Vortex-induced vibration of finite-length circular cylinders with spanwise free-ends: Broadening the lock-in envelope

The unsteady response of finite-length rigid circular cylinders with spanwise free-ends undergoing vortex-induced vibration (VIV) has been experimentally investigated in a large-scale wind tunnel. The model consists of a cylinder mounted on the upstream tip of an elastic cantilever beam, thus allowing the investigation of the effects of the aspect-ratio on the VIV dynamic response of the system. Models with different aspect-ratios have been tested, and the time dependent oscillation amplitude of the cylinder has been determined from the analysis of images obtained with a high-speed camera. The solid body rotation of the vibrating cylinder and the vorticity flux generated by this moving boundary, entering the flow field, are associated with the formation of large vortical structures and increased forcing on the cylinder. Surprisingly, the results show an unexpected broadening of the lock-in envelope by about 200% as the aspect-ratio decreases from 28.8 to 5 and an almost 230% increase in the peak oscillation amplitude. The reduced damping parameter known as the Skop-Griffin parameter has been used to interpret these novel results. The Griffin plot shows a reasonably good agreement between the present experimental data of different aspect-ratio cylinders and those of other investigators. Published by AIP Publishing.