AN EXPERIMENTAL-STUDY OF THE AEOLIAN VIBRATIONS OF A FLEXIBLE CIRCULAR-CYLINDER AT DIFFERENT INCIDENCES

This paper deals with an experimental investigation of the free vortex -induced oscillations of a flexible circular cylinder vibrating in a p lane, at an angle alpha with respect to the cross-flow direction. The dynamic response of the vibrating system (damping ratio zeta congruent -to 10(-4)) to different wind speeds shows a hysteresis, with two bran ches and two discontinuities. As alpha is increased, the hysteresis lo op is progressively reduced. Each branch of the hysteresis is associat ed with a particular vortex pattern in the near wake, either the 2S or the 2P modes of vortex shedding. The discontinuities are due to a sud den change from one of the vortex-shedding modes to the other. An incr ease of alpha also reduces the synchronization region, the maximum amp litude of vibration and the double amplitude region, which disappears for alpha greater-than-or-equal-to 30-degrees. Bifurcations in the cyl inder transient response are used to construct the critical curve that defines the boundary between the 2S and the 2P modes of vortex sheddi ng. This curve is observed to shift towards higher velocities as alpha is increased. For all planes of vibration, the onset of synchronizati on occurs at the same reduced velocity. The maximum amplitude of the c ylinder in each plane of vibration can be estimated from the projectio n, on this plane, of the maximum amplitude at alpha = 0-degree. When t he blades supporting the model are inclined at alpha = 45-degrees, vib rations in two orthogonal planes have been observed. The results obtai ned with progressive increments or decrements of the flow velocity are similar to those obtained when the model is released from rest or fro m a high amplitude.