Boundary effect of a viscous fluid on a longitudinally vibrating bar: Theory and application

A theoretical model has been developed to represent the interaction of a bar in longitudinal vibration with its surrounding viscous fluid. This model was generated to better predict the length of timber piles used in bridge foundation. Experiments were performed with sand and with several water-glycerin mixtures surrounding an aluminum bar, showing a significant effect of the bounding material on the vibration characteristics of the bar. The viscous fluid was observed to lower the measured frequencies, indicating system mass loading, while the presence of the sand caused an upward shift in frequencies, indicating increased system stiffness. Hamilton's principle was used to model the longitudinal bar motion with the fluid loading due to the viscous boundary, following the procedure of a known soil-bar interaction model. It was found that the fluid model gives an excellent prediction of the natural frequencies of the bar. The modal mass and damping were: not predicted quite as accurately, but the fluid model offered a significant improvement over a simple bar model. (C) 2001 Acoustical Society of America.