THE DEVELOPMENT OF DUAL BLUFF-BODY VORTEX FLOWMETERS

If a bluff body is placed in the path of a fluid, vortex shedding occu rs: the frequency of vortex shedding is proportional to the fluid velo city. A mechanism has been proposed by Gerrard to describe the formati on of vortices downstream of a single bluff body. Because vortex shedd ing is a natural phenomenon, which is affected by random flow turbulen ce, vortex flowmeters have limited repeatability. Previous investigati ons have shown that this repeatability is improved by using certain op timum combinations of two rectangular bluff bodies in series. Visualis ation studies on these dual combinations show that the vortex shedding mechanism is quite different to that proposed by Gerrard. This mechan ism suggests that further improvements may be obtained: (a) By having a gap in the shape of a double wedge. (b) By having a downstream bluff body with a concave or convex rear face. This means that a dual combi nation of triangular shaped rather than rectangular bluff bodies may g ive enhanced repeatability. This paper describes the results of a wind tunnel evaluation of ten dual combinations which have the above featu res. The tests were carried out at a fixed flow velocity and turbulenc e level, and for a range of bluff body separations. The paper presents results for percentage frequency standard deviation for the combinati ons tested. The results confirm that some combinations show significan t improvements in repeatability. (C) 1997 Elsevier Science Ltd.