An investigation on the suppression of flow induced vibrations of bluff bodies

• Main Author: Seto, Mae L.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/4758

Wind induced oscillations of bridges, tall buildings, smokestacks, transmission line conductors and similar bluff bodies have been of interest to scientists and engineers for a long time. Unchecked, the vortex resonance and galloping type of vibrations can cause damage to structures. Some examples include the original Tacoma Narrows Bridge, the dramatic cracking of industrial chimneys, and the destruction of building components as well as the structure itself. Apart from catastrophic destruction, this class of low frequency vibrations are known to cause undesireable working conditions leading to nausea, dizziness, disorientation and vertigo, particularly for those working at relatively greater heights as in tall buildings and air traffic control towers. Flow induced vibrations are of special relevance to engineers today because of the tendency to build taller structures and longer span bridges with ever lighter building materials. The thesis studies vortex induced and galloping type of instabilities associated with structural geometries of fundamental importance. Of particular interest is the effectiveness of energy dissipation to suppress the oscillations. To that end, a comprehensive study focuses on the design of nutation dampers and assesses their effectiveness in arresting wind induced instabilities. To begin with, a parametric study of the damper, in conjunction with frequency response tests, is used to identify important system variables contributing to significant energy dissipation. The results show that optimum combinations of the damper parameters such as the geometry, liquid height, surface seeding, and compartmenting can lead to an efficient damper, particularly if the operating conditions are conducive to wavebreaking. Among the dampers tested, the circular cylindrical geometry proved to be the most efficient. The addition of floating particles further improved the performance by around 30 %. Next, a numerical model, based on the nonlinear shallow water equations of motion, is developed to predict dissipation characteristics. The numerical results are also animated to provide better visual appreciation of the free surface wave dynamics. The agreement between numerical and experimental results is quite good considering the complex character of the flow. This is followed by construction of a fluid-structure interaction model through coupling of the nonlinear shallow water equations to a single degree of freedom structure undergoing vortex resonance. The agreement between wind tunnel experiments and the model is surprisingly good considering the nonlinear character of the fluid dynamics and structural interactions with it. The numerical algorithm should serve as a valuable tool in designing this class of dampers for practical applications. Finally, wind tunnel tests with two-dimensional models substantiate, rather dramatically, the effectiveness of the nutation dampers in arresting both vortex resonance and galloping types of instabilities. The dampers continue to be effective even for the case when the structure is located in the wake of other structures, the situation frequently encountered in practice. A visualization study, for flow within the damper and suppression of structural instabilities during the wind tunnel tests, complemented the experimental and numerical investigations. Both still photographs as well as a video were taken. Each phase of the study represents innovative contributions and the results obtained are of far-reaching consequence for a class of structures currently under design and those planned for the future. The thesis ends with some concluding comments and recommendations on rewarding avenues of research to pursue in the next phase of the work.