Disturbance-field decomposition in a transversely forced swirl flow and flame

High-amplitude combustion instabilities are a destructive and pervasive problem in gas-turbine combustors. Although much research has focused on measuring the characteristics of these instabilities, there are still many remaining questions about the fluid-mechanic mechanisms that drive the flame oscillations. In particular, a variety of complex disturbance mechanisms arise during velocity-coupled instabilities excited by transverse acoustic modes. The resulting disturbance field has two components: The acoustic-velocity fluctuation from both the incident transverse acoustic field and the excited longitudinal field near the nozzle, and the vortical-velocity fluctuations arising from acoustic excitation of hydrodynamic instabilities in the flow. In this research, the relative contribution of these two components had been explored using proper orthogonal decomposition as a methodology for decomposing the velocity-disturbance field. Although proper orthogonal decomposition is successful at decomposing these two components at certain nonreacting conditions, it fails at reacting conditions. These results show the significant interaction of velocity-disturbance modes under reacting conditions and the limitations of the proper-orthogonaldecomposition technique for extracting velocity-decomposition information.