Abstract: The vortex-induced vibration (VIV) characteristics of a circular cylinder with different degrees of surface roughness was studied numerically. The response amplitude, response frequency, vortex force, vortex shedding flow pattern and phase angle between the VIV displacement and the vortex force with different degrees of surface roughness were compared. The numerical results show that the VIV amplitude decreases with increase of surface roughness. For smooth cylinder or the cylinder with small surface roughness, the VIV response could be divided into three branches: initial branch, upper branch and lower branch. Within initial and upper branches, the vortex shedding flow pattern displays 2S mode. However, it produces 2P mode within lower branch. For the cylinder with large surface roughness, the VIV response only could be divided into two branches: initial branch and lower branch. The vortex shedding flow pattern displays 2S mode within initial branch, while it displays 2P mode within lower branch.

Key words: surface roughness, circular cylinder, vortex induced vibration, flow pattern of vortex shedding, vortex phase