The flow-induced vibration of a D-section cylinder is investigated at $\text{Re}=100$ using two-dimensional laminar flow simulations, with the cylinder restricted to transverse motion. The orientation angle is fixed at $\alpha ={60}^{\circ }$, and three mass ratios, $m^{*}=5$, 10, and 20, are considered over a range of reduced velocities. The analysis combines nonlinear numerical simulations with linear stability analysis using a reduced model identified by the eigensystem realization algorithm, structural energy share analysis, instantaneous phase analysis, and dynamic mode decomposition (DMD). At low reduced velocities, the lift and displacement remain frequency synchronized, but the lift response reaches its maximum before the displacement response. The linear stability and structural energy share results associate this offset with a change in the structural participation of the flow-related and structural modes. At higher reduced velocities, a finite transitional interval is observed in which the dominant structural and lift frequencies no longer coincide. Within this interval, the displacement amplitude decreases after the main lock-in response and subsequently shows a local recovery, while the instantaneous phase difference between lift and displacement does not remain close to either $0^{\circ }$ or $\pm {180}^{\circ }$. Representative DMD results show a corresponding shift of the dominant wake-frequency component toward a higher frequency. At still higher reduced velocities, the response approaches a low-amplitude state with the lift and displacement close to antiphase. The transitional interval becomes broader as the mass ratio increases, after which the system approaches a low-amplitude state with the lift and displacement close to antiphase. These results characterize the finite transitional response and clarify its evolution toward the low-amplitude state within the investigated range of reduced velocity.

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• Flow-structure interactions
• Vortex interactions