Irregular vortex-induced vibrations of a two-dimensional circular cylinder at a low Reynolds number

Abstract

Irregular vortex-induced vibrations (VIV) of a two-dimensional circular cylinder, characterized by the time-varying oscillation amplitude and frequency, were observed in both experiments and numerical simulations. This paper attempts to elucidate the phenomena by numerical simulations of a circular cylinder in VIV, with mass ratios of 2, 10 and 50, at a Reynolds number of 150. The lift force acting on the cylinder is assumed to be a sum of two components associated with added mass and hydrodynamic damping. The time-dependent added-mass and hydrodynamic damping coefficients are determined by the least-squares fit of lift from the cylinder displacement. The effective natural frequency varies with time due to the added mass, and the cylinder oscillation amplitudes are influenced by the hydrodynamic damping force. Consequently, VIV irregularities arise from the complex evolution of the added-mass and hydrodynamic damping coefficients. The irregular oscillations are associated with the interaction of vortices in the near wake, which results in the intermittent appearance of disorganized Karman vortex street. Moreover, the vortex interactions involve a rapid reduction of the hydrodynamic damping coefficients. Besides, the irregular oscillation region is identified in terms of the reduced velocity, and smaller mass ratio tends to have larger width of the region.