Fluid reaction surfaces (i.e. – impellers) – Support mounting – carrier or fairing structure
Reexamination Certificate
1996-07-22
2001-04-10
Verdier, Cristopher (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Support mounting, carrier or fairing structure
C416S500000, C416S24400R, C416S119000, C188S379000
Reexamination Certificate
active
06213721
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to noise emission reduction and particularly to the reduction of noise emitted by the tower of a wind turbine.
Wind turbines conventionally comprise a tall toward of circular or polygonal cross-section with a nacelle mounted at the top, the nacelle mounting a large multi-bladed propeller rotating about the horizontal axis and connected through a system of gears to an electrical generator also contained within the nacelle, the nacelle is arranged to rotate about a vertical axis on top of the tower so as to keep the propeller disc facing into the wind. All of the parts driven by the turbine mechanically are contained within the upper nacelle and electrical power generated within the nacelle is carried down the tower and away to its destination by the cables and rotary electrical connectors.
Since the wind turbine includes a generator and associated gearing the wind turbine inevitably generates noise. Due to the necessity to place wind turbines on high windswept points and due to the fact that the noise is generated by the parts at the top of the wind turbine such noise can be a problem over a large area around the wind turbine since the noise can travel along a direct straight line path to considerable distances without encountering any absorbing obstruction.
This problem is accentuated by the fact that most wind turbines generate noise at two or more discrete frequencies which are distinctive of the wind turbine design as well as generating noise across a continuous spectrum so where a farm of a number of identical wind turbines is set up their combined noise at these discrete frequencies, being at the same frequency for all of the turbines, can carry for several miles.
It has been found that a major source of this noise is mechanical vibrations generated by the moving parts inside the nacelle which transmitted into the nacelle structure and then into the tower. This mechanical vibrations causes the tower structure to vibrate and it is this vibration of the tower structure which emits the noise.
It has been attempted to solve this problem by fitting vibrating isolating mounts between the machinery is the nacelle and the actual nacelle and tower structures but this has been fully effective in eliminating this noise.
This invention was intended to eliminate or at least reduce noise emission from wind turbine towers, but may have other applications.
BRIEF DESCRIPTION OF THE INVENTION
According to a first aspect this invention provides apparatus for reducing noise emission from a structure vibrating at two or more discrete frequencies, the apparatus comprising a plurality of vibration damping tiles secured to the surface of the structure, each tile comprises a high density layer linked to the structure by a layer of plastics material and has compression and shear modes of vibration and associated resonant frequencies, the mass of the high density layer, the dimensions of the layer of plastics material and the properties of the plastics material being selected such that the compression mode and shear mode resonant frequencies correspond respectively to two of said discrete frequencies.
Employing the present invention provides a cost effective means of reducing vibrations from a structure at two frequencies, and is particularly applicable to wind turbines. Accordingly a second aspect of the invention provides a wind turbine comprising a nacelle, the nacelle generating noise in the support tower at two or more discrete frequencies or frequency bands, a surface of the support tower having at least one vibration damping tile secured to it, the at least one tile comprising a high density layer linked to the tower by a less dense plastics material and has compression and shear modes of vibration and associated resonant frequencies, the mass of the high density layer, dimensions of the layer of plastics material, and the properties of the plastics material being selected such that the compression mode and shear mode resonant frequencies correspond respectively to two or said discrete frequencies or frequencies bands of the nacelle and act to reduce the amplitude of vibration of the support tower at those frequencies or frequency bands.
Noise at the two frequencies to which the tile is tuned can be eliminated, or at least reduced, in the support structure of a wind turbine in accordance with the second aspect of the invention. The invention is particularly advantageous as the tiles can be fitted to either new or existing structures. Furthermore in the case of existing structure employing tiles which act to suppress two different frequencies halves the number of tiles and associated area that would otherwise be required. This is particularly advantageous for it enables sufficient tiles to be placed on the inner wall of a wind turbine support tower above the safety door normally about 6 feet below the top of the tower. Once in this region safety equipment used whilst climbing the tower can be removed and the tiles can then be bounded to the inner walls while standing on the safety platform.
According to a third aspect of the present invention there is provided a method of reducing vibrations in a structure comprising the steps of:
identifying two frequencies or frequency bands in the structure that it is desired to suppress;
constructing a plurality of mass damping tiles each comprising a high density layer and a layer of plastics material by which the high density layer is to be connected to the structure;
selected the mass of the high density layer, the dimensions of the plastics material, and the properties of the plastics material such that the tile has a compression mode resonant frequency corresponding to one of the identified frequency or frequency bands, and a shear mode resonant frequency corresponding to the other identified frequency or frequency band; and
securing said plurality of tiles to the structure.
The present invention arose whilst the invention were designing a tile to suppress noise at a single frequency or frequency band, corresponding to the compression resonant frequency of the tile. During the course of their work it was realised that other resonant modes may be present within the tile and that it may be possible to exploit these. However there are difficulties in getting a single tile to operate at two predetermined frequencies, for altering any one parameter of the tile affects both frequencies. However when investigated it was found that according to the theory below it should be possible for a limited range of frequencies to create a tile which would have two distant resonant frequencies, one in compression mode and one in shear mode, and that such a tile would cover the two frequencies of particular concern in a wind turbine application.
Notation: f
6
=resonant shear frequency
f
c
=resonant compression frequency
m=mass of the steel plate
A=loaded area of the plastics layer (PU)
t=thickness of the PU
E
c
=compression modulus PU
E=Young's modulus PU
G=shear modulus PU
B=bulk modulus PU
S=shape factor=loaded area/free area
K=spring stiffness
k=constant
The rigid body resonant frequencies of the tile may be calculated from the following:
f
=
1
2
π
K
/
m
(
1
)
where K=E
c
A/t for compression frequency and K=G A/t for the shear frequency.
The compression modulus is calculated from
Ec=E(1+2kS
2
)/[1+E(1+2kS)/B] (2)
This expression was taken from ‘Engineering Design with Natural Rubber’ by P. B. Lindley published by The Malaysian Producers' Research Association. Tables for values of k for a given hardness are also to be found in this publication.)
The ratio of the resonant frequencies can therefore be calculated as:
f
c
/f
s
=[E(1+2kS
2
)/G(1+E 1+2kS
2
)/B]
½
It is seen from equation (3) that the ratio of the two resonant frequencies is dependent on the shape factor since all the other parameters in the equati
Casey Donald C.
Thomson Marconi Sonar Limited
Verdier Cristopher
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