Advantages of and developments in dual bluff-body vortex flowmeters

Abstract

The paper begins by reviewing the main performance characteristics of vortex flowmeters - eg, signal stength, signal-to-noise ratio, frequency, repeatability, meter factor and blockage ratio. It then develops a performance equation which calculates the total time required to capture sufficient cycles to achieve a given overall repeatability. From this equation we see that for a typical single bluff-body vortex meter the time required to achieve an overall repeatability of 0.02% is 387min. This underlines the point that single bluff-body meters are in general not very useful for fiscal metering or custody-transfer applications. The paper then reviews the general principles of vortex shedding from two bluff bodies in tandem, explaining that there are two distinct flow regimes, depending on whether the bluff-body separation is less than, or greater than, a critical value. A summary of results is presented for initial tests on 15 combinations of two rectangular bluff bodies in tandem, together with three single bluff bodies for comparison. These initial tests were carried out in a 300 mm diameter wind tunnel at a free-stream velocity of 6.5 mls and a turbulence intensity of 0.5%. Seven combinations with optimum separations were then tested over a free-stream velocity range between 0.6 and 13.0 mls and at three turbulence levels. Detailed results showing the variation in RMS vortex velocity, signal-to-noise ratio, meter factor and percentage frequency standard deviation with free-stream velocity are presented. Certain of these combinations give a far more regular and repeatable vortex signal than single bluff bodies; the time required to achieve an overall repeatability of 0.02% is as low as 33 min. All of the above dual combinations have a small separation, so that vortex shedding takes place behind the downstream bluff body only. Detailed maps of the vortex field around certain dual combinations are presented which show that, in certain cases, the oscillations of boundary layers in the gap are strongly correlated with the oscillations of boundary layers moving around the bluff bodies. Thus the vortex shedding from the downstream bluff body is reinforced by oscillating boundary layers moving through the gap.