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