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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Mechanical Engineering

Year Degree Awarded

2015

Month Degree Awarded

May

First Advisor

Yahya Modarres-Sadeghi

Subject Categories

Acoustics, Dynamics, and Controls | Ocean Engineering

Abstract

A bluff body, i.e., an object with a blunt cross-section immersed in cross-flow forms an unstable wake, resulting in the formation of large-scale vortical structures, which induce unsteady forces on the body. If the body is flexible or flexibly mounted, vortex-induced vibration (VIV) results, which can have significant implications for a number of physical systems, from aeolian harps to power transmission lines, towing cables, undersea pipelines, drilling risers and mooring lines used to stabilize offshore floating platforms. VIV has been a major subject of research in recent years. The majority of these studies have focused on symmetric systems in which the structure, its geometry, its boundary conditions and the incoming flow are symmetric. However, in many real-world applications, VIV occurs in asymmetric systems. A flexibly-mounted vertical uniform circular cylinder which is placed in a uniform current is an example of a system with symmetric geometry, boundary conditions and wake. This geometry is usually considered as a canonical configuration in modeling structures undergoing VIV. However, many other configurations can exist in which the geometrical symmetry of the system is broken, such as an inclined cylinder in which the cylinder is placed with an angle with respect to the incoming flow direction, or a tapered cylinder in which the diameter of the circular cylinder varies along its length, or when the geometry of the cross-section changes from circular to other shapes such as square or triangle. Also, in some other cases, such as a circular cylinder which is forced to rotate about its long axis, the symmetry of the surrounding wake is broken by the rotation imposed on the cylinder. These cases among many examples are systems with broken symmetry, and are widely used in the offshore industry. The main objective of this thesis is to study VIV of systems with broken symmetry. For this purpose, experiments are conducted and the VIV response of the systems are studied in terms of amplitude and frequency of oscillations as well as the wake of the cylinder. It is shown that higher harmonics contributions of flow forces play an important role in VIV of structures. Also, various taper ratios in VIV of tapered cylinders are found to be the major parameters influencing the VIV response of tapered cylinders. Different rotation rates in VIV of a rotating cylinder, various angles of attack in VIV of a triangular cylinder and different angles of inclination in VIV of flexible inclined cylinder are found to be the influencing parameters in studying the VIV of structures with broken symmetry.

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