Abstrict A fluidic oscillator and a flow meter including such a fluidic
oscillator comprising a fluid inlet suitable for forming a two-dimensional
jet: an obstacle possessing a front portion in the form of a chamber
facing the inlet on the path of the jet, said chamber including
two walls that are symmetrical about a longitudinal plane of symmetry
passing through said inlet, the walls meeting said plane; and means
for increasing the pressure on each of the lateral flanks of the
obstacle in the vicinity of the front portion in alternation and
in a relationship with the oscillation of the jet between the walls
of the chamber.
Claims I claim:
1. A fluidic oscillator comprising:
a fluid inlet suitable for forming a two-dimensional fluid jet;
an enclosure opening out firstly to said fluid inlet and secondly
to a fluid outlet;
a single obstacle being disposed within said enclosure and occupying
the major portion of said enclosure, said enclosure having walls
cooperating with outside walls of said obstacle to form flow passages
for said fluid, said obstacle including a pair of side flanks defining
a front portion forming a chamber which both faces said inlet and
is located on the path of said jet, said front portion including
two walls forming a part of said chamber, said two walls being symmetrical
about a longitudinal plane of symmetry passing through said inlet,
said walls meeting in said plane: said fluid jet oscillating between
two points respectively symmetrically located on each said wall
of said chamber, two alternately strong and weak eddies being formed
on either side of said longitudinal plane of symmetry in said flow
passages; and
wherein said oscillator further includes means adapted to alternately
accentuate the increase in pressure on each of said side flanks
of said obstacle in the vicinity of said front portion in relation
to oscillation of said jet between said walls of said chamber.
2. A fluidic oscillator according to claim 1 wherein said walls
of said chamber are inclined substantially to form a V-shape whose
two branches flare.
3. A fluidic oscillator according to claim 1 wherein said means
to alternately accentuate the increase in said pressure includes
two main passages disposed symmetrically through said walls of said
chamber, each of said main passages being inclined relative to said
longitudinal plane and having an inlet inside said chamber and an
outlet on one of said side flanks of said obstacle.
4. A fluidic oscillator according to claim 3 wherein said main
passages are slots.
5. A fluidic oscillator according to claim 3 wherein said main
passages are circular or rectangular or square in section.
6. A fluidic oscillator according to claim 3 wherein said main
passages are tapering in section so as to accelerate said fluid
flowing through said passages, followed by a flaring section.
7. A fluidic oscillator according to claim 3 wherein said main
passages pass through said chamber at or close to a stop point of
said jet.
8. A fluidic oscillator according to claim 3 including at least
two additional fluid passages formed through said obstacle, said
additional passages being downstream from said main passages and
being symmetrically disposed relative to said longitudinal plane
of symmetry.
9. A fluidic oscillator according to claim 3 wherein at least
said main passages are bent.
10. A flow meter, comprising:
(a) a fluidic oscillator, said fluidic oscillatory including:
(i) a fluid inlet suitable for forming a two-dimensional fluid
jet;
(ii) an obstacle including a front portion forming a chamber facing
said inlet and located on the path of said jet, said front portion
including two walls forming a part of said chamber, said two walls
being symmetrical about a longitudinal plane of symmetry passing
through said inlet, said two walls meeting in said plane; two main
passages disposed symmetrically through said two walls, each of
said main passages being inclined relative to said longitudinal
plane and having a passage inlet opening to said chamber and an
outlet respectively opening on one of a lateral flank of said obstacle,
said fluid jet oscillating between said walls of the chamber at
an oscillation frequency that is characteristic of the flow rate
of said fluid jet; and
(b) means for measuring said oscillation frequency, wherein said
means for measuring said oscillation frequency includes means for
measuring variations in the flow speed of said fluid through said
main passages formed through said walls of said chamber.
11. A flow meter according to claim 10 wherein said means for
measuring variations in the flow speed include a speed sensor placed
on at least one of said main passages.
12. A flow meter according to claim 10 wherein the means for measuring
the oscillation frequency of said jet include pressure sensors placed
in said passages or in the vicinity of said passages.
13. A fluidic oscillator comprising:
a fluid inlet suitable for forming a two-dimensional fluid jet;
a single obstacle including a pair of side flanks defining a front
portion forming a chamber which both faces said inlet and is located
on the path of said jet, said front portion including two walls
forming a part of said chamber, said two walls being symmetrical
about a longitudinal plane of symmetry passing through said inlet,
said walls meeting in said plane; and wherein said oscillator further
includes two main passages disposed symmetrically through said walls
of said chamber, each of said main passages being inclined relative
to said longitudinal plane and having an inlet inside said chamber
and an outlet on one of said side flanks of said obstacle.
14. A fluidic oscillator according to claim 13 wherein the passages
are slots.
15. A fluidic oscillator according to claim 13 wherein said passages
are circular or rectangular or square in section.
16. A fluidic oscillator according to claim 13 wherein said passages
are tapering in section so as to accelerate the fluid flowing through
said passages, followed by a flaring section.
17. A fluidic oscillator according to claim 13 wherein said passages
pass through said chamber at or close to the stop point of the jet.
18. A fluidic oscillator according to claim 13 including at least
two additional fluid passages formed through said obstacle, said
additional passages being downstream from the first passages and
being symmetrically disposed relative to the longitudinal plane
of symmetry.
19. A fluidic oscillator according to claim 13 wherein at least
said main passages are bent.
Description The present invention relates to a fluidic oscillator enabling
the flow rate of a fluid, a liquid or a gas, to be measured.
BACKGROUND OF THE INVENTION
Fluidic oscillators are of considerable interest for metering the
volumes of fluids such as water and gas delivered to users. Most
flow meters presently in existence use mechanical moving parts.
This applies in particular to flow meters using a turbine or a membrane.
In comparison, fluidic oscillators do not have any moving parts
liable to wear over time, and consequently such oscillators do not
need recalibrating.
Such oscillators can be small in size and very simple in design.
They are therefore very reliable. In addition they deliver a frequency
signal which is easily converted into a digital signal. This characteristic
is particularly advantageous when meters are read remotely.
Most effort in developing such flow meters has been directed to
vortex effect flow meters and to Coanda effect flow meters.
The principle on which vortex effect flow meters operate is based
on the well-known fact that the presence of an obstacle in a duct
along which a fluid is flowing gives rise to eddies that escape
periodically. Measurement is based on detecting the frequency at
which eddies detach, which frequency is proportional to the flow
speed for an obstacle of given geometry.
The eddy frequency is measured in various ways which make it possible
to derive the mean speed of the flow and thus the flow rate. Vortex
effect flow meters are generally very sensitive to noise and to
fluid conditions upstream. In practice, a flow rectifier is used
to make the speed profile uniform. A flow meter of this type is
described, for example, in U.S. Pat. No. 3589185.
The Coanda effect as used in Coanda effect flow meters consists
in the natural tendency of a fluid jet to follow the contours of
a wall when the jet is discharged close to the wall, even if the
outline of the wall departs from the discharge axis of the jet.
A fluidic oscillator of this type includes a chamber into which
the fluid jet discharges through a converging nozzle. Two side walls
are placed in the chamber symmetrically about the discharge axis
of the jet. The jet leaving the inlet to the oscillator attaches
itself spontaneously to one of the side walls by the Coanda effect.
A portion of the flow is then bled off via a lateral channel of
the wall to which the jet attaches itself, thereby having the effect
of detaching the jet from said wall and attaching it to the opposite
wall. The phenomenon then takes place again, thus giving rise to
permanent oscillation in the incoming flow. Unfortunately, in this
type of apparatus, the range over which flow can be measured is
relatively limited and the nonlinearity of the calibration curve
is quite large. Furthermore, this type of apparatus may stop oscillating
under certain conditions relating to external disturbances, and
this gives rise to a loss of signal. To increase the range in which
measurement is possible, Okadayashi et al. have proposed, in U.S.
Pat. No. 4610162 combining two fluidic oscillators, one operating
at low flow rates and the other at high flow rates.
Because of the drawbacks encountered with vortex effect and with
Coanda effect flow meters, attempts have been made to develop other
types of fluidic oscillator which operate using principles that
are fundamentally different. Applications thereof are found in the
flow meters described in the following U.S. Pat. Nos.: 4184636
4244230 and 4843889.
For example, U.S. Pat. No. 4244230 describes a fluidic oscillator
flow meter placed in a duct on the path of the fluid, and extracting
a portion of the fluid. The oscillator has two members disposed
side by side and having facing walls that form a nozzle. An obstacle
has a frontal oscillator chamber placed facing the nozzle. The chamber
has a common inlet and outlet. The jet leaving the nozzle penetrates
into the chamber and strikes the far wall of the chamber.
The jet is put into transverse oscillation inside the chamber by
the formation of two eddies on either side of the jet. The eddies
alternate between being strong and weak, in phase opposition. The
jet leaves via the common outlet and is directed into the main flow.
Pressure sensors enable the oscillation frequency of the jet in
the chamber to be measured, which frequency is proportional to the
flow rate.
The performance of flow meters of that type is generally better
than the performance obtained using conventional fluidic flow meters.
Unfortunately, said performance is still not satisfactory, in particular
with respect to sensitivity and measurement range.
An object of the present invention is to remedy the above drawbacks.
The invention provides a fluidic oscillator and a flow meter including
such an oscillator and having performance that is better than that
of prior art flow meters.
SUMMARY OF THE INVENTION
More precisely, the invention provides a fluidic oscillator enabling
jet oscillation to be maintained even at low flow rates. The oscillator
comprises:
a fluid inlet suitable for forming a two-dimensional jet;
a obstacle possessing a front portion in the form of a chamber
facing the inlet, and located on the path of the jet, said chamber
including two walls that are symmetrical about a longitudinal plane
of symmetry passing through said inlet, the walls meeting in said
plane;
the oscillator being characterized in that it comprises means for
alternately increasing the pressure on each of the side flanks of
the obstacle in the vicinity of its front portion in relation with
oscillation of the jet between the walls of the chamber.
Advantageously, the walls of the chamber are inclined substantially
to form a V-shape whose two branches flare.
In a particular embodiment, the oscillator includes an enclosure
opening out firstly to the fluid inlet and secondly to a fluid outlet,
the obstacle being disposed within said enclosure, said enclosure
having walls co-operating with the outside walls of the obstacle
to form flow passages for the fluid.
Advantageously, the means for increasing pressure comprise two
main passages disposed symmetrically through the walls of the chamber,
each of said passages being inclined relative to the longitudinal
plane and having an inlet inside the chamber and an outlet on one
of the lateral flanks of the obstacle.
According to an advantageous feature of the invention, the said
passages are slots.
Said passages may be circular or rectangular or square in section.
In a particular embodiment, the section of the passages decreases
initially and then increases.
Advantageously, said passages pass through said chamber at or close
to the stop point of the jet.
In a variant embodiment, the oscillator includes at least two additional
fluid passages formed through said obstacle, said additional passages
being downstream from the first passages and being symmetrically
disposed relative to the longitudinal plane of symmetry.
In a particular embodiment, at least said main passages are bent.
The invention also provides a flow meter including such a fluidic
oscillator, the fluid oscillating between the walls of the chamber
at an oscillation frequency that is characteristic of the fluid
flow rate, the meter also including means for measuring the oscillation
frequency.
Said means for measuring the oscillation frequency comprise means
for measuring variations in the flow speed of the fluid through
the passages formed through the walls of the chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention are described by way of example with
reference to the accompanying drawings, in which:
FIGS. 1A and 1B show an embodiment of a flow meter including a
fluidic oscillator of the present invention, FIG. 1A showing the
oscillator in longitudinal section and FIG. 1B being a cross-section
on line AA of FIG. 1A;
FIG. 2A is a fragmentary diagrammatic view of an obstacle provided
with a passage forming a venturi in accordance with a particular
embodiment of the invention;
FIG. 2B is a diagram showing a variant embodiment of a fluidic
oscillator of the invention;
FIG. 3 shows the operating principle of the fluidic oscillator;
FIG. 4 is a graph showing how the oscillation frequency of the
jet varies as a function of the Reynolds number, with and without
slots; the Reynolds number is based on the speed of the jet and
on the width of the jet;
FIG. 5 shows the signal to noise ratio as a function of Reynolds
number with and without slots;
FIG. 6 shows head loss as a function of Reynolds number with and
without slots; and
FIG. 7 is a longitudinal section through a second embodiment of
the invention.
DETAILED DESCRIPTION
FIGS. 1A and 1B are a diagram of a flow meter including a fluidic
oscillator of the invention and a section view of the oscillator.
The fluid whose flow rate is to be measured may be a liquid or a
gas and it is represented by arrows F.
As can be seen in FIGS. 1A and 1B, the oscillator has a longitudinal
plane of symmetry 2.
The fluid penetrates into the oscillator via a chamber 1 in the
form of a cube of side equal to the height of the jet followed by
an inlet in the form of a rectangular-section convergent nozzle
3. The chamber 1 provides a transition between an axisymmetrical
flow and the two-dimensional flow required in the oscillator. The
use of a convergent nozzle 3 enables the flow speed of the fluid
to be increased.
Means other than a convergent nozzle could be used for obtaining
such a fluid jet, for example a duct of small diameter and of sufficient
length.
The fluid inlet opens out into an enclosure E. An obstacle 16 formed
with two lateral or side flanks that are symmetrically shaped about
the longitudinal plane of symmetry 2 is placed inside the enclosure
E. A convergent chamber 8 is formed in the side flanks of front
portion of the obstacle 16 facing the outlet from the nozzle 3.
In this way, the fluid jet delivered by the nozzle on penetrating
into the convergent chamber 8 gives rise to two eddies that are
symmetrical about the plane of symmetry 2.
The front portion of the obstacle 16 facing the nozzle 3 is v-shaped
with the branches of the v-shape flaring outwardly (walls 20 and
21 in FIG. 1A). The inside of the v-shape is formed by two walls
22 and 23 that are inclined and symmetrical about the plane 2. These
two walls 22 and 23 meet in the plane 2 to form a dead space 9.
The walls 20 to 23 are convex in shape except for that portion of
the walls 22 and 23 which is close to the plane of symmetry 2 with
said portions of the walls 22 and 23 being concave in shape. Each
of the walls of the obstacle 16 is pierced by a fluid passage 11
or 12. These passages are disposed symmetrically about the plane
2.
These passages are advantageously slots so as to form two islands
6 and 7 in the obstacle 16. The passages 11 and 12 may be channels
and they may be circular or rectangular or square in section. In
FIG. 1A, the section of the passages 11 and 12 is uniform.
Nevertheless, the section may decrease so as to accelerate the
fluid flowing through said passages (the venturi effect) and may
then increase so as to avoid additional head loss. FIG. 2A is a
diagram providing a fragmentary view of the obstacle 16 when provided
with a passage 11 that has a venturi shape.
The fluid may also flow through the oscillator via passages 5
15 4 and 14 that exist between the walls of the obstacle 16 and
the walls of the enclosure E.
FIG. 1B is a cross-section through the fluidic oscillator as marked
by arrows AA. It may be observed that the shape of the fluidic oscillator
in cross-section perpendicular to the plane of symmetry 2 is rectangular.
In this particular embodiment, the height H of the oscillator is
about 7 times the width of the inlet nozzle to the oscillator.
Inside the slots 11 and 12 sensors 24 and 25 enable the flow speed
of the fluid in the slots to be measured.
FIG. 3 shows how the fluidic oscillator of the present invention
operates. The flow whose flow rate is robe measured penetrates into
the oscillator via the convergent nozzle 3. This nozzle is connected,
via a chamber in the form of a cube of side equal to the height
of the oscillator, to a duct of circular section (not shown) mounted
upstream and through which the fluid flows. On passing through the
nozzle 3 the fluid is concentrated to form a jet 30. Above a threshold
value for the flow speed of the fluid in the jet 30 said jet alternates
at a certain oscillation frequency between the two positions represented
by P1 and P2 in FIG. 3.
The natural instability of the jet coming from said oscillation
is due to the asymmetry of the jet, which is usually inevitable
due to the natural instability of the jet. The asymmetry can be
accentuated by placing a small obstacle on the wall of the nozzle
3. However, this is generally not necessary.
The oscillation frequency of the jet between the two positions
P1 and P2 is directly proportional to the flow rate of the fluid
passing through the oscillator. The factor of proportionality is
determined merely by calibrating the oscillator.
When the jet is in position P2 as shown in FIG. 3 two eddies are
formed, one of them, 31 is concentrated and strong while the other
one of them, 32 is weak. The slots 11 and 12 are positioned so
as to correspond to the stop point of the jet, i.e. to the point
of impact of the jet on the obstacle 16 at maximum deviation of
the jet from the plane of symmetry 2.
In position P2 a portion of the fluid flows through the slot 12
and emerges from the outlet 13 of the slot so as to flow towards
the passage 15 thereby increasing the pressure in the passage 5
and at the outlet 17 of the oscillator. The eddy 32 gives rise to
fluid escaping towards the outlet 17 via the passages 4 and 14
but also through the slot 11. However, the quantity of fluid flowing
through the slot 11 is small compared with the quantity of fluid
flowing through the slot 12 while the jet is in position P2.
The jet oscillates between the two positions P1 and P2 for two
reasons.
Firstly, it is well known that the trajectory of a free jet is
unstable since a jet is never homogeneous in cross-section, but
is asymmetrical. When the jet encounters an obstacle, such as the
obstacle 16 it oscillates between two extreme positions, with the
frequency of oscillation being directly proportional to the flow
rate. This oscillation can be explained by the creation of the two
main eddies 31 and 32 in FIG. 2.
Secondly, the slots 11 and 12 accentuate the oscillation and increase
the frequency of oscillation. For example, in the case shown in
FIG. 3 when the jet is in position P2 high pressure zones are
established at the inlet to the slot 12 and the presence of a strong
eddy 31 practically shuts off the passage 5. Under such circumstances,
most of the fluid flows via the passages 4 and 14 with the flow
through the slot 11 being relatively small. The fluid that flows
through the slot 12 increases the pressure at the outlet thereof
and flows away via the passage 15 and also via the passage 5 to
encounter the eddy 31. This increase in pressure serves to push
the eddy 31 towards the plane of symmetry 2 and thus contributes
to displacing the jet from position P2 towards position P1. While
the jet is moving in this way, the eddy 31 weakens while the eddy
32 becomes stronger. When the jet reaches the position P1 in a
manner symmetrical to that described above with reference to FIG.
3 the eddy 32 is the stronger eddy and the major portion of the
fluid flows away via the slot 11 while the portion of the fluid
that flows away via the slot 12 is small.
Returning to FIG. 1A, the distance between the outlet 40 of the
nozzle 3 and the end wall 41 of the dead zone 9 that faces it constitutes
an important geometrical parameter. This distance must be greater
than a threshold value. Beyond said value the jet oscillates at
a frequency which depends on said distance.
Reducing the depth of the dead zone enables the oscillation frequency
to be increased. However it also increases the minimum flow rate
for oscillation. A compromise is therefore necessary.
However, if the distance is less than the threshold value, then
the fluid jet is not long enough to become unstable and it cannot
oscillate.
The distance between the inlets 42 of the slot 11 and 43 of the
slot 12 is also important: if this distance is greater than an optimum
value, then the oscillation frequency of the jet falls off when
the distance 42-43 increases. Conversely, if the distance 42-43
is too short, then the convergent chamber 8 becomes so small that
the eddies do not have enough room to move. The two eddies then
remain locked in a fixed position and no oscillation of the jet
occurs. The dimensions 40-41 and 42-43 depend on the dimensions
of the jet itself, and thus on the outlet of the nozzle 3.
To determine the optimum dimensions of the oscillator, it is possible
to proceed by digital simulation and by experimentation. For example,
to determine the optimum value of the distance 40-41 the obstacle
16 is displaced along the plane of symmetry 2 so as to find said
optimum value. Similarly, for the distance 42-43 it is possible
to proceed by experiment with different values. The distances 40-41
and 42-43 depend on the outlet dimensions of the nozzle 3.
The purpose of the inlet convergent nozzle 3 is to reduce the flow
rate threshold in the fluid to be measured and to make the speed
profile thereof more homogeneous.
when the jet moves from position P1 to position P2 the speed of
the fluid in the slot 12 is fast while the flow speed of the fluid
in the slot 11 is slow. When the jet moves from position P2 to position
P1 the opposite situation is established, i.e. the flow speed of
the fluid in the slot 11 is faster than the speed in the slot 12.
Consequently, two speed sensors 24 and 25 situated in the slots
11 and 12 can be used to detect the variations in fluid flow speed
from one slot to another. Since the speed variations in the slots
11 and 12 are in phase opposition, the output signals from the sensors
24 and 25 may be subtracted so as to obtain a signal having twice
the amplitude of the amplitudes provided by the sensors 24 and 25.
In addition, by subtracting the signals from the sensors 24 and
25 the signal to noise ratio thereof is improved.
The flow of fluid through the slots 11 and 12 is not subject to
disturbances other than the influence of the jet which gives rise
to periodic variations in the flow. This flow along the slots is
thus easy to monitor. Measuring variations in the fluid flow speed
along the slots is an advantageous manner of measuring the oscillation
frequency of the jet and consequently the flow rate.
Naturally, as in prior art devices, it is possible to measure the
oscillation frequency of the jet by measuring the pressure difference
between two specified points in the converging chamber 8. For example,
two pressure sensors may be disposed, one of them, 51 close to
the inlet 43 of the slot 12 and the other one of them, 50 close
to the inlet 42 of the slot 11.
By measuring variations in speed, it is possible to extend the
range over which flow rates can be measured. If it is desired to
maintain measurement accuracy of the oscillation frequency and thus
of the flow rate over a range of 1 to 150 the speed will vary in
the same ratio, i.e. from 1 to 150 while the pressure varies from
1 to 150 squared, i.e. from 1 to 2500.
It is very difficult to maintain the same degree of accuracy over
such a wide pressure measuring range merely by using a single pressure
sensor. In addition, since the amplitude of the pressure signals
is, to a first approximation, proportional to the square of the
speed of the jet, at low speeds variation in the pressure may be
as little as 0.1 Pascals. A pressure of this order of magnitude
cannot be detected with pressure sensors. It is therefore advantageous
to measure the oscillation frequency of the jet by means of a signal
that depends on the speed rather than the pressure of the jet so
that the dynamic range of the signal is likewise of the same order
of magnitude as the dynamic range of fluid speeds. Any type of speed
sensor may be used, for example film sensors or hot wire sensors,
thermistors, a silicon sensor, laser anemometers, or even magnetic
sensors when the fluid is an electrically conductive liquid.
So as to accelerate the speed of rotation of the eddies and consequently
the transfer of dynamic pressure by increasing a pressure in the
vicinity of the front portion of the obstacle, it is preferable
for the slots 11 and 12 to extend in the direction of the local
fluid speed at the points P1 and P2. These points P1 and P2 are
situated symmetrically about the point of symmetry 2 and in the
immediate proximity of the stop points of the jet on the obstacle
16. These positions depend very little on Reynolds numbers. By adding
additional slots (FIG. 7) this dependence can be reduced even further.
The respective outputs 10 and 13 of the slots 11 and 12 are situated
close to the passages 4 and 5 so as to increase the thrust of the
fluid leaving each slot on the larger intensity eddy so as to thrust
it back towards the plane of symmetry 2. In FIG. 1A, the slots extend
at about 90.degree. relative to the plane of symmetry 2. If this
angle of inclination is reduced (i.e. displacing the slot 12 in
FIG. 1A so that it points more towards the region 15 than towards
the region 5), then the head loss in the oscillator decreases but
the frequency of oscillation of the jet also decreases. The direction
in which the slots 11 and 12 extend is therefore a result of a compromise.
In order to further improve the thrust generated in alternation
on the eddies by the jets leaving the slots 11 and 12 the slots
11 and 12 may be bent as shown in FIG. 2B so as to move the outlets
10 and 13 of the slots 11 and 12 closer to the eddies. Thus, the
jets act directly on the eddies without being guided via the channels
4 or 5.
The upper limit on the range of flow rates that can be measured
with the oscillator depends only on the head loss to which the fluid
is subjected on passing through the oscillator. The lower limit
of the measurement range is determined by the flow rate that gives
rise to eddies of an intensity that is too small to cause the fluid
jet to oscillate. This drawback can be mitigated by placing a speed
sensor 44 on the outlet 40 of the converging nozzle 3 for the purpose
of measuring very low flow rates that cannot be measured by the
oscillator. The range over which the flow meter can measure is thus
extended. In addition, the speed sensor 44 may be regularly calibrated
by comparison with the speed or flow rate signal provided by the
oscillator when the sensor 44 and the oscillator are both operating
properly. The speed sensor 44 usually drifts over time whereas the
oscillator remains stable. This solution is described in U.S. Pat.
No. 5003810.
The oscillator of the invention thus uses an oscillating regime
based on the hydrodynamic instability that results from interaction
between a pair of eddies and a fluid jet. By measuring variations
in the speed of the fluid travelling through the slots 11 and 12
it is possible to extend the range of flow rates that can be measured
and to obtain better calibration. With the oscillator of the invention,
there is no risk of losing the signal in the available measurement
range, and the oscillator delivers a single frequency for a given
flow rate.
FIG. 7 shows another embodiment in which two additional slots 45
and 46 are added. This gives rise to greater efficiency of the oscillator
by reducing the head loss in the fluid passing through the oscillator.
It is thus possible by experiment to find the best compromise between
the orientation and the number of slots as a function of the characteristics
of the fluid whose flow rate is to be measured, and also as a function
of the range of flow rates to be measured.
FIGS. 4 5 and 6 are graphs showing experimental results obtained
using the fluidic oscillator of FIG. 1A. In order to compare the
influence of the slots 11 and 12 on the performance of the oscillator,
the experiments were performed by obstructing the slots 11 and 12
in a first series of experiments, while leaving the slots 11 and
12 open during a second series of experiments.
FIG. 4 shows the resulting oscillation frequency expressed in Hertz
as a function of the Reynolds number based on the speed of the jet
and on its width. The upper curve corresponds to experimental results
using slots while the lower curve corresponds to results without
slots. It can be seen that the oscillation frequency of the jet
is improved with the oscillator of the invention.
FIG. 5 shows how the signal-to-noise ratio, expressed in decibels,
varies as the Reynolds number varies. A very considerable improvement
in the signal-to-noise ratio can be seen when the slots 11 and 12
are not obstructed (upper curve).
FIG. 6 shows how the head loss expressed in Pascals varies as a
function of the Reynolds number with and without slots. It can be
seen that the slots 11 and 12 do not give rise to head loss. The
experiments whose results are shown in FIGS. 4 5 and 6 above were
performed using air. |