טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGendel Sergey
SubjectNonlinear Spatio-Temporal Flow-Structure Interaction of an
Elastically Attached Slender Body at High
Incidence
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Emeritus David Degani
Professor Oded Gottlieb
Full Thesis textFull thesis text - English Version


Abstract

Flow-induced vibrations are motions of flexible or elastically attached rigid-bodies that are subject to external flow. The flow field can exert an asymmetric force on the structure leading to motion transverse to the mean flow direction. An example of this phenomenon is the Von Kármán vortex street in the wake of a fixed circular cylinder that results in vortex-induced vibrations of its elastically tethered counterpart due to a periodic lift force. Flow-induced vibration problems span many different branches of engineering, such as civil structures (bridges, stacks, transmission lines), marine, aerospace and land vehicles, ocean and space structures (tethered platforms, mooring cables). In recent years, investigation of these phenomena has become increasingly important, as structures are progressively lighter and flexible, and are more prone to large amplitude flow-induced vibrations in severe environmental conditions. The goal of this research is to investigate the governing mechanisms observed in fluid-structure interaction (FSI) of elastically attached slender bodies and, conversely, the influence of the interactions on the downstream fluid flow patterns. In this research we numerically investigate the flow around an inclined tangent ogive-cylindrical rigid-body, mounted on torsion springs that can rotate in the pitching, yawing, and rolling directions. The body is subjected to three-dimensional, compressible, laminar flow at Reynolds number of 30,000 (based on body diameter) and a low Mach number of 0.2.

 A second-order implicit finite-difference scheme is employed for the flow equations, adapted to three-dimensional curvilinear coordinate system, whereas the coupled structural equations, derived using Euler angle notation, are solved by an explicit fourth-order Runge-Kutta method. The investigation is focused on the influence of the angle-of-attack, in the range of 20 to 55 degrees, on the FSI of an elastically mounted slender body.

For a fixed body with a small angle-of-attack the yaw moment was found to change smoothly with the location of circumferential disturbance, whereas for an angle of attack   the yaw moment was characterized by an almost a square-wave behavior with bifurcation points at specific angles relative to the initial vertical orientation .  For an elastically mounted body the response with small angles-of-attack resulted in large amplitude periodic yawing and pitching moments for all disturbance locations. The response for an angle of attack resulted in large amplitude moments which occurred only for disturbance locations which coincided with the bifurcation points of the fixed body. The response at the bifurcation points was found to be quasiperiodic for  periodic for  and chaotic-like for.

The direct implication of this investigation is that careful decomposition of experimental measurements is essential, particularly when the balance-mounted rigid bodies in laminar flow exhibit nonstationary and chaotic-like dynamics, which are due to nonlinear and complex fluid structure interaction. The current work assumed that the computational model is deterministic, and uncertainty in the properties of the dynamical system is ignored. This should be addressed in future work.