Multi-mode vortex and wake-induced vibrations of a flexible cylinder in tandem arrangement

Experiments with a flexible cylinder of circular cross-section towed behind a stationary circular cylinder at various distances are described in this paper. The dynamics of the model immersed in the wake of the leading one are fully characterised after data processing and computation of amplitude and frequency responses, as well as modal content as a function of reduced velocity. Reduced velocity was controlled by changing the towing speed of the system and by adjusting different pre-tensions on the model.

The amplitude response of the model has been found to be dominated by resonance or lock-in regions in the reduced velocity axis at the different structural modes of the cylinder model. Separation distance does not produce significant qualitative changes in the response amplitudes and frequencies observed when compared to the response of the same cylinder without wake interference. However, there is a clear effect of the incoming wake, because amplitude responses are considerably higher than those expected in isolated cylinders at the same reduced velocities. Also, the response has been found to be more multi-modal with responses made of large amplitude contributions of several adjacent modes.