Two-degree-of-freedom vortex-induced vibration of two mechanically coupled cylinders of different diameters in steady current

Two-degree-of-freedom vortex-induced vibration of two mechanically coupled cylinders with a diameter ratio of 0.1 in steady current is investigated numerically. The study is aimed to investigate the effects of gap ratio (G/D) between the two cylinders and the angular position (θ) of the small cylinder relative to the large one on the vibration amplitude and frequency. The force coefficients of the cylinder bundle are also investigated at typical gap ratios and angular positions of the small cylinder. It is found that the vibration frequency components, the amplitude of the vibration and the force coefficients of the two-cylinder bundle are very sensitive to the gap ratio and the angular position of the small cylinder. The maximum cross-flow vibration amplitude occurs when the cylinders are arranged in the staggered configuration (θ=67.5°) at the gap ratio of 0.1. The minimum cross-flow and in-line vibration amplitudes occur at θ=112.5° when the gap ratio is 0.3. The maximum in-line vibration amplitudes occur at the staggered positions of θ=67.5° as G/D=0.1 and θ=135° as G/D=0.2. Furthermore, it is found that the instantaneous position of the bundle relative to the last generated vortices has prominent effect on the force coefficients.