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Abstract

This work presents a numerical study on the vortex-induced vibration (VIV) phenomenon of synchronization of a vertical, flexible, circular cylinder with a length-to-diameter ratio of 475, being free to move along the in-line (IL) and cross-flow (CF) directions for Reynolds numbers of 42K, 84K and 126K. It is found that the dominant mode numbers, the maximum root mean square amplitudes, the dominant frequencies and the lift coefficient increase with the Reynolds number, but the drag coefficient decreases. The in-line response shows a main frequency component at twice the cross-flow frequency. At some Reynolds number value and riser span location, a third harmonic frequency component is observed in the CF response. The aims of this paper are to study the lock-in phenomenon and the effect of the flow-induced tension in the riser frequency spectrum. The lock-in analysis shows that when both the cross-flow riser movement and the velocity transversal component frequency values are the same, lock-in takes place. The lock-in is established at the vibration mode predominant frequency for the three Reynolds numbers. The results show also that, taking into account the tension produced by the flow, the vibration frequency spectrum will be calculated accurately. The drag force produces a flow-induced tension that makes the riser behave as a tension-dominated riser, even if the riser was not pre-tensioned.

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