Numerical Investigation Of Fluid-structure Interaction Of Circular Cylindrical System At Low Reynolds Number

Fluid flow over circular cylinder is a classic problem in fluid mechanics. Cylindrical structures are also widely used in ocean engineering, for example, the support piles of ocean platforms, deepwater risers and subsea pipelines. As the fluid passes the bluff bodies, periodic oscillating forces in cross-flow and the in-line directions are observed, which acccouts for the undesirable vortex-induced vibration (VIV). The VIV may result in fatigue damage, shortening the service life threatening the structure safety. Furthermore, in some cases, the cylindrical structures often appear in complex configuration and cluster, which involves more complex flow interference in the fluid-structure interactions. Therefore, it is necessary to conduct fundamental investigations understand the mechanism behind the physical processes.In order to study the fluid-structure interaction of cylindrical structures, a three-step finite element solver for the Navier-Stokes equations of incompressible viscous fluid in the frame of Arbitrary Lagrangian-Eulerian (ALE) is developed, which combines also structure motion prediction algorithm and computational mesh update strategy. The numerical model is validated carefully against the typical bnchmark problems and is then ulitised to investigate the fluid flow around twin tandem circular cylinders near wall, two freedom degrees VIV of multiple circulat cylinders, and rotationally oscillating circular cylinder with splitter plate, respectively. This work focuses on the basic aspects of the fluid-structure interaction the numerical simulatiosn are hence resctricted to the two-dimensional flows at low Reynolds number.First, the laminar flow over twin tandem circular cylinders near plane wall is investigated. The effect of L I D and G/D on the fluid forces and vortex shedding mode are presented, where L is the center-to-center distance of the cylinders and G is the smallest spacing gap between the cylinder surface and the wall bottom. Numerical simulations indicate that three different wake modes can be classfied with the variations of L I D and G I D. The jumps associated with the fluid forces on cylinders are observed as the wake mode transition appears.Secondly, the vortex-induced vibration of twin tandem circular cylinders is investigated. In the numerical simulations, the upstream cylinder holds fixed while the rear cylinder may performs oscillation in both transverse and stream-wise directions under specified spring stiffness and structural damping. The reduced velocities considered in the conputations vary from3.0to12.0. The effects of spacing ratio (based on the center-to-center distance of cylinders) and the mass ratio on the displacement responses and fluid forces are examined. The numerical results indicate that both of the spacing ratio and mass ratio have great influence on the VIV responses in terms of the lock-in band, displacement and hydrodynamics. All these aspects are confirmed to be closely related to the wake modes and flow interference.Finally, the fluid-structure interaction of circular cylinder with splitter plate is studied, including both the forced flapping and free flapping of the cylinder-plate system. For the forced flapping, with respect to the cylinder center, the influence of the flapping amplitude and frequence on the forces and vortex shedding mode are examined. Compared to a fixed cylinder with splitter plate, the drag force is reduced when f>0.2; while the amplitude of lift force is significantly increased. For the free flapping case, centered on the cylinder center, the fluid forces on the structure, rotatioanlly oscillating amplitude and frequency and wake modes are the concerned issues, involving various reduced velocities and damping coefficients. Numerical results show that an abrupt change of the rotation amplitude and frequency, mean drag force, lift amplitude occur at the special reduced velicity Ur=4.5. The mean flapping angle and mean lift force is approximately zero at low reduced velocity. However, the random shift with definite magnitude appears at higher reduced velocity, reffering to Ur=11.0～15.0in this work, which is shown to be atributed to the asymmetric vrtex shedding in the near wake, implying an apperance of instability in the fluid flow-induced rotational oscilation of circlar cylinder with splitter plate.