Double wing vortex flowmeter

Abstract: A vortex flowmeter has an elongated vortex generator in the flow passage of a body and transverse to the direction of flow. A first wing is located in the flow passage downstream of the generator and a second wing is located downstream from the first wing. A sensor is associated with each wing to provide a signal responsive to movement of the wing, and the signals are combined to have external noise components of the signals cancel each other.

Description: This invention relates to a meter and a method for measuring fluid flow by detecting Karman vortices, according to the preamble of claims 1 and 3, respectively. It is well known that placing an elongated obstruction transverse to the direction of fluid flow within a conduit will result in the generation of vortices in the wake of the obstruction. The vortices are induced by and shed alternately from opposite sides of the obstruction, which is called a bluff body. This has been referred to as the Karman effect. The frequency of the vortex shedding is proportional to the width of the bluff body and the velocity of flow, so that detecting the frequency can generate signals indicative of fluid flow velocity. The shedding frequency is well known and is described by the equation f=Ns V/D, where Ns is the Strouhal number, V is the fluid velocity past the bluff body, and D is the diameter or width of the bluff body. The vortices are generated in pairs, often referred to as two rows, and are disposed on either side of the longitudinal axis of the bluff body. The rotational direction of the individual vortices is such that each reinforces the other and combines with the other. As the vortices proceed away from the bluff body, the result is loss of individual character for each vortex and the creation of sinuous-like fluid motion transverse to the direction of the velocity of the fluid. In effect, the vortices form a standing transverse wave beyond the bluff body and the wavelength is I =V/f = D/Ns. The sinuous-like wave is persistent, with normally expected dissipation, unless disrupted by some mechanical means. For example, a physical member disposed longitudinally in the stream will substantially dissipate the vortex street formed by the two rows if the length of the member is sufficient. In general, the strength of the vortices increases with increased velocity and with increased fluid density in the relationship of ρV². A variety of means for detecting the vortices have been proposed, including the use of acoustic detection (U.S. Patent 3,886,794, hot wires (U.S. Patent 4,275,602 and a physical member disposed downstream of the obstruction and subject to deflection as the alternating vortices pass by. In this latter approach, the physical member often takes the form of a wing and the wing may either be pivotably mounted in the conduit (U.S. Patent 3,116,639 and U.S. Patent 4,181,020 or the wing may be fixed to the conduit (U.S. Patent 4,699,012. A major shortcoming of flowmeters designed to detect vortex shedding arises from external forces and accelerations in the attached piping systems. Because the physical detection member of a vortex flowmeter is designed to be deflected by the vortices, any other forces which would cause deflection, such as external vibration of the conduit, will be detected as well. This is especially true at lower fluid velocities when the vortex strength is diminished. The external forces (the "noise") thereby adversely affect the accuracy of the signal generated by the physical member. Although many compensating methods have been employed to minimize the noise effect, each has the potential for only small or inadequate reduction of the influences of noise. The present application is directed to an improved vortex flowmeter in which the effects of external noise are substantially eliminated. US-A-4 835 436 shows a vortex flowmeter having a wing sensor with a slit formed therein to provide an overhanging structure for the upstream half of the wing sensor connected to an impulse receiving member. US-A-4 910 994 shows a vortex flowmeter comprising a vortex generating bluff body and a pair of vortex sensing members disposed downstream of the bluff body 9. The document WO-A-91/10882 shows a vortex flowmeter according to the preamble of the independent claim. However this conventional flowmeter cannot sufficiently reduce the influences of noise on the accuracy of the signal generated by the sensor wing. It it therefore an objective of the present invention to provide a method of measuring flow through a flow passage and a flow meter having a high accuracy, wherein errors in measurement resulting from external vibration or other sources of external noise can be substantially eliminated. According to the present invention the above-mentioned objective concerning the method as indicated in the preamble of claim 1 is performed by the steps of detecting only the noise at a second position along the axis that is spaced from the first position, generating a signal at the second detection position representative of the amplitude of the noise and combining the signals to enhance the vortex signals and cancel out the effect of the noise. According to the present invention the above-mentioned objective concerning the flowmeter as indicated in the preamble of claim 3 is performed in that the first wing for detecting vortices and external noise at a first position along the longitudinal axis of the flow passage has a length in the direction of fluid flow that is about equal to a wavelength of the vortex street leaving the vortex generator so as to substantially absorb the vortex street, a second wing for detecting only the external noise is separate from the first wing and disposed downstream of the first wing, a second sensor is associated with said second wing for providing a signal in response to movement of said second wing and means are provided for combining the signals from the sensors to provide a combined signal indicative of fluid flow, with the noise signal cancelling out. Accordingly the influence of external noise components can be eliminated and the fluid flow can be measured with high accuracy. Preferred embodiments of the present invention are laid down in the dependent claims. Hereinafter the present invention is illustrated and explained in greater detail by means of a preferred embodiment thereof in conjunction with accompanying drawings, wherein: Fig. 1 is a longitudinal view in vertical section taken through a second embodiment of a flowmeter in accordance with the present invention; Fig. 2 is a view in longitudinal section taken in the plane of the line 7-7 of Fig. 1; and Fig. 3 is a schematic diagram of a circuit for combining the signals of the sensors of the embodiment according to figures 1 and 2. Fig. 1 and 2 illustrate a preferred embodiment of the double wing vortex flowmeter. A relatively rigid body 40 is adapted to be inserted in the piping for the flow of fluid to be measured. The body defines a flow passage 41 of circular cross section. An elongated bluff body 42 is disposed across the flow passage 41 and along the longitudinal axis 43 of the flow passage. A first wing 44 is disposed downstream of the bluff body 42 and a second wing 45 is disposed downstream of the first wing 44. The wings 44 and 45 are each joined at one end to a side of the body 40 and are cantilevered therefrom into the flow passage 41. The wings 44 and 45 may be cantilevered from opposite sides of the body or may be mounted rigidly to the body 40 at both ends, if desired. The wings 44 and 45 are identical in construction so that they have the same resonant frequency. As shown in Fig. 2, the vortices V shed from the vortex generator 42 will be spaced based upon the dimensions of the vortex generator 42 and the Strouhal number of the flow. Although the vortices are independently shed from either side of the vortex generator 42, Von Karman showed that the only stable shedding that can exist is if the swirl of each vortex reinforces its opposite partner. It is therefore possible to consider that two oppositely shed vortices formed together and that the paired vortices form a dynamic wave oscillating to and fro across the areas of the wings 44 and 45. Both of the wings 44 and 45 contain internally embedded sensors 50 and 51, respectively. The sensors 50 and 51 are in the form of piezoelectric transducers. The sensors 50 and 51 are of known construction and they comprise a piezoelectric ceramic which, when subjected to strain, will produce a voltage signal indicative of the strain. The first wing 45 should have a length L′ in the direction of fluid flow that is about equal to a wave length of the vortex streets generated by the bluff body 42. The first wing 44 will be deflected by the effects of the vortices passing by it as well as by external vibrations or other sources of noise. The result will be that the sensor 50 will sense and provide a signal that is indicative of both the amplitude of the vortex street and the amplitude of the noise. By providing the first wing 44 with a length L′ that is about a wave length of the vortex streets, the first wing 44 will absorb the vortex street so that fluid flow encountering the second wing 45 will be substantially free of the vortex street. Under such conditions, the second wing 45 will be deflected only by noise and the sensor 51 embedded in the second wing 45 will sense and provide a signal indicative only of deflections due to noise. By combining the signals from the two sensors, the noise component can cancel out with the result that a signal indicative solely of the amplitude of the vortex street will result. Fig. 3 shows a method of combining the signals of the sensors 50 and 51 using digital signal processing with an adaptive filter algorithm. The input signal from the first wing sensor 50 that contains both the vortex and noise components is a primary signal that is fed to a summing circuit 55. The signal from the second wing sensor 51 is a reference signal that contains only noise, and the reference signal is fed to an adaptive filter 56 whose algorithm W(t) produces a signal that is subtracted from the primary signal at the summing circuit 55 to produce an output signal substantially free of noise. Although the invention is shown as incorporating piezoelectric transducers as the sensors, other forms of sensors could be employed, including other forms of pressure sensors. For example, the movement of the wings could be detected optically or acoustically. The term "wings" as used in this application is not meant in a limiting sense and is not meant to define a particular shape or structure. Instead, the term is used to refer to any physical element imposed downstream of the vortex generator and which will be subjected to deflection by the vortex streets or noise, or both.

Claims: A method of measuring flow through a flow passage (41), comprising; generating vortex rows on either side of a longitudinal axis (43) of the flow passage (41); detecting the vortices and external noise at a first position along the axis (43); generating a signal at the first detection position representative of the amplitude of the vortices and noise, characterised by the steps of detecting only the noise at a second position along the axis (43) that is spaced from the first position; generating a signal at the second detection position representative of the amplitude of the noise; and combining the signals to enhance the vortex signals and cancel out the effect of the noise. A method as claimed in claim 1, characterised by the step of substantially absorbing the energy of the vortex street before it reaches the second detection position. A vortex flowmeter, comprising a body (40) having a flow passage (41), a vortex generator (42) in the flow passage (41) transverse to the direction of flow through the flow passage (41) and producing a vortex street, at least a first wing (44) disposed in the flow passage (41) downstream of the vortex generator (42), a first sensor (50) associated with said wing (44) for providing a signal, in response to movement of said wing, (44) representative of the combined amplitude of vortices and external noise, characterised in that the first wing (44) for detecting vortices and external noise at a first position along the longitudinal axis of the flow passage (41) has a length (L′) in the direction of fluid flow that is about equal to a wave length of the vortex street leaving the vortex generator (42) so as to substantially absorb the vortex street, a second wing (45) for detecting only the external noise is separate from the first wing (44) and disposed downstream of the first wing (44), a second sensor (51) is associated with said second wing for providing a signal in response to movement of said second wing (45), and means are provided for combining the signals from the sensors (50, 51) to provide a combined signal indicative of fluid flow, with the noise signals cancelling out. A vortex flowmeter as claimed in claim 3, characterised in that the sensors (50, 51) are each disposed within the respective wing (44, 45). A vortex flowmeter as claimed in claim 4, characterised in that the sensors (50, 51) are piezoelectric transducers. A vortex flowmeter as claimed in at least one of claims 3 to 5, characterised in that the first wing (44) and the second wing (45) are each attached to the body (40) at one end of the wing (44, 45).

Inventors: BRUNKALLA RONALD L, VANDER HEYDEN WILLIAM H, WARNE CRAIG V, BRUNKALLA, RONALD L., VANDER HEYDEN, WILLIAM H., WARNE, CRAIG V.
Applicants: BADGER METER INC, BADGER METER, INC.
Application year: 1992
Main classification: G01F1
IPC codes: G01F1