Numerical simulation of bubble-type vortex breakdown within a tube-and-vane apparatus

Numerical calculations for three-dimensional, unsteady, laminar, bubble-type vortex breakdown within a tube-and-vane-type apparatus at a Reynolds number of 2000 and circulation number Omega = 1.41 are presented. This study is unique in that rather than specifying the inlet swirl velocity through a fit to experimental data (or a Burgers profile), the swirl was induced by directing the fluid through an array of 16 turning vanes, the arrangement being similar to that employed in the original experimental works of Sarpkaya [J. Fluid Mech. 45, 545 (1971); AIAA J. 12, 602 (1974)]. The interior of the resulting breakdown bubble consisted of one primary torroidal recirculation cell, which was tilted from, and found to gyrate about, the bubble centerline. The dominant frequency of gyration was identified, and the mechanism of fluid exchange examined. Subsequent calculations were performed using fixed inlet swirl and axial velocity profiles that were obtained from the results computed using the full geometry (including the turning vanes). Results revealed no significant difference in the downstream breakdown location or structure. (C) 2000 American Institute of Physics. [S1070-6631(00)01403-3].