In this paper, transitions from convective to absolute instability in post-vortex-breakdown swirling flow with nonuniform density are investigated using linear stability analysis. Addition of swirl to flow in devices of practical interest achieves rapid mixing, but also renders such flows absolutely unstable, leading to self-excited flow oscillations. Through the spatiotemporal analysis, the onset of absolute instability is systematically analyzed in parametric space for uniform and nonuniform density cases. Five unstable azimuthal wavenumbers (, , ) studied show that, although mode first becomes absolutely unstable, it is the mode associated with the precessing vortex core that turns out to be more unstable for high swirl numbers and reduced shear layer thickness. The imposition of density field, representative of reacting flows, suppresses the absolute instability mode by shifting its onset to higher backflow velocities. Further, to investigate the cause of instabilities, a total disturbance energy equation is obtained with various energy feeding/depleting terms. Although the axial shear is the dominant production term, the alignment of pressure redistribution and density variation terms provides the necessary stabilization to weaken the absolute instabilities.
Mechanisms for Absolute Instabilities in Uniform and Nonuniform Density Swirling Flows
AIAA Journal ; 58 , 3 ; 1056-1067
2019-12-24
12 pages
Article (Journal)
Electronic Resource
English
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