A Numerical Study On Flow-Induced Vibration Of Flexible Structures

Flow-induced vibration of flexible structures is often observed as a phenomenon in our day-to-day lives. The flow-induced vibration involves the interaction of the fluid part and solid one, and the coupling process on the interface is very complicated. Despite its prevalence, however, this fundamental problem of fluid-structure interaction is not very well understood. The present work furthers the understanding of flow-induced problems for several typical flexible structures, by means of employing various kinds of computational methods. Our main objective is to explore the influence of system parameters of fluids and structures on vibration responses of the flexible bodies, and that of structure responses on flow fields of the fluid-structure system.In the first part of this dissertation, an Immersed Boundary Method (IBM) with a simple area-preserving correction scheme has been developed to solve the fluid-structure interaction (FSI) of the elastic capsule with its surrounding fluid. As a validation, the relaxation of an elastic capsule in rest fluid is firstly simulated to prove the feasibility of the proposed IBM method. Furthermore, the transient behavior of an elastic capsule in a simple shear flow at low Reynolds number is studied. The corresponding numerical results show that the shear rate promotes the capsule deformation, so does the inertia effect.In the second part, the flexible filament in a uniform flow has been numerically modeled by using the penalty immersed boundary method. The isolated flexible filament remains a straight line state or flapping state under the uniform inflow. Under sustained flapping, the trajectory of the free end presents a figure-eight type motion. The bending stiffness, mass ratio and Reynolds number are three key parameters, which impact the sustained flapping behavior of the filament. The heavier or more flexible the filament is, more wide the flapping region will be. Moreover, we have also explored a pair of tandem flexible filaments flapping in a viscous flow. From a function of flapping amplitude of the tails and drag force of the filaments to the gap of two filaments, an inverted hydrodynamic drafting can be observed in our study. The flapping amplitude of the leader filament is lower than that of the follower filament, and so is the drag force of the filament. Additionally, the change of the follower filament such as bending stiffness and mass ratio can only affect the flapping motion and drag force of its own, but have no influence on the leading one. Throughout the study, the flow field of tandem filament demonstrates similar flow patterns:small vortices shedding from two flapping filaments, and then combining into a periodically shedding vortex in the wake.Additionally, numerical simulations of a flexible circular cylinder subjected to a vortex-induced vibration (VIV) are also conducted. The Reynolds number for simulations is fixed at1000. The comparison between2D simulations and3D simulations for the flexible cylinder shows that the maximum response amplitude of the cross-flow oscillation is about0.57D for2D rigid cylinders (modeled by a spring-damper-mass model) and1.03D for flexible cylinders, respectively. The results from3D simulations are closer to previous experimental results. Furthermore, the results obtained with various frequency ratios show that different wake patterns exist according to the frequency ratio, such as2S mode,2P mode and some more complicated ones. The wake pattern is different at various sections along the cylinder length, due to the fact that the two ends of the beam are fixed. Compared to the results of a rigid cylinder, the vibration of the flexible cylinder shows much more three-dimensionality in the wake. Moreover, five different vibrating modes appear in the simulation. From the comparisons of their vortex structures, the strength of the wake flow is related to the exciting vibrating mode and different vortex patterns arise for various vibrating modes. Only2P pattern appears in the first vibrating mode, while2S-2P patterns occur in the other vibrating modes, if monitoring at different sections along the length of the cylinder. Finally, an aero-elastic problem of a isotropic flexible fixed-wing is numerically studied by a fluid structure interation solver, in which the solid part is modelled by the updated Lagrangian finite volum method. The numerical results present there are two different modes of vibration for the flexible fixed-wing under various attack angles. The corresponding stress-strain contour of a flexible fixed wing and the aero-flow field are also presented. Moreover, the effect of Reynolds number to the aerodynamic force of the flexible fixed-wing is more than that of elastic modulus. Otherwise, the effect of elastic modulus to the elastic deformation of the flexible fixed-wing is more than that of the Reynolds number.