Numerical investigation of multi-mode vortex-induced vibrations in a low-mass-ratio twin-cylinder system

This study numerically investigates vortex-induced vibration (VIV) in a twin-cylinder system with low mass ratio ( m ⁎ = 2.6) and frequency ratio ( f n 2 / f n 1 = 3 ), focusing on coupled in-line (IL) and cross-flow (CF) vibrations without wake interference. Through two-dimensional RANS simulations with the validated SST k-ω turbulence model and variational mode decomposition, bidirectional fluid-structure interactions are analyzed across reduced velocities ( Ur 1 = 2–21). Results show that strong SU-branch responses coincide with attenuation of the competing mode, and the modal interaction interval spans Ur1 = 7–10.5. Within this interval, modal interaction is characterized by added-mass detuning and changes in added damping. A response separation sub-regime ( Ur 1 = 7–8.25) is first identified, where second-mode in-line resonance breaks symmetry, inducing divergent orbital trajectories, distinct wake states, and asymmetric cycle-averaged fluid work between the cylinders. A modal competition sub-regime ( Ur 1 = 8.25–10.5) follows, where changes in CF modal fluid work accompany a gradual modal switch, while simultaneous dual-mode participation generates in-line beat-frequency excitation ( f nw 2 ± f nw 1 ), producing irregular trajectories and non-stationary 2T/2P wake patterns. These findings elucidate nonlinear multimodal coupling mechanisms and provide insights for offshore structures with closely spaced natural frequencies.