Coherent structures in unsteady swirling jet flow
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An LDA technique and phase-averaging analysis were used to study unsteady precessing flow in a model vortex burner. Detailed measurements were made for Re=15,000 and S=1.01. On the basis of the analysis of phase-averaged data and vortex detection by the λ2-technique of Joeng and Hussain (1995), three precessing spiral vortex structures were identified: primary vortex (PV), inner secondary vortex (ISV), and outer secondary vortex (OSV). The PV is the primary and most powerful structure as it includes primary vorticity generated by the swirler; the ISV and OSV are considered here as secondary vortical structures. The jet breakdown zone is the conjunction of a pair of co-rotating co-winding spiral vortices, PV and ISV. The interesting new feature described is that the secondary vortices form a three-dimensional vortex dipole with a helical geometry. The effect of coupling of secondary vortices was suggested as a mechanism of enhanced stability reflected in their increased axial extent.
KeywordsVortex Vorticity Nozzle Exit Vortical Structure Secondary Vortex
The authors are pleased to acknowledge support from the Portuguese Science and Technology Foundation (through Research Grant POCTI/34768/EME/1999 and Research Fellowship SFRH/BPD/1641/2000 provided for S.I. Shtork). The help of Mr. Eduardo Bimba in assembling the experimental setup is also gratefully appreciated.
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