Vortex-induced vibration and hydrodynamic characteristics of a round-ended cylinder

A round-ended section possesses the advantages of both circular and square sections, i.e. a large span and smooth transition at both ends. Therefore, the two-degree-of-freedom VIV response of a round-ended cylinder is numerically investigated in this paper in the reduced velocity (U_r) range of 2–16, comparing with the response of a circular cylinder of the same mass-damping ratio. The numerical results indicate that the development and separation of boundary layers determine the appearance of the minimum pressure coefficient, the vortex shedding mode and the characteristic sizes of wake structure. The reattachment and second separation of boundary layers are observed in the round-ended cylinder when U_r ≥ 6, contributing to a much downstream final separation point as compared to the circular cylinder. Both the hydrodynamic forces and the VIV response are closely associated with the vortex shedding mode, which is further illustrated by the variations of the added mass coefficient, the phase difference and the transferred energy coefficient. The turning points of the curves of hydrodynamic coefficients and response amplitudes correspond to the transition of vortex shedding mode as well as the switching of VIV branch. Compared to the circular cylinder, the oscillation of the round-ended cylinder presents an elongated lower branch and much smaller vibration amplitudes.