Wake induced vibration is a distinctive phenomena of fluid-elastic instability arising from interactions of a body in the wakes of another bluff body and characteristically different from the well-understood vortex induced vibrations. This work presents a fluid-structure interactions numerical model as an alternative tool for investigation of wake induced vibrations. In an attempt to better understand mechanisms of wake induced motions, a simplified model of two cylinders in tandem arrangement with different diameters under cross flow was considered in this work. Cross flow velocity conditions vary from moderate to high Reynolds number (Re=2 × 10~3 - 5 ×10~4 in the same range as many experiment reported recently in literature. A hybrid detached eddy simulation approach is used for turbulence modelling at those high Reynolds number conditions in order to resolve complex near body flow features as well as in the wake regions. The proposed model is first validated through extensive benchmarking with experimental studies for responses of tandem cylinders at the same flow conditions as in physical experiment. With good agreement to experimental data, the model was extended for simulations of cylinders of different diameters in tandem arrangement. For different diameters between upstream and downstream cylinders, the fundamental frequencies of shedded vortices from the cylinders are essentially different. It is observed from the present study that responses of the downstream cylinder are characterized not only the geometrical parameters such as distances and diameter differences between the cylinders but also the Reynolds number. As contrast to many experimental studies, at constant Reynolds number, downstream cylinders are found to have multiple lock-in regions in a wide range of reduced velocities. This distinctive behaviour of the cylinders at constant Reynolds numbers and diameter ratios suggests strong evidence of complicated mechanism of wake-induced vibrations phenomena. Further analysis of results from high fidelity numerical simulations were carried out for detailed investigations of force amplitudes and frequencies. The current analysis revealed multiple frequency content of the force; thus explaining high response amplitudes of the downstream cylinder at high reduced velocity.
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