Escape of a circular cylinder from a potential well via nonlinear vortex-induced vibrations: An experimental investigation

In this study, the basic characteristics of escape from a potential well is investigated experimentally for a circular cylinder undergoing vortex-induced vibrations for Reynolds number ranging between 1000 and 5000. The experimental system consisted of a circular cylinder suspended by two elastic beams and allowed to oscillate in the vicinity of two attracting magnets. The shape of the system's restoring force and the height of the potential energy barrier are controlled by changing the distance between the magnets and the oscillating cylinder. Escape occurs when the oscillating cylinder overcomes the potential barrier and collapses onto one of the attracting magnets. It is shown that, escape can occur in a range of flow velocities bounded by two speeds, U₁<U<U₂. At U₁, escape occurs due to a sudden jump in the steady-state response amplitude when the flow speed is quasi-statically increased from zero towards higher values. At U₂, escape is associated with a gradual increase in the steady-state response amplitude when the flow speed is quasi-statically decreased from high values. The range of flow speed for which escape can occur increases with the system's effective nonlinearity and decreases with the mass ratio (ratio between the mass of the cylinder and the displaced fluid). At a critical value of the nonlinearity, U₁ and U₂ merge into a single point causing escape to occur at a single speed regardless of the direction of the flow speed sweep. For values of the nonlinearity below this critical value, escape can only occur at U₂. Finally, it is shown that, results of the experiment do not change based on whether the flow speed is quasi-statically or abruptly varied. This is because no overshoot was observed in the transient response of the cylinder when the flow speed was abruptly changed.