VORTEX-INDUCED VIBRATIONS OF A LONG FLEXIBLE CIRCULAR-CYLINDER

In an experimental study of the vortex-induced oscillations of a long flexible circular cylinder, the observed stationary amplitudes describ e an hysteresis loop partially different from earlier studies. Each br anch of the loop is associated with a vortex shedding mode and, as a j ump from one branch to the other occurs, the phase difference between the cylinder displacement and the vortex shedding undergoes an abrupt change. The critical flow velocities at which the jump occurs concur w ith the flow visualization observations of Williamson & Roshko (1988) on the vortex shedding modes near the fundamental synchronization regi on. Impulsive regimes, obtained at a given flow velocity with the cyli nder initially at rest or pre-excited, and progressive regimes resulti ng from a variation of the flow velocity, are examined. The occurrence of bifurcations is detected for a flow velocity range in the case of the impulsive regimes. The coordinates of the bifurcations define a bo undary between two vortex shedding modes, a boundary that verifies the critical curve obtained by Williamson & Roshko (1988). The experiment al set-up of this study simulates half the wavelength of a vibrating c able, eliminates the end effects present in oscillating rigid cylinder set-up and has one of the lowest damping ratios reported for the stud y of this phenomenon.