Flow between concentric cylinders with the inner cylinder rotating and an axial pressure gradient imposed in the annulus reveals a rich variety of flow regimes depending on the flow conditions. The occurrence of these flow regimes was studied experimentally by both visually and optically detecting the transition from one flow regime to another over a wide range of Taylor numbers for moderate axial Reynolds numbers. Seven flow regimes of toroidal vortices were identified, including Taylor vortices, wavy vortices, random wavy vortices, modulated wavy vortices, turbulent modulated wavy vortices, turbulent wavy vortices, and turbulent vortices. The toroidal vortices in these flow regimes look similar to the corresponding vortices when there is no axial flow, except that they translate with the axial flow at a speed slightly greater than the bulk axial velocity. Three flow regimes of helical vortices were observed at low Taylor numbers, including laminar helical vortices, stationary helical vortices, and wavy helical vortices. Depending on the flow parameters, the helical vortices had both positive and negative helix angles with respect to the bulk flow and appeared either stationary or moving downstream. Another flow regime consisting of the repeating sequential appearance of turbulent wavy vortices, turbulent helical vortices, and turbulent vortices was also observed.
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