Excitable modes in forced symmetry breaking for a cylindrical container with one rotating endwall

The technical relevance of swirling flows has inspired numerous investigations into the field: swirling flow burners, trailing vortices of delta wings, separators etc. are some of the applications where such research has yielded interesting results. At the same time, geophysics-inspired general swirling flow studies provided much input to this field. A subcategory of swirling flows is that where vortex breakdown is observed. Vortex breakdown is a condition where a vortex core undergoes a transformation from a slender flow entity to a structure of a drastically different nature: in the literature, numerous manifestations of this phenomenon have been reported, along with classification proposals for the flow structures connected with vortex breakdown. A particular class of vortex breakdown types, i.e. that of bubble breakdown, has attracted a significant portion of the attention of researchers in the field, for its technical relevance, but also for a reason that is of particular interest to this work: it has been shown that steady and unsteady bubbletype vortex breakdown can be produced in a almost ideally controllable environment, i.e. in a completely enclosed cylindrical container with one spinning endwall, figure 1, (or in more intricate variations of this system). Such a geometrical configuration enjoys the benefit of being described fully by 2 parameters, i.e. the Reynolds number (based on the container diameter and the angular velocity of the rotating lid) and the aspect ratio. Additionally, there are no arbitrary or difficultly controllable boundary conditions (inflow/outflow), since all boundaries are either stationary or spinning solid walls.