A study on the FIV hydrodynamic force coefficients of two staggered flexible cylinders via an inverse method

The flow-induced vibration (FIV) of two cylindrical structures may cause serious fatigue damage, which is a major concern in engineering fields. Few studies have focused on the FIV hydrodynamic characteristics of two staggered flexible cylinders. In this paper, the hydrodynamic coefficients of two staggered flexible cylinders are calculated using the structural responses acquired from the model tests. The cross-flow (CF) spacing ratios are set as 2, 3, 4 and 6, and the in-line (IL) spacing ratios are set as 4, 6, 8; thus, a total of twelve cases are considered. The fluctuating forces in the CF and IL directions are reconstructed using a finite element model. The forgetting factor least squares method is employed to decompose the CF/IL fluctuating force into lift/varying drag and CF/IL added mass force. In the CF direction, the hydrodynamic coefficients of the two staggered cylinders are significantly different from those of the single cylinder in the mode switch regions. The CF vibrations of the two staggered cylinders enter the higher-order mode earlier, resulting in an increase in the CF added mass coefficients. The downstream cylinder has larger lift coefficients due to the vortex shedding effect of the upstream cylinders. In the IL direction, the upstream cylinder has lower IL added mass coefficients due to the higher-order harmonics in the IL direction. The varying drag coefficients of the two staggered cylinders significantly differ from those of the single cylinder owing to the subharmonic frequency components. A hybrid aspect of wakeinduced flutter and vortex-induced vibration is observed in the vibration of the downstream cylinder.