The three-dimensional structure of confined swirling flows with vortex breakdown

In a recent experimental study, Spohn, Mory & Hopfinger (1998) investigated in detail the flow in a closed cylindrical container with a rotating botto m for Reynolds numbers in the steady and unsteady regimes. Their visualizat ion photographs revealed that the stationary vortex breakdown bubbles, whic h form along the container axis within a range of governing parameters, are open, with inflow and outflow, and asymmetric at their downstream end. For Reynolds numbers within the unsteady regime, visualizations of the limitin g streamlines on the cylindrical wall showed that the Stewartson layer sepa rates asymmetrically along stationary spiral convergence lines that form be low the top cover. We study numerically the container flow, by solving the unsteady, three-dimensional Navier-Stokes equations, in order to clarify th e origin and elucidate the underlying physics of these complex, three-dimen sional flow features. The stationary vortex breakdown bubbles we simulate e xhibit all the asymmetries observed in the laboratory. By analysing the Lag rangian characteristics of the calculated flow fields, we explain the origi n of these asymmetries, clarify the experimentally documented filling and e mptying mechanisms, and show that the flow in the interior of stationary vo rtex breakdown bubbles exhibits chaotic particle paths. We also show that t he spiral separation lines observed by Spohn et al. (1998) inside the Stewa rtson layer at high Reynolds numbers are due to the growth of pairs of coun ter-rotating, spiral vortices and the interaction of these vortices with th e stationary-cover boundary layer.