Tube vibration due to vortex sheddingThe vortex formation is periodic & is a function of the fluid velocity. The tube is subject to alternating lift forces. The frequency at which the vortex is formed and shed, if coincides with tube natural frequency, the tube would vibrate.
Click belowtacoma bridge.mpeg
The fluid flow causes tube to be displaced from its initial posotion. This displacement causes a further shift in flow field changing the fluid force acting in the tubes.
Fluid elastic vibration
Turbulent buffetingEven when the karman vortices are not formed, the fluid flow produces fluctuating forces, having various frequency modes that act on tubes and force them to oscillate. Tube oscillate in the fluid in a selective forced oscillation mode at their natural frequency.