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

Vortex-induced vibrations (VIV) of a positively buoyant (light) tethered sphere in uniform flow as well as its wake characteristics were measured in a closed loop water channel. Experiments were performed at free stream velocities ranging between 0.048 and 0.22m/s, corresponding to sphere Reynolds numbers ranging from Re _D=430 to 1925. The measurements were done using high-speed sphere tracking as well as time resolved particle image velocimetry in a horizontal plane located at the sphere's center. Until the Hopf bifurcation, the sphere remained stationary and the wake was characterized by a train of hairpin vortices exhibiting near-symmetry in the vertical plane similar to stationary sphere visualization results. For our limited parameter range, the amplitude response of two different data sets (same sphere and free stream velocity but different water viscosity) collapsed better when plotted versus Re _D than when plotted versus the reduced velocity, U ^*. The amplitude response beyond the first bifurcation displayed continuously increasing rms amplitudes in agreement with the sphere's small mass parameter (<critical mass). Beyond the traditional 'lock-in' regime (U ^*=3-8), the sphere's oscillation frequency strongly increased extending the 'lock-in' phenomenon to larger Re _D. Vortex shedding characteristics were analyzed using the directional swirling strength in conjunction with the vorticity as the former enables vortex identification. Vortex shedding dynamics were similar over the whole Re _D range. However, for low Re _D clear counter-rotating vortex pairs were observed, at the highest Re _D the spatio-temporal vortex shedding pattern in the far wake was characterized by high frequency, fragmented vortical structures organized in a low frequency 'saw tooth' pattern, the latter associated with the sphere's oscillation frequency. Furthermore, phase averaged results showed a decreasing vortex pinch-off phase as Re _D increased.