Flow Induced Vibration of Power and Process Plant Components: A Practical Workbook
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The vortices exert a fluctuating reaction force on the cylinder. The component of this force in the lift direction (perpendicular to the flow direction) has a frequency equal to vortex-shedding frequency fs, while that in the drag direction (direction of flow) has a frequency equal to 2fs. In general, the force component in the drag direction is much smaller than the force component in the lift direction.
When the cylinder is flexible with characteristic natural frequencies, a phenomenon called lock-in may happen. When one of the structural modal frequencies are close to fs or 2 fs, the vortex-shedding frequency fs (or 2 fs) may actually shift from its value for a stationary cylinder to the nearest natural frequency of the cylinder, resulting in large amplitude, resonant vibration. Lock-in can occur in either the lift or the drag direction and both have been observed in power plant components, resulting in substantial financial losses.