The development of dual bluff body vortex flowmeters

Abstract

If a bluff body is placed in the path of a fluid, vortex shedding occurs: the frequency of vortex shedding is proportional to the fluid velocity. A mechanism has been proposed by Gerrard to describe the formation of vortices downstream of a single bluff body. Because vortex shedding is a natural phenomenon, which is affected by random flow turbulence, vortex flowmeters have limited repeatability. Previous investigations have shown that this repeatability is improved by using certain optimum combinations of two rectangular bluff bodies in series. Visualisation studies on these dual combinations show that the vortex shedding mechanism is quite different to that proposed by Gerrard. This mechanism suggests that further improvements may be obtained: 1. (a) By having a gap in the shape of a double wedge. 2. (b) By having a downstrean bluff body with a concave or convex rear face. This means that a dual combination of triangular shaped rather than rectangular bluff bodies may give enhanced repeatability. This paper describes the results of a wind tunnel evaluation of ten dual combinations which have the above features. The tests were carried out at a fixed flow velocity and turbulence level, and for a range of bluff body separations. The paper presents results for percentage frequency standard deviation for the combinations tested. The results confirm that some combinations show significant improvements in repeatability.