Bubble- and spiral-type breakdown of slender vortices

Bubble- and spiral-type breakdown and periodic transition between the two were visualizedby varying the volume flux in a transparent, low-speed, vertical water tunnel with a diffuser inset. A feedback model for the initiation and development of bubble-type breakdown is proposed, and the transition from bubble- to spiral-type breakdown is explained by an asymmetry of the circumferential vorticity distribution. Both are qualitatively confirmed by experimental flow visualizations. The initiation of the bubble-type breakdown and its transition to the spiral-type were successfully simulated by solving the Navier-Stokes equations for three-dimensional, unsteady, and incompressible flow with boundary conditions similar to those of an experimental investigation. Hence, for the first time it was possible to compare the results of numerical and experimental flow visualizations and therefore to draw a conclusion about the reliability of simulating vortex breakdown numerically.