Formation of Turbulent Vortex Breakdown: Intermittency, Criticality, and Global Instability

This study provides quantitative insight into the formation of vortex breakdown and the onset of global instability a turbulent swirling jet. A water jet is guided through a rotating honeycomb that imparts the rotational motion, through a contraction, and discharged into a large water tank. The flow states evolving at increasing swirl are out via time-resolved particle image velocimetry. The experimental results scale properly with the swirl based on the axial momentum flux when the commonly used boundary-layer approximations are omitted. instantaneous velocity field reveals that vortex breakdown occurs intermittently at a wide range of swirl before it appears in the mean flow. At this intermittent state, the evolving breakdown bubble oscillates between two streamwise locations where the vortex core is subcritical. Upon further increasing the swirl, the oscillations decay and a region of reversed flow appears in the mean flowfield. The formation of this socalled breakdown state is accompanied by a supercritical-to-subcritical transition of the inflowing core. The reversed flow region is found to grow linearly with increasing swirl until the flow undergoes a Hopf bifurcation to a global single-helical mode, and vortex breakdown adopts a spiral shape. The mode shape is extracted from the particle image velocimetry snapshots by means of proper orthogonal and Fourier analysis. The present experiment reveals that, at gradually increasing swirl, the jet first to an axisymmetric breakdown state that remains globally stable until a critical swirl number is exceeded. sequence of flow states agrees well with the transient formation of vortex breakdown observed in laminar flows.