Mode excitation hysteresis of a flexible cylinder undergoing vortex-induced vibrations

A series of experiments was performed to investigate the modal excitation of a tensioned flexible cylinder in a uniform flow due to flow-induced vibrations. Experiments were conducted in a recirculating flow channel using a flexible cylinder with relatively low aspect ratio such that low structural mode numbers were excited. The spanwise motion of the cylinder was measured using standard motion tracking techniques with high-speed cameras. Hysteresis is observed in the response of the cylinder dependent on whether the flow speed is increased or decreased from the previous experiment. This observation differs from predictions in the literature regarding hysteresis and is attributed to hysteresis in the transition between excited structural modes coupled with the cylinder wake. It is also found that the flexible cylinder is unable to sustain excitation of asymmetric modes in the in-line direction unless the fluid-structure interaction excites a 1:1 frequency response between the in-line (IL) and cross-flow (CF) directions, resulting in a pedaling mode response. The inability to excite asymmetric modes is consistent with the response of linear systems undergoing a symmetric drag load and is consistent with the cylinder undergoing a preferred figure eight shape motion when excited. Distributed fluid forces are derived from the structural characteristics and body motions illustrating the transition of the distribution of added mass and excitation forces on the body.