Ph.D Thesis

Ph.D StudentEshbal Lior
SubjectTomographic PIV Measurements in the Wake of a Stationary
and a Tethered Sphere undergoing Vortex-Induced
DepartmentDepartment of Energy
Supervisors ASSOCIATE PROF. Rene Van Hout
PROF. David Greenblatt
Full Thesis textFull thesis text - English Version


Vortex induced vibrations (VIV) of structures exposed to flows may be both detrimental to their structural integrity as well as beneficial in the case of energy harvesting applications. Spheres may be considered prototypical bluff bodies. Although many qualitative dye visualisations of the flow in the wake of a stationary sphere have been performed, little quantitative data are available. Furthermore, although the amplitude response of tethered spheres is well studied, little is known about the 3D flow field in the wake of an oscillating tethered sphere. In this work, the 3D flow field in the wake of a rigidly mounted sphere (diameter of 8 mm) as well as a tethered sphere (diameter of 6 mm, sphere-to-fluid mass ratio of 7.77) undergoing VIV in a water tunnel, was measured using tomographic particle image velocimetry (tomo-PIV). The position and orientation of the tethered sphere was simultaneously tracked to link the characteristic structures in the wake of the sphere to its dynamics. Reduced velocities ranged between 1.9 and 22.8, which correspond to Reynolds numbers between 230 and 2,696. Measurements in the wake of the stationary sphere provided insight into the generation of secondary vortices that were never seen in dye or smoke visualizations. For the tethered sphere, characteristic vortices that exert forces on the sphere, changed significantly for different reduced velocities, corresponding to the changing sphere oscillation dynamics. At reduced velocity 3.6, the sphere remained stationary while "omega-shaped" vortices were shed periodically. The shape of these vortices was symmetrical relative to a plane formed by the streamwise direction and the direction of gravity. At reduced velocity 5.8, "hairpin-like" vortices having a symmetry plane perpendicular to gravity were alternately shed, while the sphere experienced VIV. These pinched-off as interconnected vortex rings further downstream. Transient measurements were performed in the tethered sphere’s near wake upon crossing the Hopf bifurcation. Just prior to the onset of VIV, the near wake and vortex shedding patterns showed signs of the impending sphere oscillation. These were investigated through the instantaneous centroid positions of the velocity deficit and the relative position of counter-rotating, longitudinal vortices in the near wake. Both indicated loss of a stable, organized pattern just prior to the onset of VIV, with the velocity deficit in the wake having no preferential orientation, exploring all possible azimuthal angles. Results of this study will help in optimizing energy harvesting applications using tethered bluff bodies.