Suppression of Vortex Shedding from a Cylinder
1. Effect of cylindrical rings
We have conducted experiments to suppress the vortex shedding from a circular cylinder by attaching cylindrical rings at equal intervals along the span of the cylinder. Ring diameter, spanwise width, spanwise pitch, and Reynolds number were varied systematically. The results show that if the configuration of rings is optimized, the periodicity of the fluctuating force almost disappears with drag reduction at Reynolds numbers higher than 20,000.
A scenario of vortex shedding suppression is considered as follows: (1) A spanwise pressure gradient originating from a stepwise change in diameter induces a spanwise flow, which brings the corner vortex at the ring edge to the side of the ring. (2) This promotes turbulent transition in the shear layer separated from the side of the ring at Re > 20,000. (3) As a result, the separated flow reattaches at the rear of the ring. This causes shrinkage of the wake behind the ring. (4) This induces a pair of large transverse vortices ±ωy just behind the ring edge. (5) Consequently, this flow alternation obstructs the formation of two-dimensional spanwise vortices and suppresses periodic vortex shedding.
