Circular cylinders exposed to vortex-induced vibrations in restricted waters: VIV response from the bottom to the free surface

In this paper, we present a comprehensive experimental investigation into the effect of bottom and free surface boundaries on vortex-induced vibrations (VIV) of a freely oscillating circular cylinder. Effects of the boundaries were examined under varying gap ratios between the cylinder and these boundaries at a low mass ratio of m_r=0.827, only in the cross-flow direction, in the Reynolds number range of 1.2·10⁴≤Re≤9·10⁴. The gap ratios, varied from −0.75 to 2, correspond to an approximate Froude number (Fr) of around 0.50. We have examined the VIV response of this cylinder through thirteen tests conducted in various stages: near the bottom boundary, at a sufficient distance from all boundaries, close to the free water surface, and partially submerged. The findings show that as the cylinder approaches the free surface, the synchronization range gradually narrows, and the amplitude response diminishes. Within these cases, frequencies deviate from the general trend nearby the end of the synchronization range. Upon the cylinderʼs upper surface contacting or penetrating the free surface, the VIV synchronization starts at a higher non-dimensional velocity. In the cases of piercing cylinders, a wide synchronization range was observed where the submergence of the body was only around 25 %. The cylinders piercing the free surface showcase a distinctive frequency pattern, revealing a nearly-constant trend despite the escalating flow velocity within the synchronization range. Lower amplitude and broader range of synchronization were observed in the experiments close to the bottom boundary. As the cylinder moves further from the bottom, the boundary effect vanishes and the amplitudes get higher.