Effect of mass ratio on flow-induced vibration of a trapezoidal cylinder at low Reynolds numbers

A numerical study of the effect of the mass ratio (M*) on the flow-induced vibration of a trapezoidal cylinder at low Reynolds numbers (Re = 60–250) is presented. The response characteristics are divided into three classes with varying mass ratios (2, 5, 10, 20, 30, 50, and 100): (1) class I for low mass ratios (M* = 2), (2) class II for medium mass ratios (5 ≤ M* < 30), and (3) class III for high mass ratios (M* ≥ 30). In class I, for the vortex-induced vibration (VIV) regime, only one peak of maximum amplitude is observed at low Re (∼70). For the galloping regime, a double rise-up for amplitudes is observed, and the mean transverse displacements become positive at higher Re and increase rapidly. In class II, the double rise-up for amplitudes appears at both the VIV and galloping regimes, and the double lock-in is also found for oscillation frequency ratios. In class III, the double rise-up disappears in the VIV and galloping regimes at all considered Re. The onset Re of the galloping regime is much higher (Re > 200), and the peak amplitudes and ranges of lock-in in VIV become much smaller with an increase in M*. Among these three classes, similar distinctions are also observed in the hydrodynamic forces. In terms of X–Y trajectories, three types are found in class I, while there are only two and one in classes II and III, respectively. Wake structures are also investigated for these classes.