Abstract
In this work, a numerical simulation of flow-induced vibrations (FIV) of two tandem long flexible cylinders is performed. In this regard, a fluid-structure interaction solver that uses a partitioned iterative formulation for coupling between the nonmatching 3D fluid mesh elements and the 1D beam elements is carried out through conservative surface-to-line coupling and vice-versa. Simulations are performed for two identical cylinders with an aspect ratio of 350 and a mass ratio of 1.9 which are subjected to a uniform with a Reynolds number of 6400. Simulations are carried out for various gap ratios ranging from a low gap ratio of 8 to a very high gap ratio of 16. The displacement time history of both the flexible cylinders is computed in both cross-flow (CF) and in-line (IL) directions. It is observed that the FIV response of the upstream cylinder roughly resembles the typical vortex-induced vibration response of a single/isolated flexible cylinder. The upstream riser vibrates with lower amplitude as a result of high reduced velocity, away from the synchronization regime. However, due to the interaction of the downstream riser with the disturbed wake of the upstream riser, its amplitude is observed to be higher, indicating resonance with the vortex shedding frequency and high amplitude vortex-induced vibrations. The shielding effects experienced by the downstream cylinder due to the wake generated from the upstream cylinder and its concomitant effect on the response of the riser have been studied thoroughly for different gap ratios. Investigation on dominant frequencies, the trajectory of the riser, and flow analysis of the interacting vortices are also performed in this study.
