Vortex shedding in oscillatory flow: geometrical effects

Abstract

This paper presents a selection of the results of an extensive experimental study of the effects of in-line, periodic flow oscillations on the character of vortex shedding from sharp-edged bluff bodies. Attention is concentrated on how oscillation amplitude and the body shape alter the range of reduced velocity, U r, over which lock-in conditions occur — i.e. conditions under which changes in the upstream mean velocity no longer lead to changes in the vortex-shedding frequency. U r is defined as U o Nd, where d is the body cross-stream width, U o is the upstream velocity and N is the oscillation frequency. It is demonstrated that, with all other parameters fixed, some body shapes are much more susceptible to lock-in than others. Of the four shapes tested, the two most commonly used in industrial vortex flowmeters (triangular and T-shaped cross-sections are the most likely to suffer from measurement errors due to in-line flow oscillatins. The additional effects of high obstacle blockage and small spanwise aspect ratio are also discussed in the context of oscillatory flow.