Vortex-meter design: The influence of shedding-body geometry on shedding characteristics

The periodic vortex shedding from bluff bodies may be used in flow metering applications. However, because the bluff-body is highly confined (typically in a pipe) the shed vortices may interact with the pipe wall; causing an undesirable non-linear behaviour. An experimental investigation has been conducted; examining the vortex-shedding characteristics of highly confined bluff-bodies in pipe flow, at high Reynolds number (ReD=4.4×104 to 4.4×105). The bluff-bodies were comprised of a forebody and tail; both of which affected the primary-shedding characteristics. The shedders typically produced two unsteady modes: Mode-I was associated with the vortex shedding and mode-II resulted from a separation of the pipe-wall boundary layer. The mode-I behaviour allowed two classes of shedder to be defined: long-tails and short-tails. Modes I and II interacted, particularly for long-tailed geometries. When the length-scale of mode-II exceeded 0.8κ (where κ is the physical scale of the primary shedding vortex), mode-II disrupted mode-I, as the mode-frequency ratio (fII/fI) approached an integer value. The coupling of modes I and II caused mode-I to deviate from its preferred Strouhal number. When the deviation exceeded 25-30%, mode-I locked on to the mode-II frequency. This did not happen for short-tailed geometries, as the length-scale of mode-I was always dominant. Mode-coupling for short-tails occurred only when the mode frequencies were equal.