Patent Description:
For optimum performance under various operating conditions, wind turbine rotor blades need to fulfil complex structural and aerodynamic requirements. With regard to the profile thickness, for example, a compromise has to be made between aerodynamic and structural performance. In an inner radius section of wind turbine rotor blades, structural requirements call for profiles having a large relative thickness of more than e.g. <NUM> %, while the strong curvature of these profiles may lead to flow separation. Flow separation has also been observed in various other surface regions of wind turbine rotor blades. To counteract such flow separation, it is well known to place vortex generators on the blade surface.

The vortex generators are positioned obliquely, so that they have an angle of attack with respect to the local airflow, in order to create a vortex which draws energetic, rapidly moving outside air into the slow-moving boundary layer in contact with the blade surface. Vortex generators are used to trigger the transition from laminar flow to turbulent flow. A turbulent boundary layer is less likely to separate than a laminar one. As a consequence, a higher lift can be obtained. However, the vortex generators at the same time increase drag. For best results, vortex generators are generally positioned just before of an imaginary line where flow separation is expected.

The document <CIT> discloses a wind turbine rotor blade having a main vortex generator configured to generate a main vortex and a secondary vortex generator configured to generate a second vortex, wherein the main vortex generator has a greater height than the secondary vortex generator.

The document <CIT> discloses a wind turbine rotor blade with two rows of vortex generators. The vortex generators of both rows have a relatively small height and for this reason are called micro vortex generators.

The document <CIT> discloses a wind turbine rotor blade in accordance with the preamble of claim <NUM>. The vortex generators of the second row are placed just behind the vortex generators of the first row.

Starting therefrom, it is an object of the invention to improve the aerodynamic efficiency of a wind turbine rotor blade having two rows of vortex generators.

This object is solved by the wind turbine rotor blade with the features of claim <NUM>. Preferred aspects of the invention are given in the dependent claims.

The wind turbine rotor blade has a blade root, a blade tip, a leading edge, a trailing edge, a first row of vortex generators and a second row of vortex generators,.

• wherein the first row and the second row extend in a generally spanwise direction and the first row is arranged closer to the leading edge than the second row,
• wherein the first row comprises two adjacent vortex generators each having a longitudinal axis extending in a generally chordwise direction and two first fins arranged symmetrically to the longitudinal axis, wherein each of the two first fins has a leading end and a trailing end, wherein the trailing ends are arranged in a distance d from each other, wherein the distance d is larger than a distance between the leading ends, wherein the two longitudinal axes are arranged in a distance D from each other and define a centre line extending in a generally chordwise direction,
• wherein the second row of vortex generators comprises a second fin having a leading end and a trailing end, wherein the trailing end is arranged closer to the centre line than the leading end,
• wherein the second fin is positioned with reference to the first fin that is arranged on the same side of the centre line as the second fin and with its trailing end closer to the centre line than its leading end such that the trailing end of the second fin is arranged in a chordwise distance c from the trailing end of that first fin,
• wherein a distance x of the trailing end of the second fin from the centre line is within a range between.

The wind turbine rotor blade in particular is designed for a wind turbine rotor with a horizontal axis. The blade root can be adapted for being fastened on a rotor hub. The spanwise direction generally extends from the blade root towards the blade tip, in particular parallel to a pitch axis or longitudinal axis of the wind turbine rotor blade. The wind turbine rotor blade has an aerodynamic profile with a suction side, a pressure side and a chord extending between a profile leading edge and a profile trailing edge. The chordwise direction corresponds to the direction of this chord, and is perpendicular to the longitudinal axis/pitch axis of the wind turbine rotor blade.

The first row and the second row of vortex generators extend in a generally spanwise direction. In other words, the vortex generators of each row are placed along a line on the wind turbine rotor blade surface running from a first point closer to the blade root to a second point closer to the blade tip. These lines may be straight and/or parallel to the longitudinal axis/pitch axis of the wind turbine rotor blade. However, they may as well be inclined with regard to the longitudinal axis/pitch axis and/or comprise a curvature and/or one or more kinks. For example, the lines may be positioned at a fixed distance from the leading edge, either in absolute terms or in relation to the chord length varying in the spanwise direction. The distance from the leading edge may also vary in the spanwise direction, for example may it increase with increasing distance from the blade root. In this context, the feature that the first row is arranged closer to the leading edge than the second row means that a vortex generator of the first row at a given spanwise position is closer to the leading edge than a vortex generator of the second row at substantially the same spanwise position, so that the air flow reaches the second row only after having passed the first row.

The first row comprises a plurality of vortex generators each having a longitudinal axis arranged in a generally chordwise direction. In other words, the vortex generators are aligned with a direction of the surrounding air flow, wherein some deviation between the direction of air flow, the longitudinal axis and/or the chordwise direction is possible.

Each vortex generator of the first row comprises two first fins placed symmetrically with regard to its longitudinal axis. The first fins may be connected to each other via a base plate or via any other supporting structure mounted to the wind turbine rotor blade surface, but they may as well be mounted individually on the wind turbine rotor blade surface, without any other connecting member between the two first fins.

The first fins as well as the second fin may be essentially planar, but they may also include some curvature. They are placed at an angle with reference to the longitudinal axis and hence to the direction of air flow. This angle may be in a range of about <NUM>° to about <NUM>°, for example. The fins may be placed generally perpendicular to a wind turbine rotor blade surface, in particular on the suction side. Each of the fins has a leading end and a trailing end, based upon where the air flow passes first. The leading end is the foremost part of the fin, the trailing end is the backmost part of the fin. As the fin may have any shape, e.g. rectangular, triangular, trapezoidal and/or including some curvature, the leading end may be a point or an edge. The same holds for the trailing end. The distance d is measured between the trailing ends of the first fins of a vortex generator of the first row of vortex generators. It corresponds to the overall width of these vortex generators.

The longitudinal axes of two adj acent vortex generators of the first row define a centre line arranged halfway between the two longitudinal axes. If the longitudinal axes are exactly parallel, the centre line will be parallel to both of them. It is also possible that an angle is formed between the two longitudinal axes, should one or both of them deviate from the chordwise direction, e.g. by a few degrees. In this case, the centre line will be a bisecting line.

The vortex generators of the second row comprise a second fin having a leading end and a trailing end, wherein the trailing end is arranged closer to the centre line. For defining the position of the trailing end of the second fin, reference is made to the position of the trailing end of one of the first fins, namely of the first fin that is on the same side of the centre line and also has its trailing end closer to the centre line than its leading end. The chordwise distance between the trailing end of this first fin and the trailing end of the second fin is denoted distance c. It is measured along the wind turbine rotor blade surface, which may include some curvature. Hence, the chordwise distance c may not follow a straight line, but corresponds to an overall direction of air flow.

In accordance with the invention, the spanwise position of the trailing end of the second fin lies in a specified area defined in terms of the spanwise distance x of this trailing end from the centre line. The spanwise distance x shall be measured along the wind turbine rotor blade surface as well, as has been explained for the chordwise distance c. The range of the spanwise distance x within the specified area is defined by upper and lower limits which depend on the chordwise distance c as indicated above. Both limits include a fixed value for the spanwise distance x which applies further away from the first row, and a value depending linearly on the chordwise distance c which applies closer to the first row. Together, the upper and lower limits define a strip-shaped surface area beginning at the trailing end of the specified first fin of the vortex generator of the first row, and then broadening with increasing distance from this trailing end.

In operation, the area defined by the upper and lower limits corresponds to an extension of a vortex generated by the first fin. When the second fin has the same orientation with reference to the direction of air flow as the first fin, it tends to generate a vortex with the same rotation direction, either clockwise or counter-clockwise. Placing the trailing end of the second fin in the specified area therefore does not generate a new vortex, but rather strengthens or re-energises the vortex generated by the first fin. For this reason, it is believed that the second row of vortex generators helps avoiding flow separation while generating less additional drag than a second row of vortex generators placed outside of the specified area.

In an aspect of the invention, the upper limit xu is defined by.

This means the area in which the trailing end of the second fin is placed is narrower, so that the strengthening of the vortex generated by the first fin is even more efficient and/or applies a larger range of operating conditions. The slope of -<NUM> c corresponds to an angle α of <NUM>° (see below explanation of <FIG>).

In an aspect of the invention, the lower limit xL is defined by.

This means the area in which the trailing end of the second fin is placed is even narrower, so that the strengthening of the vortex generated by the first fin is even more efficient and/or applies to an even larger range of operating conditions. The slope of -<NUM> c corresponds to an angle β of <NUM>° (see below explanation of <FIG>).

In accordance with the invention, the chordwise distance c is larger than twice the distance d. It has been found that the vortex strengthening effect works best when the distance c is at least as large as the distance d.

In an aspect of the invention, the second row comprises a plurality of second fins, wherein each of the second fins is arranged with reference to one of the first fins as defined in claim <NUM>. In this way, each of the second fins strengthens a vortex generated by one of the first fins.

In an aspect of the invention, the two vortex generators of the first row are arranged in a longitudinal section of the wind turbine rotor blade in which the wind turbine rotor blade has an aerodynamic profile with a relative thickness of <NUM> or more. The relative thickness may also be <NUM> or larger. For this type of profiles, the second row of vortex generators is particularly useful.

In an aspect of the invention, the aerodynamic profile has a flat trailing edge. The flat trailing edge helps limiting the curvature of the suction side and therefore avoiding flow separation. In combination with the inventive positioning of the second fin, optimal aerodynamic efficiency can be obtained.

In an aspect of the invention, the flat trailing edge has a thickness of at least <NUM> % of a chord length of the aerodynamic profile. The thickness may also be at least <NUM> % or at least <NUM> % of the chord length.

In an aspect of the invention, a vortex generator of the first row comprises a base plate mounted on a wind turbine rotor blade surface, wherein the two first fins of the vortex generator are connected to the base plate. This type of vortex generator is easy to install.

In an aspect of the invention, a vortex generator of the second row comprises a base plate mounted on a wind turbine rotor blade surface, wherein a single second fin is connected to the base plate. In other words, each of the second fins of the second row of vortex generators is installed by means of a separate base plate. The defined spanwise positions of the second fins may include relatively large distances between adjacent second fins. Installing the second fins individually by means of separate, relatively small base plates instead of in pairs with relatively wide base plates can therefore lead to less disturbance of the air flow between adjacent second fins.

In the following, the invention is explained in greater detail based on an embodiment shown in figures. The figures show:.

The wind turbine rotor blade <NUM> of <FIG> has a blade root <NUM>, a blade tip <NUM> and a pitch axis <NUM>. The pitch axis <NUM> may also be referred to as longitudinal axis of the wind turbine rotor blade <NUM>. At the blade root <NUM>, the wind turbine rotor blade <NUM> has a cylindrical cross section and is adapted for attachment to a wind turbine rotor hub (not shown). The cross section with increasing distance from the blade root <NUM> transforms into an aerodynamic profile, as illustrated by the dotted lines. The surface of the wind turbine rotor blade <NUM> facing the viewer forms a suction side <NUM>, the opposite surface of the wind turbine rotor blade <NUM> forms a pressure side <NUM>. The pressure side <NUM> and the suction side <NUM> both extend from a leading edge <NUM> to a trailing edge <NUM>.

A first row <NUM> of vortex generators and a second row <NUM> of vortex generators are arranged in a generally spanwise direction in a longitudinal section <NUM> of the wind turbine rotor blade <NUM>, on the suction side <NUM>. The first row <NUM> of vortex generators is positioned closer to the leading edge <NUM> than the second row <NUM> of vortex generators. In the embodiment of <FIG>, each vortex generator of the first row <NUM> and of the second row <NUM> comprises two fins. However, this is not mandatory in particular for the second row <NUM>, in which one or more of the vortex generators may consist each of a single second fin only.

The cross section of <FIG> lies within the longitudinal section <NUM> of the wind turbine rotor blade <NUM>, close to the blade root <NUM>. At this longitudinal position, the aerodynamic profile has a relative thickness of about <NUM> %, calculated as the profile thickness <NUM> divided by the chord length <NUM>, and a flat trailing edge <NUM> with a height <NUM> of about <NUM> % of the chord length <NUM>.

On the suction side <NUM>, one can see a first fin <NUM> of a vortex generator of the first row <NUM>. The first fin <NUM> has a trapezoidal shape with a leading end <NUM> formed by a short edge arranged approximately perpendicular to the surface of the suction side <NUM>, and a trailing end <NUM> arranged approximately perpendicular to the surface of the suction side <NUM>. One can also see a second fin <NUM> of a vortex generator of the second row <NUM>. The second fin <NUM> has a trapezoidal shape with a leading end <NUM> formed by a short edge arranged approximately perpendicular to the surface of the suction side <NUM>, and a trailing end <NUM> arranged approximately perpendicular to the surface of the suction side <NUM>. The chordwise distance c is measured between the trailing ends <NUM> and <NUM> along the wind turbine rotor blade's surface.

<FIG> shows the positions of the vortex generators in greater detail, looking onto a section of the suction side <NUM> of the wind turbine rotor blade <NUM>. The leading edge <NUM> is shown to the left, the trailing edge <NUM> is shown to the right of <FIG>. The first row <NUM> of vortex generators comprises a vortex generator having a first fin <NUM> and a further first fin <NUM> arranged symmetrically to a longitudinal axis <NUM> of this vortex generator.

The distance d is indicated between the trailing end <NUM> of first fin <NUM> and the trailing end of first fin <NUM>. It corresponds to a width of the vortex generator.

Another, adjacent vortex generator of the first row <NUM> of vortex generators is arranged in a spanwise distance D. It comprises two fins <NUM>, <NUM> and a longitudinal axis <NUM>. The longitudinal axes <NUM>, <NUM> of both vortex generators of the first row <NUM> of vortex generators are arranged in a generally chordwise direction. They define a centre line <NUM> arranged halfway between the longitudinal axes <NUM>, <NUM>. The spanwise distance D is measured between the longitudinal axes <NUM>, <NUM>.

The second row <NUM> of vortex generators comprises a second fin <NUM> having a leading end <NUM> and a trailing end <NUM>. The trailing end <NUM> is arranged in a spanwise distance x from the centre line <NUM>, and in a chordwise distance c from the trailing end <NUM> of the first fin <NUM>. The trailing end <NUM> is arranged in a specific surface area <NUM> shown with a shading. The range of spanwise positions x within the surface area <NUM> is defined by an upper limit xu given by a first line <NUM> and a second line <NUM> and by a lower limit xL given by a third line <NUM> and a fourth line <NUM>.

The first line <NUM> is described by the term xu = D/<NUM> - d/<NUM> - <NUM> c and applies as an upper limit xu for the spanwise distance x where c ≤ <NUM> D - <NUM> d. The slope of -<NUM> c corresponds to an angle α = <NUM>°. The second line <NUM> is described by the term xu = <NUM> D.

The third line <NUM> is described by the term xL = D/<NUM> - d/<NUM> - <NUM> c and applies as a lower limit xL for the spanwise distance x where c ≤ <NUM> D - <NUM> d. The slope of -<NUM> c corresponds to an angle β = <NUM>°. The fourth line <NUM> is described by the term xL = <NUM> D.

When looking from the leading edge <NUM> at the air flow close to the suction side <NUM>, the first fin <NUM> will generate a vortex rotating counter-clockwise and roughly covering the surface area <NUM> when flowing towards the trailing edge <NUM>. The second fin <NUM> with its trailing end <NUM> arranged within this surface area <NUM> will not generate a new vortex, but will strengthen the vortex generated by the first fin <NUM>.

One can see from <FIG> that each of the second fins comprised in the second row <NUM> of vortex generators is arranged with reference to one of the first fins <NUM>, <NUM>, <NUM>, <NUM> of the first row <NUM> of vortex generators, as has been explained in detail for the second fin <NUM>, that is in a specific surface area shown with a shading.

Claim 1:
A wind turbine rotor blade (<NUM>) comprising a blade root (<NUM>), a blade tip (<NUM>), a leading edge (<NUM>), a trailing edge (<NUM>), a first row (<NUM>) of vortex generators and a second row (<NUM>) of vortex generators,
• wherein the first row (<NUM>) and the second row (<NUM>) extend in a generally spanwise direction and the first row (<NUM>) is arranged closer to the leading edge (<NUM>) than the second row (<NUM>),
• wherein the first row (<NUM>) comprises two adjacent vortex generators each having a longitudinal axis (<NUM>, <NUM>) extending in a generally chordwise direction and two first fins (<NUM>, <NUM>, <NUM>, <NUM>) arranged symmetrically to the longitudinal axis (<NUM>, <NUM>), wherein each of the two first fins (<NUM>, <NUM>, <NUM>, <NUM>) has a leading end (<NUM>) and a trailing end (<NUM>), wherein the trailing ends (<NUM>) are arranged in a distance d from each other, wherein the distance d is larger than a distance between the leading ends (<NUM>), wherein the two longitudinal axes (<NUM>, <NUM>) are arranged in a distance D from each other and define a centre line (<NUM>) extending in a generally chordwise direction,
• wherein the second row (<NUM>) of vortex generators comprises a second fin (<NUM>) having a leading end (<NUM>) and a trailing end (<NUM>), wherein the trailing end (<NUM>) is arranged closer to the centre line (<NUM>) than the leading end (<NUM>),
• wherein the second fin (<NUM>) is positioned with reference to the first fin (<NUM>) that is arranged on the same side of the centre line (<NUM>) as the second fin (<NUM>) and with its trailing end (<NUM>) closer to the centre line (<NUM>) than its leading end (<NUM>) such that the trailing end (<NUM>) of the second fin (<NUM>) is arranged in a chordwise distance c from the trailing end (<NUM>) of that first fin (<NUM>), characterised in that
• the chordwise distance c is larger than twice the distance d and
• a distance x of the trailing end (<NUM>) of the second fin (<NUM>) from the centre line (<NUM>) is within the range between
(i) an upper limit xU defined by:

<TAB>

(ii) a lower limit xL defined by

<TAB>