Suppression of Fluctuating Lift on a Circular Cylinder by Attaching Cylindrical Rings

Abstract

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 diameter (D), spanwise width (W), and spanwise pitch (P) on vortex shedding suppression were investigated. It was found that the periodicity of the pressure fluctuation on the sides of the cylinder disappeared at higher Reynolds numbers (Red ≥ 20,000) for the ring configuration with D/d = 1.3, W/d = 1, and P/d ≈ 3, where d is the base cylinder diameter. In this configuration, the fluctuating lift force was about 1/30 that of a 2D cylinder due to the suppression of periodic shedding and the reduction of spanwise correlation. The mechanism behind this was explored through flow visualization and particle image velocimetry and is considered to be as follows: A spanwise pressure gradient originating from a stepwise change in diameter induces spanwise flow, which brings the corner vortex to the side of the ring. This promotes laminar-turbulent transition in the shear layer separated from the ring at Red ≥ 20,000. As a result, the wake behind the ring shrinks significantly, which induces a pair of large transverse vortices just behind the ring edges. Consequently, the formation of two-dimensional spanwise vortices is obstructed, and the periodicity of the vortex shedding is suppressed.