GB2238380A

GB2238380A - Vortex shedding flowmeter

Info

Publication number: GB2238380A
Application number: GB8926473A
Authority: GB
Inventors: James Lionel West; Steven Webster
Original assignee: Hughes Microelectronics Ltd
Current assignee: Hughes Microelectronics Ltd
Priority date: 1989-11-23
Filing date: 1989-11-23
Publication date: 1991-05-29
Also published as: GB8926473D0

Abstract

Incoherent light from an LED 8 is launched into a tensioned, multimode optical fibre 2 positioned transversely across a region 6 where fluid flow velocity is to be measured. A photodetector 10 receives light transmitted by the fibre. Vortex shedding causes the fibre to vibrate and set up standing waves. These cause optical bending losses in the fibre and intensity modulation of the light beam. The frequency of modulation is detected by a frequency detector and frequency extracted as an output, 12, to provide a signal proportional to flow velocity. <IMAGE>

Description

VORTEX SHEDDING FLOWMETER This invention relates to a method and apparatus for measuring fluid flow velocities by monitoring the frequency at which vortices are shed from a bluff body suspended in the fluid.

Vortex shedding flowmeters are well known. Typical examples are described in the following US Patents: 4,656,353; 4,501,157; 4,706,502; 4,742,574; 4,206,642; 4,011,754; 4,339,661; and 4,519,259. In one type of such flowmeters, the bluff body and the element monitoring the vortices are separate entities; the monitoring element being downstream of the bluff body. A typical such example is described in US Patent 4,339,661. In a second type of such flowmeters, the bluff body is itself the monitoring element. A typical such example is described in US Patent 4,706,502.

In the latter US Patent, a tensioned single mode optical fibre is placed across the fluid flow. At certain velocities, von Karman vortex shedding occurs from the fibre which induces an oscillating strain therein. This strain is monitored by launching a coherent light beam into the fibre and detecting the optical path length changes induced by the oscillating strain using interferometric techniques.

The present invention is concerned with a vortex shedding flowmeter of the second type, which also employs a tensioned optical fibre to produce and monitor the vortices. The technique for monitoring is optical, but is much simpler than the interferometric technique disclosed in US Patent 4,706,502.

In the present invention a light beam is launched into the tensioned fibre. The latter vibrates and forms standing waves as the vortices are shed. This causes variations in the angle between the light beam and the fibre surface, which in turn causes the grazing angle to be exceeded at certain points along the fibre. These optical losses intensity modulate the light beam, and the frequency of modulation (proportional to fluid flow velocity) can be easily detected with standard electronic equipment.

According to the present invention, there is provided a vortex shedding flowmeter for measuring the velocity of a flowing fluid, comprising a tensioned multimode optical fibre for extending transversely to the fluid flow, a light source for launching a beam of light into one end of the optical fibre, photodetector means at the other end of the fibre for receiving light from the light source as transmitted by the fibre, and electrical sensing means, coupled to the photodetector means, responsive to the frequency at which the light beam is intensity modulated as a result of optical bending losses in the tensioned fibre, and for providing an output proportional to the flow velocity.

According to the present invention, there is also provided a method of monitoring the velocity of a flowing fluid, which comprises placing a tensioned multimode optical fibre transversely to the fluid flow, launching a light beam into one end of the fibre, receiving light from the light source at the other end of the fibre as transmitted by the fibre, and monitoring the frequency at which the light beam is intensity modulated as a result of optical bending losses in the tensioned fibre to provide an output proportional to flow velocity.

In order to obtain optical losses of acceptable magnitude, the fibre must be a multimode fibre. The light source is preferably of an incoherent nature.

A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which the Figure is a schematic diagram of a preferred flowmeter according to the invention.

Referring to the Figure, the flowmeter comprises a tensioned, multimode optical fibre 2 supported by means of a clamping arrangement 4 transversely across a region 6 where fluid flow is to be measured. One end of the fibre communicates with an LED 8 providing a source of incoherent light for launching into the fibre. The other end of the fibre terminates at a photodiode 10 communicating with electronic apparatus 12 for monitoring changes in light intensity as received by photodiode 10.

The electronic apparatus is conventional and measures the frequency of the light intensity modulation. It typically consists of an amplifier, a frequency detector, and a frequency to voltage converter. For convenience, to place the electronic components close together, a rigid support arm 14 returns fibre 2 from the right-hand side of region 6, to the left-hand side adjacent LED 8. The support arm 14 and clamping arrangement 4 are adjustable in length relative to one another by a micrometer (not shown) so that the tension in the fibre across region 6 may be adjusted.

In use, as the region 6 is subjected to a flowing fluid, at certain velocities vortex shedding occurs from the tensioned fibre which cause the latter to vibrate and set up standing waves. This resonance creates a series of bends in the fibre in region 6. These, in turn, create variations in the angle between the light and the fibre surface and cause the grazing angle to be exceeded for certain optical modes. These optical bending losses thus amplitude modulate the light transmitted by the fibre.

The frequency of the detected signal at photodiode 10 is proportional to fluid flow velocity.

As the flow rate changes, the response received by the frequency sensing equipment consists of a series of spaced points corresponding to the harmonics of the fundamental frequency of the fibre. This itself is dependant upon its length and tension. Provided that the adjacent resonance points are close enough together, in between such points there exist amplitude modulated signals whose modulation depth is governed by the proportions of the adjacent harmonics. The closer the interpoint spacing of harmonics (AHz) as a function of flow velocity, the more accurate is the flowmeter response. The tension on the fibre may be altered to tune the fibre, at any given steady flow rate, to a detectable resonant point, or to ensure that the harmonics are sufficiently close together that measurable interpoint modulation by mixing of adjacent harmonics occurs.As a calibration can be stored for the expected frequency for any given fluid of given viscosity and density, as a function of tension, it is possible to equate the tension and frequency of the fibre to flow velocity. The response varies with Reynolds No. (Re) and Strouhal No. (S), where: Re = p.u.d n and S = f.d u where p = density of fluid n = viscosity of fluid d = diameter of fibre f = frequency of vortex shedding u = velocity of fluid.

In typical measurements using such an above-described flowmeter, we have measured flow velocities in water and air up to about 10 metres per second with a fibre about 30 cms in tensioned length, and with tensions of 0.1 to 0.5 Newtons. Typical interpoint spacing of harmonics is 0.2 metres per second. These measurements relate to a clad multimode fibre, but we have discovered that higher flow rates can be measured with shorter fibres if the buffer coating is stripped.

The above-described apparatus is particularly suited for measuring flow rates in bulk volumes of fluid, simply by exposing the tensioned fibre in region 6 to the flowing fluid. The invention can, however, be also applied to fluid flowing through a tube by extending the tensioned fibre transversely across the tube. In such circumstances, particularly with liquids, precautions will need to be taken to prevent unwanted loss of fluid at the positions where the fibre enters and leaves the tube walls.

Claims

CLAIMS:

1. A vortex shedding flowmeter for measuring the velocity of a flowing fluid, comprising a tensioned multimode optical fibre for extending transversely to the fluid flow, a light source for launching a beam of light into one end of the optical fibre, photodetector means at the other end of the fibre for receiving light from the light source as transmitted by the fibre, and electrical sensing means, coupled to the photodetector means, responsive to the frequency at which the light beam is intensity modulated as a result of optical bending losses in the tensioned fibre, and for providing an output proportional to the flow velocity.

2. A flowmeter according to claim 1, wherein the light source is incoherent.

3. A flowmeter according to claim 2, wherein the light source is an LED.

4. A flowmeter according to any of claims 1 to 3 wherein the electrical sensing means includes a means for extracting a frequency as an output.

5. A method of monitoring the velocity of a flowing fluid, which comprises placing a tensioned optical fibre transversely to the fluid flow, launching a light beam into one end of the fibre, receiving light from the light source at the other end of the fibre as transmitted by the fibre, and monitoring the frequency at which the light beam is intensity modulated as a result of optical bending losses in the tensioned fibre to provide an output proportional to flow velocity.

6. A method according to claim 5 which comprises launching an incoherent light beam into a multimode optical fibre.

7. A vortex shedding flowmeter substantially as herein described with reference to the accompanying drawing.

8. A method of monitoring the velocity of a flowing fluid substantially as herein described with reference to the accompanying drawing.