|M.Sc Student||Katz Amit|
|Subject||Vortex-Induced Vibrations of a Tethered Sphere in a Steady|
Flow: Near-Wake Flow and Effect of Active Flow
Control on Sphere Dynamics
|Department||Department of Mechanical Engineering||Supervisors||ASSOCIATE PROF. Rene Van Hout|
|PROF. David Greenblatt|
|Full Thesis text|
Bluff bodies in a moving fluid often undergo vortex induced vibrations (VIV) as a result of asymmetric vortex shedding. When these perturbations lock in with the natural frequency of the structure they can cause significant vibrations. VIV of 3D bodies (such as tethered spheres) is not as well investigated in the literature as quasi-2D bodies such as cylinders.
The objectives of this thesis were: to investigate the near wake interactions of tethered sphere VIV, and to investigate the effect of acoustic perturbations. In this investigation “heavy” spheres (m* > 1) were mounted in a wind tunnel and tested at Reynolds numbers in the range of ReD = 500-2500 (U* ≡ U/fnD = 4-25). Two separate experiments were performed: near-wake flow field characterization of the uncontrolled sphere, and active boundary layer control by means of acoustic perturbations. High-speed Particle Image Velocimetry (PIV) (100-700Hz) was employed in the flow‑field characterization, and high-speed sphere tracking (300Hz) was employed in the acoustic control experiments.
Without acoustic perturbations, sphere tracking revealed three different bifurcation regions in agreement with previous publications. In the stationary VIV region, PIV experiments revealed periodic recirculation of vortices in the wake of the sphere. The frequency of these vortices matched the frequency detected in the wake of statically mounted (non-tethered). PIV measurements in the periodic region of VIV revealed complex periodic interactions between the shear layer and near-wake vortices. PIV measurements in the low-amplitude non-stationary region reveal that the shear layer is broken up into small-scale structures. The frequency of these structures was found to be in good agreement with the instability frequency of static held spheres.
Acoustic perturbations had a strong impact on the sphere dynamics. Excitations were applied at two different normalized excitation frequencies, fex/fn = 22 and 64. Excitations applied at fex/fn = 22 showed no effect in the periodic region of VIV. In the non-stationary region these same perturbations showed significant amplification of VIV. The applied frequency (fex = 35Hz) was near to the shear layer instability frequency observed in the PIV experiments.
Perturbations applied at fex/fn = 64 completely nullified sphere oscillations in the periodic regime. In the non-stationary region the excitation damped VIV slightly, but did not nullify it completely. Although detailed time resolved PIV was not performed for the controlled case, it is hypothesized that the acoustic perturbations interfere with the vortex shedding process.