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Title: A numerical study of instability and vortex breakdown of swirling flow
Author: Amirante, Daria
ISNI:       0000 0001 3419 6441
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2007
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Direct numerical simulations ofboth axisymmetric and three-dimensional highly swirling flows are conducted to study the vortex breakdown phenomenon and the onset of helical instabilities. The enduring debate on the physical reasons underlying the breakdown of slender v9rtices has widely involved theoretical, experimental and computational studies. In the present investigation, we are motivated by the necessity to evaluate the range of applicability of recent studies which have correlated the global response of this class of flows to their local stability characteristics. In synthesis, the dynamics of the unsteady structures developing in swirling flows are explained according to simplified theories which assume the flow to be locally parallel. These results, which might be considereg as the Cnatural extension of concepts well established for two-dimensional jets and wakes, appear to be quite surprising if applied to swirling flows in breakdown configuration. In fact, the presence of one or more large regions of recirculating flow (the vortex bubbles) renders the assumption ofnear parallelism strongly violated. Inspired by this observation, we have carried out a numerical investigation in order to study the evolution of self-sustained oscillations. For this purpose, a finite difference code has been developed and later adapted to perform linear analysis around a given parallel swirling flow. Successively, a comparative study between the global and local analysis methodologies has been conducted. The novelty of the work is rep~esented by the use of simple filtering techniques which can be implicitly activated if the cylindrical coordinates are employed. These have made possible to focus on the nonlinear evolution . of higher order modes. Following this strategy, we have identified an instability mechanism which cannot be explained by the local theory and whose existence is clearly associated with the presence of recirculating flow. The result is considered important since it provides a further contribution to the general understanding of the global modes. Throughout this thesis we have followed a bottom-up approach in terms ofthe assumptions undertaken. The general stability properties of swirling flows are initially revisited based on 1D models. The hypothesis of one-dimensionality has been later replaced by that ofaxisymmetry. Real swirling flows are examined in the final chapter for Reynolds numbers in the range of those generally employed in the physical experiments.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available