For the general damping of vibrations in such blades, it is known to use different kinds of damping arrangements in the form of mechanical or hydraulic vibration dampers which have limited effect in connection with vibrations over a wide range of frequencies. As examples of such blades, reference is made to DK 95 00222 U3 and DK 172,039 B1.

It is also known to focus on a special and quite narrow frequency range, i. e. the natural frequency of the individual blade, at which considerable vibrations often appear in con- nection with the frequent changes in load which arise during operation. These vibrations are undesirable because they have a fatiguing effect on the blade material, and in some cases the changes in load which arise are of such a magnitude that they can be considered as being completely abnormal, and which at the strong vibrations which arise are known to shorten the life of the blade to a considerable degree. The commonly-known vibration dampers are either unsuitable for precisely this damping purpose, or it is a complicated matter to have them built into the blade.

With the invention it is achieved that this problem can be considerably reduced by making use of a tuned damping ar- rangement which can exercise a strong damping by a counter- vibration effect on the blade precisely at the natural fre- quency of the blade or possibly a harmonic hereof, while in general it is otherwise more or less ineffective as vibration damper. Such a tuned vibration damper shall be understood to be an arrangement with a freely movable mass, which in an un- damped manner can be accelerated at the beginning of a vibra- tion phase, and is hereby capable of arriving at an opposing, fixed stop face just as the vibration changes to a counter-

phase sequence, whereby the reverse vibration is counteracted by the whole of the kinetic energy of the moved mass.

With the invention it has been found possible to realise this principle in a particularly simple manner while making use of a damping arrangement of the TLD type (Tuned Liquid Damper), i. e. by use of one or more liquid containers with tuned length and fluid height in order to achieve a desired transmission rate of a fluid wave therein, in that such a wave will thus constitute the said movable mass. Consequent- ly, the liquid does not need to pass through any narrowed opening to achieve a general,"soft"vibration damping, but on the contrary is allowed to swash freely between the oppos- ing ends of the liquid container. In order to have the great- est effect, the damping arrangement should be placed as closely as possible to the tip of the blade, i. e. in an area where the breadth of the blade is particularly great. How- ever, with the invention it has been ascertained that in con- nection with ordinary windmill blades it is actually possibly to work with containers which, without any special unfortu- nate changes in the shape of the blade, can be built into the hollow blades with lengths and volumes of such a magnitude that a particularly distinct damping of the type discussed can be achieved. The vibration frequencies which are relevant will typically be of some few Hz, and it has proved that a corresponding"swashing frequency"can be achieved precisely if the relevant space at disposal is, so to speak, utilised to the full.

For an effective swashing it is a condition that the con- tainers used are only approximately half-filled with liquid, which for the individual containers imposes a limitation in the weight of the moved mass. It has proved, however, that in said outer area of a normal blade there will be room for the mounting of containers with a total liquid content which can have sufficient mass and mobility to be able to counteract

the natural frequencies of the blade in an effective manner.

However, this requires that the containers have maximum breadth inside the blade, and that the liquid can swash maxi- mally unhindered inside the containers, and in both of these respects the invention distinguishes itself considerably from the two above-mentioned publications.

Here, there are 4 conditions which are of particular sig- nificance: 1) While the edgewise natural vibrations can manifest themselves with quite great amplitudes, e. g. 10-20 cm for a large and heavy windmill blade, with the tuned damping it will thus be sufficient to use a relatively small amount of liquid, in that its main effect will be to damp the actual transient sequence so that the vibration does not increase in ampli- tude. For normal windmill blades, a liquid amount of only 10-20 kg will be adequate, and such an amount can well be housed in a container system in the cramped space near the tip of the blade.

2) For good utilisation of the available space-and by and large to make it possible for a damping of the relevant type to be effected-it is necessary to work with a container system which fills out the blade breadth as much as possible, whereby the con- tainers should be configured with a cross-sectional shape which is close to that of the cross-section of the blade, i. e. be suitably converging at both of the opposite sides so that the container cross-sec- tion is distinctly oval. The somewhat limited calcu- lation models for tuned liquid dampers refer to sin- gle vessels with opposing vertical end surfaces, and with the invention it has been found conditional with the oval container shape that a liquid wave which turns upon arrival at a pointed container end

will turn slightly earlier than upon arrival at a flat end wall, which imposes a limitation of the ef- fective container breadth. It has also been found, however, that the relatively short distance between the opposite sides of the container, which in prac- tice will be the opposite, curved sides of the oval container, impose a certain braking on the swash wave, the speed of which is hereby appropriately re- duced so that the container still behaves as effec- tive with full breadth.

3) With a rotating blade, it must be taken into consid- eration that a considerable G-effect will be imposed on the liquid in the containers near the tip of the blade. This has a distinct influence on the speed of the wave, in that this is increased by a factor of the square root of the gravitational acceleration.

On the whole, this condition is decisive for the re- levant damper system being able to be used in prac- tice for the damping of vibrations with the above- mentioned frequency (a few Hz), in that the blade would otherwise have to be of an unrealistically large breadth. If, for example, the G-effect applies 19G, the wave speed will be increased to approx. 4.3 times the wave speed at standstill, i. e. under sta- tionary conditions the blade breadth should be more than four times greater. On the other hand, it is hereby a condition that a relevant tuning can only be carried out in connection with a known G-effect, i. e. in connection with blade rotors which work with constant speed of rotation. During the rotation, the G-effect will change with plus/minus gravitation, i. e. 1G, but this is not thought to be of any sig- nificance for the efficiency of the damping.

4) Since the liquid containers are disposed in an area which will normally be converging outwards, the con- tainers must be configured with decreasing operative length, whereby the wave frequency will be increased in relation to the innermost containers which, out of regard for good liquid filling, should be as large as possible. In order to maintain the desired frequency, this increase, which to some degree is supported by the slightly greater G-effect which arises, must be compensated for by using a reduced liquid height, in that this has a reducing effect on the speed of the wave. The outermost containers, which are thus both smaller and have lower liquid height, will consequently only to a modest degree contribute to the overall mass of the movable me- dium.

For further information concerning the TLD damping, reference is made to"Journal of Wind Engineering and Industrial Aerodynamics"41-44 (1992), pp 1883-1894,"Model- ling of Tuned Liquid Damper" ; Elsevier Science Publishers B. V.

In the following, the invention will be explained in more detail with reference to the drawing, in which Fig. 1 is a plan view of a windmill blade which is pro- vided near its top area with built-in containers for vibra- tion damping in accordance with the invention, Fig. 2 is a perspective view of the relevant area, partly in section, and Fig. 3 is a schematic perspective view of just one of the relevant containers.

Near its outer end, the windmill blade 2 shown in fig. 1 is provided with a number of built-in, transversely-exten- ding liquid containers 4 which more or less fill out the whole of the internal cavity in the blade in the relevant

area. In this area, the breadth of the blade decreases out- wards, and for precisely this reason use is made of several containers which can be of a breadth or transverse length which will be decreasing outwards. Similarly, these contain- ers will be of a breadth which also decreases outwards.

This is seen in more detail in fig. 2, which shows the shell construction of the blade with the outer end of an in- ternal main beam 6 and with a number of containers 4 placed opposite the end of this beam. The containers 4 more or less fill out the inner cavity in the blade, whereby outwardly they decrease in both breadth and height. In fig. 3 only three containers are shown, but in practice a considerably higher number can be involved, e. g. up towards fifty or more containers. The object will be for all of the containers to- gether to hold an amount of liquid which is so great that by its said swashing it can manifest sufficient kinetic energy to effectively counteract the natural frequency of the blade.

In practice it has been found that a total liquid con- tent of 10-20 1 in the containers will be sufficient to achieve a good vibration damping against the natural vibra- tions contemplated, and precisely such a content will actu- ally be able to be housed in half-filled containers 4 lying near the tip area of the blade.

One of the containers 4 is shown schematically in fig.

3, in a situation where a liquid wave at the natural fre- quency of the blade has reached all the way forward to the left-hand end of the container. From here, the wave will re- turn to a congruent formation at the opposite end, and hereby influence this almost in a blow-like manner for coun- teraction of the vibration. When no vibrations are gener- ated, the liquid will position itself like a cake with sur- face 6 in the position shown, which due to the G-effect dur- ing the rotation of the blade will be maintained regardless

of the position of the blade. The relevant liquid cake will thus stand vertical in the position shown in fig. 2, pressed out against the respective outwardly-facing side surfaces of the containers 4.

When the frequency of the edgewise natural vibration for the relevant blade is known, together with the G-effect to which the container will be exposed when in operation, each individual container 4 must be filled with liquid for the tuning, e. g. in a vibration machine.

When the G-effect is not known, the filling should be effected for tuning to a swash frequency which is the fre- quency of the natural vibration divided by the square root of the estimated G-effect.

The damping containers are preferably built into the outermost end of the blade, e. g. in a pivotal tip part of the blade as indicated in fig. 1, and they can be completely built in because with said embodiment they can be provided as long-life units which do not require any form of mainte- nance. In areas with possible frost, the filling with liquid should naturally be frost-proofed, e. g. by using glycol.