WO1995019500A1

WO1995019500A1 - Windmill

Info

Publication number: WO1995019500A1
Application number: PCT/DK1995/000017
Authority: WO
Inventors: Peter Grabau
Original assignee: Lm Glasfiber A/S
Priority date: 1994-01-12
Filing date: 1995-01-11
Publication date: 1995-07-20
Also published as: DK172218B1; DE19580147T1; DK73796A; AU1413595A; DE19580147B3

Abstract

A device for reducing the noise of windmills. In recognition of the situation that a large amount of the noise and other harmful influences on windmills are caused by vortices at the trailing edge of the wing, a flexible trailing edge is used which may adjust to local variations of pressure. A preferred embodiment utilizes a woven trailing edge (6) of strong artificial fibres which are cemented into the rear edge (4) of the wing profile at the joint between the two halves (1, 2). Particular advantages may be obtained by unravelling the strips, so that a large amount of individual fibres constitute the flexible trailing edge.

Description

WI NDMI LL

The invention concerns a device for reducing noise at the wings of windmills comprising a frontal edge and a trailing edge with a wing profile therebetween and a wing tip.

Windmills take energy from the flowing air and changes the direction of the air which leaves each wing. During the running of windmills an annoying noise may be noticed which comprises noise from the gearbox which is fitted between the rotor shaft and the generator which is driven by the windmill and aerodynamic noise which in addition sometimes takes the form of a pure tone or whistle which predominantly stems from the influence of the wind on the wings. Noise from the wings is in particular generated by pressure differences where two currents of air meet, i.e. where the flow from the front and rear sides of the wing meet at the rear edge and at the wing tip. The pressure differences give rise to cross flows and vortices, and these may become audible when the flow is across sharp edges, which is the traditional shape of the trailing edge of wings. The same vortex generation which causes the noise also influences the stability of the wing and of the whole windmill and its stalling characteristics, as well as a power loss because some of the power is radiated as acoustic power.

It has been considered that a part of the noise is caused by the laminar/turbulent flow across the profile of the wing gives rise to vortex generation at the trailing edge of the wing. These may because of non-linearities feed energy back to flow which is nearer the frontal edge whereby the laminar flow is disturbed over a larger area of the wing. Thereby the wing becomes an aerodynamically worse profile, and not only is the efficiency reduced, but the wing as such is subjected to vibrations. Furthermore the noise obtains an acoustic line spectrum which is perceive_d as a loud howling noise. It has been suggested to attack this non-linear feedback mechanism by giving the trailing edge a special configuration, but the best results have been obtained in the so-called "owl wing" where the frontal edge of the wing profile is fitted with a serrated edge. Apparently the laminar flow is attacked n such a way that the action of a non-linear feedback mechanism may be reduced. However, the frontal edge is subjected to wear, in particular due to precipitation and sand drift, and it would be preferable to influence the vortex shedding directly at the trailing edge.

It is the purpose of the invention to influence the flow at the trailing edge of a windmill wing in such a way that the harmful generation of vortices is avoided and a further purpose is to increase the stability of a wing.

The pressure differences may be equalized faster if the flow caused thereby is used either for contributing to the work in moving the wing or may be braked diffusively.

It would not be sufficient to attempt to change the direction of the flow, as this would only increase the tendency for vortex formation and influence the stalling properties and stability and efficiency.

The invention is based on the recognition that it is possible to let the trailing edge carry a material which is able to conform its shape to the pressure difference between upper and lower side of the wing at the trailing edge, possibly to let parts of the generated flow pass through this material. Due to this recognition a device for limiting the noise of a windmill wing particular in that the trailing edge is provided with a surface which is compliant along essentially the whole of the trailing edge.

An advantageous embodiment is obtained by letting this surface consist of an elastomeric material which may adapt itself to local pressure conditions caused by turbulence.

A further advantageous embodiment is obtained by letting the surface consist of a strip of woven cloth in a strong material, but a knitted strip will be more compliant, as is well known, and this constitutes a further embodiment. A further advantageous embodiment having even more compliance and a certain degree of damping of the flow is provided by letting the surface consist of a large number of smaller strips perpendicular to the trailing edge so that they may move independently and thereby further adjust to local pressure conditions.

The maximum of mobility in reaction to local pressure conditions is obtained in an advantageous embodiment where individual fibres collectively make up the desired trailing edge. Further embodiments provide particular advantages by means of the structure of the fibres. Smooth fibres provide relatively spoken a large stiffness perpendicular to the trailing edge for each fibre, while corrugated fibres give an increased possibility of damping the flows which appear due to local pressure variations.

The invention will be described in further detail in the following with reference to the drawing in which

Fig. 1 shows a cross section in a wing profile with an elastic trailing edge, Fig. 2 shows a cross section in a wing profile with a local deflection of the elastic trailing edge,

Fig. 3 shows an enlarged image of the joint between the wing profile and the elastic trailing edge,

Fig. 4 shows a wing profile with a trailing edge fitted with fibres, and

Fig. 5 shows an enlarged image of the joint between the wing profile with a trailing edge built of fibres.

In Fig. 1 is seen a hollow wing profile which consists of two halves 1 and 2. They are each made in the traditional manner by building-up in a mould and by way of example utilising alternating layers of a polyester resin and glas cloth, with a smooth gel-coat as the first layer, i.e. closest to the smooth mould part. During assembly a special cement is supplied to the front edge 3 and to the trailing edge 4, and according to the invention a strip of elastomeric material 5 is simultaneously cemented into it during assembly of the wing profile. In Fig. 3 it is shown how a sufficiently large area for cementing is provided for fastening the elastomer strip securely. In Fig. 2 it is shown how the strip 5 is locally bent upwards in reaction to a local underpressure on the top side, and as there is internal friction in the strip a considerable part of the energy which would be converted to a vortex tube parallel to the trailing edge is converted to heat instead. In this way it is partly avoided that energy is fed back along the wing profile towards the front edge, partly that a strong vortex tube is created.

The strip 5 of elastomeric material may advantageously have a profile of its own which is pointed towards the rear, and it may be reinforced in order that it is not worn to shreds during use. In Fig. 4 is correspondingly shown how a fibre strip 6 is fixed by cementing between the two halves l and 2 at the trailing edge 4, and in Fig. 5 is shown this cementing-in-place is shown in greater detail. At this stage of manufacture it is immaterial whether the strip is the last step in the manufacturing process, such as the use of a knitted strip or if the finished wing is provided with slits in the woven material or if the strip is taken completely to pieces to obtain individual fibres. The important anchoring of the fibres is in the present case performed by cementing between the two wing profile halves, but it might also be obtained by disposing them as "eye lashes" in conjunction with the lamination of either of the two wing profile halves. The important is that there is no sharp edge against which the fibres might be broken.

In case it is individual fibres which like the hairs of a brush terminate the trailing edge, the strongest construction would be by lenghtwise slitting of woven ribbon which contains strong fibres, preferably aramide fibres. This typically consists of a warp of cotton thread with a weft of aramide threads. The lengthwise slitting enables one selvedge or list to be cemented or laminated to the wing, whereby a strong anchoring is obtained due to the loops. When the strip is mounted the warp threads are removed by unravelling, and a very large number of aramide fibres remain which have a slight corrugation. In case a harder warp is used, the corrugation will be stronger, and it may be influenced by heat treatment after the weaving operation.

Claims

P A T E N T C L A I M S

1. A device for reducing noise at the wings of windmills comprising a frontal edge and a trailing edge with a wing profile therebetween and a wing tip c h a r a c t e r i s e d i n that the trailing edge is provided with a surface which is compliant along essentially the whole of the trailing edge.

2. A device according to claim 1, c h a r a c t e r i s e d i n that the surface is constituted by a thin elastomer strip.

3. A device according to claim 1, c h a r a c t e r i s e d i n that the surface is constituted by a woven ribbon, preferably of aramide.

4. A device according to claim 3, c h a r a c t e r i s e d i n that the ribbon is is a knitted ribbon, preferably of aramide thread.

5. A device according to claim 2,. c h a r a c t e r i s e d i n that the ribbon is broken along at least a part of its extent by cuts perpendicular to the trailing edge.

6. A device according to claim 1, c h a r a c t e r i s e d i n that the surface is con¬ stituted by a very large number of aramide fibres which are disposed essentially perpendicular to the trailing edge.

7. A device according to claim 5, c h a r a c t e r i s e d i n that the threads are straight. •

8. A device according to claim 5, c h a r a c t e r i s e d i n that the threads are corrugated.

9. A process for the manufacture of a device for reducing noise according to any of the preceding claims, c h a r a c t e r i s e d i n that a strip of com¬ pliant material is cemented-in in connection with the assembly of the two halves of the wing at its trailing edge.

10. A process for the manufacture of a device for reducing noise according to any of the claims 1, 3 - 8, c h a r a c t e r i s e d i n that a strip cut from aramide cloth is laminated into one or several of the layers of the trailing edge facing the top surface, and that weft threads or warp threads respectively are removed from the strip, leaving warp threads or weft threads, respectively.