Verification Study of an FEA Hydroelastic Modeling Method

The analysis and design of fluid-filled structures is a common problem in mechanical engineering. The fluid will add mass, stiffness, and damping properties to the structure that are not present in the dry configuration, and that alter the dynamic response of the structure. Understanding the fluid-structure dynamics can be an essential part of designing a fluid container, particularly if large structural deflections or dynamic motion are expected. For many structures, it is important to appreciate the modal harmonics of the structure and how the fluid will influence them. For instance, the modal frequencies of the tank change with the fluid level, requiring a study of frequency response for different fluid levels. Actual testing to confirm these effects can be expensive and impractical so it is useful to have an accurate model to supplement or replace physical data. Finite element analysis (FEA) codes are well suited to modeling the hydroelastic coupling between the fluid and container. FEA is commonly used to model the structural dynamics of complex systems and to generate the modal frequencies and shapes. One such application is the design of the fuel tank on a flight vehicle, where pressure fluctuations in the fuel line can lead to oscillations in thrust at the engine and high vibrations in the structure. If the frequency of these vibrations matches a natural frequency of the fluid-tank structure, a feedback loop forms coupling the structural response to the fluid oscillations through the engine thrust. This phenomenon is often referred to as “pogo” instability in flight vehicles. The resulting vibrations can cause the vehicle to break apart. The fuel tank must be designed to avoid natural frequencies in the range likely to be excited vehicle vibrations. This report describes the use and verification of hydroelastic modeling capabilities using FEA software to model a fluid-filled fuel tank. Test data from previous experimental work is used to validate the natural frequencies and mode shapes of the tank at different fluid fill levels. The FEA model compares very well to experimental data and theoretical calculations, providing good validation for the modeling technique. The FEA modeling method is thus authenticated as a valuable tool for predicting the dynamics of fluid-tank systems.