BUBBLE-TYPE AND SPIRAL-TYPE BREAKDOWN OF SLENDER VORTICES

Bubble- and spiral-type breakdown and periodic transition between the two were visualized by varying the volume flux in a transparent, low-s peed, vertical water tunnel with a diffuser inset. A feedback model fo r 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 a re qualitatively confirmed by experimental flow visualizations. The in itiation of the bubble-type breakdown and its transition to the spiral -type were successfully simulated by solving the Navier-Stokes equatio ns for three-dimensional, unsteady, and incompressible flow with bound ary conditions similar to those of an experimental investigation. Henc e, for the first time it was possible to compare the results of numeri cal and experimental flow visualizations and therefore to draw a concl usion about the reliability of simulating vortex breakdown numerically .