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 0.22 m/s, corresponding to sphere Reynolds numbers ranging from ReD=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 ReD 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 (

► Lower than critical mass tethered sphere was exposed to uniform water flow.
► High-speed tethered sphere tracking revealed sphere dynamics up to ReD=1925.
► Time-resolved PIV revealed simultaneous vortex shedding and sphere dynamics.
► Amplitude response increases with ReD and asymmetric vortex shedding is observed.
► Vortex pinch-off phase decreases with increasing ReD.