On the transition process of a swirling vortex generated in a rotating tank

In this study, we investigate the transition of a swirling vortex from a one-celled to a two-celled vortex structure in a rotating tank. The main idea is to initiate the flow by siphoning fluid out of the tank and then to lift the siphoning mechanism out of the water within a short period of time. Before it reaches a state of quasi-two-dimensionality, the core region of the vortex can be roughly divided into three stages. (1) A siphoning stage induces the formation of the one-celled vortex. (2) A downward jet impingement stage triggers the transition of the vortex into the two-celled one. (3) A detachment stage of the inner cell leads to a cup-like recirculation zone, which is pushed upward by an axial flow from the boundary layer. This eventually develops into a stable quasi-two-dimensional barotropic vortex. The core region is enclosed by an outer region, which is in cyclostrophic balance. In the siphoning stage, the flow pattern can be well fitted by Burgers’ vortex model. However, in the post-siphoning stage, the present data show a flow pattern different from the existing two-celled models of Sullivan and Bellamy-Knights. Flow details, including flow patterns, velocity profiles, and surface depressions were measured and visualized by particle tracking velocimetry and the dye-injection method with various colors. The one-celled and two-celled flow structures are also similar to the conceptual images of the one- and two-celled tornadoes proposed in the literature.