Vortex Shedding Suppression for a Circular Cylinder by Attaching Cylindrical Rings (A Consideration of the Mechanism)

Omnidirectional reductions in drag and fluctuating forces can be achieved for a circular cylinder subjected to cross-flow by attaching cylindrical rings along its span at an interval of several diameters. In this work, the effects of ring configuration, the diameter D, spanwise width W, and spanwise pitch P, on the vortex shedding suppression was investigated. As a result, it was found that the periodicity in the pressure fluctuation on the sides of the cylinder disappeared for Red ≥ 20000 at ring configurations of D/d = 1.3, W/d = 1 and P/d ≈ 3. At this configuration, the fluctuating lift force reduced markedly to about 1/30 of a 2D cylinder due to the suppression of the periodic shedding together with the weakening of the spanwise correlation. The mechanism of this was explored through flow visualizations and PIV measurements, which was considered as follows: A spanwise pressure gradient originated from a stepwise change in the diameter induces a spanwise flow, which brings the corner vortex to the side of the ring. This promotes the turbulent transition in the shear layer separated from the ring for Red ≥ 20000. As a result, the wake behind the ring markedly shrinks, which induces a pair of large transverse circulations just behind the ring edges. Consequently, two-dimensional spanwise vortices are obstructed to form, resulting in the suppression of the periodicity in the vortex shedding.