We experimentally investigate the unstable behavior of a large scale hollow-type streamwise vortex which has a solid-rotation type distribution of vorticity existing only in an annular region initially, focusing on how the large scale vortex breaks down into small scales downstream. With increasing the intensity of the initial vorticity, the large scale hollow vortex undergoes significant transition from a steady-state to nonlinear stages governed by coherent vortices through the primary linear instability. When the periodic motions due to the primary instability grow and redistribute the vorticity, a rotating vorticity field of polygonal shape begins to develop. This is an important precursor to the nonlinear evolution of the instability of hollow-type streamwise vortex. The flow structures we obtained in this work allow the fluid mixing to be significantly enhanced even under low Reynolds numbers. The present study also shows that the ratio of maximum azimuthal velocity to mean axial velocity is important as a governing flow-similarity parameter. This finding is reasonable as the cited swirl velocity ratio is directly proportional to the ratio of vortex Reynolds number to axial flow Reynolds number.
