Abstract
An immersed boundary method is employed to simulate vortex-induced vibrations (VIV) of a circular cylinder in two dimensions. The Reynolds number is 150, and the cylinder mass ratios of 2 and 10 are considered. The synchronization regions for these two mass ratios are determined by the simulations. It is found that the cycle-averaged added mass is about zero at the reduced velocity of 6.1. The instantaneous frequency, which is obtained by Hilbert transformation of the cylinder oscillating displacement, exhibits an important feature whereby the cylinder oscillation in the VIV synchronization region is modulated with a frequency twice the displacement prevailing frequency. The cylinder displacement could still be well approximated by a sine function with a constant frequency and amplitude. However, the lift force acting on the cylinder cannot be estimated in the same manner. In fact, both the lift force amplitude and frequency are modulated. The suggested expression provides a better approximation of the lift force. Moreover, it reveals that the presence of the higher harmonics in the lift force is the result of the amplitude and frequency modulation.
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