Fluid-structure interaction (FSI) occurs in all fluid-carrying pipe systems. More often than not, its consequences are minor and may be neglected for practical design purposes. In some cases, however, FSI can be responsible for unacceptable operational behaviour - e.g. sustained or sudden noise - or the failure of the pipe or its support system.
The underlying mechanisms of FSI in pipes are well understood and, in principle, the phenomenon can be analysed with good accuracy. Unfortunately, there is an important proviso, namely that the actual accuracy is strongly dependent upon the correctness of the input data defining geometrical and physical properties of pipes, valves, pumps and supports, etc. In reality, it is rare to be able to prescribe these with high accuracy in a practical application. In fact, it is even rare to be able to do so in controlled, laboratory experiments.
Some interactions between fluids and pipes occur in steady flows whereas other exist only in unsteady flows. Likewise, some occur with flows inside pipes whereas others are associated with flows outside pipes. A typical example of the latter is the vibration of pipes (or pipe groups) in a cross-flow.
(from http://www.win.tue.nl/fsi/)