GB2119514A

GB2119514A - Vortex-shedding flowmeters

Info

Publication number: GB2119514A
Application number: GB08211593A
Authority: GB
Inventors: Martin John Collier; David Glyndwr White; Ian Murdoch
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1982-04-22
Filing date: 1982-04-22
Publication date: 1983-11-16
Also published as: GB2119514B

Abstract

In a vortex-shedding flowmeter a vane 14 having an enlarged free end 16 extends from a first bluff body 12. The vane may have a reduced width portion 17 passing through a second bluff body which is defined by two parts 13A and 13B. Vortices are produced by a fluid flowing past the flowmeter in the direction shown by arrow 11, causing flexing of the vane 14 which is detected by a piezoelectric ceramic plate 18. Further modifications are disclosed in Figures 5 to 7 (not shown). <IMAGE>

Description

SPECIFICATION Vortex-shedding flowmeters This invention relates to vortex-shedding flowmeters.

When an obstacle, e.g., as shown at 1 in Figure 1 (which is a horizontal axial cross section through a pipe carrying a flow of fluid in the direction shown by the arrow) is placed in a flowing fluid, the fluid boundary layer adjacent to the obstacle can only follow its contour at low flowrates or Reynolds Numbers. At Reynolds Numbers over approximately 100, the fluid boundary layer separates from the body contour somewhere along its iength. Friction and the pressure field behind the body combine to decelerate the boundary layer, causing it to reverse direction and curl upon itself forming a vortex 2. This vortex grows in size and then breaks free, moving downstream. Obstacles which produce such vortices are sometimes called "bluff bodies".

With a symmetrical bluff body the changes in local velocity and pressure associated with a first vortex like that shown at 2 interact with a second vortex 3 forming on the opposite side of the obstruction, causing it to break free. The process repeats itself and a stable pattern of shedding alternately from opposite sides of the bluff body is quickly established so that the downstream wake becomes a staggered pattern of vortices, generally referred to as a "von Karman vortex street".

A bluff body thus constitutes a fluid dynamic oscillator, which, like all oscillators, depends on some form of feedback to regulate and maintain its frequency. In the case of a vortex sheddng bluff body the feedback is provided by the transfer, across the wake, of some of the energy associated with each vortex. The frequency at which vortices are produced is related in a known manner to the velociy of flow.

The invention arose in the design of a bluff body vortex shedding flowmeter in which certain features have been introduced to attempt to improve the regularity of shedding.

In this Specification the term "flowmeter" is to be interpreted as including instruments which detect a particular flow rate or a change of flow rate through a critical value in addition to instruments which produce an output which varies with varying flow rates.

It is known, e.g., as shown in Figure 2 (which is a vertical axial cross-section through another pipe carrying a flow of fluid) to attach a vane 4 to the downstream side of a bluff body 5 to detect, by observing movement of the vane 4, the frequency of vortex shedding. Figure 2 shows a second bluff body 6 downstream of the first bluff body 5. The inventors have performed a series of experiments showing that in such an arrangement, having the bluff bodies, the optimum bluff body separation is iess than the optimum vane length. It was therefore found necessary to pass the vane through a slot in the downstream bluff body to allow the vane 4 to flex freely. An alternative design, which is shown in Figure 3 (which is a view similar to that of Fig. 2), avoids this slot, but gave an inferior performance and was not pursued.In this design a vane 7 attached to a first bluff body 8 is shorter than the separation between the first bluff body 8 and a second bluff body 9. However a further vane 7A is attached to the second bluff body 9.

This invention provides a vortex shedding flowmeter comprising a bluff body arranged to generate vortices when fluid flows past it and a vane arranged to be deflected by the vortices thereby giving an indication of the frequency of generation of the vortices and therefore a measure of the flow, the vane having one end fixed to the bluff body and an enlarged free end extending laterally with respect to the direction of flow.

Preferably there is a second bluff body past which the van may extend. It is also preferable that the vane has a region of reduced width extending through the second bluff body. A third bluff body may be aligned with the second bluff body and attached to the vane.

One way in which the invention may be performed will be described by way of example with reference to Figures 4, 5, 6 and 7 of the accompanying drawings in which: Figure 4 is a schematic axial cross-section showing part of a vortex shedding flowmeter in accordance with the invention; Figure 5 is a schematic axial cross-section showing another embodiment; Figure 6 is another schematic axial crosssection of another embodiment; and Figure 7 is a schematic axial crosssection of yet another embodiment.

A pipe 10 contains a fluid which flows in the direction shown by the arrow 11. A first bluff body 12 extends across the pipe in such a way that it allows the fluid to flow past it on opposite sides. A second bluff body is in two parts 1 3A and 13B and is located downstream of the first bluff body 12.

The two parts 1 3A and 1 3B are aligned with each other and arranged so that there is a gap between them. A cantilevered vane 14 of flexible material has one end 1 5 fixed to the first bluff body 1 2 and an enlarged free end 1 6 which extends laterally with respect to the dection of flow of the fluid.

There is a region 1 7 of the vane which is of reduced width and lies adiacent to the enlarged free end 1 6. The region 1 7 of reduced width lies between the two parts 1 3A and 13B of the second bluff body. A piezoelectric ceramic plate 1 8 is attached to the vane 14 to given an indication of the flexing of the vane. Thus, when the fluid flows past the flowmeter and vortices result it is possible to measure the velocity of the fluid.

A further embodiment is shown in Figure 5.

This is similar to the embodiment as shown in Figure 4 and parts on Figure 5 which are similar to parts on Figure 4 are denoted by identical reference numerals. In the Figure 5 embodiment there is no region 1 7 of reduced width of the vane 14 but there is a third bluff body 1 9. This is attached to the vane 14 and is aligned with parts 20A and 20B of a second bluff body lying in the the gap between them which is wider than that shown in Figure 4.

Figure 6 shows a further embodiment which is similar to that shown in Figure 5 and in which parts similar to the parts of Figure 5 are denoted by identical reference numerals. In Figure 6 there is no third bluff body 19.

Figure 7 shows another embodiment similar to that of Figure 6 but where the second bluff body 20A, 20B is formed in a single part having a slot through which the vane passes.

Claims

1. A vortex-shedding flowmeter comprising a bluff body arranged to generate vortices when fluid flows past it and a vane arranged to be deflected by the vortices thereby giving an indication of the frequency of generation of the vortices and therefore a measure of the flow, the vane having one end fixed to the bluff body and an enlarged free end extending laterally with respect to the direction of flow.

2. A vortex-shedding flowmeter as claime,d in claim 1 and including a second bluff body past which the vane extends.

3. A vortex-shedding flowmeter as claimed in claim 2 and wherein the vane has a region of reduced width which extends through the second bluff body.

4. A vortex-shedding flowmeter substantially as illustrated in and described with reference to Figures 4, 5, 6 and 7 of the accompanying drawings.

4. A vortex-shedding flowmeter as claimed in claim 2 and including a third bluff body which is aligned with the second bluff body and attached to the vane.

5. A vortex-shedding flowmeter substantially as illustrated in and described with reference to Figures 4, 5 and 6 of the accompanying drawings.

New claims or amendments to claims filed on 3.5.83 Superseded claims 1-5 New of amended claims:

1. A vortex-shedding flowmeter comprising a first bluff body arranged to generate vortices when fluid flows past it, a second bluff body located downstream of the first and a vane extending laterally to the flow having one end fixed at the first bluff body and an enlarged free end arranged downstream of the second bluff body, such that the vane is arranged to be deflected by the vortices, thereby giving an indication of the flow.

2. A vortex-shedding flowmeter as claimed in claim 1 and wherein the vane has a region of reduced width which extends through the second bluff body.

3. A vortex-shedding flowmeter as claimed in claim 1 and including a third bluff body which is aligned with the second bluff body and attached to the vane.