This study examines the vortex-induced vibrations (VIV) and hydrodynamic characteristics of a rotating circular cylinder with two degrees of freedom (2DOF). The effects of varying mass ratios (M*), rotation rates (α) and reduced velocities (U*) on the cylinder’s vibration responses, hydrodynamic properties and near-wake structures are explored. The results indicate that the cylinder rotation significantly suppresses vibrations in the cross-flow direction, with the inhibitory effect intensifying with the increasing rotation ratio α. Additionally, contrary to the non-rotating cylinder, in which f x * = 2f y * , the rotating cylinder shows congruence in vibration frequencies in both the X - and Y - directions. In the motion trajectory, with the non-rotating cylinder exhibiting an “8” pattern, the rotating cylinder traces a single closed-loop shape. The mass ratio notably influences the displacement amplitude, trajectory, and phase portrait. Higher mass ratios (M* = 6, 8) suppress the growth of time-averaged displacements in both the X - and Y - directions and the incidence of “lock-in” phenomena in the Y direction. Particularly, as M* = 2, α = 1.0, 1.5, the displacement in the Y - direction is notably higher than that under other conditions. The higher mass ratio also modifies the motion trajectory of the cylinder, shifting it from a closed-loop droplet shape to a single closed loop, with the frequency ratio between lift force and cross-flow displacement transitioning from 3:1 to 1:1. Vortex structure analysis reveals patterns such as “2P”, “P+S”, and “2S”, with high mass ratios showing a U-shaped mode and an extended “2S” region in the wake structure.
