Transition of the Global Mode in Coaxial Swirling Jets After Vortex Breakdown

The interaction of two swirling jets in a coaxial injector, a configuration widely used in aero-engine combustors, is investigated using a combination of experiments and linear stability analysis. The influence of the outer-to-inner bulk velocity ratio, [Formula: see text], on the flow dynamics is systematically examined. Coherent structures and their spectral characteristics are identified from experimental data using spectral proper orthogonal decomposition (SPOD). At low [Formula: see text], the flow is dominated by a precessing vortex core (PVC) confined to the inner swirling jet. In this regime, increasing the outer-stream bulk velocity strengthens the oscillation amplitude but does not fundamentally modify the nature of the global instability. At high [Formula: see text], the PVC undergoes a transition to a single helical mode that spans both jets, driven by substantial changes in the mean-flow topology resulting from jet–jet interaction. Global linear stability analysis performed on the mean flow shows good agreement with the SPOD results, indicating that the observed dynamics are predominantly governed by linear global modes. Wavemaker analysis further reveals that the PVC originates near the centerbody, whereas the helical mode is generated in the lip-wake region. These distinct origins are reflected in their sensitivity tensor characteristics: the PVC involves comparable sensitivity across different velocity components, while the helical mode is predominantly sensitive to azimuthal velocity, implying that mode switching may be associated with azimuthal momentum feedback.