6 Vortex-Induced Vibration

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 f_s, while that in the drag direction (direction of flow) has a frequency equal to 2f_s. 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 f_s or 2 f_s, the vortex-shedding frequency f_s (or 2 f_s) 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.