NARROW PROFILE VORTEX SHEDDING BODY

Abstract: A body (11) for improved shedding of vortices has a short downstream length, e.g. 0.3 cm, yet is capable of excellent flow metering performance. The body includes an upstream elongate thin plate (19) spanning the inner diameter of a conduit, and one or more downstream sections (21). Generally, these sections are in the form of cylindrical rods of various cross-sections, with their longitudinal axes parallel to that of the plate. These sections may be separated from the plate, or rigidly attached to the downstream side thereof. By simultaneously maintaining the ratio of the width (HF) of the thin plate to the overall width (HT) of the downstream sections within the range from 1.0 to 2.2, and the ratio of the width of the thin plate to the diameter of the conduit within the range from 0.1 to 0.3, enhanced linearity of signal is achieved. This results in a narrow profile flowmeter assembly which occupies the same amount of space as a conventional orifice plate.

Description: Narrow profile vortex shedding body The present invention relates generally to a vortex shedding bluff body, and more particularly to such a bod having a minimal downstream extension, usable in a narro profile flow eter configuration. It is well known that the presence of a bluff, or non streamlined, body within a moving fluid stream cause vortices to be shed alternately from opposite sides o the body, the frequency of production of the vortice being related to the flow rate of the fluid. Th dynamics of the so-called Von Karman vortex street whic emanates from the bluff body have been extensivel studied, and the principles have been effectively applie in numerous commercial flow-rate measuring devices. Although a single thin, flat plate is recognized as being capable of shedding vortices (see, for example U.S. Patent No. 3,116,639), this shape has seen limited com¬ mercial use. Bluff bodies typically used in commercial flow meters have a more complex structure, and a down- stream length which is a substantial fraction of the transverse width, generally resulting in a flowmeter of considerable length. For example, U.S. Patent No. 3,572,117 shows a bluff body which extends downstream for a distance of one to two times its width. Similarly, in U.S. Patent No. 3,948,097 the bluff body has a down¬ stream length between 0.5 and 0.9 times its width, while U.S. Patent No. 3,810,388 teaches that a width-to-length ratio of 1.5 (i.e., the length is 2/3 the width) yields maximum energy for vortex formation. ζJϊlE OMPI " WΪP U.S. Pat. No. 4,297,898 discloses the use of "plate-lik upstream and downstream obstacles but the thickness the respective obstacles and the gap therebetween yiel a configuration whose overall downstream length is on t order of 1.2 times its width. The use of a bluff body of appreciable downstream leng has restricted the utility of a vortex flowmeter certain applications. For example, a user of a narro profile flowmeter, such as an orifice plate, general cannot replace his existing installation with the highe performance vortex flowmeter without extensive modific tion to the piping system to accommodate the considerab longer vortex flowmeter housing. Although in U. Patents Nos. 4,171,643 and 4,186,599 a narrow-profi vortex shedding configuration is formed from a th plate, multiple vortex-shedding members are employe spaced transversely across the diameter of the pipe. In view of the above, it is an object of the present i vention to generate strong, linear vortex streets from bluff body without the need for a minimum downstre length. It is a further object of the invention provide a narrow-profile vortex flowmeter using such bluff body which is suitable for replacement of an or fice plate flow measuring system. An embodiment of a body for shedding vortices within fluid flowing through a conduit of inside diameter D, accordance with the present invention, includes an u stream elongate member spanning the interior of the co duit, having an upstream-facing surface aligned norm to the flow direction. The surface has a width Hp as measured perpendicular to the direction of span of th upstream member. The body further includes a down stream member, which also spans the interior of th conduit, its direction of span being parallel to that o the upstream member. This downstream member has a widt H τ in the direction normal* to the flow direction, a measured perpendicular to its direction of span, and a extension L along the direction of flow, as measure from the upstream-facing surface. The ratio H tr / H is within the range from 1.0 to 2.2, while the rati H F /D is in the range from 0.1 to 0.3, and the rati L/H_ is less than 0.4. In a particular embodiment of the present invention, th downstream member consists of elongate convex projection integrally formed with the back surface of the upstrea member. These projections, in addition to being indi vidually parallel to the direction of span of the up stream member, are also parallel to one another. Ther are one or more such projections. In the case of mul tiple projections, the dimension H τ represents th overall width of the combination of projections, i.e. the transverse distance between the outermost edges o the two outermost projections. These projections ca have various cross-sectional shapes, Including rectangu lar, semi-circular and U-shaped. In an alternate embodiment, the downstream portions ar elongate plates or rods which are separated by a narro gap from the back surface of the upstream plate. As i the previous embodiment, the rods or plates are paralle to each other as well as being parallel to the directio of span of the upstream member, and similarly they can OMPI . IPOj have square, rectangular, circular, oval, etc. cros sections. The overall width H- of such a configuration is the same as defined above for the convex projections The limitation of the overall downstream extension L the bluff body structure to - a maximum of 0.4 times t width H p , allows the overall length of a flowmet utilizing such a body to be no greater than the typic thicknesses of orifice plate installations, for an a preciable range of conduit inner diameters. This co straint makes the narrow profile flowmeter suitable f replacement of orifice plates. The novel features and advantages of the present inve tion will become apparent to those skilled in the a from the following description of the preferred embod ments in conjunction with the accompanying drawings which: FIG. 1 is a perspectiv . _.ew of a bluff body in acco dance with the present invention; FIG. 2 is a cross-sectional view of the bluff body of FIG. 1, taken along the line 2-2; FIG. 3 is a schematic of a flowmeter apparatus utilizi the bluff body of FIG. 1; FIG. 4 is a graph depicting the variation in vort linearity with variations in the ratio Hp/H- and FIG. 5A, 5B through 10A, 10B are, respectively, perspe tive and sectional views of several alternate embodimen JO of the bluff body, the sectional views being taken alon the lines 5B-5B through 10B-10B respectively. Referring now to FIG. 1, an elongate bluff body 11 span a circular opening 13 within a thin housing 15. The bod is rigidly attached to the housing at both ends." Th opening is equal in size and shape to the bore of a con duit (not shown) within which the bluff body is place for the purpose of shedding vortices within a stream o fluid. The direction of fluid flow, once the bluff bod is placed within the conduit, is in the direction towar the viewer, as indicated by an arrow 17. The bluff body 11 is made up of two components, an up stream portion 11A and a downstream portion 11B. Th upstream portion is an elongate thin plate 19, with flat upstream-facing surface 19A (see FIG. 2) aligne normally to the direction of fluid flow. The non streamlined contours of the body cause vortices to b shed alternately from its opposite sides, in a manne well known to the art. The thickness, or downstrea extending dimension, of the plate is minimal, bein determined primarily by the structural requirement necessary to withstand the forces generated by the flui flowing past the plate. Preferably, the thinner th plate the better, since according to the present inven tion, desired stability and linearity characteristics o the vortex street can be maintained without the adde downstream length taught by the prior art. Thus, extra downstream length means wasted volume and material. The downstream portion 11B comprises a second elongate thin plate 21 parallel to the upstream plate 19 and stretching across the inner opening 13 of the housing i the same direction as the upstream plate. As with t first plate, the second plate is rigidly attached to th housing. Referring now to FIG. 2, there is shown a cross-section view of the bluff body 11 of FIG. 1 as viewed perpe dicular to its longitudinal axis (longitudinal axis bei the same as direction of span across the opening 13) This view shows quite clearly the parallel arrangeme of the upstream and downstream portions. The width o the upstream plate 19, that is, the dimension which normal both to the flow direction and to the longitudina axis of the bluff body, is labeled as f- . The wid of the downstream plate 21 is similarly defined and i given the label H τ . Experiments have shown that strong vortices having frequency which is linearly proportional to the velocit of flow are achieved by maintaining the ratio H p / within specified limits (hereinafter described), whil simultaneously keeping the ratio of Hp to the diamet D of the opening 13 within the range 0.1 to 0.3, a rang of values already known to the prior art. (It should b remembered that the diameter D is equal to the diamete of the conduit through which the fluid flows.) Th linearity is sustained over a range of velocities fro approximately 0.15 to 3.0 meters per second (i.e., 20:1 rangeability) . These results are achievable eve if the total downstream length L of the bluff body, measured from the upstream-facing surface 19A, is allowe to fluctuate over a range of values, namely the range which L/Hp 0.4. FIG. 3 is a schematic representation of a flowmete apparatus on which these experimental results were ob .tained. The bluff body 11 and its attached housing 1 are installed in a conduit 23 carrying fluid in th direction indicated by an arrow 25. The presence of th body within the stream generates a periodic vorte street, indicated generally by reference numeral 27 which progresses downstream with the moving fluid. At downstream location, a pair of ultrasonic transducers, transmitting transducer 29 and a receiving transduce 31, are disposed on opposite sides of the conduit. A oscillator 33, typically operating at a frequency of MHZ, drives the transmitting transducer so as to projec an acoustic signal through the conduit and the fluid, toward the receiving transducer. The interaction of th acoustic signal with the vortex-carrying fluid strea causes a modulation of the acoustic signal, at the re petitive frequency of the vortices. The receivin transducer detects the modulated signal and transfers i to an amplifier 35. The detected signal is passe through a demodulator 37 to yield a measurement signal having a frequency equal to that of the vortex street. Finally the measurement signal passes through a filter 41 to an electronic counter 43, producing an output indicative of the vortex generation frequency. Since acoustic or ultrasonic detection of vortices and vortex frequencies are well known in the prior art, no detailed description of the detection circuitry need be given. However it should be noted that any of a variety of well known vortex detection schemes, including non-acoustic systems, may be adaptable to the measurement of vortices produced by the bluff body of the present invention. visualizations of the flow around bluff bodies built i accordance with the present Invention have shown tha due in part to. the abbreviated downstream length, vorte formation occurs downstream of the entire body. Th fully formed vortex is not shed from the upstream portio alone, but from the upstream and downstream portio acting in concert. Although the upstream and downstrea portions, if viewed separately, each have vortex shedding shapes, when coupled together as describe herein they ■ define a unitary vortex-shedding structur Thus, it has been found that attempts to detect th vortices alongside, or at a point too near, the body m yield signals having unsatisfactory characteristics e.g., poor stability and linearity. Optimum results a obtained by locating the ultrasonic transducers suff ciently downstream of the bluff body that the vortic have broken away from the separation streamline produce by the body, and have become fully defined. Typicall this is a distance of approximately two-thirds of th pipe diameter downstream from the body. By externally setting the flow velocity of the stream t a predetermined, calibrated value and measuring t corresponding vortex frequency, a proportionality betwee the flow velocity and the frequency can be determined Varying the flow velocity, while maintaining the dime sions of the bluff body constant, yields a series proportionality values, and the percent deviation in t propo tionality over a range of velocities equals t percent non-linearity of the vortex generating scheme The generally accepted unit for expressing proportiona ity is the Strouhal number S, which is generally define as the ratio of the vortex frequency F times the face width Hp of the bluff body divided by the flow velocity V. Because of the unique geometry of the bluff body o the present invention, it is more appropriate to use th corrected Strouhal number S*, as the indicator of propor tionality, S-w being given by the equation: The curve shown in FIG. 4 represents a general summar of the experimentally observed variations in linearit occurring with changes in Hp/H,, for a variety o bluff body configurations built in accordance with th present invention. These data were generated with constant, and 0. l^Hp/D<0.3. It can readily be see that a particularly desirable degree of linearity, i.e., non-linearity limited to within +_0.75%, can be expecte when Hp/H τ is in the range from approximately 1.0 t 2.2. Although the downstream portion of the embodiment o FIG. 1 is a single, thin plate of rectangular cross section, this is not the only type of downstream arrange ment which performs adequately in the context of the present invention. For example, consider the bluff body shown in FIGS. 5A and 5B, in which the downstream portion is a rod, or cylinder, 44 of circular crosssection. Although experimentation was not performed on downstream bodies of every conceivable shape, it is generally expected that bodies having square, triangular or even irregularly shaped cross-sections will perform in accordance with the teachings of the present invention as long as the constraints on the ratio of H F /H T and Hp/D are observed. . CΓREA OMPI It should be noted that isolated deviations from th desirable _+0.75% non-linearity shown in FIG. 4 may occu for a particular Hp/H τ for a body configuration o the type shown in FIGS. 1 and 5A, i.e., a single down stream body separated from -the upstream portion. Eve in such a case, however, the body configuration continue to function as an effective vortex shedder. However these deviations can be eliminated by altering slightl one of the dimensions, while keeping the ratios withi the prescribed limits. Multiple downstream bodies also are functional, as in th case of the structures shown in FIGS. 6A, 6B and 7A, 7B . It should be pointed out, however, that in the case o multiple downstream bodies, H τ does not merely refe to the combined widths of the two individual bodies, bu rather is a measurement of the overall width of th bodies, as measured between their outermost edges, a shown in FIGS. 6B and 7B . Although in each of the previously discussed embodiment there is a separation between the back surface of th upstream portion and the downstream portions, the princi ples underlying the present invention are found not t be restricted to such a configuration. Indeed, as lon as the dimensional ratios are maintained as previousl discussed, the downstream portions can be attached to or integrally formed with, the upstream portion, and th same vortex-generating characteristics are maintained Referring now to FIGS. 8A and 8B, the downstream portion are In the form of discrete elongate protuberances o projections 45, having a convex outer surface, whic depend from the back of the upstream portion. As in the OMPI case of the earlier embodiments, the longitudinal axe of these projections are parallel to the longitudina axis of the upstream plate, as well as being parallel t each other. Although these projections are shown to b generally rectangular in cross-section, semi-circula projections 47 (FIGS. 9A and 9B) , and U-shaped projec tions 49 (FIGS. lOA and 10B), also have proven accept able. Again, although no more than two downstream por tions are shown in any of the embodiments, it is to b understood that multiple such- bodies can be used, subjec to the dimensional limitations. The present invention establishes that appreciable down stream length L (i.e., L/H p >0.4) of the bluff body i not necessary for producing commercially acceptabl vortex streets, a notion which is contrary to tha espoused by the prior art. Thus, the myriad bluff bodie described or suggested herein are particularly useful i a flowmeter having a narrow profile. In other words, th total length L can be made small enough to allow th bluff body and its surrounding housing 15 (see FIG. 1) to be no thicker than the spacing typically associate with orifice plates. For pipe sizes up to 8 inch dia¬ meter, a standard orifice plate generally is 1/8 inc thick; for larger pipe diameters, the orifice plates may range from 0.6 to 1.3 cm thick. By inserting such a narrow profile flowmeter between two sections of pipe in the place of an orifice plate, and utilizing downstream ultrasonic transducers functioning in the manner of transducers 29, 31 (see FIG. 3), a traditional orifice plate installation can be converted to a more accurate, reliable and more linear vortex flowmeter installation with little if any modification to the piping system. In lieu of the electronic counter 43, appropriate elec tronic circuitry for converting the measurement frequenc into a volumetric flow rate, or other appropriate units can be substituted. Again such circuitry is well know to those skilled in the electronics art, and will not b elaborated upon in the present application. Of all the embodiments described herein, the preferre embodiment is as shown in FIGS. 8A and 8B, with th attached rectangular projections. Experimentation ha shown this embodiment to shed vortices effectively i both liquid and gaseous flows. When combined with vortex detecting device, particularly an ultrasonic type it provides an accurate and sensitive flow etering capa bility. An embodiment which is particularly amenable t a vortex detecting scheme which depends on a physica interaction with the vortices, such as a piezoelectri detector, is the embodiment of FIGS. 5A and 5B. Suffi cient energy is transferred between the vortices and th downstream circular rod to permit measurement of th vortex frequency through observation of the rod's move ments. This bluff body configuration can be adapte readily to the external vortex sensing scheme disclose in co-pending U.S. application serial number 236,416 which has a common assignee as the present application. Clearly, certain modifications and substitutions to th disclosed embodiments may become apparent to thos skilled in the art, but which do not depart from th spirit of the present invention. For example, differen cross-sectional shapes for the downstream portions of th bluff body, or alternate vortex detection schemes may b proposed. It is intended, however, that such modifica- tions be included within the scope of the followin claims. OMPI Λ, -vrSo £

Claims: Claims : 1. A body for shedding vortices within a fluid flowi through a conduit of inside diameter D, of the typ having both upstream and downstream members, characte ized by: said upstream member (19) being elongate and spanni the interior of said conduit, having an upstream-facin surface (19A) aligned normal to the flow direction, sa surface having a width H F as measured perpendicular t its direction of span; and said downstream member (21) being elongate and spa ning the interior of said conduit, its direction of spa being parallel to the direction of span of said upstre member, having a width H τ , . as measured perpendicul to its direction of span, and an extension L along t flow direction, as measured from said upstream-faci surface, the ratio Hp/H τ being within the range fr 1.0 to 2.2, the ratio Hp/D being in the range from 0. to 0.3, and the ratio L/H F being less than 0.4. 2. The body as set forth in claim 1, further characte ized by said downstream member being integral with sa upstream member. 3. The body as set forth in claim 1, further characte ized by said downstream member being separate from sai upstream member. 4. The body as set forth in claim 2, further characte ized by said downstream member being an elongate conve projection (45), aligned parallel to the direction span of said upstream member and fixed to the downstre side of said upstream member. 5. The body as set forth in claim 2, further character ized by said downstream member comprising a plurality o elongate convex projections, each aligned parallel to th direction of span of said upstream member and fixed t the downstream side of said upstream member. 6. The body as set forth in claim 4, further character ized by said convex projection having a generally rec tangular cross-section along its length. 7. The body as set forth in claim 4, further character ized by said convex projection having a generally semi circular cross-section along its length. 8. The body as set forth in claim 4, further character ized by said convex projection having a U-shaped cross- section along its length. 9. The body as set forth in claim 3, further character¬ ized by each of said convex projections having a general¬ ly rectangular cross-section along its length. 10. The body as set forth in claim 5, further character- ized by each of said convex projections having a general- ly semi-circular cross-section along its length. 11. The body as set forth in claim 5, further character¬ ized by each of said convex projections having a U-shaped cross-section along its length. * V OMΪ 12. The body as set forth in claim 3, further character ized by said downstream member comprising an elongat rod having a generally rectangular cross-section alon its length. 13. The body as set forth in claim 3, further character ized by said downstream member comprising an elongate ro (44) having a generally circular cross-section along it length. 14. The body as set forth in claim 3, further character ized by said downstream member comprising an elongate ro having a generally oval cross-section along its length. 15. The body as set forth in claim 3, further character ized by said downstream member comprising a plurality o elongate rods, each rod being aligned parallel both t the direction of span of said upstream member and to eac other, and lying in a plane transverse to the flow di rection. 16. The body as set forth in claim 15, further character ized by each of said rods having a generally rectangula cross-section along its length. 17. The body as set forth in claim 15, further character ized by each of said rods having a generally circula cross-section along its length. 18. The body as set forth in claim 15, further character ized by each of said rods having a generally oval cross section along its length. 19. A narrow profile vortex-shedding assembly, of th type suitable for insertion between two sections of con duit, characterized by: a thin, annular housing (15) having an inner openin which matches the bore of said sections of conduit; a first elongate member (19) spanning said inne opening and being rigidly attached to said housing, sai first member having a width H p as measured perpendicu lar to its direction of span; and a second elongate member (21) spanning said inne opening downstream of said first member, its directio of span being parallel to the direction of span of sai first member, and being rigidly attached to said housing said second member having a width H τ as measured per pendicular to its direction of span, the performanc characteristics of the vortices shed by said assembl being dependent on the magnitude of the ratio Hp/H-. 20. A narrow profile vortex-shedding assembly, of th type suitable for insertion between two sections of con duit, characterized by: a thin, annular housing (15) having an inner openin which matches the bore of said sections of conduit; a first elongate member (19) spanning said inne opening and being rigidly attached to said housing, sai first member having a width H p as measured perpendicu lar to its direction of span; and a second elongate member (21) spanning said inne opening downstream of said first member, its directio of span being parallel to the direction of span of sai first member, and being rigidly attached to said housing, said second member having a width H τ as measured per pendicular to its direction of span, the ratio Hp/H τ being within the range from 1.0 to 2.2, the. total thic ness of said assembly being not greater than that of standard size orifice plate for said conduit. *ξτj _ O

Inventors: MARSH DAVID W, MARSH, DAVID, W.
Applicants: FOXBORO CO, THE FOXBORO COMPANY
Application year: 1983
Main classification: G01F1
IPC codes: G01F1