Combustion dynamics in aero-engines exhibit complex thermoacoustic behavior involving multiple interacting modes. Recent research on the so-called intrinsic thermoacoustic modes has provided new impetus for exploring the physical origins of these modes and their mutual interactions. The present study concerns the interplay between the intrinsic thermoacoustic modes and duct acoustic modes in a fundamental combustion configuration. The biglobal linear stability analysis of a ducted premixed flame is carried out by exploiting the linearized compressible reactive flow equations. The pure intrinsic thermoacoustic mode is first identified and characterized. Parameter variations are conducted to study the behaviors of eigenmode trajectories in the proximity of an exceptional point. Near the exceptional point, trajectory veering and mode switching are observed. We find that different parameter variation paths can lead to inconsistent results in mode origin identification. Hence, we propose to characterize the thermoacoustic modes based on the flow structures. Specifically, the Helmholtz decomposition is employed to extract the potential and solenoidal components of the thermoacoustic modes and we find the intrinsic thermoacoustic modes and duct acoustic modes exhibit distinct decomposed flow structures.
Chen et al. (Wed,) studied this question.