|M.Sc Student||Eshbal Lior|
|Subject||Time Resolved Measurements of Vortex-Induced Vibrations of|
a Positively Buoyant Tethered Sphere in Uniform
|Department||Department of Mechanical Engineering||Supervisor||Dr. Rene Van Hout|
|Full Thesis text|
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 - 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. Up till the first 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 (< 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 ReD. 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 ReD range. However, whereas for low ReD clear counter-rotating vortex pairs were observed, at the highest ReD 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. Phase averaged results showed a decreasing vortex pinch-off phase as ReD increased. Furthermore, in analogy with wing trailing vortices, estimation of the fluid forces on the sphere (by applying vortex tracking algorithm on the phase averaged results) showed that maximum forces are applied during pinch-off.