Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- 1 - R. H. Barnard 3X (Rev.)
VORTEX FL~WMETER
BACKGROUND OF THE INVENTION
. _ _ . . _
This invention relates to a device which employs vortex
shedding for measuring the relative speed between the device and
a surrounding fluid.
Such devices may be used to determine the relative speed
of a body through a fluid, as in a ship's log, or to determine
the relative speed of a fluid past a stationary body, as in a
fluid flowmeter. The relative speed is determined from
measurements of the frequency at which vortices are shed from an
obstruction in the fluid stream. The term "fluid" employed
herein is hereby defined to include both liquids and gases.
PRIOR ART STATEMENT
Various vortex flowmeters are well known. ~ypically
they comprise a bluff body disposed in a fluid stream, and means
for detecting the frequency of shedding of vortices produced by
the interaction of the fluid with the body. As the principles
of operation of vortex flowmeters are well known in the art and
have been exploited commercially for a number of years, such
principles of operation will not be described herein.
SUMMARY OF THE PRESENT INVENTION
Although many prior art bluff body configurations will
shed vortices at a frequency that is very nearly directly
related to the flow rate, the present invention resides in
certain geometric configurations that have been found to give
exceptionally good, very strong and highly regular shedding of
the vortices. The configuration of the present invention
produces a regular shedding of vortices with a lower loss of
total energy in the fluid stream than that given by other
published designs of similar geometric hlockage. The bluff body
of the present invention is also well suited for use with some
preferred versions of the type of vorte~ shedding frequency
sensor employing fiber optics.
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According to the present invention, there is provided
velocity measurement apparatus comprising: a passageway; an
elongate vortex shedding bluff body extending across said pas-
sageway, said passageway being adapted to carry a flow of a
fluid; and a sensor to sense vortices produced by said bluff
body, said sensor producing an output signal directly propor-
tional to the speed of said fluid relative to said bluff body,
said bluff body having a cross-sectional shape including a
tapered head with the,taper pointing away from said sensor
and a tail portion downstream of said head and having a width
along the entire length thereof, said tail portion width being
smaller than the width of said head, said head having an axial
length not greater than half of said head width.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings which illustrate exem-
plary embodiments of the present invention:
Figure 1 is a longitudinal sectional view of a bluff
body arranged in a passageway; and
Figure 2 is a side elevational view of an alternative
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In Figure l a bl,uff body 1 eY~tends as a bar in front
of a conventional vortex sensor 20 across a pipe 12 defining
a passage for fluid flow in the direction of an arrow A. Lead
faces 2 of bluff body 1 are arranged to make an included angle
that is greater than 90, but less than 180. This fea-ture
has been found to reduce significantly the total energy loss
through a flowmeter incorporating such a passage and body in
comparison to flowmeters using bodies of a similar configuration,
i.e., bodies having the same ratio of bluff body cross-stream
depth (head width) d to pipe diameter D, but with a flat leading
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face. If the included angle ~ between the leading faces 2
is less than 90 and the other dimensions are within the ranges
specified below, then the vortex shedding becomes ill-defined
in frequency, and irregular in terms of the response that it
generates in sensor 20. As angle ~ approaches 180, the energy
loss increases and the energy loss reduction
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-` ~179528
- 3 - ~. H. Barnard 3X (Rev.)
advantage is gradually lost. For ~ = 90 x the axial length
of the head portion of the bluff body is half the head width d,
for ~ greater than 90 x is less than d/2.
The side faces 3 are preferably parallel as shown in the
drawing, but may be inclined. With the side faces 3 parallel,
as illustrated, however, this bluff body configuration is
particularly well suited for integral use with some versions of
certain ~iber-optic vortex sensors (pressure sensors). Such
sensors involve the use of parallel diaphragms. It is an
1~ advantage that such diaphragms be flush with the bluff body side
faces 3 so as to prevent the formation of crevices in which dirt
or other deposits may accumulate. However, the bluff body may
alternatively be employed with other sensors which sense the
vortices and produce an output signal representative of the
speed of fluid flow through the pipe 12. Many different sensors
exist, and are well known in the art.
The streamwise distance L between the corners 4 and the
corners 5 may be anywhere in the range of one-half to twice that
of the maximum cross-stream dimension d. The most regular
shedding of vortices occurs, however, when distance L is in the
region of 1.4 times that of dimension d. The distance 1 between
the corners 5 ~tail width) may vary between a value approaching
zero and a valùe equal to 1/2 d with the sides preferably
parallel. Not all combination of dimensions d, L and 1 within
the specified ranges will give good regular vortex shedding, but
with the dimensions d and L in the region of the ratio 1:1.4 and
the dimensions d and 1 simultaneously in the region of the
ratio 1:0.33, good regular vortex shedding occurs.
A meter using a bluff body of the type described above
(Fig. 1) is hereinafter referred to as having an arrow shape.
In the case of triangular cross-sectioned tapered heads,
the angle ~ between the leading faces thereof is preferably, but
not exclusively, in the range 100 to 140.
Bluff body sections other than those shown in Fig. 1 may
be employed. It is not necessary that the head of the bluff
body be pointed or he arrow-shaped.
79S~8
- 4 - R. H. Barnard 3X (Rev.)
Fig. 2 shows an alternative bluff body in which a
head 13, while tapering in the upstream direction, is rounded
rather than pointed at its tip 14. Other symmetrical tapering
configurations are possible such 25 those which include more
rounded heads, or heads whose sections comprise a series of
curves or lines joining their tips to the maximum width
portions. For all of these configurations, includin~ those
shown in Figs. 1 and 2, the axial length of the head x is equal
to or less than half the head width d.
1~ The invention has been described with respect to a
vortex flowmeter in which the speed of fluid flow through a
passage is determined. It may alternatively be employed to
determine the relative speed of a body in a fluid as in a ship's
log.
ADS:rw
3/22/82
