Flow-induced vibrations of two cylinders of different natural frequencies

The paper presents an experimental investigation on characteristics of flow-induced vibrations of two tandem cylinders of different natural frequencies. Both cylinders are allowed to vibrate in the cross-flow direction only. Six different natural frequency ratios f_n^* (= f_n,u/f_n,d) = 0.6, 0.8, 1.0, 1.2, 1.4 and 1.6 are considered, where f_n,u and f_n,d are the natural frequencies of the upstream and downstream cylinders, respectively. The spacing ratio L/D (where L is the spacing between the two cylinder centers and D is the diameter) is considered as 1.5 and 2.0. Simultaneous measurements of vibration and frequency responses and vortex shedding frequencies are conducted using laser vibrometers and hotwires, respectively. The results indicate that in the galloping vibration regime there is a critical reduced velocity at which the amplitude of the downstream cylinder drastically jumps and that of the upstream cylinder may drop, depending on f_n^*. The jump/drop is connected to a lock-in of the vortex shedding with the fifth harmonics of f_n,d. The different natural frequencies of the cylinders may suppress both vortex-excited and galloping vibrations of the cylinders at different reduced velocity ranges. The vibration response and its connections with the frequency ratio are elucidated. How the different natural frequencies of the upstream and downstream cylinders affect the vortex shedding frequency in the wake is also illustrated.