Flow-induced vibrations of rectangular cylinders

The flow-induced streamwise vibrations of rectangular cylinders with cross-sectional aspect ratios from 0.25 to 5 in a parallel approach flow of water are investigated. Experimental results are presented. The root-mean-square amplitude of the vibrations of cylinders whose cross sections are nearly square is very sensitive to small changes in reduced velocity, when the reduced velocity is near to the reciprocal of twice the Strouhal number. To study this behavior, records of the cylinder motion and photographs of the wake were taken simultaneously, which allowed a correlation of the mode of vortex shedding with the vibration amplitude. The vortex shedding changes continuously, but not periodically, from alternate to symmetric and vice versa. Correspondingly, the vibration amplitude varies markedly, being small when the vortex shedding is alternate and large when it is symmetric. Vortex shedding and the cylinder motion interact with each other to excite and amplify the streamwise oscillations.