Flow-induced vibrations of two side-by-side circular cylinders: Asymmetric vibration, symmetry hysteresis and near-wake patterns

The immersed boundary method was utilised to numerically investigate the flow-induced vibrations (FIV) of two elastically mounted side-by-side circular cylinders in a uniform flow with low Reynolds numbers. Six distinct near-wake patterns were observed; the irregular (IR) pattern, the in-phase synchronized (IS) pattern, the anti-phase synchronized (AS) pattern, the biased anti-phase synchronized (BAS) pattern, the out-of-phase flip-flopping (OFF) pattern, and the hybrid (HB) pattern. A detailed analysis on the asymmetric vibration and symmetry hysteresis phenomena was conducted by focusing on the near-wake patterns and the interaction between the cylinders. Results show that the asymmetric vibrations of the cylinders are closely related with the stably biased gap flow and the resulting narrow-wide near-wake pattern. While the symmetry hysteresis is caused by the coexistence of two distinct near-wake patterns - the IS and the BAS patterns. The transition processes of BAS to IS and IS to BAS were illustrated by using the long-time histories of the lift coefficients, the combined lift coefficient and the phase differences of lift and displacement. Results on hydrodynamic forces and the vibration responses show that the HB pattern is a combination of IS, OFF and AS patterns with a very long period.