The contrast in phase angles between forced and self-excited oscillations of a circular cylinder

The problem of vortex-induced vibration (VIV) on a circular cylinder has historically been considered from the viewpoints of both self-excited and forced oscillations. In an intermediate Reynolds number range, the self-excited case typically has an irregular displacement response that occurs without a constant amplitude or frequency. In a forced oscillation of constant amplitude and frequency in this intermediate range, the applied force can either lead or lag the cylinder oscillation with a constant phase angle. In a self-excited oscillation, the phase angle could possibly vary a great deal because of a more irregular oscillation. This study presents a set of phase angles determined from 2-D CFD/LES calculations of the self-excited oscillations of a cylinder at Re=8000 over a range of lock-on conditions. The results show that the phase angles determined from the self-excited case do not have a consistent trend but behave in an irregular manner, not at all like those in a forced sinusoidal oscillation. Both a spectral analysis and a complex demodulation analysis of the displacement and lift coefficient confirm that this self-excited behavior does not occur at a single frequency or at a constant phase angle at the Reynolds number for this study.