טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentKrakovich Alexander
SubjectVortex-Induced Vibrations of a Heavy Tethered Sphere in
Uniform Flow: Time-Resolved Measurement of
Self-Excited Sphere Oscillations and
Wake Charecteristics
DepartmentDepartment of Mechanical Engineering
Supervisors Dr. Rene Van Hout
Professor Oded Gottlieb
Full Thesis textFull thesis text - English Version


Abstract

A fluid flow interacting with a structure often produces Vortex-Induced Vibrations (VIV) that are of great interest due to their potential damage (e.g. failure of the Takoma Narrows bridge) as well as their potential for energy harvesting (conversion of VIV to electrical power). VIV are nonlinear self-excited oscillations that emerge beyond a critical flow velocity threshold and as the flow velocity is increased, periodic, quasi-periodic and non-stationary chaotic-like oscillations often take place. In this research, measurements of a heavy, tethered sphere mounted in a transparent, closed-loop water channel (cross-section 50x50 mm2) are presented. High-speed measurements of sphere motion and Particle Image Velocimetry (PIV) were used to track the sphere as well as quantitatively measure the dynamics of the corresponding wake. Measurements were taken in a horizontal plane at Re = 340 to 5,600 and VR = 2.8 to 31.1, where Re is the sphere Reynolds number and VR is the reduced velocity, with recording rates up to 1,200 fps. Spatio-temporal dynamics of the flow-field were investigated using the out-of plane vorticity and swirling strength whereas sphere dynamics were studied using spectral analysis and Poincaré maps. Three distinct bifurcation regions were identified, each region characterized by a typical vortex shedding pattern and sphere motion. In region I the sphere remained stationary and the wake was characterized by a train of hairpin vortices symmetrical in the vertical plane. Region II was characterized by almost periodic transverse finite amplitude sphere oscillations (up to Ay = 0.6D where Ay is the transverse amplitude and D is the sphere’s diameter) and asymmetrical vortex shedding where the vortex pinch-off phase decreased with increasing VR. In Region III, non-stationary sphere dynamics (with maximum amplitudes of Ay = 0.75D) and vortex shedding were characterized by high frequencies associated with shear layer instabilities causing pinch-off of small scale vortices. In addition, large scale undulations of the wake associated with the sphere motion were observed.  It was also found that the drag coefficient for a tethered sphere was up to 100% larger than that of a stationary sphere.