Linear stability of reacting swirling shear flows.

Many modern combustion systems leverage the phenomenon of vortex breakdown for flame stabilization and mixing enhancement. Yet, vortex breakdown introduces shear flow features which may promote other flow instabilities, manifested by helical vortex shedding, precession of the swirling vortex core, and full-blown combustion instability. Theoretical studies of incompressible flows have revealed the nature of these bifurcations and used linear stability analysis to predict their occurrence. However, the role of combustion in these dynamics is not clear. Flame-induced baroclinity and compressibility explicitly modifies the flow’s vorticity distribution, alters interactions among regions of concentrated vorticity, and couples the thermodynamic and hydrodynamic states. This numerical investigation explores the linear stability of normal disturbance modes with respect to a model axisymmetric swirling base flow. The results are interpreted in terms of the asymptotic spatiotemporal behavior of the disturbance modes and a budget of terms in the vorticity equations is discussed and compared with the incompressible flow case.