A numerical method is developed to simulate the coupled phenomena in a fluid-flexible-structure system. Specifically, a two-dimensional panel method is used to calculate the hydrodynamic forces and a modal superposition method is adopted to solve the governing equation of an Eulerian beam. The stability boundary of a single flexible beam in a uniform flow is calculated and compared with previous results to verify the validity of the code. The flow-induced flapping of a single and two flexible bodies for S=1.0, U * =7.0 are investigated. For the flow-induced vibration of a single beam, the oscillation frequency is close to the secondary natural frequency of a cantilever. For two parallel flexible beams, they oscillate in phase when the non-dimensional separating distance H<0.25. When H>0.25, the out-of-phase mode occurs with a jump in frequency. When H>1, the interaction between the two beams decouples and the frequency and forces of each beam revert to behavior associated with a single beam in the same flow. Simulations of coupled-flapping of two tandem flexible structures proved that the drag acting on the upstream body is reduced while for that downstream drag is obviously increased when the structures are closely arranged. The numerical results obtained in the present work are qualitatively consistent with early experimental results. fluid-flexible-structure coupling, panel method, modal superposition method Citation: Wang S Y, Yin X Z. A numerical method to simulate the coupled oscillations of flexible structures in flowing fluids.The flapping of flags in the wind, the waving of wheat at harvest time and the movement of seaweed underwater are all ubiquitous phenomena in everyday life, and are also prototypical instances of flexible bodies immersed and interacting with a flowing fluid. There are also many similar fluid-flexible-structure interactions in industrial applications. For example, the severe paper fluttering caused by high operating speeds of newsprint machines creating wrinkles, folds and even paper rips. Also, the uncontrolled undulation of cables and pipes in oceans can arouse security concerns. In reference to humans, snoring is a result of flow-induced vibration of the soft palate. Moreover, animals propagate through air or water by swaying their tails, fins or wings. These are also results of interactions between fluids and deformable bodies. The only distinction is that muscle *Corresponding author (email: xzyin@ustc.edu.cn) contractions and nerve control in live animals make for active rather than passive deformation. Theoretically, the interaction between a moving fluid and a deformable structure depends not only on speed and direction of oncoming flow, but also on shape and rigidity of the flexible body. When a fluid flows over a flexible body, fluctuating hydrodynamic forces acting on the body change its shape. In turn, the deformations and motion of the flexible body also change the hydrodynamic forces. This results in a strongly-coupled process involving both the dynamics of the fluid an...