Vortex-induced Vibrations ( VIVs ) of a flexible cylinder subjected to multi-directional flows have been studied based on a wake oscillator model. The multi-directional flow comprises two slab of flows in different directions, with each slabs having uniform uni-directional profile. The dynamics of the flexible cylinder is described based on the linear Euler-Bernoulli beam theory and a wake oscillator model is uniformly distributed along the cylinder to model the hydrodynamic force acting on it.

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... The dynamics of the coupled system has been solved numerically using the finite element method and simulations have been conducted with the cylinder subjected to multi-directional flows with different angles between the two slabs. A large number of different initial conditions have been applied and more than one steady state responses have been captured for each flow condition. The steady state responses exhibit two different patterns, one is characterized by two waves travelling in the opposite directions, while the other is dominated by a single travelling wave. It has been found that the cross-flow VIV primarily occurs in the local cross-flow direction and a transition of its vibrating direction happens at the interface of the two flows. Such transition is not observed in the inline VIV and significant vibrations at the double frequency appear in both local cross-flow and inline directions. Energy analysis shows that this transition is boosted by a specific energy transfer pattern between the structure and the flow, which excites the vibration of the cylinder in some directions while damps it in others. It has also been found that this energy transfer is related to certain motion trajectories.