Vortex Shedding and Galloping of Parabolic Bluff Bodies

Abstract

The present study was prompted by a desire to further understand a serious, resilient instability found by Veljkovic (2001) in his study of the flow induced vibration of a parabolic bluff body. Veljkovic (2001) found that when the center of mass was moved forward of the elastic axis by 95 mm parabolic bluff bodies began to experience what appeared to be galloping. Quantitative data seemed to suggest the body was first experiencing vortex shedding which triggered galloping. The purpose of this study was to improve our understanding of the flow excitation of open parabolic bluff bodies, and how the separation distance between the center of mass and the elastic axis affect the critical velocity. To this end an experiment was designed to verify the excitation mechanism, and secondly to study the relationship between the change in critical velocity with changes in separation distances between elastic axis and center of mass. An experiment was designed to be performed in the water tunnel with two different separation distances between elastic axis and centre of mass: 250 mm and 95 mm. The most efficient way to study the excitation mechanism was to use flow visualization in a water tunnel. The method of flow visualization selected for this experiment was seeding the flow with aluminum particles. The flow velocity of the water was increased at incremental steps with the amplitude and frequency of vibration measured at each step. This experiment was carried out over a Reynolds number range of 1.6xl04 to 9.97x104 which is similar to Veljkovic’s (2001) range and in the range where the wake is in transition to turbulence. The Strouhal number for this experiment was 0.12, which is similar to Veljokovic’s (2001) 0.13. By using a water tunnel the flow visualization capabilities were enhanced, though the damping was significantly increased. The 95 mm experiment appeared to suffer from high damping and flow velocities sufficiently low that no obvious vortex shedding excitation was observed. The 250 mm experiment appeared to mimic Veljkovic’s (2001) 95 mm test and seemed to be excited by vortex shedding and subsequently galloping. From seeding the flow, the aluminium tracers clearly showed that the excitation mechanism for the 250 mm, and therefore Veljkovic’s (2001) 95 mm experiment, was vortex shedding followed by galloping. Flow reattachment to the after body clearly distinguishes galloping from vortex shedding.