Loss of lock-in in VIV due to spanwise variations of diameters

Abstract Elongated cylindrical structures with non-uniform diameters, under cross-flow, are often encountered in marine engineering and energy harvesting devices. In this work, the vortex-induced vibrations of elastically supported circular tapered and wavy cylinders with different cross sections areas are studied numerically using a wake oscillator model. The effect of the tapering ratio on the amplitude of oscillation is particularly analyzed. For tapered cylinders it is found that the amplitude of oscillation under flow decreases strongly when a critical tapering is reached. This is consistent with experimental data. A similar effect is found in wavy cylinders. A third system, consisting of a two-section cylinder, also elastically supported and under cross-flow is then considered and the drop in amplitude is related to the ratio between diameters. For this system, a linear stability analysis of the equations of the coupled fluid-solid system allows giving a good approximation of the critical diameter ratio, consistently with the tapered cylinder case. It is concluded that the critical diameter ratio corresponds to the maximum variation in vortex shedding parameters that the coupled wake-cylinder system can support along its length while keeping a consistent vortex shedding.