Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1986-02-27
1987-11-17
Goldstein, Herbert
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
250227, G01F 132
Patent
active
047065028
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a flowmeter for measuring the velocity of flow of a fluid, such as, a liquid, gas or vapour phase, by detecting the vortex shedding frequency of the fluid flow and, more particularly, to such a flowmeter which utilises a fibre optic sensing element to detect vortex shedding.
Flow rate measurement based on the natural phenomenon of vortex shedding is known and has been realised using a variety of techniques for monitoring the vortex shedding frequency. When a bluff (that is, non-streamlined) body is sited in a fluid flow, it will, under certain conditions, cause a regular stream of vortices to form in the fluid downstream of the body. These vortices leave alternately from opposite sides of the body. Moreover, as each vortex is generated and shed, it produces a lateral force on the body and, if the body is sufficiently flexible, it will oscillate.
The pattern of flow past the bluff body is governed by the Reynolds number (Re). The transition from a steady laminar flow to an unsteady turbulent flow occurs at Re.about.40. For Re>40 wakes appear behind the body and eddies are formed. When Re exceeds 100, the boundary layer separates and the eddies are shed alternately from opposite sides of the body, resulting in the Karman vortex sheet. This vortex shedding occurs with a regular periodicity in the turbulent flow region, except in the transition regions when 200<Re<400 and 3.times.10.sup.5 <Re<3.times.10.sup.6. This breakdown is mainly due to transition from a laminar to a turbulent boundary layer state.
The vortex shedding frequency f is related to the flow velocity v by the equation body. Generally s is a function of the Reynolds number, and the study of the flow velocity dependence of s is an established branch of research in hydrodynamic studies. For large values of Re the turbulence of the vortices has a stabilising effect on the value of s and it is effectively constant. Hence, the flow velocity may be directly determined from a measurement of the vortex shedding frequency.
The linearity and wide dynamic range of the vortex shedding process have been exploited in a number of commercial flowmeter designs. Vortex shedding detection techniques include temperature, pressure and strain sensing, with the sensing element either being sited on the bluff body or being disposed down the vortex sheet. Also, a flowmeter of this type has been proposed which uses a multimode optical fibre sensing element, as the bluff body, and in which the oscillating strain induced in the fibre sensing element, as a result of the vortex shedding effect, is detected by the fibredyne technique. This proposal is described in the publication "Electronics Letters" of Mar. 19, 1981 at page 244. The fibredyne technique suffers from random fading and the generation of large numbers of harmonics of the fundamental pertubation. Whilst the technique adequately determines the vortex shedding frequency, it shows the harmonics of the fundamental frequency in the output spectrum and does not give the absolute amplitude of the strain.
SUMMARY OF THE INVENTION
It is an object of the present invention to avoid the problems experienced with the previously proposed arrangement of fibre optic vortex shedding flowmeter and to provide such a flowmeter having a single mode optical fibre sensing element and utilising interferometric monitoring techniques.
To this end, the invention consists in a flowmeter for monitoring the vortex shedding frequency of a fluid flow, comprising a tensioned optical fibre sensing element arranged to extend transversely to the fluid flow so as to oscillate in response to vortex shedding, a light source for illuminating the optical fibre, and a photodetector for sensing the interference pattern at an output of the fibre sensing element and producing an electrical signal corresponding to the vortex shedding frequency, characterised in that the sensing element comprises at least part of a single mode optical fibre forming the signal arm of an interferom
REFERENCES:
patent: 4416159 (1983-11-01), Williamson et al.
patent: 4530603 (1985-07-01), Shaw et al.
patent: 4534222 (1985-08-01), Finch
patent: 4613752 (1986-09-01), Davis
Bucaro et al, "Acoustic-Optic Sensor Development", in Conf. IEEE Electronics & Aerospace Systems, pp. 572-580, 10/79.
Lyle et al, "Vortex Shedding Fluid Flowmeter Using Optical Fiber Sensor", in Electronics Letters, 3/81, vol. 17, #6, pp. 244-245.
Jackson David A.
Jones Julian D. C.
Leilabady Pedram A.
Goldstein Herbert
Kent Scientific and Industrial Projects Limited
LandOfFree
Vortex shedding flowmeter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Vortex shedding flowmeter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Vortex shedding flowmeter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1442081