Effects of inflow conditions on low mass-damping cylinder subjected to vortex-induced vibrations

This work analyses the effects of the inflow conditions on the bifurcations characteristics of a two-degree-of-freedom low mass-damping cylinder undergoing vortex-induced vibrations. A two-dimensional RANS model was implemented to simulate the fluid–structure interaction problem. The cylinder response was analysed under different inflow conditions and systematic decrements of the inflow acceleration. The results showed a bifurcated response throughout the upper branch, dependent on the inflow conditions. A low-amplitude state was observed at high inflow accelerations, whereas a high-amplitude state was reached when the inflow acceleration decreased below a certain threshold. A proper selection of the inflow conditions led to a significant increment of the minimum inflow acceleration required to trigger the upper branch, reducing up to 60% of the computational time per simulation. The bifurcated response was divided based on its temporal stability, where the cylinder transitioned from a high- to a low-amplitude state after several high-amplitude oscillations. Unstable responses were associated with multi-frequency fluid forces that interfere with the fluid–cylinder energy transfer, precipitating a state transition. A systematic analysis of the unstable region showed that slight differences in the initial inflow conditions and the inherent simplifications of the tested two-dimensional RANS model significantly impacted the cylinder response stability.