Numerical simulation on vortex-induced vibration of two tandem square cylinders with medium spacing at a low Reynolds number

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. Copyright ©2020 Journal of Harbin Institute of Technology.All rights reserved.