Vortex breakdown in an unconfined vortical flow

Flow visualization experiments were conducted on an unconfined vortex formed on a stationary impermeable base. At the low circulation range, all seven modes of vortex breakdown previously documented for a confined flow could be observed, in spite of the vast differences in the two sets of apparatus. The unconfined flow also revealed the existence of a conical breakdown. This occurrence was first reported by the authors (5th ISOPE Conf. 11-15 June 1995) and has since been recently observed under confined swirling flows. It is characterized by a conical breakdown envelope and occurs at the high swirl regions. At the higher circulation range, the vortex structure erupted from the base into a large-scale turbulent flow. Results are presented on a chart using parameters related to the intrinsic flow properties of the vortical flow-circulation, extent of vortex stretching, and core size. (C) Elsevier Science Inc., 1997.