Time-Resolved Particle Image Velocimetry Measurements of Vortex Breakdown

Vortex breakdown occurs in many flow applications, introducing unsteadiness that is often undesired. Recently, researchers have suggested mechanisms of stabilization and prevention of breakdown through active control. While vortex breakdown has been previously investigated, researchers have struggled to efficiently characterize flows due to the flow sensitivity to intrusive probes. A proven vortex breakdown rig design was modified to allow for time-resolved non-intrusive flow measurements which could be used to study breakdown onset and possible control mechanisms. In the present study, vortex breakdown is investigated to characterize its behavior and identify key parameters for future studies. High speed flow visualization images were taken to determine when breakdown occurs. 2D Particle Image Velocimetry (PIV) measurements of a vortex in the rig were taken, with a range of swirls and Reynolds numbers that includes the breakdown regime. Instability in the vortex core position was observed regardless of whether breakdown was exhibited. The core axial velocity increased relative to the no swirl velocity for cases with no breakdown, whereas it decreased significantly when breakdown was present. A resulting change in shear layer polarity was observed through vorticity calculations, which is proposed as a simple criterion for identifying breakdown with 2D PIV. Rotational momentum is estimated for each case and found to dominate the flow for the breakdown regime. The findings provide a framework for future PIV investigations of vortex breakdown.