Wake Modification During Vibration Generation Of A Cantilever Cylinder In Line With Another Of Smaller Diameter

This thesis presented the cross-flow induced response of a cantilever cylinder(diameter D) in line with another of smaller rigid cylinder of diameter d. There are mainly two cases in this thesis, in which the diameter ratio d/D is 0.4 and 0.6, the cylinder spacing L/d is 1.5 and 1.0 respectively, in which L is the line distance between the centers of the upstream cylinder and the forward stagnation point of the downstream cylinder. The vibration motion and the pressures around the vibrating cylinder were measured by laser vibrator and pressure scanner respectively. In addition, I had captured the photo of the wake by means of PIV.A lateral violent vibration of the downstream cylinder is observed for both of the two cases. Once the cylinder begins to vibrate, the vibration amplitude y/D increases rapidly with increasing Ur(Ur = U∞D/fn, where U∞ is the free-stream velocity and fn is the natural frequency of the cylinder system). This is due to as the Ur increasing, the energy transfer from fluid to cylinder Wf is larger than the energy dissipated by damping Wd, energy is therefore transferred to the cylinder system. In the process of the cylinder kept rest and released to vibrate the added mass and effective damping of the downstream cylinder varied intensely, and reached stable when the cylinder keeps constant vibration at last. POD(Proper Orthogonal Decomposition) analysis is applied to examine the downstream cylinder wake. The POD lower modes coefficient is calculated using Fast Fourier Transform. The result shows that the different flow structures corresponding to different dominant frequencies. When the cylinder keeps free vibration, the first POD mode represents the large-scale coherent structures, and its power spectra show the peaks corresponding to the natural frequency. The mode 2 and mode 3 represent the periodically Karman vortex, and their power spectra show the dominant frequency of vortex shedding frequency. The higher POD modes(mode>4) represent the small-scale turbulent structures.