| This dissertation describes a combined theoretical and experimental study of spiral vortex breakdown. The primary objective of the research was to examine the role that the characteristic spiral geometry plays in the breakdown process. The study began with the description of a theoretical spiral vortex model based on the supposition of self-induced stagnation of the spiral structure; experiments were designed and conducted to test the validity of the theory. The experiments allowed for the control and measurement of the vortex flow field independent of the external, or breakdown-inducing, flow field. The results of the measurements were used in a new static spiral vortex model, developed to incorporate the detailed core-vorticity information, which showed that the self-induced velocities on the spiral geometry were insufficient to stagnate the spiral structure in the flow field, relative to a fixed reference frame. Based on these results, a dynamic, temporally- and spatially-evolving segmented vortex model was developed to examine the kinematics of an idealized breakdown pulse. Results of the dynamic modeling showed that the spiral breakdown has the characteristics of a self-induced upstream-travelling wave combined with a clearing of fluid particles downstream through the spiral structure. Through the self-induced upstream wave propagation in a downstream-convecting flow field (thus giving the appearance of a stagnated structure), the characteristic spiral geometry was found to be responsible for its own continuation. Other results include the experimental observation of a "pseudo spiral breakdown" mode and the constancy of the non-dimensional circulation (vortex circulation divided by local spiral diameter and local flow velocity) over a variety of vortex and breakdown flow conditions. |
