The breakdown of a columnar vortex with axial flow

The interaction between a columnar vortex and external turbulence 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 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. Its growth rate agrees with that of the linear analysis of Mayer and Powell. 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.