טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShoshani Oriel
SubjectFluid-Structure Interaction and Orbital Instabilities of
Elastically Tethered Bluff Bodies
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Oded Gottlieb
Full Thesis textFull thesis text - English Version


Abstract

Vortex-induced vibration (VIV) is a critical phenomenon that needs to be considered in the design of many structural engineering systems that interact with an incident fluid field. Examples include chimneys, bridges, skyscrapers, ocean platforms, and pipelines in severe environmental conditions. The practical significance of VIV has led to a large number of fundamental studies which consider as a paradigm an elastically tethered rigid-body subject to time-dependent wake-induced drag, lift and transverse forces and moments. These studies have focused primarily on water or wind tunnel experiments and on numerical simulation of the coupled fluid-structure interaction. However, even for the case of a tethered circular cylinder in two-dimensional uniform flow, VIV consists of coexisting self-excited periodic and complex non-stationary dynamics. The complexity and high-cost of both experiments and spatio-temporal numerics, and their inability to predict the multiple VIV stability thresholds in a general framework motivates this research which includes two systematic modeling approaches that are analyzed asymptotically. In the first approach a reduced-order cubic wake-oscillator is derived directly from the governing two-dimensional Navier-Stokes equations near the onset of the von-Kármán vortex street for a fixed cylinder. The second approach is based on a phenomenological high-order wake-oscillator model that is derived to account for secondary bifurcations which have been documented numerically and experimentally. In both approaches the tethered structure is modeled via a Lagrangian approach to consistently account for geometric and material nonlinearities which govern the translational and rotational internal resonances. Both models are resolved using a temporal multiple-scales model-based estimation procedure and their validity is shown by comparison with documented experimental and numerical studies.  Analysis of the resulting slowly-varying evolution equations augmented by numerical integration of the corresponding nonlinear systems enables construction of a comprehensive bifurcation structure that incorporates non-stationary chaotic-like dynamics, demonstrates the influence of rigid-body rotations, and sheds light on the mechanisms that govern the onset of orbital instabilities.