|M.Sc Student||Kovalev Daniel|
|Subject||Tomographic PIV Measurements of the Flow Field in the Wake|
of a Tethered Sphere Undergoing Vortex Induced
|Department||Department of Mechanical Engineering||Supervisor||Dr. Rene Van Hout|
|Full Thesis text|
Flow-Structure interactions is a widely observed phenomenon that can have results both disastrous (e.g Tacoma Narrows bridge) and productive, such as harvesting of renewable energy. One of the least researched bodies in this field of study is the tethered sphere. Studying vortex induced vibrations (VIV) of a tethered sphere will help in developing systems for green energy generation. In this study, the 3D flow field in the wake of a negatively buoyant (“heavy”) tethered sphere (diameter D = 5.97 mm) having a mass parameter of m* = 7.8 undergoing VIV in a water tunnel, was measured using tomographic particle image velocimetry (tomo-PIV). The flow field was investigated for different reduced velocities, U* = U/ fn D, where U denotes the free stream velocity, fn the natural frequency of the tethered sphere in the fluid. The reduced velocities ranged between 3.6 < U* < 37.7, corresponding to Reynolds numbers ranging between 382<Re (=UD/ν)< 3965. The dynamics of the sphere at each U* were measured and the relation between the dynamics and the flow field in the wake of the sphere while undergoing VIV was investigated. At U* = 3.6, the sphere’s oscillations were negligible. However, the power spectra revealed a peak in the frequency corresponding to the natural frequency of the sphere. The shed vortices resembled a train of “hairpin” heads having a vertical plane of symmetry that developed into vortex rings further downstream. At U* = 5.8, corresponding to the “lock-in” region, the sphere’s oscillations were highly periodic with large transverse amplitudes, which was further observed by a very clear peak in the power spectra. “Omega-shaped” vortices having a horizontal plane of symmetry were shed from alternating sides of the sphere, transforming into vortex rings further downstream. The vortex rings were still connected to upstream structures by a “bridging-vortex”, i.e. a thin vortex filament, while the “omega-shaped” vortices were connected by “twisted” longitudinal vortices. The “twist” probably originated from the rotation of the sphere. The rotation consisted of two frequencies; one which was identical to the oscillation frequency, and another slower frequency, which could also be noticed in the corresponding power spectra. At higher U* values, U* = 7.9 and 13.2, the sphere’s transverse amplitude response was intermittent and vortices in the sphere’s wake were characterized by longitudinal and vertical “struts” as well as “arches”, with seemingly little organization. The 3D organization and type of the detected vortical structures in the wake of the sphere varied with the oscillation amplitude. When the oscillations were relatively small, the shed structures were more organized and resembled previously seen structures, whereas when the oscillations increased, the structures were less clearly ordered.