Fluid-structure interaction

Fluid-structure interaction (FSI) occurs when a fluid interacts with a solid structure, exerting pressure on it which may cause deformation in the structure and thus alter the flow of the fluid itself. Such interactions may be stable or oscillatory, and are a crucial consideration in the design of many engineering systems, especially aircraft. Failing to consider the effects of FSI can be catastrophic, especially in large scale structures and those comprising materials susceptible to fatigue. The Tacoma Narrows suspension bridge is probably one of the most infamous examples of large-scale failure.

Aside from its destructive potential, FSI is responsible for countless useful effects in engineering. It allows fans and propellers to function; sails on marine vehicles to provide thrust; aerofoils on racecars to produce downforce, and our lungs to inflate when we breathe.

Broadly speaking, fluid-structure interactions can be classified into three groups - zero strain interactions, such as the transport of suspended solids in a liquid matrix; constant strain steady flow interactions, e.g. the constant force exerted on an oil-pipeline due to viscous friction between the pipeline walls and the fluid; and oscillatory interactions, where the strain induced in the solid structure causes it to move such that the source of strain is reduced, and the structure returns to its former state only for the process to repeat. It is the latter of these that allows reed instruments to actually produce sound, in which case the systems of equations governing their dynamics have oscillatory solutions. The act of "blowing a raspberry" is another such example.

Traditionally, fluid and solid dynamical systems have been solved independently. However, for problems where there is sufficient coupling between the two systems, such separation is not possible and the resultant systems are invariably too complex to solve analytically. Computational fluid dynamics is essential in predicting the behaviour of such systems, and extensive research is ongoing in this now well-established field.