Kinematic analysis of the movement of a tethered sphere immersed in a free surface flow

This paper studies the flow-structure interaction between a tethered sphere and a free surface flow. In several flow conditions, the displacement of the sphere and the characterization of the flow field downstream the sphere itself are provided and discussed. The experiments have been performed in a open water channel where the sphere was placed with low value of relative submergence, h* (ratio between the distance from the free surface to the surface of the sphere and the diameter of the sphere). The tethered sphere was characterized by a low value of the mass ratio, m*~1 (ratio between the mass of the system and the added mass) and a low value of damping. The movements of the sphere have been measured using by means of an analog laser displacement sensor and by image analysis of the sphere movement CCD acquisitions. The flow field reconstruction downstream of the sphere has been provided by a PIV (Particle Image Velocimetry) technique specifically developed for hydraulic applications which has allowed the characterization of some of the main flow structures involved by the interaction between the sphere and the free surface flow. On the base of the experimental evidence we have discussed the influence of the free surface flow on the sphere movement.