Test arrangement and method for fatigue testing a wind turbine blade

Abstract

A test arrangement for fatigue testing a wind turbine blade, including a floor-mounted test rig having a fixing device for fixing the wind turbine blade to the test rig and an excitation assembly for exciting the wind turbine blade at a test frequency, wherein the test rig includes a liquid tank having a chamber containing a predefined liquid, wherein the liquid in the liquid tank has a resonance frequency depending on the amount of liquid in the liquid tank and the chamber geometry is provided.

Claims

1. A test arrangement for fatigue testing a wind turbine blade, comprising: a floor-mounted test rig having a fixing device for fixing the wind turbine blade to the floor-mounted test rig; and an excitation assembly for exciting the wind turbine blade at a test frequency; wherein the floor-mounted test rig comprises a liquid tank having a chamber containing a predefined liquid, the predefined liquid in the liquid tank having a resonance frequency depending on an amount of predefined liquid in the liquid tank and a chamber geometry.

2. The test arrangement according to claim 1, further comprising an actuating device for adjusting the resonance frequency of the predefined liquid, the actuating device being a liquid amount adjusting device and/or a chamber geometry adjusting device, wherein the amount of predefined liquid and the chamber geometry are chosen such that the resonance frequency of the predefined liquid is at least essentially equal to the test frequency.

3. The test arrangement according to claim 2, further comprising a control device configured to control the actuating device such that the resonance frequency of the predefined liquid becomes at least essentially equal to the test frequency.

4. The test arrangement according to claim 1, wherein the floor-mounted test rig comprises a massive main body mounted on a floor via a foundation.

5. The test arrangement according to claim 4, wherein the liquid tank is mounted laterally to the main body and comprises or is attached to the fixing device opposite to the main body.

6. The test arrangement according to claim 4, wherein the liquid tank is mounted on top of the main body.

7. The test arrangement according to claim 1, wherein the chamber has a cuboidal or cylindrical form and/or that the liquid tank is made of steel and/or a fibre reinforced material.

8. The test arrangement according to claim 1, wherein the predefined liquid comprises water and/or oil and/or a material having a viscosity larger than 1 Pa*s.

9. The test arrangement according to claim 1, further comprising additional liquid tanks and/or the liquid tank comprises multiple chambers for different oscillation directions and/or test frequencies.

10. A method for fatigue testing a wind turbine blade using a test arrangement according to claim 1, the method comprising: choosing the test frequency for the wind turbine blade that at least essentially matches the resonance frequency of the wind turbine blade; adjusting the amount of the predefined liquid in the chamber and/or the chamber geometry depending on the test frequency such that the resonance frequency of the predefined liquid in the chamber is at least essentially equal to the test frequency; and exciting the wind turbine blade at the test frequency.

11. The method according to claim 10, wherein that a cuboidal chamber for water as the predefined liquid is used, wherein the amount of water in the chamber is determined according to the equation $f_n=\frac{1}{2\pi }\sqrt{\frac{n\pi g}{L}\tanh \left(\frac{n\pi h}{L}\right)},$ wherein f.sub.n is the resonance frequency of the n-th mode, g is the gravity constant, h is the height of the water in the liquid tank and L is the length of the cuboidal chamber.

12. The test arrangement according to claim 1, wherein the liquid tank is a damping device for damping oscillations of the wind turbine blade introduced into the test rig.

Description

BRIEF DESCRIPTION

(1) Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

(2) FIG. 1 depicts a schematical drawing of a first embodiment of a test arrangement;

(3) FIG. 2 depicts a frontal view of the test rig of the test arrangement of FIG. 1;

(4) FIG. 3 depicts a graph showing resonance frequencies of water depending on its amount and chamber geometry;

(5) FIG. 4 depicts a schematical drawing of a second embodiment of a test arrangement;

(6) FIG. 5 depicts a frontal view of the test rig of the test arrangement according to FIG. 4; and

(7) FIG. 6 depicts a liquid tank having two chambers.

DETAILED DESCRIPTION

(8) FIG. 1 shows a first, an exemplary embodiment of a test arrangement according to the embodiment of the present invention. The test arrangement 1 is adapted for fatigue testing of a wind turbine blade 2 also indicated in FIG. 1, but not forming part of the test arrangement 1.

(9) The test arrangement 1 includes a test rig 3 to which the wind turbine blade 2 may be fixed via a fixing device 4, for example a flange having the same fixing means as a rotor hub of a wind turbine where the wind turbine blade 2 can be employed. The test arrangement 1 further includes an excitation assembly 5 only schematically shown, which is adapted to excite the wind turbine blade 2 using a test frequency, which may be equal or close to a resonance frequency of the wind turbine blade 2, at a position away from the ground-mounted test rig 3. The test rig 3 is thus unmoveable and fixed to the floor so that moments introduced from the blade route 6 are at least partly transferred to the ground via the test rig 3.

(10) The test rig 3 includes a main body 7 made of concrete, which may optionally be attached to a foundation 8 also made of concrete. However, the fixing means 4 in this first embodiment is not directly mounted on the main body 7, but a liquid tank 9 defining a chamber 10 containing a liquid 11, in this case water, is mounted between the fixing device 4 and the main body 7. The liquid tank 9 can be understood as a damping device, since moments introduced from the blade root 6 via the fixing device 4 due to oscillations of the wind turbine blade 2 excite the liquid 11 inside the chamber 10. Hence, energy is dissipated and less load is exerted onto the main body 7, since the blade root 6, the fixing device 4, the liquid tank 9 and the main body 7 are arranged in series.

(11) In the embodiment of FIG. 1, the resonance frequency of the liquid 11 in the chamber 10 is adaptable to the test frequency used by the excitation assembly 5. In particular, in this embodiment, the chamber 10 is a cuboid, that is, a rectangular box. The resonance frequency of water as the liquid 11 in the chamber 10 depends on the chamber geometry, in particular its length L, and the amount of liquid 11, in this case the height h of the water inside the chamber 10, according to the above-discussed formula (1). The graph of FIG. 3 shows the frequency f.sub.1 depending on h and L. As can be seen, the resonance frequencies of the liquid 11 in the chamber 10 have values also typical for test frequencies used for fatigue testing blades 2.

(12) While it is in principle conceivable to have a predetermined amount of water and a predetermined chamber geometry adapted to a certain test frequency employed for fatigue testing of wind turbine blades 2 of a certain size, in particular length, in this embodiment, an actuating device 12 comprising a liquid amount adjusting device 13 is provided. The liquid amount adjusting device 13 includes a reservoir 14 for water, a pump 15 and valves which are not shown in FIG. 1 for simplicity. Using the actuating device 12, the amount of water in the chamber 10 can be configured such that a certain resonance frequency of the water in the chamber 10 results, which is at least essentially equal to a test frequency used, but may also, for example, relate to other modes of the wind turbine blade 2. To allow this kind of adaption to a wind turbine blade 2 to be tested, the test arrangement 1 also includes a control device 16 adapted to control the excitation assembly 5 as well as the actuating device 12. Since the test frequency used by the excitation assembly 5 is therefore known to the control device 16, the actuation device 12, in particular the liquid amount adjusting device 13, can be controlled to adapt the amount of liquid 11 in the chamber 10 such that the resonance frequency of the liquid 11 is at least essentially equal to the test frequency used by the excitation assembly 5, or equal to another frequency to be damped. The amount of water can, for example, be determined according to formula (1).

(13) In the embodiment shown in FIGS. 1 and 2, the liquid tank 9 is made of steel and secured to the main body 7 by bolts 17. In the figures, long bolts 17 are shown, however, shorter bolts 17 may also be used. A thick steel plate (not shown) attached to the main body 7 may be used as an attachment aid for the liquid tank 9. In this manner, a stable, robust construction is achieved.

(14) FIGS. 4 and 5 show a modified second embodiment of a test arrangement 1 according to the embodiment of the present invention. In this case, the fixing device 4 is directly attached to the main body 7, wherein the liquid tank 9 is arranged on top of the main body 7. In this embodiment, the load received from the blade root 6 of the wind turbine blade 2 steal acts on the main body 7 before being reduced by exciting the liquid 11 in the liquid tank 9.

(15) In the case that two or more test frequencies are used, for example for exciting the wind turbine blade 2 in two different modes, a liquid tank construction as shown in FIG. 6 may be used. The liquid tank 9 in FIG. 6 includes two chambers 10a, 10b, both containing a certain amount of liquid 11 and having a chamber geometry such that one of the test frequencies is, for example, matched by the resonance frequencies of the liquid 11 in the respective chamber 10a, 10b. In this case, for example, an actuation device 12 may be associated with each chamber 10a, 10b, so that both resonance frequencies of the respective liquid 11 may be adjusted.

(16) It is noted that the chamber 10, 10a, 10b may have different geometrical shapes in other embodiments, for example a cylindrical shape. Further, the liquid 11 used can also be or includes another material, for example oil or slime. It is finally noted that the actuating device 12 may also include a chamber geometry adjusting device such that the resonance frequency of the liquid 11 may also be adjusted by changing the chamber geometry, for example the length L.

(17) Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

(18) For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.