Experimental evidence for the mechanism of subcritical vortex-induced vibration

The vortex-induced vibration (VIV) of circle cylinder can occur at subcritical Reynolds numbers as low as 20, which is called subcritical VIV. Recent numerical research suggests that the interaction of fluid mode and structural mode is the primary cause of its occurrence. Due to the limitations of experimental techniques and data analysis methods, researchers failed to find experimental evidence to support this view. To this end, we conducted subcritical VIV tests in a rotating channel and discovered that the flow is unstable when subcritical VIV occurs. Subsequently, an experimental data-driven model for unsteady flow is constructed. By coupling the unsteady flow model with the structural dynamic equations, we constructed the FSI model for subcritical VIV. By performing linear stability analysis on the FSI model, we found that there are two key modes in the FSI system, namely fluid mode and structural mode. These two modes will couple when the structural natural frequency is close to the characteristic frequency of the flow, resulting in the instability of structural mode and thus inducing subcritical VIV. This experimental finding clarifies that subcritical VIV is essentially flutter as a result of structural mode instability, providing the first experimental evidence for the mechanism of subcritical VIV.