Influence of mass and damping ratios on VIVs of a cylinder with a downstream counterpart in cruciform arrangement

Previous wind tunnel experiments by the present authors showed that two kinds of longitudinal vortex induce cross-flow vibrations (LVIVs) on a circular cylinder accompanied by the vortex-synchronization (lock-in) by setting another cylinder downstream with proper gaps in cruciform arrangement, like the well-known Karman vortex induced vibration (KVIV) of a single cylinder. In this work, experiments on the three types of VIVs are carried out using a water tunnel to investigate influences of the mass ratio MR and the damping ratio ζ on the oscillation response. It is shown that the two LVIVs, i.e. the trailing- and the necklace-VIVs, occur in water flow over respective ranges of flow velocity U in spite of large differences in values of MR and ζ as compared with experiments in air flow. However, their influences on oscillation responses of the two LVIVs are largely different from each other, and also from the KVIV. They are compared by referring to the de-synchronization prediction based on the dimensional analysis and the equation on cylinder motion. The variation of behaviors among the three types of VIV are attributed to the differences among the three vortices in their threshold functions for synchronization and the vortex-synchronization functions which express the feedback effect of the cylinder oscillation on the alternating lift generated by the vortex.