Streamwise Oscillations of Cylinders

Slender structures, such as marine piles, have been observed to experience oscillations in a streamwise direction when exposed to an external flow under appropriate circumstances. This phenomenon is explored both theoretically and analytically with a view to explaining the basis of the phenomenon and of providing a method of calculating the response amplitude. The flow around a vibrating body, is assumed to be quasisteady. From this assumption a stability criterion, analogous to the Den Hartog criterion for transverse oscillation, is derived. This stability criterion indicates that instability is to be expected only over a range of Reynolds number near the critical value which corresponds to a sudden drop in the drag coefficient and an increase in the Strouhal number. Using a linear oscillator model for the body, an analysis is performed to seek solutions for the vibration amplitude. The results of this analysis are compared with observations of prototype oscillations of marine piles which occurred at Reynolds numbers high enough to be in the critical range.