Fluid-Structure Coupling between Two Tandem Elastic Cylinders

Abstract

The paper presents an experimental investigation on characteristics of flow-induced vibrations of two circular cylinders in tandem arrangement. Both cylinders are allowed to vibrate in the cross-flow direction only. The spacing ratio L/D (where L is the spacing between the two cylinder centers and D is the diameter) is varied as 1.2, 1.5, 2.0, 2.5, 3.0, 4.0 and 6.0. Four regimes are identified. In regime I (L/D ≤ 1.5), the two cylinders experience vortex excitation (VE) combined with ‘galloping. In regime II (1.5 <L/D < 2.5), again both cylinders undergo VE followed by galloping, but the VE and galloping are separated. Regimes III (2.5 ≤ L/D ≤ 3.0) and IV (L/D > 3.0) are characterized by separated VE and galloping for the downstream cylinder and only VE for the upstream cylinder. Compared to that of the downstream cylinder, in regime I the vibration amplitude of the upstream cylinder is larger, and in regime II it is larger at a smaller reduced velocity Ur and smaller at a larger Ur. On the other hand, regimes III and IV correspond to a larger vibration amplitude for the downstream cylinder and very smaller amplitude for the upstream cylinder. Vibration of the downstream cylinder is however violent in the former regime and weaker in the latter. The wake structure of vibrating cylinders is demonstrated by PIV measurements.