Vortex-Induced Vibration of Circular Cylinders Using Multi-Block Immersed Boundary-Lattice Boltzmann Method

Despite decades of research, vortex-induced vibration (VIV) of circular cylinders is still a topic of strong interest in fluid mechanics, as it is of great importance in many engineering disciplines, such as bridges, nuclear reactors and high-rise buildings. In order to provide an in-depth understanding of complex fluid-structure interaction during VIV, this thesis considers the following physical scenarios using an in-house code developed based on immersed boundary-lattice Boltzmann method (IB-LBM). First, a system with two fixed cylinders with an intermediate centre-to-centre spacing is considered. It is found that the frequency component of the force on each individual cylinder changes from a single value to multiple ones, then to a large number of discrete ones and eventually to a broadband continuous spectrum, as the alignment angle increases. Second, the vibration of a cylinder may occur due to fluid-structure interaction, and thus the free motion is investigated using the results from the corresponding forced oscillation. It is shown that when a cylinder is in periodic free motion, its motion will remain the same if the combined mass-damping parameter remains unchanged and the variations of body mass and stiffness follow a particular pattern. Here, the damping ratio is redefined using the motion frequency of the body instead of the commonly adopted natural frequency of the body. Third, large-eddy simulation as turbulence model is implemented in the computer code and multi grids are adopted in IB-LBM to improve computation efficiency and accuracy. Turbulent flow is then studied. The results show that the effect of the Reynolds number on the well-known three response branches at different reduced velocities, or initial, upper and lower branches, is significant. When Reynolds number is fixed, at its lower range calculated, there are only initial and upper branches, and at higher range, there are only upper and lower branches.