Streamwise fluctuations of vortex breakdown at high Reynolds numbers

Abstract

This thesis deals with the characterization of the dependence on the flow geometry of the streamwise fluctuations of the stagnation point of vortex breakdown in axisymmetric tubes and over delta wing aircraft. The statistical analysis presented herein shows that in an axisymmetric tube the 'darting' about the mean stagnation point are distributed normally for the two Reynolds numbers: ReD = 230,000 and 300,000 (independently of the Reynolds number in the range noted). The darting over a delta wing is not only non-Gaussian but also exhibits rather large localized fluctuations (Strouhal numbers ranging from 0.04 to 0.1), presumably due to the strong influence of the surrounding flow and the geometrical conditions: increase of circulation along the trailing edge, the abrupt separation of flow at the base of the delta wing, and other protuberances that emerge from the upper and lower surfaces of the wing (support elements in laboratory and stabilizers on delta wing aircraft). It is concluded that the behavior of vortex breakdown is strongly dependent on the surrounding geometry and that only experiments in axisymmetric tubes can provide the purest form of vortex breakdown for numerical simulations and analytical studies towards the understanding of the internal turbulence and its spectrum within the breakdown bubble for theoretical and industrial purposes.