Numerical investigation on two degree-of-freedom flow-induced vibration of three tandem cylinders

Numerical simulations of two-degree-of-freedom flow-induced vibration of three elastically mounted tandem cylinders are performed for a low Reynolds number of Re = 150. The spacing ratio L/D is varied from 2 to 5, where L is the center-to-center between the cylinders and D is the cylinder diameter, and the reduced velocity Vr is varied from 3 to 14. The results show a typical vortex-induced vibration response of the upstream cylinder, except for those at L/D=2, where the response amplitudes are about twice those of the single cylinder. Both the midstream and downstream cylinders have large response amplitudes at all the examined spacing ratios. Depending on the flow interferences among three cylinders, the large amplitudes at high reduced velocities are contributed to the gap flow mechanism in the proximity regime and wake-induced vibration mechanism in the co-shedding regime, respectively. The periodic beating phenomenon occurs on both the midstream and downstream cylinders and the range of Vr for beating phenomenon increases with L/D. Depending on the spacing ratios and reduced velocities, six different flow regimes are identified: single bluff body flow, alternating shear layer reattachment flow, deflected gap flow, two parallel vortex streets with co-shedding flow, chaotic wake and 2S wake flow