Gaseous fluid flow meter utilizing karman vortex street

A gaseous fluid flow meter utilizing a Karman vortex street has a conduit having opposed flat walls and through which a gaseous fluid to be measured flows, a vortex generating member disposed perpendicularly to the direction of flow of the fluid to generate the Karman vortex street downstream thereof, a vortex detector disposed on the conduit and having a transmitter in one flat wall for transmitting a continuous ultrasonic wave across the Karman vortex street and a receiver in the other flat wall positioned opposite the ultrasonic wave transmitter in a direction perpendicular to the direction of the flow of the gaseous fluid through the conduit for receiving the continuous ultrasonic wave to detect the changes in phase of the ultrasonic wave indicating the number of vortices of the Karman vortex street generated in a unit time, and a sound absorbing material on only the portion of the inner surfaces of the flat walls of the conduit around the transmitter and around the receiver and extending sufficiently far along the walls from the transmitter and receiver for preventing the generation of standing waves in front of the walls due to the reflection of the ultrasonic wave.

BACKGROUND OF THE INVENTION 
This invention relates to a gaseous fluid flow meter utilizing a Karman 
vortex street. 
Gas flow meters utilizing the Karman vortex street include a vortex 
generating rod immersed in a gaseous fluid flowing through a conduit 
perpendicularly to the direction of flow of the fluid to generate the 
Karman vortex street downstream of the rod and employ an ultrasonic wave 
to detect the Karman vortex street thereby to measure a flow rate of the 
gaseous fluid. In order to detect the Karman vortex street, it has been 
proposed to dispose an ultrasonic transmitter and an ultrasonic receiver 
in opposed relationship in the conduit through which a measured gaseous 
fluid flows so that an ultrasonic wave transmitted from the ultrasonic 
transmitter is modulated by the vortices of the Karman vortex street and 
then received by the ultrasonic receiver. The ultrasonic wave is 
continuously transmitted to the receiving side and the modulation is a 
change in phase of the ultrasonic wave due to vortices is first sensed. 
Such a system is, on the one hand, advantageous in that when the energy of 
the ultrasonic wave is increased, the output from the receiving side 
becomes high and influence of multi reflection and diffused reflections 
within the conduit can be fully disregarded. On the other hand, the 
arrangement is disadvantageous in that a standing wave is generated 
between the opposed transmitting and receiving elements due to resonance, 
and this standing wave has a node coinciding with the position where the 
receiving element is mounted. This affects the change in phase due to the 
effect of the vortices, which in turn makes the sensing of an accurate 
flow rate difficult or impossible. Particularly, when the flow rate of air 
is being measured, the ultrasonic wave is propagated through the air at a 
propagation velocity which changes with a change in the air temperature. 
If the ultrasonic wave being used has a constant frequency, then this 
change in propagation velocity is attended by a variation in the wave 
length thereof. This has resulted in the disadvantage that a standing 
ultrasonic wave is formed at a certain temperature of the air which causes 
the ultrasonic receiver not to receive in a normal fashion the ultrasonic 
wave transmitted from the ultrasonic transmitter. 
OBJECT AND BRIEF SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a new and 
improved flow meter utilizing a Karman vortex street to measure the flow 
rate of a gaseous fluid over a wide range and at temperatures extending 
from a low to an elevated temperature. 
To sense accurately the vortices in the fluid flow, sound absorbing 
material for absorbing the resonance energy is provided only on the 
opposite portions of the inner surface of the duct around the transmitting 
and receiving elements, thereby to extinguish only the standing wave and 
the node. 
The present invention provides a flow meter utilizing a Karman vortex 
street and comprising a conduit having a measured gaseous fluid flowing 
therethrough, a vortex generation member disposed perpendicularly to the 
direction of flow of the fluid within the conduit to generate a Karman 
vortex street downstream thereof, a vortex detector disposed in the 
conduit and utilizing an ultrasonic wave to detect the number of vortices 
of the Karman vortex street generated in a unit time, and a sound 
absorbing material for absorbing and attenuating the ultrasonic wave on 
the inner wall of the conduit only around the receiver and around the 
position of the transmitter and extending a predetermined distance 
therefrom.

Throughout the figures like reference numerals designate the identical or 
corresponding components. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 of the drawings, there is illustrated one 
embodiment of a flow meter of the present invention utilizing a Karman 
vortex street. The arrangement comprises a conduit 10, having opposed flat 
walls, and a vortex generating rod 14 fixedly secured in the conduit 
perpendicularly to the direction of flow of the gaseous fluid within the 
conduit 10. In the illustrated embodiment the rod has a triangular 
cross-section with one side of the regular triangle located at right 
angles to the direction of flow of the gaseous fluid upstream of the 
longitudinal axis of the rod 14. Thereby the vortex generating rod 14 
generates a Karman vortex street 16 downstream thereof. 
The conduit preferably has a rectangular cross-section having a ratio of 
height to breadth of 1 to 2. The breadth L of the conduit can be 
determined from the base triangular rod. The triangular rod having a base 
of a dimension d generates, downstream thereof a Karman vortex street 
including two parallel arrays of vortices at a pitch of l and a spacing h 
between the two parallel arrays. Since h/d.apprxeq.1.2.about.1.3 and 
h/l=0.281 are theoretically known, the pitch l is calculated to be equal 
to (4.27.about.4.62)d by dividing h/d by h/l. Also breadth L.gtoreq.l must 
hold. Therefore the breadth of the conduit can be determined by the base d 
of the triangular rod. A typical conduit can be determined by the base d 
of the triangular rod. A typical conduit is 26.times.52 mm with a rod with 
a base of 0.91 mm. The rod may be of a circular cross-section. In the 
latter case the dimension d designates the diameter of the rod. 
An ultrasonic transmitter 18 is disposed on the conduit 10 immediately 
downstream of the vortex generating rod 14 so that the ultrasonic 
transmitting surface thereof is slightly outwardly of the inner surface of 
the wall of the conduit 10. The transmitter 18 is connected to an 
ultrasonic oscillator 20 and a continuous ultrasonic wave is transmitted 
therefrom. Preferably the power of the transmitter is from several to tens 
of milliwatts. An ultrasonic receiver 24 is disposed on the conduit 10 
directly opposite to the ultrasonic transmitter 18 with the ultrasonic 
receiving surface thereof similarly projecting slightly from the inner 
surface of the conduit 10. The receiver 22 is connected to an ultrasonic 
receptor 24. 
The ultrasonic receiver 22 preferably has the structure shown in FIG. 2. As 
shown in FIG. 2, the ultrasonic receiver 22 comprises an electrically 
insulating base plate 30, a plurality of supporting rods 32 (only two of 
which are illustrated) formed of a resilient material such as rubber and 
mounted on the base plate 30 and a bimorph type ultrasonic vibrator 34 
supported to the supporting rods 32. A hollow inverted cone 36 is 
connected at the apex to the ultrasonic wave-receiving surface of the 
ultrasonic vibrator 34. The cone 36 forms a resonator for the ultrasonic 
wave involved and serves as a combined deflecting and reflecting member. 
The vibrator 34 is electrically connected to the ultrasonic receptor 24 
(see FIG. 1) through a pair of electrodes 38 sealed in and extending 
through the base plate 30. 
The assembly formed as above described in surrounded by a housing 40 and 
the base plate 30 is connected in sealed relationship to the open end of 
the housing 42 for completing the ultrasonic receiver 22. The resonator 36 
has the larger diameter end facing an ultrasonic wave-receiving surface 
disposed on the other or closed end of the housing 42. 
When the ultrasonic transmitter 18 operates to transmit a continuous 
ultrasonic wave, particularly when the energy thereof is high, because the 
receiver 22 is directly opposite the transmitter, reflections will occur 
and resonance takes place, resulting in the setting up of a standing wave 
with a node coinciding with the position of the receiver 22. Such a 
standing wave with the node positioned to coincide with the receiver will 
substantially prevent detection of any phase change of the transmitted 
ultrasonic wave due to the passage therethrough of the vortices 16. In 
order to prevent the formation of the standing wave, the area of the inner 
wall of the conduit 10 around the transmitter and around the receiver is 
lined with a sound absorbing material 12 formed for example of unwoven 
cloth, preferably of polyester fiber. The thickness of the sound absorbing 
material 12 may preferably be 0.8 mm and is such that it surrounds the 
portions of the transmitter 18 and receiver 22 projecting from the inner 
surface of the wall of the conduit 10, so that the transmitting and 
receiving surfaces of the transmitter and receiver are flush with the 
inner surface of the sound absorbing material 12. It is not necessary to 
cover the entire inner surface of the conduit 10 with the sound absorbing 
material. It is sufficient to cover only the area around the transmitting 
and receiving surfaces of the transmitter and receiver, respectively. The 
sound absorbing material need extend only 150 mm upstream and 50 mm 
downstream from the respective transmitting and receiving surfaces. 
In order for the sound absorbing material to absorb the sound, the product 
of the density .rho. of the gaseous fluid flowing in the conduit 10 and 
the velocity C of the sound through the fluid must equal the product of 
the density .rho.' of the sound absorbing material and the velocity C' of 
sound through the sound absorbing material, i.e. 
.rho..times.C=.rho.'.times.C'. For a gaseous fluid, such as air, the 
product of the density of the gaseous fluid and the velocity of sound 
therethrough is in the range of 40-43 microbars/cm/sec. The sound 
absorbing material should therefore be a material which is porous, such as 
unwoven cloth, foamed polyethlyene or the like. 
In operation the measured gaseous fluid, for example, air, flows through 
the interior of the conduit 10 in the direction of the arrow shown in FIG. 
1 and the vortex generating rod 14 partly obstructs the flowing fluid to 
generate the Karman vortex street 16 downstream thereof. On the other 
hand, the ultrasonic transmitter 18 driven by the ultrasonic transmitter 
18 driven by the ultrasonic oscillator 20 transmits a continuous 
ultrasonic wave through the flowing gaseous fluid perpendicularly to the 
direction of the flow of the fluid and toward the ultrasonic receiver 22. 
While the ultrasonic wave is propagated through the flowing fluid it is 
modulated by the vortices of the Karman vortex street to change the phase 
of the ultrasonic wave and then the modulations in the ultrasonic wave 
received by the ultrasonic receiver 22 are detected and converted to a 
corresponding electrical signal. This electrical signal is applied to the 
ultrasonic receptor 24. The ultrasonic receptor 24 detects the number of 
vortices of the Karman vortex street generated in a unit time thereby to 
measure the flow rate of the gaseous fluid in the manner well known in the 
art. 
As above described, the continuous ultrasonic wave emitted from the 
ultrasonic transmitter 18 propagates through the flowing gaseous fluid 
while being directed thereinto. Accordingly, the ultrasonic wave reaches, 
in addition to the receiving surface of the ultrasonic receiver 22, that 
portion of the inner surface of the wall of the conduit 10 located 
adjacent to the receiver 22. However, since that inner wall surface is 
covered with the sound absorbing material 12, it does not reflect the 
ultrasonic wave. As a result, no standing wave is formed and a stable 
measurement can be made without being affected by reflected ultrasonic 
waves and a standing ultrasonic wave. 
It will be readily be understood that the sound absorbing material 12 is 
required only to be applied to that portion of the inside of the conduit 
10 extending sufficiently far from the receiving surface of the receiver 
22 to extinguish the standing wave. Preferably the material extends about 
150 mm upstream and 50 mm downstream of the receiver. 
The provision of the sound absorbing material around the transmitter 18 is 
to insure that no standing wave is produced by any reflected waves. Again, 
the sound absorbing material need extend no farther from the transmitter 
18 than is sufficient to extinguish any standing wave. 
The arrangement illustrated in FIGS. 3 and 4 comprise a conduit with a 
rectangular cross-sectional profile divided into a pair of parallel 
conduit portions 10 and 50. Only the conduit portion 10 includes the 
components 12, 14, 18, 20 and 22 as shown in FIG. 1 with a laminar flow 
producing means in the form of a rectifier 26 being disposed at the inlet 
thereof. 
The flow rate of the gaseous fluid flowing through the conduit portion 10 
is measured in the manner as above described in conjunction with FIG. 1 
and the overall flow rate of the fluid flowing through both conduit 
portions 10 and 50 can be determined by the measured flow rate. 
The arrangement shown in FIGS. 3 and 4 is advantageous over that shown in 
FIG. 1 in that in FIGS. 3 and 4 the distance between the ultrasonic 
transmitter and receiver 18 and 22 can be reduced to permit the use of a 
low power ultrasonic wave. Further the amount of sound absorbing material 
12 can be reduced because of a decrease in area over which the particular 
ultrasonic wave reaches the inner wall surface of the conduit portion 10. 
If desired, the conduit may be divided into more than two conduit portions 
only one of which is constructed substantially as illustrated in FIG. 1. 
The arrangement illustrated in FIG. 5 is different from that shown in FIG. 
1 only in that, in FIG. 5, the conduit 10 is connected at the downstream 
end to an expanded pipe 52 having a transverse dimension greater than that 
of the conduit 10. 
The conduit 10 has previously been required to include a portion in the 
form of a straight pipe extending downstream of the vortex generating rod 
14 a distance L (see FIG. 5) equal to at least five times the transverse 
dimension D thereof (see FIG. 5). In the arrangement of FIG. 5, however, 
this length L can be equal to or smaller than twice the transverse 
dimension D. This results in a decrease in the overall dimension of the 
resulting flow meter. 
In the arrangement of FIG. 5 it is seen that the ultrasonic wave from the 
transmitter 18 may reach the inner wall of the expanded pipe 52. It has 
been found, however, that the ultrasonic wave reflected from the inner 
wall of the expanded pipe 52 almost completely decays after it enters into 
the conduit 10 and before it reaches the ultrasonic receiver 22. As a 
result, the expanded pipe 52 does not adversely affect the measurement of 
the flow rate. 
If it is desired to bend the expanded pipe 52 downstream of the straight 
portion of the conduit 10 then the bent portion thereof can have the inner 
wall irregularly corrugated as shown by the reference character A in FIG. 
6. The irregularly corrugated walls 56 diffusely reflect the ultrasonic 
wave incident thereupon to prevent the ultrasonic wave reflected from the 
inner wall of the expanded pipe 52 from directly reaching the ultrasonic 
receiver 20. 
From the foregoing it is seen that the present invention provides a flow 
meter utilizing the Karman vortex street which prevents a transmitted 
ultrasonic wave from reflecting from an inner wall of a conduit containing 
the flow being measured and therefore prevents a standing ultrasonic wave 
from being formed within the conduit due to the reflection of the 
ultrasonic wave. 
Further the ultrasonic receiver shown in FIG. 2 is advantageous in that the 
inverted cone-shaped resonator is operable to diffuse and reflect the 
ultrasonic wave from the transmitter reaching the same but not directly 
toward the transmitter thereby preventing a standing ultrasonic wave from 
being formed due to the ultrasonic wave from the transmitter interferring 
with that reflected from the inverted cone-shaped resonator. 
Therefore it is seen that the inverted cone-shaped resonator 36 cooperates 
with the sound absorbing material 12 to permit a more accurate measurement 
of the flow rate. 
While the present invention has been illustrated and described in 
conjunction with a few preferred embodiments thereof it is to be 
understood that numerous changes and modifications may be resorted to 
without departing from the spirit and scope of the present invention. For 
example, the conduit may be formed of the sound absorbing material as 
above described. Also the resonator 36 is not required to be in the form 
of a hollow inverted-cone and it may be irregularly corrugated or 
wedge-shaped so as to reflect diffusely the ultrasonic wave falling 
thereon. Further a net of suitable meshes may be disposed in front of both 
the ultrasonic transmitter and receiver and the hollow inverted 
cone-shaped resonator can be omitted.