POD and DMD analysis of dynamic flow structures in the recirculation region of an unconfined swirl cup

The flow field of the swirl-stabilized combustor plays a significant role in fuel atomization and flame stability. The experimental investigation of the non-reacting flow field downstream of a swirl cup with no confinement is carried out by means of particle image velocimetry measurements. The statistical uncertainty is calculated to evaluate the turbulence convergence and projection errors. The flow fields provide a compelling picture of the basic characteristics of the swirl flow, while the root mean square velocity analysis illustrates the upward and downward fluctuations of the emanating jet. The proper orthogonal decomposition (POD) modes reveal the most pronounced features of the flow, namely the central recirculation zone and the precessing vortex core (PVC) at its boundaries, as well as a significant feature that occurs several times in the modes, i.e., the entrainment of the surrounding atmosphere as an alternative to the corner recirculation zone. Furthermore, the dynamic mode decomposition (DMD) modes in the low-frequency region characterize the slow change ($S_t=0.0026$) that occurs when the emanating jet is shifted upward as well as the PVC oscillations ($S_t=0.113$) in the flow. The DMD modes in the high-frequency then characterize the high-frequency oscillations induced by vortex shedding in the swirl flow. The research is helping to provide a clear picture of the flow downstream of the swirl cup without any confinement.