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

Numerical calculations for three-dimensional, unsteady, laminar, bubble-typ e vortex breakdown within a tube-and-vane-type apparatus at a Reynolds numb er 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 f it to experimental data (or a Burgers profile), the swirl was induced by di recting the fluid through an array of 16 turning vanes, the arrangement bei ng 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 th e resulting breakdown bubble consisted of one primary torroidal recirculati on cell, which was tilted from, and found to gyrate about, the bubble cente rline. The dominant frequency of gyration was identified, and the mechanism of fluid exchange examined. Subsequent calculations were performed using f ixed inlet swirl and axial velocity profiles that were obtained from the re sults computed using the full geometry (including the turning vanes). Resul ts revealed no significant difference in the downstream breakdown location or structure. (C) 2000 American Institute of Physics. [S1070-6631(00)01403- 3].