Vortex-induced inline vibration of a circular cylinder at intermediate Reynolds numbers

This study presents three-dimensional direct numerical simulations of a cylinder undergoing vortex-induced vibration (VIV) inline with a free stream. The incoming flow Reynolds number (Re) ranged from 250 to 1500, covering a range not extensively explored. The mass-damping ratio of the cylinder is limited to m*\zeta = 4.89E-3, which is applicable to offshore cylindrical structures (such as cables and pipelines). Several novel inline VIV phenomena were uncovered. Firstly, two VIV regions (or branches) were found to be dependent on Re. The primary lock-on region consistently occurred at a reduced velocity U* (defined based on the natural frequency of the structure in vacuum) greater than 2.0 across all Re, contrasting with the secondary lock-on region that developed at smaller U* values ranging from 1.1 to 1.8 for Re = 550. The relationship between the peak response amplitude and Re in each branch should be well described by an exponential decay function. Secondly, each branch exhibited various wake patterns, such as the 2S, P+S, and 2P wakes in the primary branch and the alternating wake and symmetric alternating wake in the secondary branch. Notably, the wake transitions from 2S via P+S to 2P in the primary branch was newly identified and has not been reported in previous inline VIV studies. Thirdly, the ordered vortex shedding in the lock-on wake suppressed the initiation of the three-dimensional instability to higher Re = 1000. Understanding of these new phenomena is essential for comprehending the complexities of VIV in two or multiple degrees of freedom.