Vortex Shedding from Linearly Tapered Cylinders

FLOW VISUALIZATION OF THE SPANWISE VORTEX SHEDDING PATTERN IN THE WAKE OF AN OSCILLATING TAPERED CYLINDER

LeadPercipitation Visualization of the Tapered Cylinder (40:1)
Oscillating with A/d =0.5, Re = 400 and St = 0.162. Transition in shedding
pattern from two vortices (upper cell) to four vortices per cycle.

Shear flow and vortex shedding behind circular cylinders have many implications for marine cables, moorings, and offshore structures. The problem of vortex shedding behind cylindrical bluff bodies has been a subject of investigation for many years. However, the majority of existing work, on vortex shedding from circular cylinders in shear flow, focuses on non-oscillating cylinders. Presented here are flow-visualization experiments performed to investigate vortex shedding from oscillating cylinders in shear flow configurations.

To model linear shear flow, linearly tapered cylinders are used. Here a cylinder, with taper ratios 40:1, was towed in a uniform body of water at Reynolds numbers Re=450 and Re=1500. The goal is to obtain a qualitative and quantitative understanding of the vortex shedding behind the tapered cylinder in an oscillating trajectory. Two methods were employed to visualize the shedding. First, the electrolytic precipitation method offered good qualitative insight. Whereas the second method, digital particle image velocimetry (DPIV), allowed for a quantitative analysis of the instantaneous, two-dimensional velocity field and the associated vorticity. Experiments were performed to observe the three dimensional structures in the wake and the spanwise shedding patterns associated with the forced oscillations.

Three-dimensional structures in the wake of the cylinder were apparent. The presence of longitudinal vortices and dislocations in the vortex lines were also observed. Williamson and Roshko (1988) proposed that the shedding pattern changes depending on the ratios of the oscillation amplitude and frequency to the cylinder diameter, for the case of the oscillating, uniform cylinder. As the diameter of the tapered cylinder changes along its length, so do these ratios, A/d and f/d. Thus, for any given oscillation amplitude and frequency, the tapered cylinder can shed vortices in more than one pattern. Here it was found that when two different patterns dominated, a complex dislocation and connection of vortex lines occurred at the point where the patterns changed. The spot at which the dislocation occurred changed with oscillation amplitude, frequency and Reynolds number.