Oscillations and vortex-induced vibrations of a tethered sphere in a flow parallel to the thread
Résumé
The behavior of a tethered sphere in a steady uniform flow has been investigated for two different configurations. Firstly, we consider a flow parallel to the tether in a small vertical water channel and, secondly, we perform experiments in a wind tunnel where the flow is perpendicular to the tether. Above a critical value Uc of the free-stream velocity, the sphere exhibits oscillations induced by the periodic vortex shedding. When the flow is parallel to the tether, new modes of oscillations of the sphere along circular or elliptic trajectories have been observed. The transition from steady to oscillatory movement of the sphere is shown to be hysteretic. The frequency and the amplitude of the oscillations, always locked to the natural frequency of the pendulum constituted by the tethered sphere, have been measured as a function of the reduced velocity. When the flow is perpendicular to the tether, the trajectories of the sphere have been recorded. The amplitude and frequency of the movement as well as the spectral density of the fluctuations of the location have been deduced by image processing.