Experimental investigation on the vortex-induced vibration of a circular cylinder partially covered with moss

This paper reports the water-flume experimental results of the vortex-induced vibration (VIV) of a circular cylinder partially covered with marine biofouling with various coverage ratios. The elastically mounted cylinder model is partially covered with real moss, which extends from the bottom to top at uniform intervals. The two-degree-of-freedom (2-DOF) vibration and the wake characteristics of the roughed cylinder were monitored with two laser displacement sensors and a particle image velocimetry, respectively. The VIV response was examined in the reduced velocity range of 1.29–11.32, and the coverage ratio (C_R) ranging from 0 % to 60 % with an increment of 15 %. The experimental results indicate that the maximum cross-flow amplitude of the roughed cylinder decreases in comparison with the smooth cylinder, suggesting the vibration suppression. Nevertheless, the lock-in region is significantly widened with the attachment of moss. The roughed cylinder with coverage ratio of 30 % exhibits the widest lock-in region. The shift of response from the initial branch to the lower branch is related to the jump of phase difference from 0° to 180°. Compared to the smooth cylinder, the jump of φ shift to occur at a higher U_r for roughed cylinders. As C_R increases, more vortices are shed from the roughed cylinder, contributing to the occurrence of 2P (two pairs of vortices are alternately shed from two sides per cycle) and 2T (three vortices are shed from each side of the cylinder per cycle) modes. The shift of vibration branch is also associated with the evolution of vortex shedding pattern.