Vortex induced vibrations of a cylinder released from rest

Transient vortex induced vibrations occur in engineering applications where time-varying free stream conditions and structural properties are ubiquitous. The present investigation models an extreme case of structural properties variation as a discontinuous change in reduced velocity. The ensuing transient dynamics of an elastically-mounted rigid cylinder are examined when subject to a change in reduced velocity from 0 to 4.5-11 while maintaining a constant Reynolds number (Re ≈ 4,400). The results reveal a significant difference in transient structural and wake dynamics depending on the quasi-steady end-state response branch the system settles on. The transient amplitude response in the initial branch is characterized by a distinct amplitude overshoot compared to quasi-steady state. Conversely, in the upper and lower branches, the oscillation amplitudes continuously increase up to quasi-steady state. The transient amplitude changes are shown to be related to the instantaneous phase difference between the forcing and displacement. Quantitative analysis links the observed changes in structural response to the associated changes in wake vortex shedding.