WO1989008216A1

WO1989008216A1 - Vibration suppression device

Info

Publication number: WO1989008216A1
Application number: PCT/GB1989/000205
Authority: WO
Inventors: William Simon Lamb; Alastair B. Sinker
Original assignee: The British Hydromechanics Research Association
Priority date: 1988-03-02
Filing date: 1989-03-02
Publication date: 1989-09-08
Also published as: GB8804923D0

Abstract

Vibration in a structure may be suppressed by mounting one or more bodies on the structure, the or each subtending no less than 45° around the axis of the structure. When a plurality of bodies are used, they may be mounted end-to-end, adjacent bodies being displaced around the axis. They may be displaced by an angle not less than the angle subtended by a body so that they do not overlap. The bodies may be used to give a buoyant effect to the structure by choosing their density to be less than the density of the fluid flowing past the structure.

Description

VIBRATION SUPPRESSION DEVICE

When fluid flows past a fixed structure, it may cause vibrations in that structure. When this is undesirable, the problem can be overcome by strengthening the structure, as has been done by affixing a helical rib to sheet metal chimney flues which are subject to the flow of air. GB-A-691259 is concerned with devices for damping vibrations in pipes, although the vibrations appear to arise from a flow of fluid within the pipe, rather than past the exterior of the pipe. In this prio specification, a tubular spring element is fitted over the pipe in the form of a metal ribbon wound on a mandrel so that each turn of the ribbon partially overlaps the previous turn. The spring element therefore extends completely around the structure whose vibrations are to be damped. Strengthening ribs on chimneys on the other hand subtend only a few degrees at the axis of the chimney.

We have found that vibrations in a structure subjected to a fluid flow can be suppressed by mounting on the exterior of the structure a body which extends only partially around the structure but subtends an angle of at least 45° at the centre of the structure. Good results have been achieved with bodies which extend around half of the structure, and with a plurality of such bodies, adjacent bodies being mounted at longitudinal intervals along the structure and in different directions in relation to the centre.

The overall specific density of the body is preferably below that of the fluid, so that a buoyancy effect is achieved. The body preferably has smooth surfaces, and sharp corners.

The invention has particular application in suppressing vibrations in structures such as pipes extending through the sea, where vibrations are induced by movement of seawater past the exterior of the structure. Examples of the invention will now be described with reference to the accompanying drawings in which:

Figures 1A, IB, 2A, 2B and 2C are side elevations of different embodiments of the invention.

Figures 3 - 7 show the relationship of y/D to Vr for various bodies.

In Figure 1A, a cylindrical structure is provided with vibration suppressing bodies on its exterior surface. Each body subtends an angle 90° at the centre of the cylinder. The exterior surfaces of the bodies are cylindrical and coaxial with the structure and of an outer radius of approximately double that of the radius of the structure. The end surfaces are radial in relation to the common axis, and the edges form right angles. There are two bodies in Figure 1A, located on opposite sides of the structure one being displaced by the length of one body in relation to the other.

In the example of Figure IB, there are four bodies, each extending around l8θ° of the structure, each displaced from the adjacent body by 90° in azimuth and by the length of one body axially. The orientation of the bodies progress regularly around the structure, forming a spiral along the axis of the structure. Figure 5 shows the relationship of y/D to Vr for this device for various angles of incidence phi, to be compared with Figure 3 which shows the same relationship for a bare cylinder. The full line in Figure 5 repeats the bare cylinder curve of Figure 2, and this curve is also repeated in Figures 4_> 6 and 7-

In Figure 2A, each body subtends lSθ° at the axis of the structure and adjacent bodies are displaced by l8θ°. Figure 4 shows the relationship of y/D to Vr for this device. In Figure 2B, each body subtends l8θ°, and adjacent bodies are displaced by 60°. Figure 6 j -

shows the relationship of y/D to Vr for this device. The angle of displacement can be varied to suit requirements, but it has been found that an angle of 45° or 135° gives best results.

In Figure 2C, the bodies of Figures IB, 2A and 2B are cut diagonally and one half is displaced l8θ° around the structure in relation to the other. The reassembled halves of the body so displaced are mounted in alignment with other reassembled halves to form the arrangement shown in Figure 2C. In this arrangement, it is important that the flow is directed in the interface plane of the two half bodies. Figure 7 shows in dashed lines the relationship of y/D to Vr for this device for this one angle of incidence. In Figure 1A, the flow is directed symmetrically between the two bodies and no y/D to Vr curve is given. In all the examples described, the bodies subtend at least 45° at the centre of the cylinder.

The bodies are preferably made of foam material or at least filled with foam material, in order to have a specific gravity less than that of the surrounding fluid, in order to achieve a buoyancy effect.

Variations of the illustrated embodiments are possible. The outer radius of the bodies can be as little as 1.05 times the inner radius. The outer surface need not be co-axial with the structure, and indeed the structure need not be cylindrical. The bodies need not be annular, but could be "wedge-shaped", i.e. the outer surface could spiral outwards starting from at or near the structure.

The bodies are securely mounted on the structure, in order to control the response of the structure to forces imposed by the passage of fluid therepast.

Claims

CLAIMS:

1. A device for suppressing vibrations in a structure having a central axis and subjected to a fluid flow comprising a body mounted on the exterior of the structure which body extends only partially around the structure that subtends an angle of at least 45° 3* the central axis measured transversely to that axis.

2. A devices as claimed in Claim 1 comprising a plurality of said bodies arranged end-to-end on said structure, each being displaced in relation to the adjacent bodies around said axis.

3- A device as claimed in Claim 2 wherein said bodies are displaced by 45° in relation to the adjacent bodies.

4. A device as claimed in Claim 2 wherein said bodies are displaced by 135° in relation to the adjacent bodies.

5. A device as claimed in any one of claims 2 to 4 wherein adjacent bodies are displaced around the structure an angle no less than the angle subtended by each body so that they do not overlap around the structure.

6. A device as claimed in any one of the preceding claims wherein the or each said body is a part toroid.

7- A device as claimed in Claim 6 wherein the outer diameter of the toroid is twice the inner diameter.

8. A vibration suppression device substantially as herein described with reference to the accompanying drawings.

9. A method of suppressing vibrations in a structure having a central axis and subjected to a fluid flow comprising mounting a body mounted on the exterior of the structure which body extends only partially around the structure that subtends an angle of at least 45° at the central axis measured transversely to that axis.

10. A method as claimed in Claim 9 comprising selecting the body to have a density less than that of the fluid.