| dc.description.abstract | The dynamics of vortex shedding from bluff bodies has been investigated
experimentally for many decades, with the simple geometric case for the
investigation being a uniform circular cylinder aligned with its axis normal to the
flow. Even though much information regarding the dynamics of the flow has been
accumulated over the years, the actual understanding of the phenomenon has
progressed slowly.
The motivation for the present study comes from the fact that the vast
majority of previous work has been concerned with the shedding of vortices from
uniform cylinders with the flow normal to its axis. This classic arrangement is seen
to produce vortices that are parallel to the axis of the cylinder, depending on the endconditions.
Even this seemingly simple symmetric arrangement is seen to produce
results characterised by large discrepancies and varied interpretations. The question
that one could ask now is what would happen if there were a slight variation in the
geometry of the cylinder. A linear variation of the diameter of the body along its
span would raise additional problems, but on the other hand might provide new and
useful insight to the problem of vortex shedding itself, since the cross-section would
still be circular.
In this work a comprehensive study of the effect of the introduction of a slight
spanwise taper on the phenomenon of vortex shedding is presented. The study
involved the extensive use of experimental data obtained using hot-wire anemometry and particle image velocimetry techniques. The effect of the taper on the onset of
vortex shedding and the variation of vortex shedding along the span are two of the
topics that are investigated in the present work. Even though the cross-section of
these cones was circular the onset of vortex shedding was delayed, with the extent
depending on the severity of taper.
The results of the study of onset characteristics were also seen to be of
importance in the investigation/prediction of transonic buffet onset on twodimensional
airfoils. It is known that near the critical conditions for the onset of
transonic buffet, there is flow separation followed by large scale lift oscillations.
Global flow instability has been shown to be a source of this unsteadiness (Crouch et
al., 2009). Crouch et al. (2007) considered a generalised approach to predicting the
onset of flow unsteadiness based on the global-stability theory. In order to assess the
generalised approach they studied the onset of vortex shedding about a cylinder
cross-section as a limiting case in 2-D. A study of the onset of unsteadiness about
more complex geometries such as cones using full 3-D unsteady Navier-Stokes
simulations incorporating global stability analysis (Garbaruk et al., 2009) is seen to
be more analogous to flows of practical interest, viz. flow about a tapered wing. The
present work provided important data pertaining to the dynamics of vortex shedding
from cones, with particular interest in the frequency of vortex shedding at the onset
of unsteadiness.
It is also shown that the taper ratio does have a major effect on the vortex
shedding process, with the normal periodic shedding being replaced by a deeply
modulated form. This non-linear amplitude modulation was found to be a global
process controlling the vortex shedding all along the span of body, especially those
with small taper ratios.
Finally an attempt has been made to mathematically model the vortex
shedding process in terms of non-linear oscillators, with a coupling that represents
the interaction of the shed vortices along the span. It turns out that the modelling
techniques using a series of spanwise oscillators with a simple coupling term, as seen
in the literature, is not sufficient to fully represent the flow. | |
