Vortex-induced vibration of a flexible cantilever

This study is concerned with the vortex-induced vibrations of a flexible cantilever in a fluid flow. Our cantilever comprises a leaf spring encased within a rubber flexible cylinder, restricting the vibrations of the body in a water channel flow to principally transverse motion. It is found that the transverse amplitude response of the cantilever has a marked similarity with transverse vibrations of an elastically mounted rigid cylinder, in that there is a clear initial branch extending to high amplitudes, with a jump to a lower branch response, as normalized velocity is increased. The continuous initial branch suggests that a distinct upper branch does not exist for the cantilever, as is found for a rigid cylinder under similar conditions of low mass and damping. Good agreement is found between the response amplitude and frequency for two "identical" cantilevers, one set up by Pesce and Fujarra, where strain is measured to infer the body dynamics, and the other arrangement by Flemming and Williamson, where the tip motion is measured using optical techniques. An interesting large-amplitude response mode is found at higher normalized velocities (U* > 12) outside the principal synchronization regime (typically U* = 4-8), which is observed for an increasing velocity, or may be triggered by manual streamwise disturbances of the body. This vibration mode is due to a coupled streamwise-transverse motion, where the streamwise amplitude becomes non-negligible, and may be related to a further vibration mode at high normalized speed, found for a vibrating pivoted rod, by Kitagawa et al. (1999). (C) 2001 Academic Press.