Experimentally investigating the instability onset in closed polygonal containers

The article investigates the instability onset in the vortex flow in the stationary container with a rotating lid and polygonal cross-sections. Transition to unsteady flow regime is determined in closed containers with square, pentagonal and hexagonal cross-sections. The onset of flow unsteadiness is measured by combining the high spatial resolution of PIV and the temporal accuracy of LDA. A detailed transition scenario from steady and axisymmetric flow to unsteady flow with discrete rotational symmetry is investigated for the aspect ratio of 2.5 ≤ h ≤ 6.0. The results show that at steady conditions there are no asymmetric distortions of the flow topology due to the non-axisymmetric effect of the polygonal geometry of the used containers. It is found that reducing the number of cross-section angles shifts the instability onset to smaller aspect ratios and the vortex breakdown bubble stabilizes and shifts the flow instability onset to higher Reynolds numbers. The features of the flow structure formation in the polygonal containers are shown to be similar to those in the axisymmetric configuration. It is proved that the onset of the unsteadiness in polygonal configurations is based on the appearance of rotating vortex multiplets, which is similar to the cylindrical container.