Analysis of effects of the free surface on the movement of a tethered sphere immersed in a steady flow

In this work, we study the effects of the free surface on the sphere immersed in a steady flow through the analysis of stream wise and transverse displacements. The range of relative submergence, h*, is between 0 and 0.75 and the tethered sphere is characterized by a low value of the mass ratio m*~1 and a low value of damping ratio (ζ=0.007). The movements of the sphere have been measured using two methods: (a) by means of an analogue laser displacement sensor, and (b) by image analysis of the sphere movement CCD acquisitions. The second method provided the 2D reconstruction of the trajectory. The experimental data have highlighted a significant influence of free surface flow both on the transverse oscillation amplitudes and on the oscillation frequencies of the sphere.