METHOD AND DEVICE FOR REDUCTION OF VIBRATIONS IN A WINDMILL BLADE

Abstract: The present invention relates to a method for reduction of vibrations in a windmill blade so that the loads on the blade and on the remaining part of the mill structure are reduced. According to the invention this is achieved by changing the aerodynamic properties of the blade as a function of the acceleration and/or speed in the direction of rotation and/or out of the rotor plane of a slidable device. The invention also relates to a device for changing the vibrations of a windmill blade, said device in a first embodiment being characteristic in that the device is a mass (13) which via a lever system (14) is pivotably mounted about an axis (14a) extending in the longitudinal direction of the blade so that by pivoting in a direction crosswise of the rotor plane the mass (13) will move a flap (15) through the lever system (14) for reduced lift on the blade.

Description: METHOD AND DEVICE FOR REDUCTION OF VIBRATIONS IN A WINDMILL BLADE The invention relates to a method for reduction of vibra¬ tions in a windmill blade so that the loads on the blade and on the remaining part of the mill structure are re¬ duced. The invention also relates to a device for use in carrying out the method. It is generally known that windmills are exposed to heavy, varying aerodynamic loads when in operation. The loads vary stochastically since the air current is turbulent. More¬ over, the loads vary systematically since the speed profile varies with the altitude above ground level and since the mill tower usually provides wind shadow, even when the rotor is placed on the windward side of the tower. Further- more, under certain aerodynamic conditions the blades may have little or negative aerodynamic damping whereby so¬ called stalling vibrations may occur. These stalling vibra¬ tions may become substantial and may seriously reduce the life of the windmill. It is also generally known that the lift properties of a given blade profile may be changed in various manners. Lowering of a flap close to the rear edge of the profile, lifting a slot above the leading edge of the blade and placing of vortex generators on the suction side of the blade will normally increase the lift of the profile. Presence of a turbulator close to the leading edge of the blade or opening of channels for air flow across the pro¬ file will normally reduce the lift of the profile. The lift properties of the entire blade may be changed by adjusting the pitch angle of the blade. A number of known methods and devices exist whereby the aerodynamic properties of windmill blades are changed in one or more of the above-mentioned manners with a view to reducing the aerodynamic loads on a windmill blade. Many stalling adjusted windmills accordingly have turbulators on the blades in order to reduce the maximum lift. Pitch ad¬ justed windmills change the pitch angle of the blades in response to various parameters. These parameters are often limited to the supplied power, but there are also systems where the loads on the blade root are measured and applied in the adjusting algorithm. When vibrating, the greatest loads occur when the structure is fully deformed. That is to say that a comparatively ad- vanced form of adjustment will be required if the size of the loads are to be substantially reduced on the basis of a measurement of the size of the loads. Such an adjustment may for example be made by means of a D-joint (differentia¬ tion of the measuring signal). However, this is difficult to make as a mechanical solution and will normally require that the adjustment is active and based on electronic sig¬ nal processing. It is therefore the object of the present invention to pro- vide a method which may in a simple manner reduce any vibrations in a windmill blade by changing its aerodynamic properties in that there is also provided a device for use in carrying out the method. This object is achieved by a method of the kind described in the introduction, said method according to the invention being characteristic in that the aerodynamic properties of the blade are changed as a function of the acceleration and/or speed in the direction of rotation and/or out of the rotor plane of a slideable device. The superior principle of the invention is to provide a simple, direct connection between the acceleration and/or speed of the outer part of the blade and the aerodynamic properties of the blade. In a particularly advantageous manner this will reduce any vibrations. The basis of the invention is the fact that the accelera¬ tion and/or speed of the outer part of a windmill blade is a good indication of how the loads will develop by structural vibrations. As opposed to a measurement of the size of the loads, where the maximum signal is not obtained till the occurrence of the situation itself, which is to be avoided, a measurement of the acceleration and/or speed will provide the maximum signal already at the time when a vibration starts to develop. It should be noted that during rotation the individual parts of the blade are always accelerated inward. This centripetal acceleration is ir¬ relevant to the invention, and the term acceleration in this specification means acceleration in the direction of rotation (edgewise acceleration) or out of the rotor plane ( flapwise acceleration) . The vibration damping may be achieved by suspending a mass within or on the blade and connect same to one or more de- vices changing the lift properties on the blade. The mentioned aerodynamic properties, which are changed, are not only limited to the lift of the blade, but may also refer to the resistance and pitch moment of the blade. The lift of the blade may accordingly be increased as well as decreased, for example by the use of a flap which will normally increase the lift when it is lowered. As far as the orientation of the lift change in relation to the direction of rotation and/or out of the rotor plane is concerned (where the lift is not the only aerodynamic pro¬ perty, which is of relevance), it is not certain that spe¬ cific accelerations will advantageously provide specific lift changes. The device according to the invention is by a first embodi¬ ment characteristic in that the device is a mass which via a lever system is mounted pivotably about an axis extending in the longitudinal direction of the blade so that by pivoting in a direction crosswise of the rotor plane and through the lever system, the mass will move a flap for re¬ duced lift of the blade. The independent claims relate to advantageous embodiments of the device according to the invention. The invention will now be further explained in the follow¬ ing with reference to the drawing, wherein: fig. 1 is a diagram of the dynamic loads on a windmill blade, fig. 2a-g is a number of examples of known methods for increasing the lift of a blade, fig. 3a-c is a number of examples of known methods for reducing the lift of a blade, fig. 4 is an embodiment of a device according to the in- vention where the flapwise acceleration of a sus¬ pended mass changes the lift of a blade by means of a flap, fig. 5 is an embodiment of a device according to the in- vention where the edgewise acceleration of a sus¬ pended mass changes the lift of a blade by means of a turbulator, fig. 6 is an embodiment of a device according to the in¬ vention where the acceleration of a suspended mass changes the lift of a blade via a viscous medium, fig. 7 is an embodiment of a device according to the in¬ vention where the accelerating mass itself is a viscous medium, which can fill or drain an inflat- able flap of a blade, fig. 8 is an embodiment of a device according to the in¬ vention where the accelerating mass is connected to a hydraulic valve controlling the pitch angle of the entire windmill blade, and fig. 9 is a diagram showing how the dynamic loads on a windmill blade can be reduced by a device according to the invention. Fig. 1 is a diagram showing the dynamic loads on a windmill blade. The diagram is the result of computerized simulation in terms of the bending moment of the blade root over time. A slow vibration 1, which is due to the variation of the wind profile with the altitude above ground level during one rotor rotation, is overlaid by a quicker vibration 2, which is due to a random excitation of the structure emanating from turbulence. Fig. 2a-g show examples of known methods for changing the aerodynamic properties of a windmill blade, such as an in¬ crease in the blade lift. The shown methods comprise various embodiments: Plain flap or aileron 2a, split flap 2b, external airfoil flap 2c, slotted flap 2d, double slotted flap 2e, leading edge slat 2f and vortex generators 2g. Fig. 3a-c show examples of known methods for changing the aerodynamic properties of a windmill blade, such as a re¬ duction of the blade lift. The shown methods comprise turbo tape 10, stall strip 11 and ventilation 12. The mentioned aerodynamic properties which are changed are not only limited to the blade lift, but may also refer to the resistance and pitch moment of the blade. A solution is therefore a pure spoiler effect, which will only relate to the resistance. The lift of the blade may accordingly be increased as well as decreased, for example by using a flap, which will nor¬ mally increase the lift when it is lowered. As far as the orientation of the lift change in relation to the direction of rotation and/or out of the rotor plane is concerned (where the lift is not the only aerodynamic pro¬ perty, which is of relevance), it is not certain that spe- cific accelerations will advantageously provide specific lift changes. Situations may well occur where the connec¬ tion is different from the one immediately expected. These more unexpected connections will only effectively be ex¬ plained by an aero-elastic calculation. Fig. 4 shows an embodiment of a device according to the in¬ vention. A mass 13 is suspended pivotably about a point 14a so as to allow the mass to be moved in a flapwise direction (i.e. crosswise of the chord). By means of a lever system 14 the mass 13 is connected to a flap 15. If the blade is accelerated away from the wind 16, which will be the case when a sudden gust of wind increases the aerodynamic loads, the mass 13 will move towards the driving side 17 of the blade and will thereby take the flap 15 in a direction to- wards lesser lift, whereby the loads on the blade are re¬ duced. Fig. 5 shows another embodiment of a device according to the invention, where a mass 18 is suspended pivotably about a point 19a so as to allow the mass to be moved in an edge¬ wise direction (i.e. along the chord). By means of a lever system 19 the mass 18 is connected to a turbulator 20. If the blade is accelerated forwards in the direction of rota¬ tion 21, which will be the case during edgewise stalling vibrations when the lift varies in an unstable manner due to the proper motion of the blade, the mass 18 will move towards the rear edge 22 of the blade and thereby lift the turbulator 20 above the surface 23 of the blade, whereby the lift is again reduced, and the unstable state ceases. Fig. 6 shows a third embodiment of a device according to the invention, where a mass 24 is mounted within a cylinder 25 and centered by to springs 26. The cylinder 25 is filled with a viscous medium 27 and is connected by means of a pipe system 28 to a cylinder 29, which can activate a lift changing device 30. The mass 24 is designed as a piston in the cylinder 25 and is adapted to allow the viscous medium to pass by and/or through itself from one end to the other, in that the mass 24 for example has a smaller diameter than the internal diameter of the cylinder 25 or is provided with longitudinal bores not shown in the drawing. By suit- ably adapting the drainage in the fit between the mass 24 and the cylinder 25 or dimensioning the bores and perhaps providing a further drainage 31, a desired damping may be achieved. The advantage of this embodiment is that by stationary conditions the lift changing device 30 will be unloaded by the vibration damper and may position itself in a position being the result of the aerodynamic reaction forces. Fig. 7 shows a fourth embodiment of a device according to the invention where a mass consists of a viscous liquid 32, which iε substantially held within a container 33, and by means of a pipe system 34 connected to a lift damping de¬ vice 35 having here the shape of an inflatable flap 36. A resilient pipe wall 37 will allow for the required change in volume. Fig. 8 shows yet another embodiment of a device according to the invention. In this embodiment a mass 38 is connected by means of a lever system to a hydraulic valve 39 forming part of a hydraulic system indicated by the letter P. The valve 39 leads the hydraulic medium to a cylinder 40, which in its turn adjusts the pitch angle of the entire blade. Fig. 9 is a diagram showing the dynamic loads on a windmill blade when a device according to the invention, as shown in fig. 4, has been implemented. It is clearly seen that the quick loads from the random excitation (compare with vibra¬ tion 2 in fig. 1) have been reduced. The slow vibration, which is due to the variation of the wind profile with the altitude above ground level during one rotor rotation (com- pare with vibration 1 in fig. 1), has not been noticeably reduced, but is also of less importance to the life of the mill.

Claims: PATENT CLAIMS 1. Method for reduction of vibrations in a windmill blade, characterised in that the aerodynamic properties of the blade are changed as a function of the acceleration and/or speed in the direction of rotation and/or out of the rotor plane of a slideable device (13, 18, 26, 32, 38). 2. Device for use in carrying out the method according to claim 1 for reduction of vibrations in a windmill blade, characterised in that the device is a mass ( 13 ) which via a lever system (14) is mounted pivotably about an axis (14a) extending in the longitudinal direction of the blade so that by pivoting in a direction crosswise of the rotor plane, the mass (13) will move a flap (15) through the lever system (14) for reduced lift on the blade. 3. Device according to claim 2, characterised in that the device is a mass (38) which via a lever system (41) is mounted pivotably about an axis (41a) extending in the longitudinal direction of the blade so that by pivoting in a direction crosswise of the rotor plane, the mass will actuate through the lever system (41 ) a hydraulic valve (39 ) controlling a hydraulic medium for respective ends of a cylinder (40), which is adapted to change the pitch angle of the blade. 4. Device according to claim 2, characterised in that the device is a mass (18) which via a lever system (19) is mounted pivotably about an axis (19a) extending in the longitudinal direction of the blade so that by pivoting in the direction of rotation of the rotor, the mass (18) will move through the lever system (19) a lift changing device in the form of a turbulator (20). 5. Device according to claim 2, characterised in that the device is a viscous liquid (32) contained within a cylinder (33) extending perpendicularly to the longitudinal direc¬ tion of the blade, and the front end of which in the direc¬ tion of rotation of the rotor has a resilient wall (37) and the rear end of which via a pipe (34) is connected to a lift changing device (35) in the form of a flap (36), which can be inflated by the viscous liquid (32). 6. Device according to claim 2, characterised in that the device is a mass (24) which is slideably arranged within a first cylinder (25) as a piston and in a position un¬ affected by accelerations is centered within the cylinder (25) by means of springs (26) arranged at either end of the mass (24), that the cylinder (25) contains a viscous medium and via pipes (28) arranged at either end is connected to the ends of a second cylinder (29) having a piston (29a) and piston rod (29b), the protruding end of which is con¬ nected to a lift changing device (30), such as a flap or a turbulator. 7. Device according to claim 6, characterised in that the mass (24) has a smaller sectional area than the sectional area of the first cylinder (25) and that the two ends of the first cylinder (25) are connected to a pipe (31a) pro- vided with an adjustable throttle valve (31).

Inventors: ENEVOLDSEN PEDER BAY, STIESDAL HENRIK, ENEVOLDSEN, PEDER, BAY, STIESDAL, HENRIK
Applicants: BONUS ENERGY AS, ENEVOLDSEN PEDER BAY, STIESDAL HENRIK, BONUS ENERGY A/S, ENEVOLDSEN, PEDER, BAY, STIESDAL, HENRIK
Application year: 1996
Main classification: F03D11
IPC codes: F03D1, F03D11