Time resolved measurements of vortex-induced vibrations of a tethered sphere in uniform flow

The motion of a heavy tethered sphere and its wake were measured in a closed loop water channel using a time resolved, high-speed particle image velocimetry technique in a horizontal plane. Measurements were performed for nondimensional reduced velocities ranging from 2.8 to 31.1 that include three bifurcation regions. In order to analyze the vortex shedding characteristics, the directional swirling strength parameter was computed in addition to the vorticity as the former enables vortex identification. In the first bifurcation region, the sphere remained stationary and the wake was characterized by a train of hairpin vortices exhibiting symmetry in the vertical plane similar to visualization results obtained for stationary spheres. The second bifurcation region was characterized by large amplitude periodic oscillations transverse to the flow. Phase-averaged results for the swirling strength showed that although the shedding mechanism was identical for several reduced velocities, the phase at which vortices were shed increased with V_R. Spatiotemporal swirling strength characteristics revealed counter-rotating vortex pairs in the far wake of the sphere. In addition to primary vortex pairs, secondary weaker vortical structures were also observed. In the third bifurcation region, nonstationary vortex shedding was characterized by high frequencies associated with shear layer instabilities causing pinch-off of small scale vortices. In addition, large scale undulations of the wake associated with the sphere motion were observed.