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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.

Ahigher-order method is developed that models continuous source and doublet singularity distributions over three-dimensional curved surfaces. The singular on-surface influence coefficients are treated by a robust desingularisation algorithm, whereas off-surface coefficients are calculated by means of an efficient subdivision and variable cubature scheme. Whilst higher-order methods have previously been developed fo! thick bodies and Dirichlet boundary conditions, this method is also capable of modelling continuous geometry and singularity surfaces over thin bodies and wakes that require Neumann boundary conditions. The Continuous Surface Method (CSM) has a number of advantages over conventional constant panel methods (CPMs). Firstly, as curved geometries are represented exactly, changing the order of the solution does not modify the physical shape of the configuration. Furthermore, as singularity solutions are continuous, the significant grid-dependency of CPMs does not arise. Finally, the continuous singularity distributions allow velocities to be evaluated accurately across the entire surface without interpolation: this enables the calculation of continuous pressure distributions and the construction ofstreamlines and wakes flowing very close to surfaces, without any problems of divergence. Numerical results comparing the CSM to a CPM have shown that for equal run times, the CSM obtains greater accuracy in pressure distributions than a CPM, and produces much smoother velocity fields. However the CSM was not able to improve upon the efficiency ofthe CPM in determining total aerodynamic forces. A wake relaxation scheme in which wakes are modelled as curved B-spline patches is developed, and is convergent for simple geometries. For.a more complex example of wakes shed from two closely overlapping sails, the wake relaxation converges to within around 0.5% of total aerodynamic load, but the low panel resolutions employed in the CSM are insufficient to model the detail of . the wake roll-up effectively. Three alternative schemes to address this problem are evaluated.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.