| Summary: | 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. === Applied Science, Faculty of === Mechanical Engineering, Department of === Graduate
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