Active fuel forcing effects on nonlinear interaction and three-dimensional structural evolution of helical vortices in a non-reacting swirl combustor

The dynamics of coherent helical structures in a lean, partially premixed, non-reacting swirl combustor subjected to periodic fuel-feeding (f_f) excitation are investigated using large-eddy simulation (LES). In the absence of forcing, the flow exhibits self-excited hydrodynamic instabilities characterized by multi-frequency oscillations and phase-drifting behavior, whereas periodic fuel excitation progressively organizes the flow toward a forcing-driven response. Proper orthogonal decomposition (POD) is employed to identify the dominant coherent structures, while Dynamic Mode Decomposition (DMD) provides frequency-resolved characterization, revealing that fuel excitation modifies the relative temporal persistence of the precessing vortex core (PVC) and the double helical vortex (DHV) dynamics. The results show that external fuel forcing modifies the interaction between the dominant PVC and secondary DHV rather than simply exciting individual coherent modes. Increasing forcing frequency promotes frequency-locking between the two structures, redistributes coherent kinetic energy from the PVC toward the DHV, strengthens the secondary helical mode, and progressively modifies the phase relationship and amplitude response of the coupled system. Phase-resolved reconstruction further reveals that the PVC maintains a dominant single-helical core under weak forcing, whereas stronger forcing induces radial deformation of the PVC and upstream expansion with increased radial spread of the DHV. These structural changes demonstrate the transition from weakly perturbed vortex dynamics to a strongly forced coupled system governed by nonlinear modal interaction. These findings demonstrate the potential of fuel-flow excitation for actively manipulating coherent vortex dynamics and support the development of active flow-control strategies in swirl-stabilized combustors.