3D Oscillations and vortex-induced vibrations of a tethered sphere in a flow parallel to the thread.
Résumé
We have investigated the wake of a tethered sphere in a steady uniform flow, which is a typical and basic example of fluid-structure interaction. Experiments have been conducted in a vertical water channel with a circular “plexiglass” test section of diameter 8 cm and of length 100 cm. The free upper surface of water is kept at a constant level and the current in the channel is created by gravity. The sphere, made of glass of density ?sphere = 2430 kg/m3, and of diameter D = 16.6 mm, is attached with glue to a thread supported by a small horizontal metallic pipe. The velocity of the flow is deduced from flow-rate measurement, after corrections taking into account the development of boundary layers along the wall of the channel and the blockage effect due to the sphere. The length L of the thread has been varied from 15 cm to 35 cm. A mirror has been positioned behind the channel with an angle of 45° to the front direction in order to get on the same image two perpendicular views Images have been recorded thanks a Camescope Sony on a numeric memory and then transferred to Mac for post-treatment. The software i-movie has been used to select the best parts of the continuous movement, which was sequenced by QuickTime at the rate of 5 to 8 images per second. When increasing the velocity of the upstream fluid U above a critical value, we have observed oscillation of the sphere induced by the periodic vortex-shedding. Different modes of oscillation of the sphere along circular, straight or elliptic trajectories are possible. Moreover, the transition from steady to oscillatory movement of the sphere reveals an hysteresis when increasing or decreasing the velocity. Moreover, experiments performed in a horizontal larger channel water have shown the existence of different oscillation modes and the desynchronization of the vortex shedding process.