Flow pattern exchange in the Taylor-Couette system with a very small aspect ratio.

Numerical investigation is carried out on the flow pattern exchanges found in Taylor-Couette flows between two concentric rotating cylinders. The inner cylinder rotates while the outer cylinder and both end walls are stationary. The aspect ratio (column length/gap width) is small, and its range is from 0.5 to 1.6. Previous experimental results for this range of the aspect ratio showed that the steady flow patterns are classified into three groups: the normal two-cell mode, anomalous one-cell mode and twin-cell mode. All modes found by experiments are predicted in the present numerical calculation. Besides these three flow modes, an unsteady mode is predicted, which is time dependent and fully developed. The existence of the unsteady mode is also confirmed by our experiments. When the inner cylinder starts to rotate from rest, vortices at the corners of the inner cylinder and both end walls develop, and they induce the normal two-cell mode. The flow of the anomalous one-cell mode or twin-cell mode appears after an abrupt breakdown of symmetric two-cell mode flows. During the gradual deceleration of the inner cylinder, the transitions of flow modes occur. We observed mode transitions between the normal two-cell mode and anomalous one-cell mode and mode transitions from the twin-cell mode to the normal two-cell mode, anomalous one-cell mode, and unsteady mode. The critical loci where these mode transitions appear are determined. The numerical confirmation of the twin-cell mode is a different result obtained in the present study.