Effects of boundary conditions on vortex-induced vibration of a fully submerged flexible cylinder

Vortex-induced vibration of a fully submerged, highly flexible circular cylinder placed horizontally in the test section of a re-circulating water tunnel and fixed at both ends is studied experimentally. The cylinder is tension-dominated with an aspect ratio of 73 and a low-mass ratio of a 1.2. Two synchronized high-speed cameras recorded the oscillations of the cylinder in the crossflow (CF) and inline (IL) directions for a reduced velocity range of U* = 4.8–29.0, corresponding to a Reynolds number range of Re = 250–1525. A modal analysis-based technique was implemented to reconstruct a continuous response of the cylinder from limited measurement points. Mono- and multi-frequency responses as well as transition from low mode numbers to high mode numbers are observed. The effect of symmetry breaking at the boundaries of the cylinder is studied using two different, carefully controlled boundary conditions. The response of the system is observed to be highly sensitive to any minimal asymmetry introduced to the system, in particular at higher reduced velocities where higher modes of oscillations are excited. Regardless of the type of boundary conditions, both odd and even mode shapes are excited in the crossflow and inline directions. The amplitude of oscillations along the length of the cylinder in both the CF and IL directions is observed to be asymmetric even in the case of symmetric boundary conditions.