Abstrict A Karman vortex flow meter including an upstream columnar body
having a substantially isosceles triangular cross section and a
downstream columnar body having a substantially isosceles trapezoidal
cross section. The downstream columnar body has a base surface which
is substantially equal in width to the base surface of the upstream
columnar body. The two bodies are positioned in a stream of a fluid
with their base surfaces disposed at a distance from each other
in mutually parallel relation and at right angles to the stream
for generating Karman vortices to measure the flowing speed or rate
of the fluid based on the frequency of the vortices. The upstream
columnar body has a pair of rounded edges along its base surface.
Claims What is claimed is:
1. A Karman vortex flow meter for measuring the flow rate of a
fluid by detecting the frequency of Karman vortex streets, said
flow meter comprising:
a pipeline for leading the flow of the fluid therein;
vortex generating means provided in said pipeline for generating
Karman vortices around both sides thereof;
vortex detecting means for detecting the frequency of the Karman
vortices generated around said vortex generating means; and
said vortex generating means comprising an upstream columnar body
having a substantially isosceles triangular cross section and a
downstream columnar body having a substantially isosceles trapezoidal
cross section, said downstream columnar body having a base surface
which is substantially equal in width to a base surface of said
upstream columnar body, each base surface of said upstream and downstream
columnar bodies being spaced apart from the other by a predetermined
distance in parallel to the other, said upstream columnar body having
a pair of rounded longitudinal edges along the base surface thereof.
2. A Karman vortex flow meter as claimed in claim 1 wherein said
upstream columnar body has a pair of narrow lateral surfaces lying
along the base surface thereof in parallel to the stream of the
fluid and each having a corner defining one of said rounded longitudinal
edges.
3. A Karman vortex flow meter for measuring the flow rate of a
fluid by detecting the frequency of Karman vortex streets, said
flow meter comprising:
a pipeline for leading the flow of the fluid therein;
vortex generating means provided in said pipeline for generating
Karman vortices around both sides thereof;
vortex detecting means for detecting the frequency of the Karman
vortices generated around said vortex generating means; and
said vortex generating means comprising an upstream columnar body
having a substantially isosceles triangular cross section and a
downstream columnar body, said downstream columnar body having a
base surface which is substantially equal in width to a base surface
of said upstream columnar body and having a cross-section defined
by an arcuately curved surface and its base surface, said curved
surface forming an arc of a circle, each base surface of said upstream
and downsteam columnar bodies being spaced apart from the other
by a predetermined distance in parallel to the other and being at
right angles to the stream of the fluid.
4. A Karman vortex flow meter as claimed in claim 3 wherein said
arcuately curved surface has a radius which is at maximum 0.75 times
as large as the width of the base surface thereof, said downstream
body having between said base surface and said arcuately curved
surface a height which is 0.3 to 0.6 times as large as the width
of the base surface thereof.
5. A Karman vortex flow meter as claimed in claim 3 or 4 wherein
said upstream columnar body has a pair of rounded longitudinal edges
along the base surface thereof.
6. The Karman vortex flow meter of claim 4 wherein said predetermined
distance is about 0.2 to 0.3 times the width of said base surface.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a Karman vortex flow meter for measuring
the flow rate of a fluid by detecting the frequency of Karman vortex
streets or trains which are regularly produced downstream of a columnar
body placed in a stream of the fluid at right angles thereto.
2. Description of the Prior Art
A Karman vortex flow meter of the type which is schematically shown
in FIG. 1 is known in the art and is disclosed in, for example,
Japanese Laid-Open Patent Application No. 184512/1983. The flow
meter comprises a pair of columnar vortex generating bodies 2 in
a pipeline 1 and a vortex detecting device 4 associated therewith.
The vortex detecting device 4 essentially consists of an upper housing
21 a lower housing 22 and a vibrator 24. The columnar bodies 2
consist of an upstream columnar body 5 having a cross section which
is substantially equal to that of an isosceles triangle, and a downstream
columnar body 6 having a cross section which is substantially equal
to that of an isosceles trapezoid, as shown in FIG. 2. They are
placed in a stream of fluid at right angles thereto. The pressure
fluctuations of the Karman vortices 23 which are generated around
both sides of the columnar bodies 5 and 6 are transmitted to the
vortex detecting device 4 through open spaces 3A and 3B. The upstream
columnar body 5 has a width or base length d.sub.1 which is equal
to the width or base length d.sub.2 of the downstream columnar body
6. They are spaced apart from each other by a predetermined distance
as shown at a, and arranged at right angles to the direction of
the fluid stream in the pipeline 1. The columnar bodies 2 provide
a flow meter capable of giving results of good linearity over a
wide range of measurement without causing any substantial pressure
loss of fluid.
The flow meter as shown in FIGS. 1 and 2 has, however, a serious
problem which will now be explained with reference to FIG. 3. The
problem occurs in a transitional stage during which a stream of
fluid makes a sudden change. It has been found through a visual
test that small vortices 23A having a higher frequency are generated
from the longitudinal edges of the upstream columnar body 5 in addition
to the Karman vortices 23 which are normally generated around the
downstream columnar body 6. Due to their high frequency, the vortices
23A grow more rapidly than the Karman vortices 23 especially the
fluid has a very widely varying flowing speed due to pulsation.
There is every likelihood that the vortices 23A may disturb the
Karman vortices 23 and make it difficult to measure the flow rate
of the fluid.
SUMMARY OF THE INVENTION
Under these circumstances, it is an object of this invention to
provide a Karman vortex flow meter which can effectively overcome
the drawbacks of the prior art as hereinabove pointed out and ensure
the stability of Karman vortices even during any transitional change
in the flowing speed of a fluid due to, for example, pulsation,
and which is simple in construction and easy to manufacture.
This object is attained by a flow meter essentially comprising
an upstream columnar body having a cross section shaped substantially
like an isosceles triangle and a downstream columnar body having
a cross section shaped substantially like an isosceles trapezoid,
the upstream columnar body having a base surface spaced apart from
the downstream columnar body, the downstream columnar body having
a base surface spaced apart from the base surface of the upstream
columnar body and lying in parallel thereto, the base surfaces of
the columnar bodies being substantially of the same width, characterized
in that the upstream columnar body has a pair of rounded longitudinal
edges each located on the opposite side of its base surface from
the other.
The upstream columnar body preferably has a pair of parallel narrow
lateral surfaces each located on the opposite side of its base surface
from the other and having a rounded corner defining one of its rounded
longitudinal edges.
The cross section of the downstream columnar body preferably has
a substantially semicircular shape defined by its base surface and
an arcuately curved lateral surface. The downstream columnar body
preferably has between its base and lateral surfaces a maximum thickness
or height which is about 0.3 to 0.6 time as large as the width of
its base surface, while its substantially semicircular cross section
preferably has a radius which is at maximum 0.75 time as large as
the width of its base surface.
Moreover, the downstream columnar body preferably has a pair of
isosceles surfaces having an angle of about 60.degree. therebetween.
Other features and advantages of this invention will become apparent
from the following detailed description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a conventional Karman vortex
flow meter;
FIG. 2 is a schematic illustration in cross section of the columnar
bodies in the flow meter of FIG. 1;
FIG. 3 is a view similar to FIG. 2 but showing the vortices generated
around the columnar bodies;
FIG. 4 is a schematic illustration in cross section of a flow meter
embodying this invention;
FIG. 5 is a view similar to FIG. 4 but showing the vortices generated
around columnar bodies;
FIGS. 6(A) and 6(B) are graphic representations of the Karman vortices
generated in a pulsating stream by the columnar bodies shown in
FIG. 5;
FIG. 7 is a schematic illustration in cross section of a flow meter
according to another embodiment of this invention;
FIG. 8 is a schematic illustration in cross section of a flow meter
according to still another embodiment of this invention;
FIG. 9 is a schematic illustration in cross section of a flow meter
according to still another embodiment of this invention;
FIG. 10 is a schematic illustration in cross section of a flow
meter according to still another embodiment of this invention;
FIG. 11 is a view showing the Karman vortices generated by the
columnar bodies in the flow meter of FIG. 9;
FIG. 12 is a view showing the Karman vortices generated by the
columnar bodies in the flow meter of FIG. 10;
FIG. 13 is a schematic illustration in cross section of a flow
meter according to a further embodiment of this invention;
FIG. 14 is a graph showing the rate of pulsation in relation to
the angle between the isosceles surfaces of a downstream columnar
body in the flow meter of FIG. 13;
FIG. 15 is a graph explaining the rate of pulsation;
FIG. 16 is a graph showing the relationship between the flowing
speed of a fluid and the Strouhal number as obtained when downstream
columnar bodies having different angles between their isosceles
surfaces are employed;
FIG. 17 is a graph showing the relationship between the flowing
speed of a fluid and the Strouhal number as obtained when downstream
columnar bodies having an angle of 60.degree. between their isosceles
surfaces have different trapezoidal heights;
FIG. 18 is a graph showing the minimum flowing speed of a fluid
at which vortices can be detected, in relation to the distance between
upstream and downstream columnar bodies; and
FIG. 19 is a graph showing the relationship between the flowing
speed of a fluid and the Strouhal number as obtained when upstream
and downstream columnar bodies having different distances therebetween
are employed.
DETAILED DESCRIPTION OF THE INVENTION
The invention will now be described more specifically with reference
to a number of embodiments or modifications which are shown in the
drawings. Referring first to FIG. 4 a flow meter embodying this
invention comprises a columnar body 7 consisting of an upstream
columnar body 8 and a downstream columnar body 9. The upstream columnar
body 8 has a cross section which is substantially equal to an isosceles
triangle. It includes a base surface 10 having a pair of rounded
longitudinal edges R. The downstream columnar body 9 has a cross
section which is substantially equal to an isosceles trapezoid.
The downstream columnar body 9 has a base surface 11 spaced apart
from a base surface 10 of the upstream columnar body 8 by an appropriate
distance a in parallel relation thereto and having a width d.sub.2
which is equal to a width d.sub.1 of the base surface 10. The columnar
bodies 8 and 9 lie in a stream of a fluid at right angles thereto.
They form Karman vortices 23 as shown in FIG. 5 but do not form
any such small vortices 23A as shown in FIG. 3.
FIGS. 6(A) and 6(B) show the characteristics of the Karman vortices
which are produced in a pulsating stream. In FIGS. 6(A) and 6(B),
V is the velocity of a fluid pulsating at an amplitude .DELTA.V.sub.1
or .DELTA.V.sub.2 .DELTA.Pa is the pressure of the fluid created
by the Karman vortices which are produced around the columnar body
7 of this invention shown in FIG. 4 and .DELTA.Pb is the pressure
created by the Karman vortices produced around the conventional
columnar body 2 shown in FIG. 2. FIG. 6(A) shows the pressures .DELTA.Pa
and .DELTA.Pb resulting from the pulsation of the fluid at an amplitude
.DELTA.V.sub.1 and FIG. 6(B) shows the pressures .DELTA.Pa and Pb
resulting from the pulsation of the fluid at an amplitude .DELTA.V.sub.2
which is greater than .DELTA.V.sub.1 . In FIG. 6(A), both of the
pressures .DELTA.Pa and .DELTA.Pb vary properly in accordance with
the amplitude .DELTA.V.sub.1 of pulsation of the fluid. In FIG.
6(B), however, the pressure .DELTA.Pb fails to vary with the greater
amplitude .DELTA.V.sub.2 as shown by a circle in a chain line, while
the pressure Pa still follows the amplitude .DELTA.V.sub.2 properly.
These results have been obtained experimentally. FIG. 6(B) confirms
that the columnar body 7 according to this invention enable the
stable generation of Karman vortices even in a fluid pulsating at
an increased amplitude.
Another device embodying this invention is shown in FIG. 7. It
includes a columnar body 12 consisting of an upstream columnar body
13 and a downstream columnar body 9 which is identical in construction
to that shown in FIG. 4. The upstream columnar body 13 has a pair
of narrow lateral surfaces b lying at right angles to its base surface
14 and in parallel to a stream of a fluid and a pair of rounded
corners R each formed between one of its narrow lateral surfaces
b and one of its isosceles surfaces joining each other at an apex
c. The provision of the narrow lateral surfaces b ensures the accurate
formation of the base surface 14 having an appropriate width d.sub.1
and the stable generation of Karman vortices 23 irrespective of
any transitional change in the flowing speed of a fluid due to,
for example, pulsation.
Another device embodying this invention is shown in FIG. 8. It
is a modified form of the device shown in FIG. 7. It includes a
columnar body 18 consisting of an upstream columnar body 19 and
a downstream columnar body 9 which is identical in construction
to that shown in FIG. 7. The upstream columnar body 19 has a pair
of narrow lateral surfaces b.sub.1 lying in parallel to a stream
of a fluid and a pair of rounded corners R each formed between one
of its narrow lateral surfaces b.sub.1 and its base surface 20.
The provision of the narrow lateral surfaces b.sub.1 ensures the
accurate formation of the base surface 20 having an appropriate
width d.sub.1 and the stable generation of Karman vortices 23.
Another embodiment of this invention is shown in FIG. 9. It includes
a columnar body 30 consisting of an upstream columnar body 31 and
a downstream columnar body 33. The upstream columnar body 31 has
a cross section which is substantially equal to an isosceles triangle
and the downstream columnar body 33 has a substantially semicircular
cross section. The base surfaces 32 and 34 of the upstream and downstream
columnar bodies 31 and 33 respectively, are substantially of the
same width d, and spaced apart from each other by an appropriate
distance a and lie in parallel to each other. They lie at right
angles to a stream of a fluid.
The upstream columnar body 31 has a pair of equilateral surfaces
and a pair of narrow lateral surfaces each formed between its base
surface 32 and one of its equilateral surfaces. The downstream columnar
body 33 has an arcuately curved surface. It has been experimentally
found that stable von Karman's vortices 23 can be generated without
causing any appreciable pressure loss if the equilateral surfaces
of the upstream columnar body 31 have an apex angle of about 90.degree.
and if the downstream columnar body 33 has a height h which is equal
to 0.3 to 0.6d between its base surface 34 and its arcuately curved
surface and its arcuately curved surface has a radius r not exceeding
0.75d, while the distance a between the bodies 31 and 33 is about
0.2d to 0.3d.
The columnar body 30 shown in FIG. 9 are effective for stabilizing
the generation of Karman vortices 23 in a transitional state in
which a fluid suddenly increases its flowing speed. It has, however,
been found through a visual test that the device of FIG. 9 is likely
to generate small vortices 23A having a higher frequency from the
edge of the base surface 32 of its upstream columnar body 31 in
addition to the Karman vortices 23 formed around its downstream
columnar body 33 as shown in FIG. 11 when the fluid pulsates and
suddenly increases its flowing speed. The vortices 23A prevent the
stable generation of Karman vortices 23.
Another device embodying this invention is shown in FIG. 10. It
is an improvement over the device shown in FIG. 9. It includes an
upstream columnar body 36 having a pair of rounded edges R between
its base surface 37 and its isosceles surfaces, respectively, while
its downstream columnar body 33 is equal in construction to that
shown in FIG. 9. The rounded edges R substantially prevents the
generation of small vortices 23A to thereby ensure the stable generation
of Karman vortices 23 as shown in FIG. 12.
Japanese Patent Publication No. 37511/1979 discloses a device which
comprises an upstream columnar body having a trapezoidal cross section
and a downstream columnar body having a semicircular cross section.
This device has, however, the disadvantage of causing a large loss
of fluid pressure. This is due to the fact that the upstream columnar
body has a front surface facing a stream of a fluid at right angles
thereto. A part of the fluid is forced by the front surface of the
upstream columnar body to change its flowing direction and impinge
upon the remaining fluid to create a turbulent flow which gives
rise to the pressure loss.
On the other hand, the device of this invention ensures the stable
generation of vortices without causing any appreciable pressure
loss, as its upstream columnar body has a substantially isosceles
triangular cross section. It has been found that particularly good
results can be obtained, whether a stream of a fluid may have a
normal or transitional state, if the isosceles surfaces of the upstream
columnar body have an apex angle of about 90.degree. therebetween.
Attention is now directed to FIG. 13 showing still another embodiment
of this invention. It is substantially equal in construction to
some of the embodiments which have already been described. It includes
a columnar body 40 consisting of an upstream columnar body 41 having
a substantially isosceles triangular cross section and a downstream
columnar body 43 having a substantially isosceles trapezoidal cross
section. The base surfaces 42 and 44 of the columnar bodies 41 and
43 respectively, are substantially of the same width as shown at
d.sub.1 and d.sub.2 and spaced apart from each other by an appropriate
distance a and lie in parallel to each other at right angles to
a stream of a fluid.
The downstream columnar body 43 has a pair of isosceles surfaces
forming an angle of about 60.degree. therebetween. It has been found
through a visual test that when a fluid flows at a low rate, the
shape of the vortices generated therein and their interference depend
on the angle between the isosceles surfaces of the downstream columnar
body. A test has, therefore, been conducted to ascertain the rate
of pulsation of a fluid at which vortices are satisfactorily produced,
in relation to the angle .beta.. The results are shown in FIG. 14.
As is obvious therefrom, the best results have been obtained when
the angle is about 60.degree.. The rate of pulsation is based on
the relationship between the velocity V of the fluid and the amplitude
.DELTA.V of its pulsation as shown in FIG. 15 and is expressed as
(.DELTA.V/2V).times.100 (%), in which V is the average velocity
of the fluid.
It has hitherto been considered that the velocity of a fluid and
the frequency of the vortices generated therein show a mutual relationship
of high linearity over a wide velocity range when the angle .beta.
does not exceed 40.degree.. However, it has subsequently been found
that the linearity of their relationship is less likely to be affected
by the angle .beta. and can be maintained substantially at the same
level as shown in FIG. 16 without having any adverse effect on
other characteristics, either, if the angle .beta. does not exceed
80.degree. and if the pipeline has an appropriately throttled inlet.
The Strouhal number St shown in FIG. 16 is expressed as (the frequency
of vortices).times.(their typical length)/(velocity of the fluid).
Apart from the optimum angle .beta. of about 60.degree., the following
has been found experimentally:
(1) It is possible to generate vortices of the highest stability
and detect them at a high SN ratio if the upstream and downstream
columnar bodies have substantially the same typical base width (d.sub.1
=d.sub.2 =d).
(2) It is possible to generate stable vortices without causing
any substantial loss of fluid pressure if the isosceles surfaces
of the upstream columnar body have an apex angle .alpha. which is
not smaller than 90.degree., and not larger than 120.degree..
(3) If the downstream columnar body has a trapezoidal height h
which is equal to, or smaller than, a half of its base width d,
it is possible to generate stable vortices and achieve a relationship
of good linearity even when a fluid flows at a low velocity, as
shown in FIG. 17.
(4) It is possible to obtain a wide range of measurement and results
of good linearity if the upstream and downstream columnar bodies
have a distance a of 0.2d to 0.3d therebetween, as is obvious from
FIGS. 18 and 19.
The various embodiments of this invention which have hereinabove
been described have a variety of advantages. The rounded edges or
corners formed on the upstream columnar body enable the stable generation
of Karman vortices even if a fluid may change its flowing speed
transitionally due to, for example, pulsation. The downstream columnar
body having an angle of about 60.degree. between its isosceles surfaces
contributes to the realization of a Karman vortex flow meter which
is simple in construction, and yet can generate stable Karman vortices
even if a fluid may change its flowing speed transitionally due
to, for example, pulsation. Moreover, the downstream columnar body
having a substantially semi-circular cross section enables the generation
of Karman vortices following a fluid velocity even if a fluid may
have a greatly varying velocity, since the vortices which have once
been generated leave it so quickly that they do not prevent the
growth of the vortices which will thereafter be generated.
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