WO2009025549A1

WO2009025549A1 - Wind turbine and rotor blade with reduced trailing edge noise

Info

Publication number: WO2009025549A1
Application number: PCT/NL2008/050550
Authority: WO
Inventors: Arne Van Garrel
Original assignee: Stichting Energieonderzoek Centrum Nederland
Priority date: 2007-08-17
Filing date: 2008-08-15
Publication date: 2009-02-26
Also published as: EP2179172A1; NL2000821C2

Abstract

A wind turbine comprises a rotor having a number of rotor blades, which each have leading edge and a trailing edge. At least one rotor blade is provided with a series of openings, which are near the trailing edge. The rotor blade has fluid-displacement means which, in use, alternately force fluid out of and into said openings. This resul in vortices on the trailing edge which reduce the noise produced.

Description

Wind turbine and rotor blade with reduced trailing edge noise

The invention relates to a wind turbine comprising a rotor having a number of rotor blades, which each have a leading edge and a trailing edge.

In use, a wind turbine produces noise. The noise production may cause a nuisance, in particular when the wind turbine is built on land. The noise pollution depends on the distance from the wind turbine — by placing the wind turbine a greater distance away, the noise level is reduced. However, the result thereof is that the surface area which is available for wind energy is reduced. In addition, the rotary speed of the wind turbine can be limited in order to reduce the noise production, which adversely affects the efficiency of the wind turbine and the power produced.

A considerable part of the noise production of a wind turbine is due to the noise of the trailing edges of the rotor blades during rotation of the rotor — the so-called trailing edge noise. NL9301910 discloses a wind turbine having a number of rotor blades, in which the trailing edges are in the shape of a saw tooth in order to reduce the noise production. However, the saw tooth shape leads to loss of power and increased noise under conditions which deviate from the design specifications.

It is an object of the invention to provide a wind turbine which reduces the trailing edge noise while the power remains substantially unchanged.

According to the invention, this object is achieved by a wind turbine comprising a rotor having a number of rotor blades, which each have a leading edge and a trailing edge, in which each rotor blade has a chord in cross section, which extends between the leading edge and the trailing edge, and in which at least one rotor blade is provided with a series of openings, which are arranged at a distance from the trailing edge which is less than 10% of the chord, and fluid-displacement means for alternately forcing fluid out of and into said openings.

The fluid-displacement means according to the invention generate so-called synthetic jets from the openings near the trailing edge, that is to say at a distance from the trailing edge which is less than 10% of the chord, for example in the trailing edge. According to the invention, the synthetic jets are used to reduce the trailing edge noise. A synthetic jet comprises a series of vortices which are formed by alternately blowing fluid out of an opening and drawing fluid into an opening. Each time mass is ejected a vortex comes out of the opening as a result of separation, whereas the opening acts as a drain when mass is flowing in. Every opening directs such a series of vortices into the flow around the rotor blade. The vortices of the synthetic jets influence said flow in such a manner that the structure of the turbulent boundary layer and the wake behind the trailing edge of the rotor blade is changed. As a result, the noise production of the trailing edge is reduced. As, apart from that, the flow around the rotor blade hardly changes, the power produced by the wind turbine remains substantially unchanged.

A further advantage is that the risk of damage is reduced, compared to the known rotor blades having relatively delicate saw tooth-shaped trailing edges.

It should be noted that a wind turbine with synthetic jets is known from WO00/50778. In this case, slots for generating the synthetic jets are provided near the leading edge of the rotor blades. The synthetic jets are used to influence the separation point. With all other embodiments which are described in this publication the slots for generating synthetic jets are also situated near the leading edge, so that the boundary layer of the flow remains attached downstream of the slots. However, according to the invention, synthetic jets are not used to influence the separation point, but to reduce the noise production of the trailing edge. Therefore, the openings for generating synthetic jets are according to the invention arranged near the trailing edge of the rotor blade.

It should furthermore be noted that a wind turbine is known from EP1674723, in which synthetic jets are arranged in the leading edge of the tip of the rotor blade. The tip of the rotor blade produces noise as a result of the tip vortex. The synthetic jets in the leading edge of the tip are substantially directed outwards in the direction of the span, so that the tip vortex is moved away from the tip. However, the synthetic jets according to the invention are not used to reduce the noise of the tip, but to reduce the trailing edge noise. In addition, it should be noted that a wind turbine is disclosed in EP 1780408, in which the rotor blades are each provided with a series of outlet openings. Air can be blown through the outlet openings into the flow. In this case, the mass flow is continuous, whereas the mass flow of the synthetic jets through the openings according to the invention is substantially equal to zero.

The rotor blade has, for example, a root end which is connected to a hub of the rotor. The rotor blade extends radially outwards from the root end across a span up to a tip end. The longitudinal direction or span direction of the rotor blade extends between the root end and the tip end. The rotor blade has an aerodynamic profile with a chord or chord line in cross section, which is defined by a straight line between the leading edge and the trailing edge of said profile. Incidentally, the profile may differ in the longitudinal direction of the rotor blade. In that case, the profile and the chord depend on the distance in the span direction from the tip end of the rotor blade. For example, the length of the chord between the tip end and the root end of the rotor blade varies. The rotor blade is usually of tapering design. In addition, it is possible for the profiles to be rotated with respect to one another.

According to the invention, the openings can be designed in various ways. Each opening has, for example, an outlet direction and the outlet directions of adjacent openings are directed towards one another. The synthetic jets according to the invention leave the openings in the outlet direction. The synthetic jets of adjacent openings are mixed with one another if the outlet directions of adjacent openings are directed towards one another. This significantly reduces the trailing edge noise.

For example, the synthetic jets emanating from in each case two adjacent openings influence one another, that is to say the synthetic jets are directed towards one another in pairs. In addition, the outlet directions of in each case three or more openings of the series of openings can be directed towards one another.

In addition, it is possible for the rotor blade to have a chord plane which is defined by the chords of the rotor blade, and in which the outlet directions of two or more adjacent openings which are directed towards one another extend substantially parallel to the chord plane. The term chord plane is understood to mean the plane which is delimited by the chords of the aerodynamic profiles which define the rotor blade. The chord plane may be flat, but in practice is usually curved due to the feet that the profiles are rotated with respect to one another. The curvature of the chord plane extends from the tip end to the root end. If adjacent openings are directed towards one another parallel to the chord plane, the synthetic jets mix in the wake of the trailing edge. This results in a significant reduction in trailing edge noise.

The outlet directions of two or more adjacent openings which are directed towards one another may extend at an angle or transversely with respect to the chord plane. The rotor blade is for example provided with several pairs of openings which generate synthetic jets which cross one another parallel to the chord plane and several pairs of openings which emit synthetic jets into the flow at an angle or transversely with respect to the chord plane. As a result, a three-dimensional mixture of synthetic jets is produced, which is particularly suitable for reducing the trailing edge noise. Instead of distributing the series of openings in pairs, it is possible to group in each case more than two openings together, whose outlet directions are directed towards one another.

Incidentally, it is possible for the outlet direction of the openings to comprise a component parallel to the chord. The synthetic jets then have, for example, substantially no velocity component transversely to the chord plane of the rotor blade. The vortices of the synthetic jets are emitted substantially in line with the flow. In this case, the vortices may comprise a velocity component in the longitudinal direction of the rotor blade.

It is also possible for the outlet direction in which the synthetic jets are forced out of the openings to be substantially parallel to the chord. In this case, the openings are directed to the rear, that is to say the fluid emitted from the openings flows substantially parallel to the flow around the rotor blade.

In one embodiment, the rotor blade has a root end and a tip end which define a span of the rotor blade, and the openings are provided at least in that portion of the rotor blade which is at a distance from the root end which is 50% to 90% of the span. If the openings for generating the synthetic jets are situated substantially at least in the outer half of the rotor blade, the trailing edge noise can be efficiently reduced.

In one embodiment, the openings are provided at a distance from one another in the span direction of the rotor blade. For example, the openings are substantially equidistant from one another. The distance between in each case two adjacent openings is in this case substantially equal. As a result, the openings are evenly and/or regularly distributed in the longitudinal direction of the rotor blade.

According to the invention, it is possible for the distance between the openings to be substantially equal to 0.1-5% of the chord, for example 0.5-2% of the chord. The distance between the openings is such that the synthetic jets from the openings influence one another. The trailing edge noise is efficiently reduced through the interaction of the synthetic jets from different openings.

In one embodiment of the invention, the fluid-displacement means are designed to alternately force fluid out of and into the openings at a frequency of 50-5000 Hz, for example 50-500 Hz. At these frequencies, the synthetic jets are particularly efficient in reducing trailing edge noise.

According to the invention, it is possible for the fluid-displacement means to be controlled as a function of a signal which is detected by a sensor. The fluid- displacement means can be controlled by a control unit The sensor and/or the control unit may be situated inside the rotor blade, but may also be arranged outside the latter.

In one particular embodiment, each rotor blade is provided with a sensor, and the fluid- displacement means of each rotor blade can be controlled as a function of the signal which has been detected by the sensor of said rotor blade. The rotor blades each have a local sensor. The fluid-displacement means of the respective rotor blades are operated on the basis of parameters which have been determined locally. The control unit can actuate the fluid-displacement means in various ways. For example, the fluid-displacement means can be switched on or switched off. It is also possible to control the frequency of the fluid-displacement means.

The sensor may be designed for detecting the wind speed and/or wind direction. At high wind speeds, the trailing edge noise does not rise above the ambient wind noise. The synthetic jets can then be switched off. In some cases, the trailing edge noise is only a nuisance with specific wind directions, for example if there are only buildings in a limited area around the wind turbine. With those wind directions, the use of synthetic jets is particularly useful.

In one embodiment, each rotor blade has an azimuth angle which is defined by the angle from the vertical which extends upwards from the axis of rotation up to said rotor blade, viewed in the direction of rotation, and the sensor is designed to detect the azimuth angle, and the control unit is designed to switch on the fluid-displacement means at an azimuth angle between 0-180°, preferably between 60-180°, and switching off the fluid-displacement means at an azimuth angle which is outside this range. The rotor blades produce most noise in the first and second quadrant, i.e. when the rotor blades move downwards. In that region, the control unit may switch on the fluid- displacement means for generating synthetic jets, while no synthetic jets are emitted when the rotor blades move upwards.

However, the sensor may also be designed differently. For example, it is also possible for the sensor to be designed to detect the rotary speed of the rotor. The average wind speed affects the rotary speed of the rotor. The noise produced increases as the rotary speed increases. In this case, the synthetic jets can be controlled as a function of the rotary speed which has been detected.

In one embodiment, the control unit is designed to determine the frequency of the fluid- displacement means. For example, the control unit comprises an electrical actuating means. The control unit may be designed to impose a fixed frequency. Alternatively, the control unit can vary and/or adjust the frequency, for example on the basis of a signal of the sensor as described above. In one embodiment, the fluid-displacement means are designed to force fluid out of a first group of the openings and simultaneously force fluid into a second group of the openings during a first time period, and to force fluid into the first group of the openings and simultaneously force fluid out of the second group of the openings during a second time period which follows the first time period. As a result thereof, the influence of the synthetic jets on the flow around the rotor blade can be adjusted in an accurate manner.

The fluid-displacement means may be designed in various ways. For example, the fluid-displacement means are provided with at least one fluid chamber, which is provided inside the rotor blade and connected to at least one opening, the fluid chamber being provided with means for changing the volume of the fluid chamber in order to force fluid out of and into the associated opening. In this case, it is possible for several fluid chambers to be provided, each of which is connected to in each case one opening or several openings.

For example, the means for changing the volume of the fluid chamber comprise a flexible membrane. Each fluid chamber is formed by a hollow inner space in the rotor blade. Each fluid chamber has a volume, which is, for example, delimited by the opening and the flexible membrane. The flexible membrane can be actuated. By deforming the flexible membrane towards the opening, i.e. to the outside, the volume is reduced. In this case, an amount of fluid is pushed out of the fluid chamber in order to create a vortex. While it is being emitted, the fluid flows "straight" out of the opening. Then, the flexible membrane is reshaped, so that the volume of the fluid chamber increases. This results in a reduced pressure in the fluid chamber, so that fluid is drawn in from outside the opening. This leads to a mass flow into the fluid chamber. In this case, the fluid flows along the surface of the rotor blade towards the opening and turns off to the inside. The net mass flow flux through the opening equals zero. Thereafter, the flexible membrane can move outwards again in order to generate a further vortex. The succession of vortices forms a synthetic jet. The fluid-displacement means may, instead of the flexible membrane, comprise a piston which can reciprocate in the fluid chamber in order to generate vortices. Other embodiments for generating synthetic jets are also possible according to the invention.

According to the invention, it is possible for one fluid chamber or several fluid chambers to be provided. For example, several openings are connected to a common elongate fluid chamber.

In one embodiment, the means for changing the volume of the fluid chamber, such as the flexible membrane or another pushing element, divide the fluid chamber into two sub-chambers, the first sub-chamber being connected to a first opening and the second sub-chamber being connected to a second opening. When the flexible membrane is deformed in one direction, the volume of the first sub-chamber decreases and the volume of the second sub-chamber increases. As a result, the first opening, which is connected to the first sub-chamber, emits an amount of fluid. Simultaneously, the second sub-chamber draws in fkdd via the second opening. If die flexible membrane is then displaced in an opposite direction, fluid flows into the first sub-chamber through the first opening, while the second opening emits a vortex.

The invention also relates to a rotor having a number of rotor blades, which each have a leading edge and a trailing edge. The invention also relates to a rotor blade having a leading edge and a trailing edge. The rotor blade has a chord in cross section, which extends between the leading edge and the trailing edge. According to the invention, a series of openings is provided, which are arranged at a distance from the trailing edge which is less than 10% of the chord, for example in the trailing edge, and fluid- displacement means for alternately forcing fluid out of and into said openings.

The invention will be explained in more detail below merely by way of example with reference to the attached drawing, in which: Fig. 1 shows a perspective view of a wind turbine comprising a rotor having a number of rotor blades according to the invention;

Fig. 2 shows a perspective view, partially in cross section, of a rotor blade of the wind turbine shown in Fig. 1; Fig. 3 shows a partially cut-away top view of a second embodiment of a rotor blade according to the invention;

Fig.4a shows a rear view of a third embodiment of a rotor blade according to the invention; Fig.4b shows a top view of the rotor blade shown in Fig.4b.

The wind turbine shown in Fig. 1 is denoted overall by reference numeral 1. In this exemplary embodiment, the wind turbine 1 is built on land. The wind turbine 1 comprises a mast 8 and a rotor 2, which is connected to the mast 8 so as to be able to rotate about an axis of rotation 10.

The rotor 2 comprises a hub 9 and a number of rotor blades 3, 4, 5. Although the rotor 2 in this exemplary embodiment has three rotor blades, more or fewer rotor blades may be provided. Each rotor blade 3, 4, 5 has a root end and a tip end. The root end is attached to the hub 9, while the opposite tip end is unattached. Each rotor blade 3, 4, 5 has a longitudinal direction which extends from the root end to the tip end.

Each rotor blade 3, 4, 5 comprises a leading edge 7 and a trailing edge 6, viewed in the direction of rotation of the rotor 2. When the rotor 2 rotates, the trailing edges 6 of the rotor blades produce noise. The trailing edge noise is caused by reflection of turbulence in the boundary layer on the trailing edge 6 of each rotor blade. The noise production increases as the thickness of the turbulent boundary layer increases. In order to reduce the trailing edge noise, each rotor blade 3, 4, 5 has a series of openings 12 which, in this exemplary embodiment, are provided in the trailing edge 6. The openings may, however, be provided near the trailing edge 6 in the rotor blade surface on the low- pressure side and/or high-pressure side (not shown). A combination with openings in the trailing edge 6, as illustrated in Fig.2, is also possible.

Synthetic jets, i.e. a succession of vortices, flow from the openings 12. The vortices affect the flow around the rotor blade in such a manner that the thickness of the turbulent boundary layer on the trailing edge 6 of each rotor blade 3, 4, 5 decreases. This leads to a reduction in the noise production of the trailing edge 6 when the rotor 2 rotates. As₅ apart from that, the flow around the rotor blade hardly changes, the power produced by the wind turbine remains substantially the same.

The synthetic jets on the trailing edge 6 thus form noise reduction means for reducing the noise caused by the rotor blade during rotation of the rotor 2. Tn order to generate the synthetic jets, the rotor Wades 3, 4, 5 comprise fluid-displacement means for alternately forcing fluid out of and into the openings 12.

In this exemplary embodiment, the fluid-displacement means comprise several fluid chambers 15, each of which is connected to the openings 12 via a duct 14. Each fluid chamber 15 is provided with a flexible membrane 16, which can be deformed by a control unit 17 (see the dotted line and the solid line of the flexible membrane 16 in Figs.2 and 3). The control unit 17 causes the flexible membrane 16 to vibrate. The vibration frequency is, for example, between 50-300 Hz. When the flexible membrane 16 of a fluid chamber 15 moves towards the opening 12 which is connected thereto, the volume of the fluid chamber 15 decreases. As a result, a quantity of fluid is forced out of said opening 12. This leads to a small vortex near the opening 12, which opens on the trailing edge 6.

After this vortex has been emitted, the flexible membrane 16 moves away from the opening 12, as the flexible membrane 16 vibrates. This means that the volume of the fluid chamber 15 increases, and fluid is drawn in through the opening 12 from outside the rotor blade. As a result thereof, the fluid in the fluid chamber 15 is replenished, so that the mass flow flux through the opening 12 is substantially equal to zero.

Subsequently, the control unit 17 moves the flexible membrane 16 back in the direction of the opening 12 in order to produce a further vortex. The vibration of the flexible membrane 16 results in a succession of vortices from the openings 12. The fluid- displacement means determine the frequency at which the vortices are emitted from the opening 12. As a result of interaction of the vortices, each succession forms a synthetic jet. The fluid forced out of the opening 12 is formed by the fluid which surrounds the rotor blades 3, 4, 5, such as air. As is illustrated in Fig.2, the openings 12 are at equal distances a from one another. The distance a between the openings is, for example, approximately 1% or 1.5% of the chord c, which is denned by the distance from the leading edge 7 to the trailing edge 6. The synthetic jets from adjacent openings 12 influence one another, so that the trailing edge noise is counteracted in an effective manner.

The openings 12 are directed in such a manner that fluid which is emitted from the fluid chamber 15 flows substantially parallel to the chord c in the flow around the rotor blade. This is advantageous for maintaining the power produced by the wind turbine 1. The fluid which flows out of the openings 12 may, however, have a speed component in the longitudinal direction of the rotor blade. It is even possible for the fluid from the openings 12 to flow slightly upwards or downwards (not shown).

Fig. 3 shows an embodiment in which each of the fluid chambers 15 comprises two sub-chambers 19, 20, which are separated from one another by the flexible membrane 16. While the membranes 16, which are controlled by the control unit 17, vibrate, fluid is pressed from the sub-chambers 19 out of a first group of openings 12 and simultaneously fluid is drawn into the sub-chambers 20 from outside the rotor blade via a second group of openings 12. This results in a shift in the frequency of the vortices from adjacent openings 12 with respect to one another.

Fig. 4a and 4b show an embodiment in which the openings 12 are designed in such a manner that the synthetic jets of adjacent openings 12 influence one another. The synthetic jets can cross each other in the chord plane of the rotor blade (see Fig.4b), but can also flow up or down with respect to the chord plane (see Fig. 4a). This leads to a significant reduction in trailing edge noise.

The invention is not limited to the exemplary embodiments illustrated in the figures. For example, each rotor blade comprises one or more fluid chambers which are connected to in each case one or several openings. Also, the flexible membrane may be replaced by any pushing element for forcing fluid out or means for changing the volume of the fluid chamber, such as a piston which is displaceable in the fluid chamber. In addition, the invention relates to any aerodynamic object which rotates in a fluid and thereby produces noise, such as a rotor blade of a propeller, helicopter or jet engine.

Claims

1. Wind turbine (1) comprising a rotor (2) having a number of rotor blades (3, 4, 5), which each have a leading edge (7) and a trailing edge (6), in which each rotor blade (3, 4, 5) has a chord in cross section, which extends between the leading edge (7) and the trailing edge (6), and in which at least one rotor blade (3, 4, 5) is provided with a series of openings (12), which are arranged at a distance from the trailing edge (6) which is less than 10% of the chord, and fluid-displacement means for alternately forcing fluid out of and into said openings (12).

2. Wind turbine according to Claim ϊ , in which each opening (12) is arranged in Hie trailing edge (6).

3. Wind turbine according to Claim 1 or 2, in which each opening (12) has an outlet direction, and in which the outlet directions of adjacent openings (12) are directed towards one another.

4. Wind turbine according to Claim 3, in which the rotor blade (3, 4, 5) has a chord plane which is defined by the chords of the rotor blade (3, 4, 5), and in which the outlet directions of two adjacent openings (12) which are directed towards one another extend substantially parallel to the chord plane.

5. Wind turbine according to Claim 3 or 4, in which the rotor blade (3, 4, 5) has a chord plane which is defined by the chords of the rotor blade (3, 4, 5), and in which the outlet directions of two adjacent openings (12) which are directed towards one another extend at an angle or transversely with respect to the chord plane.

6. Wind turbine according to one of the preceding claims, in which each opening (12) has an outlet direction which runs substantially parallel to the chord.

7. Wind turbine according to one of the preceding claims, in which the rotor blade (3, 4, 5) has a root end and a tip end which define a span of the rotor blade (3, 4, 5), and in which the openings (12) are provided at least in that portion of the rotor blade (3, 4, 5) which is at a distance from the root end which is 50% to 90% of the span.

8. Wind turbine according to one of the preceding claims, in which the openings (12) are provided at a distance from one another, and in which me distance between the openings (12) is substantially equal to 0.1-5% of the chord, for example 0.5-2% of the chord.

9. Wind turbine according to one of the preceding claims, in which the fluid- displacement means are designed to alternately force fluid out of and into the openings (12) at a frequency of 50-5000 Hz, for example 50-500 Hz.

10. Wind turbine according to one of the preceding claims, in which the fluid- displacement means can be controlled as a function of a signal which is detected by a sensor.

11. Wind turbine according to Claim 10, in which each rotor blade (3, 4, 5) is provided with a sensor, and in which the fluid-displacement means of each rotor blade (3, 4, 5) can be controlled as a function of the signal which has been detected by the sensor of that rotor blade (3, 4, 5).

12. Wind turbine according to Claim 10 or 11, in which the fluid-displacement means of each rotor blade (3, 4, 5) can be switched on or switched off.

13. Wind turbine according to one of Claims 10- ϊ 2, in which the sensor is designed to detect the wind speed and/or wind direction.

14. Wind turbine according to Claim 11 or 12, in which each rotor blade (3, 4, 5) has an azimuth angle which is defined by the angle from the vertical up to said rotor blade (3, 4, 5), viewed in the direction of rotation, and in which the sensor is designed to detect the azimuth angle, and the control unit (17) is designed to switch on the fluid-displacement means at an azimuth angle between 0-180°, preferably between 60-180°, and switching off the fluid-displacement means at an azimuth angle which is outside this range.

15. Wind turbine according to one of the preceding claims, in which the fluid- displacement means are designed to force fluid out of a first group of the openings (12) and simultaneously force fluid into a second group of the openings (12) during a first time period, and to force fluid into the first group of the openings (12) and simultaneously force fluid out of the second group of the openings (12) during a second time period which follows the first time period.

16. Wind turbine according to one of the preceding claims, in which the fluid- displacement means are provided with at least one fluid chamber (15), which is provided inside the rotor blade (3, 4, 5) and connected to at least one opening (12), and in which the fluid chamber (15) is provided with means for changing the volume of the fluid chamber (15) in order to force fluid out of and into the associated opening (12).

17. Wind turbine according to Claim 16, in which the means for changing the volume of the fluid chamber (15) comprise a flexible membrane (16).

18. Wind turbine according to Claim 16 or 17, in which the means for changing the volume of the fluid chamber (15) divide the fluid chamber (15) into two sub- chambers (19, 20), and in which the first sub-chamber (19) is connected to a first opening (12) and the second sub-chamber (20) is connected to a second opening (12).

19. Rotor having a number of rotor blades (3, 4, 5), which each have a leading edge (7) and a trailing edge (6), in which each rotor blade (3, 4, 5) has a chord in cross section, which extends between the leading edge (7) and the trailing edge (6), and in which at least one rotor blade (3, 4, 5) is provided with a series of openings

(12), which are arranged at a distance from the trailing edge (6) which is less than 10% of the chord, and fluid-displacement means for alternately forcing fluid out of and into said openings (12).

20. Rotor blade comprising a leading edge (7) and a trailing edge (6), in which the rotor blade (3, 4, 5) has a chord in cross section, which extends between the leading edge (7) and the trailing edge (6), and in which the rotor blade (3, 4, 5} is provided with a series of openings (12), which are arranged at a distance from the trailing edge (6) which is smaller than 10% of the chord, and fluid-displacement means for alternately forcing fluid out of and into said openings (12).

21. Method for operating a wind turbine according to one of Claims 1-18.

22. Method according to Claim 21 , in which a parameter, such as wind speed and/or wind direction, is detected by a sensor, a signal which corresponds to said parameter detected by the sensor is transmitted to a control unit, and the fluid- displacement means are controlled by the control unit as a function of said signal.