Abstrict An air flow meter (10) has an air flow channel (11) with a vortex
generator or strut (13). The cross sectional area of the air flow
adjacent the vortex generator is varied to change the flow velocity
and hence to change the vortex frequency. The cross sectional area
can be changed by rotating strut (13), moving a flexible plate (24)
in a sidewall (22), or moving a vortex generator (33) to a region
of different cross sectional area.
Claims I claim:
1. A device for generating vortices in a bounded fluid stream comprising:
a fluid flow duct means for passing a fluid stream;
a generator means for generating vortices positioned in said duct
in the fluid stream;
an adjustment means for varying the cross sectional area of the
fluid flow adjacent said generator means; and wherein said generator
means and said adjustment means include:
an elongated plate member having a rotationally movable axis of
elongation with respect to the direction of flow of the fluid, so
that a different cross section can be presented to the fluid flow.
2. A device for generating vortices as recited in claim 1 wherein
said elongated plate member has an elliptical cross sectional area
and the axis of rotation is perpendicular to the cross sectional
area.
3. A device for generating vortices as recited in claim 1 further
comprising:
a sensor means for generating a control signal for varying the
cross sectional area of the fluid flow adjacent said generator means;
and
an actuator means coupled to said sensor means and said elongated
plate member for rotating said elongated plate member as a function
of the output of said sensor means.
4. A device for generating vortices in a bounded fluid stream comprising:
a fluid flow duct means for passing a fluid stream;
a generator means for generating vortices positioned in said duct
in the fluid stream;
an adjustment means for varying the cross sectional area of the
fluid flow adjacent said generator means; and wherein:
said generator means includes a narrow, elongated plate member
having an axis of elongation parallel to the direction of flow of
said fluid stream;
said fluid flow duct means having a cross sectional area of different
magnitudes along the direction parallel to the fluid flow; and
said generator plate member being longitudinally movable to different
cross sectional magnitudes in said duct.
5. A device for generating vortices as recited in claim 4 wherein
said fluid flow duct means has at least a first cross sectional
area and a second cross sectional area smaller in magnitude than
said first cross sectional area.
6. A device for generating vortices as recited in claim 5 further
comprising a transitional region between said first and second cross
sectional areas, said transitional region having a decreasing cross
sectional area going from said first to said second cross sectional
area.
7. A device for generating vortices as recited in claim 6 wherein
the cross section of said transitional region increases linearly
with distance along said fluid flow duct means.
8. A method for changing the rate of generation of vortices by
a vortex generator positioned in a fluid flow duct carrying a fluid
stream including varying the cross sectional area of the fluid flow
adjacent the vortex generator by moving the vortex generator to
a position of the fluid flow duct having a different magnitude cross
section.
Description TECHNICAL FIELD
Embodiments of the invention hereinafter described pertain to devices
for the generation of vortices.
BACKGROUND ART
It is known to position a generator plate in an air flow to generate
Karman vortices. Detection of the frequency of these vortices can
be used as an indication of the mass of air flowing by the generator
plate. For example, U.S. Pat. No. 4312237 issued Jan. 26 1982
discloses various embodiments of narrow, elongated generator plates
used for the generation of Karman vortices in a fluid stream. In
one embodiment of U.S. Pat. No. 4312237 a generator plate has
a shedding section wherein an aperture intersects side surfaces
of the generator plate to form a leading aperture edge which serves
as a shedding corner for the generation of vortices.
Also, U.S. Pat. No. 4312236 issued Jan. 26 1982 discloses a
vortex generating device having a narrow generator plate which is
immersed in a fluid flow to generate Karman vortices. The generator
plate has side surfaces parallel to the direction of fluid flow.
The side surfaces have a shedding region therein forming shedding
corners at which vortices detach from the generator plate.
Air flow meters using such vortex generators have produced stable
vortices over a wide range of fluid flow rates. Such devices have
functioned particularly well and operated in accordance with the
acoustical techniques disclosed in U.S. Pat. No. 3680375 to Joy
et al.
However, there has been a problem of adjusting the vortex frequency
to a precise value at a given flow for flow meters that do not have
precise mechanical dimensions.
The vortex flowmeter has the characteristic of having a quasi-digital
output where each output pulse represents the passage of a certain
amount of fluid. This amount of fluid is determined by the flow
duct area and the cross section width of the vortex generator. This
relationship is defined by:
where
f is the vortex frequency
S is the Strouhal constant
Q is the volumetric flow rate
d is the vortex generator width and
A is the flow duct area
In order to produce flowmeters having an identical vortex frequency
to fluid flow relation, the mechanical duct area must be held to
precise tolerances.
For some applications where tight tolerances cause a cost penalty,
such as in the automotive field, a means is needed to adjust some
mechanical dimension to change the frequency-to-flow relation in
order to offset manufacturing tolerances.
DISCLOSURE OF INVENTION
In accordance with principles of this invention, a change in the
flow velocity and, hence a change in the vortex frequency, is caused
by changing the flow duct area which is unobstructed by the vortex
generator cross section. Such a change in the unobstructed flow
duct area can be caused by either changing the width of the vortex
generator or by adjusting the boundaries of the flow duct area.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of an air flow meter in accordance with an
embodiment of this invention including provisions for varying the
vortex generator width;
FIG. 2 is a top view of an air flow meter in accordance with an
embodiment of this invention including means for varying the boundaries
of the air flow ducts; and
FIG. 3 is an air flow meter in accordance with an embodiment of
this invention wherein the vortex generator is longitudinally movable
to sections of the air flow duct having different interior boundaries.
BEST MODES FOR CARRYING OUT THE INVENTION
Referring to FIG. 1 an air flow meter 10 has an air flow channel
11 with walls 12. Positioned within air flow channel 11 is an elongated
plate number such as a vortex generator or strut 13 having an elliptical
cross section. The vortex strut 13 dimension that governs the shed
frequency is the width of the strut, measured at right angles with
respect to the flow. Vortex strut 13 can be rotated about an axis
14. This will cause the cross sectional area of strut 13 exposed
to the air flow to change. The change in vortex frequency correlates
very closely with the expected change based upon the projected width.
Therefore, an elliptical strut with a ten percent difference in
its major and minor axes will cause a total frequency change of
ten percent when rotated through 90.degree..
Referring to FIG. 2 an air flow meter 20 has a flow channel 21
bounded by walls 22. An elongated plate member such as a vortex
generator 23 is positioned within the flow channel 21. Sidewall
22 includes a flexible plate 24 which can be deflected by an adjustment
screw 25. Plate 24 is elongated and is secured to wall 22 by pins
26A and 26B. Advantageously, plate 24 can move longitudinally with
respect to pin 26B. Although screw 25 can be turned manually, the
position of screw 25 can be made to be a function of a sensor, such
as a pressure temperature sensor 27 which provides an electrical
signal to an actuator 28 which is mechanically coupled to screw
25. The variable flow duct area provided by plate 24 in air flow
meter 20 is advantageous where a simple vortex strut cannot be used.
For example, when vortex pulse to pulse stability is important,
a simple strut such as 13 shown in FIG. 1 cannot be used. As a result,
one or more adjustable air duct sidewalls which can be distorted
by exerting a force on wall 22 are used to adjust the duct area.
Strut 23 can be of any advantageous cross section. As adjustment
screw 25 is turned in, flexible plate 24 distorts since it is constrained
to sidewalls 22 at both ends, and causes a reduction in area of
the duct air flow path 21. This change in area causes a compensating
increase in the velocity which produces a higher vortex frequency.
Referring to FIG. 3 an air flow meter 30 includes an air flow
path 31 with bounding walls 32. An elongated plate member such as
a vortex strut or generator 33 is positioned within air flow path
31 and is movable longitudinally. Opposing interior surfaces of
walls 32 have a variable spacing so that the cross sectional area
of air flow path 31 is different at different longitudinal positions.
Because strut 33 is movable longitudinally, it can be positioned
in different size cross sectional areas thereby changing the vortex
frequency. Advantageously, a reduction in the cross sectional duct
area can be created by a gradual decrease in one or more of the
duct sidewalls such as shown in region 34. This causes a gradual
increase in the velocity and the vortex strut 33 may be moved through
this region to cause a change in the detected vortex frequency to
fluid flow relation. Advantageously, the change in cross sectional
area along region 34 is linear. Again, this structure will perform
satisfactorily with a variety of complex vortex generators. Changes
in the duct area have been found to cause corresponding changes
in the vortex frequencies.
The structures of FIGS. 1 2 and 3 show apparatus for changing
the output frequency of a vortex flow meter. Such a change is useful
for compensating for variations in mechanical dimensions of the
flow meters due to manufacturing tolerances. However, the apparatus
may be used for different purposes. For example, as shown in FIG.
2 the mechanical motion causing a change in the vortex frequency
may be a function of the output of pressure and/or temperature sensors
to offset changes in gas density and hence cause the air flow meter
to indicate mass flow rather than volumetric flow. Other methods
could have the mechanical motion be a function of a process such
as a programmed fuel enrichment and engine control systems, which
could be applied during accelerating conditions and leaning of the
fuel/air mixture during decelerating conditions.
Various modifications and variations will no doubt occur to those
skilled in the various arts to which this invention pertains. For
example, the particular tapering of the flow channel cross section
and the construction of the movable plates in the flow channel may
be varied from that disclosed herein. Further, in addition to using
a sensor to actuate screw 25 a sensor can be used to actuate the
rotation of vortex strut 13 about axis 14 and to actuate the longitudinal
movement of vortex strut or generator 33. That is, the vortex frequency
can be adjusted either manually or as a function of a sensed parameter.
These and all other variations which basically rely on the teachings
through which this disclosure has advanced the art are properly
considered within the scope of this invention.
INDUSTRIAL APPLICABILITY
This invention has various industrial applications including, but
not limited to, the measurement of air flow coming into an internal
combustion engine and providing a signal for use by electronic engine
controls governing engine operation. In particular, the measurement
of the air flow can be used to govern the air fuel ratio supplied
to the internal combustion engine.
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