Modal linear stability analysis has proven very successful in the analysis ofcoherent structures of turbulent flows. Formally, it describes the evolution ofa disturbance in the limit of infinite time. In this work we apply modal linearstability analysis to a turbulent swirling jet undergoing a control parametertransient. The flow undergoes a transition from a non-vortex breakdown state toa state with a strong recirculation bubble and the associated global mode.High-speed Particle Image Velocimetry (PIV) measurements are the basis for alocal linear stability analysis of the temporarily evolving base flow. Thisanalysis reveals that the onset of the global mode is strongly linked to theformation of the internal stagnation point. Several transition scenarios arediscussed and the ability of a frequency selection criterion to predict thewavemaker location, frequency and growth rate of the global mode are evaluated.We find excellent agreement between the linear global mode frequency and theexperimental results throughout the entire transient. The corresponding growthrate qualitatively conforms to experimental observations. We find no indicationfor a nonlinear global mode as proposed by previous studies.
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