Abstract
Flow-induced vibrations (FIV) of two tandem cylinders are the major concerns in many engineering fields. However, little attention is paid to the FIV mechanisms of two tandem flexible cylinders, especially for the hydrodynamic features. To investigate the FIV hydrodynamic characteristic of two tandem flexible cylinders, the fluid forces on the flexible cylinders were reconstructed by a finite element model using the response displacements acquired from model tests. Considering the coupling effect of multi-frequency vibrations, the lift coefficients, varying drag coefficients, cross-flow (CF) and in-line (IL) added mass coefficients were obtained utilizing the Forgetting Factor Least Squares (FF-LS) method. To investigate the influence of the spacing ratio T/D (where T is the center-to-center separation distance between the two cylinders and D is the cylinder diameter) on the FIV hydrodynamic features, five spacing ratios (T/D = 4.0, 6.0, 8.0, 10.0 and 16.0) were considered. The upstream cylinder and the single cylinder have CF hydrodynamic coefficients that share similar trends versus the reduced velocity, except for T/D = 4.0. The downstream cylinder may disturb the vortex shedding from the upstream cylinder, leading to the decrease in mean lift coefficients and mean varying drag coefficients on the upstream cylinder. Both the wake shielding effect and the vortex shedding effect have a significant influence on the hydrodynamic features of the downstream cylinder. Under the wake shielding effect, the mean lift coefficients and mean varying drag coefficients on the downstream cylinder are reduced, while the mean CF and IL added mass coefficients are increased. Owing to the vortex shedding effect, the mean lift coefficients and mean varying drag coefficients calculated using the local flow velocity are higher than those on the single cylinder, which increases the contribution of the adjacent modes to the vibrations.
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