A direct prediction method for vortex-induced vibration based on modal force balance is improved to consider the added mass effects. Adopting a non-iterative root-search method in modal space, the equation set gets solved directly without power-balance iterations. Using this method, the VIV of a flexible riser under uniform flow of 2.6m/s are predicted. Compared with the experiments, the VIV displacement response amplitude with variable is more conservative, while the added mass coefficients and dominant frequency match well with the experiments. Further, the difference of this VIV displacements between predicted and experimental results are further studied. It shows the inaccurate of the predicted methods derives from the difference between the hydrodynamic coefficients under VIV and these obtained from forced oscillation experiments.
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