Vortex breakdown in a two-fluid confined flow generated in a cylindrical container

Two-fluid swirling flows are rich in features of fundamental and practical interest. Among them, the most intriguing phenomenon that is important for applications is the formation of local circulation cells (vortex breakdown, VB). Our experimental study explores flows of two immiscible liquids in a sealed vertical cylindrical container driven by the rotating lid. The lid rotation generates the meridional circulation of an adjacent fluid, which serves as a rotating "liquid lid" for the lower fluid. The upper fluid goes from the cylinder axis to the periphery near the lid and back near the interface. This centrifugal circulation tends to propel the circulation in the main-fluid domain - in the lower fluid. As the angular speed of lid rotation increases, vortex breakdown occurs in the lower fluid. Using visualization of the flow and measurement of velocity by PIV method, we reveal common features of the VB development in the lower fluid at different thicknesses of the upper fluid. We show that the range of Reynolds numbers, where VB occurs, might be different depending on the volume of the upper liquid and its kinematic viscosity.