There are several recognized phenomena associated with flow-induced vibration. These include vortex shedding, fluid elastic instability, turbulent buffeting, parallel-flow eddy formation, and acoustic vibration. Since any one of these can produce a flow-induced vibration problem, each must be considered in any comprehensive vibration analysis for a shell-and-tube heat exchanger.

A. Vortex shedding

Flow across a tube produces a series of vortices in the downstream wake formed as the flow separates alternately from the opposite sides, of the tube as shown in Figure 1. This shedding of vortices produces alternating forces, which occur more frequently as the velocity of flow increases. For a single cylinder the tube diameter D_o, the flow velocity u, and the frequency of vortex shedding f_vs, can be described by the dimensionless Strouhal number Sr:

\[\label{eq1} \mbox{Sr} = \frac{f_{vs}D_o}{u}\tag{1}\]

or