To investigate the vortex-induced vibration characteristics and coupling mechanisms of two tandem square cylinders with typical spacing ratio

vortex-induced vibration of two tandem square cylinders was numerically simulated at Reynolds number Re=150 with two spacing ratios of 2.0 and 4.0

and variations of the vibration responses with reduced velocity were studied. The flow pattern and the evolution of the flow field around the cylinders were discussed

and the energy input mechanism for the downstream cylinder was emphatically analyzed. The vibrations of the two cylinders were mainly in the transverse direction for the two spacing ratios. When the spacing ratio was 2.0

the “soft-lock-in” phenomenon occurred in the co-shedding regime for both cylinders (with the lock-in frequency much less than 1.0 in the vibration lock-in region). The maximum transverse amplitudes of the two cylinders both appeared in the vibration lock-in region

and the maximum amplitude of the upstream cylinder was larger. In the vibration lock-in region

due to the abrupt increase of the mean drag force of the upstream cylinder

the mean distance between the two cylinders severely decreased

which disturbed the energy input of the lift force to the vibration of the downstream cylinder. Thus

the vortex in the wake region of the downstream cylinder became unstable. When the spacing ratio was 4.0

the “lock-in” phenomenon was only observed for the downstream cylinder in the reattachment regime. The maximum transverse amplitudes of the two cylinders appeared outside the vibration lock-in region

and the maximum amplitude of the upstream cylinder was much smaller than that of the downstream cylinder. Caused by the mixing of the vortex generated by the two cylinders

the energy input of the lift force for the downstream cylinder was enhanced

enlarging its transverse amplitude. The transverse distance between the two cylinders increased accordingly

which resulted in the parallel vortex street mode of the co-shedding regime in the wake region of the downstream cylinder. In addition

the transverse amplitudes of the downstream cylinder had obvious extreme values in the reattachment regime

and the corresponding wake modes were all parallel vortex street mode of the reattachment regime.