Flow-induced vibrations of two circular cylinders in tandem arrangement. Part 1: Characteristics of vibration

This paper presents the results of an investigation on the flow-induced vibration characteristics of two circular cylinders in tandem arrangement, with L/D=0.1–3.2 and reduced velocity Ur=1.5–26, where L is the gap spacing between the cylinders and D is the cylinder diameter. The cylinder vibration was restricted to a plane normal to the incident flow. Three different experimental conditions were examined: (i) both cylinders were allowed to vibrate; (ii) the downstream cylinder only was allowed to vibrate with the upstream cylinder fixed; and (iii) the upstream cylinder only was allowed to vibrate with the downstream cylinder fixed. Five Regimes I–V were identified, depending on L/D, fluctuating lift forces and vibration characteristics of the cylinders. In Regimes I (0.1≤L/D<0.2) and IV (2≤L/D<2.7), the cylinder vibration is absent. In Regime II (0.2≤L/D<0.6), both cylinders vibrate violently for Ur>6, including a divergent vibration of the upstream cylinder. In this regime, the vibration amplitude of the downstream cylinder is strongly dependent on whether the upstream cylinder is vibrating or fixed, whereas that of the upstream cylinder is weakly dependent on the downstream cylinder. In Regime III (0.6≤L/D<2), the convergent vibrations of the two cylinders occur at and around Ur≈6.7. In this regime, the upstream cylinder vibration is completely suppressed when the downstream cylinder is fixed, but the downstream cylinder vibration is almost independent on the upstream cylinder. Regime V corresponds to L/D≥2.7, where the two cylinders are separated sufficiently far, thus each vibrating like an isolated cylinder at and around Ur≈6. In this regime, the downstream cylinder vibration is strongly dependent on the upstream cylinder, but the upstream cylinder vibration is almost insensitive to the downstream cylinder condition.