On the fluid dynamics of circular cylinders with span-wise waviness

Abstract eng:
The current paper numerically deals with the fluid dynamic properties of the circular cylinder with span-wise sinusoidal waviness. Three aspects, i.e., the fixed configuration, vortex induced vibration and forced vibration, have been considered at low Reynolds numbers. It is revealed that the current wavy cylinder exhibits a stable-unstable-stable transition in the studied Reynolds number range. The dynamic mode decomposition-based linear stability analysis has been utilized to extract the linear global modes associated with different flow states. The vortex induced vibration of the wavy cylinder at these three flow states is studied next. The results show that even the complete suppression of the Karman vortex shedding does not eliminate the free vibrations. To further shed light upon this finding, the forced vibration tests are conducted. The lift spectrum of the wavy cylinder was found to exhibit two peaks at most of the excitation frequency except an interruption by a typical lock-on phenomenon. This observation is very similar to that of the normal cylinder, where the coherent Kamran frequency constantly interferes with the external excitations. This indicates that the Karman instability that has been totally suppressed by the stationary wavy cylinder revives in the dynamic cases, and plays a role in determining the vortex induced vibrations when the cylinders are flexibly mounted.

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