Transition from spiral- to bubble-type vortex breakdown

Numerical calculations employing the fully three-dimensional, time-dependent Navier-Stokes equations have been utilized to compute a transition from spiral- to bubble-type vortex breakdown for an unconfined longitudinal vortex. The transition was initiated through a small increase in the magnitude of the free-stream axial velocity deceleration. The resulting bubble structure, which consisted of a single toroidal recirculation cell, ultimately grew unstable. Transition back to a stronger spiral breakdown followed. Results are in qualitative agreement with experimental observations. (C) 1996 American Institute of Physics.