronting relation to define a gap (50) between said respective outwardly facing surfaces (29); and the gap (50) has a minimum width Wg perpendicular to the flow direction (Da); and the outwardly open flow region (101) is common to all of said device elements (10) and bounded by a portion (7") of the aerofoil surface (7) in the gap (50).

3. A boundary layer energisation device (100) according to claim 2, wherein the adjacent struts (20) of each adjacent pair of device elements (10) are mirror-symmetric about a plane of symmetry (P4) extending centrally through the gap (50) in the flow direction (Da), perpendicular to the aerofoil surface (7").

4. A boundary layer energisation device (100) according to claim 2, wherein the width Wg of the gap (50) is from 0.25 CC to 2 Cc.

5. A boundary layer energisation device (100) according to claim 2, wherein the device (100) includes at least three of said device elements (10).

6. A boundary layer energisation device (100) according to claim 1, wherein the struts (20) of the at least one device element (10) are mirror-symmetric about the first plane (Pl).

7. A boundary layer energisation device (100) according to claim 1, wherein, when considered in the first plane (Pl), the flowpath (11) has a height H between the inwardly facing surface (48) of the crosspiece (40) and the bounded portion (7') of the aerofoil surface; and the height H, when considered along the flow direction (Da) between the leading and trailing edges (42, 43) of the crosspiece (40), is invariate or varies by not more than + / - 20% of the length Cc of the chord (46) of the crosspiece (40).

8. A boundary layer energisation device (100) according to claim 1, wherein, when considered in the second plane (P2), the crosspiece (40) blends into each of the struts (20) at a radiused junction (70).

9. A boundary layer energisation device (100) according to claim 1, wherein, when considered in the second plane (P2), a central portion (41) of the crosspiece (40) extends in parallel with the bounded portion (7') of the aerofoil surface for a width Wcc, wherein the width Wcc is from 0.5 Cc to 3 Cc.

10. A boundary layer energisation device (100) according to claim 1, wherein, when considered in the first plane (Pl), the crosspiece (40) defines an aerofoil profile having a suction side (44) and a pressure side (45), the chord (46) of the crosspiece (40) being a chord of said aerofoil profile; and the pressure side (45) of the crosspiece (40) forms the inwardly facing surface (48) of the crosspiece (40).

11. A boundary layer energisation device (100) according to claim 1, wherein, when considered in a third plane (P3a, P3b, P3c, P3d) parallel with the bounded portion (7') of the aerofoil surface, each of the struts (20) defines an aerofoil profile; the aerofoil profile of the strut (20) defining a leading edge (22), a trailing edge (23), a suction side (24), a pressure side (25), and a chord (26) being a straight line between said leading and trailing edges (22, 23).

12. A boundary layer energisation device (100) according to claim 1, wherein, when considered in each of two, upper and lower third planes (P3b, P3d), said third planes (P3b, P3d) being mutually parallel and parallel with the bounded portion (7') of the aerofoil surface, the upper third plane (P3b) being defined between the crosspiece (40) and the lower third plane (P3d), each of the struts (20) defines a leading edge (22), a trailing edge (23), and a chord (26) being a straight line between said leading and trailing edges (22, 23); wherein a twist angle A2 is defined between the chord (26) of a respective strut (20) in the lower third plane (P3d) and the chord (26) of the same respective strut (20) in the upper third plane (P3b) when projected onto the lower third plane (P3d); wherein the twist angle A2 is greater than 0° and not more than 10°.

13. A boundary layer energisation device (100) according to claim 12, wherein, when considered in each of said upper and lower third planes (P3b, P3d), each of the struts (20) defines an aerofoil profile having a suction side (24) and a pressure side (25), the chord (26) of said respective strut (20) being a chord of the aerofoil profile.

14. A boundary layer energisation device (100) according to claim 1, wherein, when considered in the second plane (P2), an outwardly facing boundary (49, 29) of the or each device element (10), terminating at the aerofoil surface (7), is substantially convex without re-entrant surfaces.

15. A boundary layer energisation device (100) according to claim 1, wherein a respective fin (80) is arranged to project outwardly from the outwardly facing surface (29) of each respective one of the struts (20).

16. An aerofoil (1) including a boundary layer energisation device (100) mounted in the use position according to claim 1 or claim 2; wherein, when the length Ca of the aerofoil chord (6) is defined in the first plane (Pl), the length Cc of the crosspiece chord (46) is from 0.01 Ca to 0.20 Ca.

17. An aerofoil (1) including a boundary layer energisation device (100) mounted in the use position according to claim 1; wherein, when considered in the first plane (Pl), the flowpath (11) has a height H between the inwardly facing surface (48) of the crosspiece (40) and the bounded portion (7') of the aerofoil surface; and, when the length Ca of the aerofoil chord (6) is defined in the first plane (Pl), the average height H of the flowpath (11) in the first plane (Pl) is from 0.02 Ca to 0.08 Ca.

18. An aerofoil (1) including a boundary layer energisation device (100) mounted in the use position according to claim 1; wherein an angle Al is defined in the first plane (Pl) between the chord (46) of the crosspiece (40) and the chord (6) of the aerofoil (1); and the angle Al lies in a range from a negative angle Al of -30° through a parallel orientation of 0° to a positive angle Al of +15°, wherein the angle Al is defined as negative if the chords (46, 6) are divergent in the airflow direction, as zero if the chords (46, 6) are parallel in the airflow direction, and as positive if the chords (46, 6) are convergent in the airflow direction.

19. A horizontal axis wind turbine rotor blade (111) including an aerofoil (1) having a boundary layer energisation device (100) mounted in the use position according to claim 1 or claim 2.

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