An insight into the vortex-induced vibration and near-wall vortex evolution of a roughed circular cylinder with truncated conical shape protrusions

The vortex-induced vibrations of a roughed circular cylinder with truncated conical shape protrusions to simulate the attachment of barnacles are numerically investigated in this paper. The coverage ratio (C_R) of protrusions varies from 0% to 80% with an increment of 20%, and the simulation is conducted in the reduced velocity range of U_r = 1.43-11.00. The numerical results indicate that the boundary layer development of the cylinder is continuously disrupted by the protrusions, generating inter-rib vortices. Five near-wall vortex structures are identified, including the main vortex, merged main vortex, subordinate vortex, inter-rib quasi-stagnation vortex and dynamic inter-rib vortex. The evolution of the near-wall vortex is dependent on both the U_r and C_R. As C_R grows, the numbers of boundary layer separation and reattachment increase, and the associated points become to be more concentrated on the front surface of the cylinder. The vortex formation length and wake width are closely related to the location of boundary layer separation, significantly influencing the hydrodynamic coefficients. The emergence of merged main vortices leads to an increase in the vortex intensity, thereby affecting the hydrodynamic coefficients. The vibration response of the cylinder with protrusions of C_R = 20% is significantly enhanced with the accompany of and broadened lock-in region, which is attributed to the transformation of the vortex shedding mode.