Some experimental observations of secondary motions in a confined vortex flow

Three decades have passed since vortex breakdown was first identified as a natural fluid flow phenomenon. Three key theories have been proposed to explain the phenomenon: hydrodynamic instability, conjugate states and flow stagnation. Despite a considerable amount of theoretical and experimental investigation, there is still nothing approaching a completely satisfactory theory of vortex breakdown. In addition, there is no agreement on a complete physical description of the structure of vortex breakdown. The present experimental investigation may substantiate a few earlier conjectures. We discuss an experimental finding that might help clarify the phenomenon through the use of flow visualization and laser-Doppler velocimetry. Experimental measurements substantiate earlier measurements and theoretical calculations of the velocity field. The evidence suggests that there is a connection between criticality and instability.