Diagnostics of bubble-mode vortex breakdown in swirling flow in a large-aspect-ratio cylinder

We report for the first time on the possible formation of regions with counterflow (bubble-mode vortex breakdown or explosion) at the center of strongly swirling flow generated by a rotating endwall in a large-aspect-ratio cylindrical cavity filled with a liquid medium. Previously, the possibility of bubble-mode breakdown was studied in detail for cylindrical cavities of moderate aspect ratio (length to radius ratios up to $H/R\sim$ 3.5), while flows in large-aspect-ratio cylinders were only associated with regimes of self-organized helical vortex multiplets. In the present study, a regime with nonstationary bubble-mode vortex breakdown has been observed in a cylindrical cavity with $H/R$ = 4.5.