Description: In certain applications it may be necessary to predict the mechanical response of a liquid-filled or a gas-filled structure. Examples include pressure vessels, hydraulic or pneumatic driving mechanisms, and automotive airbags. A primary difficulty in addressing such applications is the coupling between the deformation of the structure and the pressure exerted by the contained fluid on the structure. Figure 4–9 illustrates a simple example of a fluid-filled structure subjected to a system of external loads. The response of the structure depends not only on the external loads but also on the pressure exerted by the fluid, which, in turn, is affected by the deformation of the structure. The surface-based fluid cavity capability provides the coupling needed to analyze such situations. The cavity is assumed to be completely filled with fluid with the same properties and state; that is, effects such as sloshing and wave propagation through the fluid cannot be modeled with this feature.