Abstract
The precessing vortex core (PVC) is a self-excited flow oscillation state occurring in swirl nozzles. This is caused by the presence of a marginally unstable hydrodynamic mode that induces precession of the vortex breakdown bubble (VBB) around the flow axis. We examine the impact of a centrebody on PVC dynamics in a non-reacting flow in a swirl nozzle combustor. Time resolved high speed stereoscopic PIV measurements are performed for two swirl numbers, S=0.67 and 1.17 and three centrebody diameters, 9.5mm, 4.73mm and 0 (i.e. no centrebody). The bulk flow velocity at the nozzle exit is kept constant as Ub=8m/s for all cases (Re~20,000). The data is analyzed using a new modal decomposition technique that combines the wavelet transform and proper orthogonal decomposition (WPOD). This gives insight into globally intermittent flow dynamics. A coherent PVC is present in the flow when there is no centrebody. Introducing a centrebody makes the PVC oscillations intermittent. The WPOD results show two qualitatively different intermittent behaviours at S=0.67 and 1.17. For S=0.67, the axial position of the VBB suggests that turbulence destabilizes the PVC mode by causing intermittent separation of the VBB and centrebody wake, resulting in PVC oscillations. For S=1.17, the VBB engulfs the centrebody and stabilizes the PVC mode. Therefore, in this case, PVC oscillations appear to be the flow response to broadband stochastic forcing of the time averaged flow by turbulence.