Vortex-induced vibration (VIV) is a major cause of structural damage in engineering, thus effective control methods need to be developed to suppress it. In the present work, the control effect on VIV by using a finite-span flexible splitter plate attached to the rear stagnation point of a circular cylinder is experimentally studied. The reduced velocity is in the range $2.43<U^{*}<12.46$, corresponding to the Reynolds numbers over the range $1910<\mathrm{Re}<9800$. The finite-span flexible splitter plate has a fixed streamwise length of 2.$5D$ based on the cylinder diameter and different widths (spanwise length) of $2.5\%L$ to $30\%L$ based on the submerged cylinder length, aiming to find a critical width where the desired control effect can be achieved. The results show that the suppression efficiency of the flexible splitter plate is gradually improved with an increase in its width. Though the difference in total lift fluctuation between the baseline and control cases is not obvious at the lower branch, the vortex lift fluctuation and the energy transfer from fluid to structure motion are reduced for the control cases, which is beneficial to the suppression of VIV. The phenomenon, that the flexible splitter plate of $20\%L$ increases the lift fluctuation of the stationary circular cylinder but suppresses VIV, inspires us that the reduction of lift fluctuation of a stationary circular cylinder may not necessarily lead to the suppression of VIV. In order to understand the spanwise effect of the finite-span flexible splitter plate, its dynamic motion as well as the flow field at different spanwise cross sections are measured for both the stationary and elastically mounted circular cylinders. When the width of the flexible splitter plate is small, such as $2.5\%L$, the plate vibrates synchronously with the circular cylinder with a good periodicity. The wake evolution at the plate region is efficiently controlled, however, the vibration characteristics of the entire cylinder are not changed. For the flexible splitter plate with a large width, such as $20\%L$, although the flow field, the force, and the vibration of the cylinder are coupled, the plate flapping is chaotic. Moreover, the wake evolution in the plate region is obviously changed, and the control effect can be transmitted to the cross section away from the plate, though the effect is weakened. This indicates the possibility that a finite-span flexible splitter plate can suppress VIV of a circular cylinder that has a much larger spanwise length.

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• Flow-structure interactions
• Vortex flows