Flow-induced Vibration Of A Cylinder In The Wake Of Another Of Smaller Diameter

Flow-induced vibration(FIV), due to its large amplitude and frequent occurrence, is a serious problem associated with the cooling/heat-exchanger systemsin nuclear plants.This thesis presents the cross-flow induced response of a both-end-springmounted circular cylinder(diameter D) placed in the wake of a rigid circular cylinder of smaller diameter d. While the diameter ratio d/D is varied from 0.2 to 1.0, the cylinder spacing L/d is from 1.0 to 5.5, where L is the distance between the center of the upstream cylinder to the forward stagnation point of the downstream cylinder. A lateral violent vibration of the cylinder is observed for d/D = 0.2 ~ 0.8 at L/d = 1.0, for d/D = 0.24 ~ 0.6 at 1.0 < L/d ≤ 2.5, for d/D = 0.2 ~ 0.4 at 2.5 < L/d ≤ 3.5, and for d/D = 0.2 at 3.5 < L/d ≤ 5.5, but not for d/D = 1.0. A decreasing d/D generates vibration for a longer range of L/d. Once the vibration starts to occur, the vibration amplitude A/D increases rapidly with increasing Ur. This is due to Wf(the energy transfer from fluid to cylinder) > Wd(energy dissipated by damping), energy is therefore transferred to the cylinder supported system. For Wf is increasing faster with increasing A/D, Wf is at last equal to Wd. The input energy transferred during the A/D increasing offers to the system with energy, and Wf counteracts Wf equally during constant vibration. It is further noted that the flow behind the downstream cylinder is characterized by two predominant frequencies, corresponding to the cylinder vibration frequency and the natural vortex shedding frequency of the cylinder, respectively. While the former persists downstream, the latter vanishes rapidly. At a small d/D, the upstream cylinder wake narrows, and the shear-layer reattachment approaches the front stagnation point. It therefore can flow alternately along two different sides of the cylinder, exciting the downstream cylinder.