The vortex breakdown of a columnar vortex with axial flow

The vortex breakdown of a columnar vortex with axial flow (q-vortex[1]) is investigated numerically. The formation of turbulent eddies around the columnar vortex and the vortex-core deformations are studied in detail by visualizing the flow field. In the marginally stable case with q = -1.5, small thin spiral structures are formed inside the vortex core. In the unstable case with q = -0.45, the linear unstable modes grow until the columnar vortex completes one turn. The data for the growth rate agree with that of the linear analysis [2]. After the vortex completes two turns, a secondary instability is excited which causes the collapse of the columnar q-vortex, after which many fine scale vortices appear spontaneously.